COMPRESSION OF DECOMPOSED REPRESENTATIONS OF A SOUND FIELD

Description

[0001] This application claims the benefit of U.S. Provisional Application No. 61 / 828,445 filed 29 May 2013, U.S. Provisional Application No. 61 / 829,791 filed 31 May 2013, U.S. Provisional Application No. 61 / 899,034 filed 1 November 2013, U.S. Provisional Application No 61 / 899,041 filed 1 November 2013, U.S. Provisional Application No. 61 / 829,182 filed 30 May 2013, U.S. Provisional Application No. 61 / 829,174 filed 30 May 2013, U.S. Provisional Application No. 61 / 829,155 filed 30 May 2013, U.S. Provisional Application No. 61 / 933,706 filed 30 January 2014, U.S. Provisional Application No. 61 / 829,846 filed 31 May 2013, U.S. Provisional Application No. 61 / 886,605 filed 3 October 2013, U.S. Provisional Application No. 61 / 886,617 filed 3 October 2013, U.S. Provisional Application No. 61 / 925,158 filed 8 January 2014, U.S. Provisional Application No. 61 / 933,721 filed 30 January 2014, U.S. Provisional Application No. 61 / 925,074 filed 8 January 2014, U.S. Provisional Application No. 61 / 925,112 filed 8 January 2014, U.S. Provisional Application No. 61 / 925,126 filed 8 January 2014, U.S. Provisional Application No. 62 / 003,515 filed 27 May 2014, and U.S. Provisional Application No. 61 / 828,615 filed 29 May 2013.TECHNICAL FIELD

[0002] This disclosure relate to audio data and, more specifically, compression of audio data.BACKGROUND

[0003] A higher order ambisonics (HOA) signal (often represented by a plurality of spherical harmonic coefficients (SHC) or other hierarchical elements) is a three-dimensional representation of a soundfield. This HOA or SHC representation may represent this soundfield in a manner that is independent of the local speaker geometry used to playback a multi-channel audio signal rendered from this SHC signal. This SHC signal may also facilitate backwards compatibility as this SHC signal may be rendered to well-known and highly adopted multi-channel formats, such as a 5.1 audio channel format or a 7.1 audio channel format. The SHC representation may therefore enable a better representation of a soundfield that also accommodates backward compatibility. EP2469741 A1 discloses Principal Component Analysis (PCA) of a sound field representation in order to compute principal and ambient sound components.SUMMARY

[0004] In general, techniques are described for compression and decompression of higher order ambisonic audio data. The invention is set forth by the appended claims.

[0005] In one aspect, a method comprises obtaining one or more first vectors describing distinct components of the soundfield and one or more second vectors describing background components of the soundfield, both the one or more first vectors and the one or more second vectors generated at least by performing a transformation with respect to the plurality of spherical harmonic coefficients.

[0006] In another aspect, a device comprises one or more processors configured to determine one or more first vectors describing distinct components of the soundfield and one or more second vectors describing background components of the soundfield, both the one or more first vectors and the one or more second vectors generated at least by performing a transformation with respect to the plurality of spherical harmonic coefficients.

[0007] In another aspect, a device comprises means for obtaining one or more first vectors describing distinct components of the soundfield and one or more second vectors describing background components of the soundfield, both the one or more first vectors and the one or more second vectors generated at least by performing a transformation with respect to the plurality of spherical harmonic coefficients, and means for storing the one or more first vectors.

[0008] In another aspect, a non-transitory computer-readable storage medium has stored thereon instructions that, when executed, cause one or more processors to obtain one or more first vectors describing distinct components of the soundfield and one or more second vectors describing background components of the soundfield, both the one or more first vectors and the one or more second vectors generated at least by performing a transformation with respect to the plurality of spherical harmonic coefficients.

[0009] In another aspect, a method comprises selecting one of a plurality of decompression schemes based on the indication of whether an compressed version of spherical harmonic coefficients representative of a sound field are generated from a synthetic audio object, and decompressing the compressed version of the spherical harmonic coefficients using the selected one of the plurality of decompression schemes.

[0010] In another aspect, a device comprises one or more processors configured to select one of a plurality of decompression schemes based on the indication of whether an compressed version of spherical harmonic coefficients representative of a sound field are generated from a synthetic audio object, and decompress the compressed version of the spherical harmonic coefficients using the selected one of the plurality of decompression schemes.

[0011] In another aspect, a device comprises means for selecting one of a plurality of decompression schemes based on the indication of whether an compressed version of spherical harmonic coefficients representative of a sound field are generated from a synthetic audio object, and means for decompressing the compressed version of the spherical harmonic coefficients using the selected one of the plurality of decompression schemes.

[0012] In another aspect, a non-transitory computer-readable storage medium has stored thereon instructions that, when executed, cause one or more processors of an integrated decoding device to select one of a plurality of decompression schemes based on the indication of whether an compressed version of spherical harmonic coefficients representative of a sound field are generated from a synthetic audio object, and decompress the compressed version of the spherical harmonic coefficients using the selected one of the plurality of decompression schemes.

[0013] In another aspect, a method comprises obtaining an indication of whether spherical harmonic coefficients representative of a sound field are generated from a synthetic audio object.

[0014] In another aspect, a device comprises one or more processors configured to obtain an indication of whether spherical harmonic coefficients representative of a sound field are generated from a synthetic audio object.

[0015] In another aspect, a device comprises means for storing spherical harmonic coefficients representative of a sound field, and means for obtaining an indication of whether the spherical harmonic coefficients are generated from a synthetic audio object.

[0016] In another aspect, anon-transitory computer-readable storage medium has stored thereon instructions that, when executed, cause one or more processors to obtain an indication of whether spherical harmonic coefficients representative of a sound field are generated from a synthetic audio object.

[0017] In another aspect, a method comprises quantizing one or more first vectors representative of one or more components of a sound field, and compensating for error introduced due to the quantization of the one or more first vectors in one or more second vectors that are also representative of the same one or more components of the sound field.

[0018] In another aspect, a device comprises one or more processors configured to quantize one or more first vectors representative of one or more components of a sound field, and compensate for error introduced due to the quantization of the one or more first vectors in one or more second vectors that are also representative of the same one or more components of the sound field.

[0019] In another aspect, a device comprises means for quantizing one or more first vectors representative of one or more components of a sound field, and means for compensating for error introduced due to the quantization of the one or more first vectors in one or more second vectors that are also representative of the same one or more components of the sound field.

[0020] In another aspect, a non-transitory computer-readable storage medium has stored thereon instructions that, when executed, cause one or more processors to quantize one or more first vectors representative of one or more components of a sound field, and compensate for error introduced due to the quantization of the one or more first vectors in one or more second vectors that are also representative of the same one or more components of the sound field.

[0021] In another aspect, a method comprises performing, based on a target bitrate, order reduction with respect to a plurality of spherical harmonic coefficients or decompositions thereof to generate reduced spherical harmonic coefficients or the reduced decompositions thereof, wherein the plurality of spherical harmonic coefficients represent a sound field.

[0022] In another aspect, a device comprises one or more processors configured to perform, based on a target bitrate, order reduction with respect to a plurality of spherical harmonic coefficients or decompositions thereof to generate reduced spherical harmonic coefficients or the reduced decompositions thereof, wherein the plurality of spherical harmonic coefficients represent a sound field.

[0023] In another aspect, a device comprises means for storing a plurality of spherical harmonic coefficients or decompositions thereof, and means for performing, based on a target bitrate, order reduction with respect to the plurality of spherical harmonic coefficients or decompositions thereof to generate reduced spherical harmonic coefficients or the reduced decompositions thereof, wherein the plurality of spherical harmonic coefficients represent a sound field.

[0024] In another aspect, a non-transitory computer-readable storage medium has stored thereon instructions that, when executed, cause one or more processors to perform, based on a target bitrate, order reduction with respect to a plurality of spherical harmonic coefficients or decompositions thereof to generate reduced spherical harmonic coefficients or the reduced decompositions thereof, wherein the plurality of spherical harmonic coefficients represent a sound field.

[0025] In another aspect, a method comprises obtaining a first non-zero set of coefficients of a vector that represent a distinct component of the sound field, the vector having been decomposed from a plurality of spherical harmonic coefficients that describe a sound field.

[0026] In another aspect, a device comprises one or more processors configured to obtain a first non-zero set of coefficients of a vector that represent a distinct component of a sound field, the vector having been decomposed from a plurality of spherical harmonic coefficients that describe the sound field.

[0027] In another aspect, a device comprises means for obtaining a first non-zero set of coefficients of a vector that represent a distinct component of a sound field, the vector having been decomposed from a plurality of spherical harmonic coefficients that describe the sound field, and means for storing the first non-zero set of coefficients.

[0028] In another aspect, a non-transitory computer-readable storage medium has stored thereon instructions that, when executed, cause one or more processors to determine a first non-zero set of coefficients of a vector that represent a distinct component of a sound field, the vector having been decomposed from a plurality of spherical harmonic coefficients that describe the sound field.

[0029] In another aspect, a method comprises obtaining, from a bitstream, at least one of one or more vectors decomposed from spherical harmonic coefficients that were recombined with background spherical harmonic coefficients, wherein the spherical harmonic coefficients describe a sound field, and wherein the background spherical harmonic coefficients described one or more background components of the same sound field.

[0030] In another aspect, a device comprises one or more processors configured to determine, from a bitstream, at least one of one or more vectors decomposed from spherical harmonic coefficients that were recombined with background spherical harmonic coefficients, wherein the spherical harmonic coefficients describe a sound field, and wherein the background spherical harmonic coefficients described one or more background components of the same sound field.

[0031] In another aspect, a device comprises means for obtaining , from a bitstream, at least one of one or more vectors decomposed from spherical harmonic coefficients that were recombined with background spherical harmonic coefficients, wherein the spherical harmonic coefficients describe a sound field, and wherein the background spherical harmonic coefficients described one or more background components of the same sound field.

[0032] In another aspect, a non-transitory computer-readable storage medium has stored thereon instructions that, when executed, cause one or more processors to obtain, from a bitstream, at least one of one or more vectors decomposed from spherical harmonic coefficients that were recombined with background spherical harmonic coefficients, wherein the spherical harmonic coefficients describe a sound field, and wherein the background spherical harmonic coefficients described one or more background components of the same sound field.

[0033] In another aspect, a method comprises identifying one or more distinct audio objects from one or more spherical harmonic coefficients (SHC) associated with the audio objects based on a directionality determined for one or more of the audio objects.

[0034] In another aspect, a device comprises one or more processors configured to identify one or more distinct audio objects from one or more spherical harmonic coefficients (SHC) associated with the audio objects based on a directionality determined for one or more of the audio objects.

[0035] In another aspect, a device comprises means for storing one or more spherical harmonic coefficients (SHC), and means for identifying one or more distinct audio objects from the one or more spherical harmonic coefficients (SHC) associated with the audio objects based on a directionality determined for one or more of the audio objects.

[0036] In another aspect, a non-transitory computer-readable storage medium has stored thereon instructions that, when executed, cause one or more processors to identify one or more distinct audio objects from one or more spherical harmonic coefficients (SHC) associated with the audio objects based on a directionality determined for one or more of the audio objects.

[0037] In another aspect, a method comprises performing a vector-based synthesis with respect to a plurality of spherical harmonic coefficients to generate decomposed representations of the plurality of spherical harmonic coefficients representative of one or more audio objects and corresponding directional information, wherein the spherical harmonic coefficients are associated with an order and describe a sound field, determining distinct and background directional information from the directional information, reducing an order of the directional information associated with the background audio objects to generate transformed background directional information, applying compensation to increase values of the transformed directional information to preserve an overall energy of the sound field.

[0038] In another aspect, a device comprises one or more processors configured to perform a vector-based synthesis with respect to a plurality of spherical harmonic coefficients to generate decomposed representations of the plurality of spherical harmonic coefficients representative of one or more audio objects and corresponding directional information, wherein the spherical harmonic coefficients are associated with an order and describe a sound field, determine distinct and background directional information from the directional information, reduce an order of the directional information associated with the background audio objects to generate transformed background directional information, apply compensation to increase values of the transformed directional information to preserve an overall energy of the sound field.

[0039] In another aspect, a device comprises means for performing a vector-based synthesis with respect to a plurality of spherical harmonic coefficients to generate decomposed representations of the plurality of spherical harmonic coefficients representative of one or more audio objects and corresponding directional information, wherein the spherical harmonic coefficients are associated with an order and describe a sound field, means for determining distinct and background directional information from the directional information, means for reducing an order of the directional information associated with the background audio objects to generate transformed background directional information, and means for applying compensation to increase values of the transformed directional information to preserve an overall energy of the sound field.

[0040] In another aspect, a non-transitory computer-readable storage medium has stored thereon instructions that, when executed, cause one or more processors to perform a vector-based synthesis with respect to a plurality of spherical harmonic coefficients to generate decomposed representations of the plurality of spherical harmonic coefficients representative of one or more audio objects and corresponding directional information, wherein the spherical harmonic coefficients are associated with an order and describe a sound field, determine distinct and background directional information from the directional information, reduce an order of the directional information associated with the background audio objects to generate transformed background directional information, and apply compensation to increase values of the transformed directional information to preserve an overall energy of the sound field.

[0041] In another aspect, a method comprises obtaining decomposed interpolated spherical harmonic coefficients for a time segment by, at least in part, performing an interpolation with respect to a first decomposition of a first plurality of spherical harmonic coefficients and a second decomposition of a second plurality of spherical harmonic coefficients.

[0042] In another aspect, a device comprises one or more processors configured to obtain decomposed interpolated spherical harmonic coefficients for a time segment by, at least in part, performing an interpolation with respect to a first decomposition of a first plurality of spherical harmonic coefficients and a second decomposition of a second plurality of spherical harmonic coefficients.

[0043] In another aspect, a device comprises means for storing a first plurality of spherical harmonic coefficients and a second plurality of spherical harmonic coefficients, and means for obtain decomposed interpolated spherical harmonic coefficients for a time segment by, at least in part, performing an interpolation with respect to a first decomposition of the first plurality of spherical harmonic coefficients and the second decomposition of a second plurality of spherical harmonic coefficients.

[0044] In another aspect, a non-transitory computer-readable storage medium has stored thereon instructions that, when executed, cause one or more processors to obtain decomposed interpolated spherical harmonic coefficients for a time segment by, at least in part, performing an interpolation with respect to a first decomposition of a first plurality of spherical harmonic coefficients and a second decomposition of a second plurality of spherical harmonic coefficients.

[0045] In another aspect, a method comprises obtaining a bitstream comprising a compressed version of a spatial component of a sound field, the spatial component generated by performing a vector based synthesis with respect to a plurality of spherical harmonic coefficients.

[0046] In another aspect, a device comprises one or more processors configured to obtain a bitstream comprising a compressed version of a spatial component of a sound field, the spatial component generated by performing a vector based synthesis with respect to a plurality of spherical harmonic coefficients.

[0047] In another aspect, a device comprises means for obtaining a bitstream comprising a compressed version of a spatial component of a sound field, the spatial component generated by performing a vector based synthesis with respect to a plurality of spherical harmonic coefficients, and means for storing the bitstream.

[0048] In another aspect, a non-transitory computer-readable storage medium has stored thereon instructions that when executed cause one or more processors to obtain a bitstream comprising a compressed version of a spatial component of a sound field, the spatial component generated by performing a vector based synthesis with respect to a plurality of spherical harmonic coefficients.

[0049] In another aspect, a method comprises generating a bitstream comprising a compressed version of a spatial component of a sound field, the spatial component generated by performing a vector based synthesis with respect to a plurality of spherical harmonic coefficients.

[0050] In another aspect, a device comprises one or more processors configured to generate a bitstream comprising a compressed version of a spatial component of a sound field, the spatial component generated by performing a vector based synthesis with respect to a plurality of spherical harmonic coefficients.

[0051] In another aspect, a device comprises means for generating a bitstream comprising a compressed version of a spatial component of a sound field, the spatial component generated by performing a vector based synthesis with respect to a plurality of spherical harmonic coefficients, and means for storing the bitstream.

[0052] In another aspect, a non-transitory computer-readable storage medium has instructions that when executed cause one or more processors to generate a bitstream comprising a compressed version of a spatial component of a sound field, the spatial component generated by performing a vector based synthesis with respect to a plurality of spherical harmonic coefficients.

[0053] In another aspect, a method comprises identifying a Huffman codebook to use when decompressing a compressed version of a spatial component of a plurality of compressed spatial components based on an order of the compressed version of the spatial component relative to remaining ones of the plurality of compressed spatial components, the spatial component generated by performing a vector based synthesis with respect to a plurality of spherical harmonic coefficients.

[0054] In another aspect, a device comprises one or more processors configured to identify a Huffman codebook to use when decompressing a compressed version of a spatial component of a plurality of compressed spatial components based on an order of the compressed version of the spatial component relative to remaining ones of the plurality of compressed spatial components, the spatial component generated by performing a vector based synthesis with respect to a plurality of spherical harmonic coefficients.

[0055] In another aspect, a device comprises means for identifying a Huffman codebook to use when decompressing a compressed version of a spatial component of a plurality of compressed spatial components based on an order of the compressed version of the spatial component relative to remaining ones of the plurality of compressed spatial components, the spatial component generated by performing a vector based synthesis with respect to a plurality of spherical harmonic coefficients, and means for string the plurality of compressed spatial components.

[0056] In another aspect, a non-transitory computer-readable storage medium has stored thereon instructions that when executed cause one or more processors to identify a Huffman codebook to use when decompressing a spatial component of a plurality of spatial components based on an order of the spatial component relative to remaining ones of the plurality of spatial components, the spatial component generated by performing a vector based synthesis with respect to a plurality of spherical harmonic coefficients.

[0057] In another aspect, a method comprises identifying a Huffman codebook to use when compressing a spatial component of a plurality of spatial components based on an order of the spatial component relative to remaining ones of the plurality of spatial components, the spatial component generated by performing a vector based synthesis with respect to a plurality of spherical harmonic coefficients.

[0058] In another aspect, a device comprises one or more processors configured to identify a Huffman codebook to use when compressing a spatial component of a plurality of spatial components based on an order of the spatial component relative to remaining ones of the plurality of spatial components, the spatial component generated by performing a vector based synthesis with respect to a plurality of spherical harmonic coefficients.

[0059] In another aspect, a device comprises means for storing a Huffman codebook, and means for identifying the Huffman codebook to use when compressing a spatial component of a plurality of spatial components based on an order of the spatial component relative to remaining ones of the plurality of spatial components, the spatial component generated by performing a vector based synthesis with respect to a plurality of spherical harmonic coefficients.

[0060] In another aspect, a non-transitory computer-readable storage medium has stored thereon instructions that, when executed, cause one or more processors to identify a Huffman codebook to use when compressing a spatial component of a plurality of spatial components based on an order of the spatial component relative to remaining ones of the plurality of spatial components, the spatial component generated by performing a vector based synthesis with respect to a plurality of spherical harmonic coefficients.

[0061] In another aspect, a method comprises determining a quantization step size to be used when compressing a spatial component of a sound field, the spatial component generated by performing a vector based synthesis with respect to a plurality of spherical harmonic coefficients.

[0062] In another aspect, a device comprises one or more processors configured to determine a quantization step size to be used when compressing a spatial component of a sound field, the spatial component generated by performing a vector based synthesis with respect to a plurality of spherical harmonic coefficients.

[0063] In another aspect, a device comprises means for determining a quantization step size to be used when compressing a spatial component of a sound field, the spatial component generated by performing a vector based synthesis with respect to a plurality of spherical harmonic coefficients, and means for storing the quantization step size.

[0064] In another aspect, a non-transitory computer-readable storage medium has stored thereon instructions that when executed cause one or more processors to determine a quantization step size to be used when compressing a spatial component of a sound field, the spatial component generated by performing a vector based synthesis with respect to a plurality of spherical harmonic coefficients.

[0065] The details of one or more aspects of the techniques are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of these techniques will be apparent from the description and drawings, and from the claims.BRIEF DESCRIPTION OF DRAWINGS

[0066] FIGS. 1 and 2 are diagrams illustrating spherical harmonic basis functions of various orders and sub-orders. FIG. 3 is a diagram illustrating a system that may perform various aspects of the techniques described in this disclosure. FIG. 4 is a block diagram illustrating, in more detail, one example of the audio encoding device shown in the example of FIG. 3 that may perform various aspects of the techniques described in this disclosure. FIG. 5 is a block diagram illustrating the audio decoding device of FIG. 3 in more detail. FIG. 6 is a flowchart illustrating exemplary operation of a content analysis unit of an audio encoding device in performing various aspects of the techniques described in this disclosure. FIG. 7 is a flowchart illustrating exemplary operation of an audio encoding device in performing various aspects of the vector-based synthesis techniques described in this disclosure. FIG. 8 is a flow chart illustrating exemplary operation of an audio decoding device in performing various aspects of the techniques described in this disclosure. FIGS. 9A-9L are block diagrams illustrating various aspects of the audio encoding device of the example of FIG. 4 in more detail. FIGS. 10A-10O(ii) are diagrams illustrating a portion of the bitstream or side channel information that may specify the compressed spatial components in more detail. FIGS. 11A-11G are block diagrams illustrating, in more detail, various units of the audio decoding device shown in the example of FIG. 5. FIG. 12 is a diagram illustrating an example audio ecosystem that may perform various aspects of the techniques described in this disclosure. FIG. 13 is a diagram illustrating one example of the audio ecosystem of FIG. 12 in more detail. FIG. 14 is a diagram illustrating one example of the audio ecosystem of FIG. 12 in more detail. FIGS. 15A and 15B are diagrams illustrating other examples of the audio ecosystem of FIG. 12 in more detail. FIG. 16 is a diagram illustrating an example audio encoding device that may perform various aspects of the techniques described in this disclosure. FIG. 17 is a diagram illustrating one example of the audio encoding device of FIG. 16 in more detail. FIG. 18 is a diagram illustrating an example audio decoding device that may perform various aspects of the techniques described in this disclosure. FIG. 19 is a diagram illustrating one example of the audio decoding device of FIG. 18 in more detail. FIGS. 20A-20G are diagrams illustrating example audio acquisition devices that may perform various aspects of the techniques described in this disclosure. FIGS. 21A-21E are diagrams illustrating example audio playback devices that may perform various aspects of the techniques described in this disclosure. FIGS. 22A-22H are diagrams illustrating example audio playback environments in accordance with one or more techniques described in this disclosure. FIG. 23 is a diagram illustrating an example use case where a user may experience a 3D soundfield of a sports game while wearing headphones in accordance with one or more techniques described in this disclosure. FIG. 24 is a diagram illustrating a sports stadium at which a 3D soundfield may be recorded in accordance with one or more techniques described in this disclosure. FIG. 25 is a flow diagram illustrating a technique for rendering a 3D soundfield based on a local audio landscape in accordance with one or more techniques described in this disclosure. FIG. 26 is a diagram illustrating an example game studio in accordance with one or more techniques described in this disclosure. FIG. 27 is a diagram illustrating a plurality game systems which include rendering engines in accordance with one or more techniques described in this disclosure. FIG. 28 is a diagram illustrating a speaker configuration that may be simulated by headphones in accordance with one or more techniques described in this disclosure. FIG. 29 is a diagram illustrating a plurality of mobile devices which may be used to acquire and / or edit a 3D soundfield in accordance with one or more techniques described in this disclosure. FIG. 30 is a diagram illustrating a video frame associated with a 3D soundfield which may be processed in accordance with one or more techniques described in this disclosure. FIGS. 31A-31M are diagrams illustrating graphs showing various simulation results of performing synthetic or recorded categorization of the soundfield in accordance with various aspects of the techniques described in this disclosure. FIG. 32 is a diagram illustrating a graph of singular values from an S matrix decomposed from higher order ambisonic coefficients in accordance with the techniques described in this disclosure. FIGS. 33A and 33B are diagrams illustrating respective graphs showing a potential impact reordering has when encoding the vectors describing foreground components of the soundfield in accordance with the techniques described in this disclosure. FIGS. 34 and 35 are conceptual diagrams illustrating differences between solely energy-based and directionality-based identification of distinct audio objects, in accordance with this disclosure. FIGS. 36A-36G are diagrams illustrating projections of at least a portion of decomposed version of spherical harmonic coefficients into the spatial domain so as to perform interpolation in accordance with various aspects of the techniques described in this disclosure. FIG. 37 illustrates a representation of techniques for obtaining a spatio-temporal interpolation as described herein. FIG. 38 is a block diagram illustrating artificial US matrices, US 1 and US 2 , for sequential SVD blocks for a multi-dimensional signal according to techniques described herein. FIG. 39 is a block diagram illustrating decomposition of subsequent frames of a higher-order ambisonics (HOA) signal using Singular Value Decomposition and smoothing of the spatio-temporal components according to techniques described in this disclosure. FIGS. 40A-40J are each a block diagram illustrating example audio encoding devices that may perform various aspects of the techniques described in this disclosure to compress spherical harmonic coefficients describing two or three dimensional soundfields. FIG. 41A-41D are block diagrams each illustrating an example audio decoding device that may perform various aspects of the techniques described in this disclosure to decode spherical harmonic coefficients describing two or three dimensional soundfields. FIGS. 42A-42C are each block diagrams illustrating the order reduction unit shown in the examples of FIGS. 40B-40J in more detail. FIG. 43 is a diagram illustrating the V compression unit shown in FIG. 40I in more detail. FIG. 44 is a diagram illustration exemplary operations performed by the audio encoding device to compensate for quantization error in accordance with various aspects of the techniques described in this disclosure. FIGS. 45A and 45B are diagrams illustrating interpolation of sub-frames from portions of two frames in accordance with various aspects of the techniques described in this disclosure. FIGS. 46A-46E are diagrams illustrating a cross section of a projection of one or more vectors of a decomposed version of a plurality of spherical harmonic coefficients having been interpolated in accordance with the techniques described in this disclosure. FIG. 47 is a block diagram illustrating, in more detail, the extraction unit of the audio decoding devices shown in the examples FIGS. 41A-41D. FIG. 48 is a block diagram illustrating the audio rendering unit of the audio decoding device shown in the examples of FIGS. 41A -41D in more detail. FIGS. 49A-49E(ii) are diagrams illustrating respective audio coding systems that may implement various aspects of the techniques described in this disclosure. FIGS. 50A and 50B are block diagrams each illustrating one of two different approaches to potentially reduce the order of background content in accordance with the techniques described in this disclosure. FIG. 51 is a block diagram illustrating examples of a distinct component compression path of an audio encoding device that may implement various aspects of the techniques described in this disclosure to compress spherical harmonic coefficients. FIGS. 52 is a block diagram illustrating another example of an audio decoding device that may implement various aspects of the techniques described in this disclosure to reconstruct or nearly reconstruct spherical harmonic coefficients (SHC). FIG. 53 is a block diagram illustrating another example of an audio encoding device that may perform various aspects of the techniques described in this disclosure. FIG. 54 is a block diagram illustrating, in more detail, an example implementation of the audio encoding device shown in the example of FIG. 53. FIGS. 55A and 55B are diagrams illustrating an example of performing various aspects of the techniques described in this disclosure to rotate a soundfield. FIG. 56 is a diagram illustrating an example soundfield captured according to a first frame of reference that is then rotated in accordance with the techniques described in this disclosure to express the soundfield in terms of a second frame of reference. FIGS. 57A-57E are each a diagram illustrating bitstreams formed in accordance with the techniques described in this disclosure. FIG. 58 is a flowchart illustrating example operation of the audio encoding device shown in the example of FIG. 53 in implementing the rotation aspects of the techniques described in this disclosure. FIG. 59 is a flowchart illustrating example operation of the audio encoding device shown in the example of FIG. 53 in performing the transformation aspects of the techniques described in this disclosure. DETAILED DESCRIPTION

[0067] The evolution of surround sound has made available many output formats for entertainment nowadays. Examples of such consumer surround sound formats are mostly 'channel' based in that they implicitly specify feeds to loudspeakers in certain geometrical coordinates. These include the popular 5.1 format (which includes the following six channels: front left (FL), front right (FR), center or front center, back left or surround left, back right or surround right, and low frequency effects (LFE)), the growing 7.1 format, various formats that includes height speakers such as the 7.1.4 format and the 22.2 format (e.g., for use with the Ultra High Definition Television standard). Non-consumer formats can span any number of speakers (in symmetric and non-symmetric geometries) often termed 'surround arrays'. One example of such an array includes 32 loudspeakers positioned on co-ordinates on the corners of a truncated icosohedron.

[0068] The input to a future MPEG encoder is optionally one of three possible formats: (i) traditional channel-based audio (as discussed above), which is meant to be played through loudspeakers at pre-specified positions; (ii) object-based audio, which involves discrete pulse-code-modulation (PCM) data for single audio objects with associated metadata containing their location coordinates (amongst other information); and (iii) scene-based audio, which involves representing the soundfield using coefficients of spherical harmonic basis functions (also called "spherical harmonic coefficients" or SHC, "Higher Order Ambisonics" or HOA, and "HOA coefficients"). This future MPEG encoder may be described in more detail in a document entitled "Call for Proposals for 3D Audio," by the International Organization for Standardization / International Electrotechnical Commission (ISO) / (IEC) JTC1 / SC29 / WG11 / N13411, released January 2013 in Geneva, Switzerland, and available at http: / / mpeg.chiariglione.org / sites / default / files / files / standards / parts / docs / w13411.zip.

[0069] There are various 'surround-sound' channel-based formats in the market. They range, for example, from the 5.1 home theatre system (which has been the most successful in terms of making inroads into living rooms beyond stereo) to the 22.2 system developed by NHK (Nippon Hoso Kyokai or Japan Broadcasting Corporation). Content creators (e.g., Hollywood studios) would like to produce the soundtrack for a movie once, and not spend the efforts to remix it for each speaker configuration. Recently, Standards Developing Organizations have been considering ways in which to provide an encoding into a standardized bitstream and a subsequent decoding that is adaptable and agnostic to the speaker geometry (and number) and acoustic conditions at the location of the playback (involving a renderer).

[0070] To provide such flexibility for content creators, a hierarchical set of elements may be used to represent a soundfield. The hierarchical set of elements may refer to a set of elements in which the elements are ordered such that a basic set of lower-ordered elements provides a full representation of the modeled soundfield. As the set is extended to include higher-order elements, the representation becomes more detailed, increasing resolution.

[0071] One example of a hierarchical set of elements is a set of spherical harmonic coefficients (SHC). The following expression demonstrates a description or representation of a soundfield using SHC: p i t r r θ r φ r = ∑ ω = 0 ∞ 4 π ∑ n = 0 ∞ j n kr r ∑ m = − n n A n m k Y n m θ r φ r e jωt ,

[0072] This expression shows that the pressure p i at any point {r r , θ r , φ r } of the soundfield, at time t, can be represented uniquely by the SHC, A n m k . Here, k = ω c , c is the speed of sound (~343 m / s), {r r , θ r , φ r } is a point of reference (or observation point), j n (·) is the spherical Bessel function of order n, and Y n m θ r φ r are the spherical harmonic basis functions of order n and suborder m. It can be recognized that the term in square brackets is a frequency-domain representation of the signal (i.e., S(ω),r r , θ r , φ r )) which can be approximated by various time-frequency transformations, such as the discrete Fourier transform (DFT), the discrete cosine transform (DCT), or a wavelet transform. Other examples of hierarchical sets include sets of wavelet transform coefficients and other sets of coefficients of multiresolution basis functions.

[0073] FIG. 1 is a diagram illustrating spherical harmonic basis functions from the zero order (n = 0) to the fourth order (n = 4). As can be seen, for each order, there is an expansion of suborders m which are shown but not explicitly noted in the example of FIG. 1 for ease of illustration purposes.

[0074] FIG. 2 is another diagram illustrating spherical harmonic basis functions from the zero order (n = 0) to the fourth order (n = 4). In FIG. 2, the spherical harmonic basis functions are shown in three-dimensional coordinate space with both the order and the suborder shown.

[0075] The SHC A n m k can either be physically acquired (e.g., recorded) by various microphone array configurations or, alternatively, they can be derived from channel-based or object-based descriptions of the soundfield. The SHC represent scene-based audio, where the SHC may be input to an audio encoder to obtain encoded SHC that may promote more efficient transmission or storage. For example, a fourth-order representation involving (1+4) 2< (25, and hence fourth order) coefficients may be used.

[0076] As noted above, the SHC may be derived from a microphone recording using a microphone. Various examples of how SHC may be derived from microphone arrays are described in Poletti, M., "Three-Dimensional Surround Sound Systems Based on Spherical Harmonics," J. Audio Eng. Soc., Vol. 53, No. 11, 2005 November, pp. 1004-1025.

[0077] To illustrate how these SHCs may be derived from an object-based description, consider the following equation. The coefficients A n m k for the soundfield corresponding to an individual audio object may be expressed as: A n m k = g ω − 4 πik h n 2 kr s Y n m * θ s φ s , where i is − 1 , h n 2 ⋅ is the spherical Hankel function (of the second kind) of order n, and {r s , θ s , φ s } is the location of the object. Knowing the object source energy g(ω) as a function of frequency (e.g., using time-frequency analysis techniques, such as performing a fast Fourier transform on the PCM stream) allows us to convert each PCM object and its location into the SHC A n m k . Further, it can be shown (since the above is a linear and orthogonal decomposition) that the A n m k coefficients for each object are additive. In this manner, a multitude of PCM objects can be represented by the A n m k coefficients (e.g., as a sum of the coefficient vectors for the individual objects). Essentially, these coefficients contain information about the soundfield (the pressure as a function of 3D coordinates), and the above represents the transformation from individual objects to a representation of the overall soundfield, in the vicinity of the observation point {r r , θ r , φ r }. The remaining figures are described below in the context of object-based and SHC-based audio coding.

[0078] FIG. 3 is a diagram illustrating a system 10 that may perform various aspects of the techniques described in this disclosure. As shown in the example of FIG. 3, the system 10 includes a content creator 12 and a content consumer 14. While described in the context of the content creator 12 and the content consumer 14, the techniques may be implemented in any context in which SHCs (which may also be referred to as HOA coefficients) or any other hierarchical representation of a soundfield are encoded to form a bitstream representative of the audio data. Moreover, the content creator 12 may represent any form of computing device capable of implementing the techniques described in this disclosure, including a handset (or cellular phone), a tablet computer, a smart phone, or a desktop computer to provide a few examples. Likewise, the content consumer 14 may represent any form of computing device capable of implementing the techniques described in this disclosure, including a handset (or cellular phone), a tablet computer, a smart phone, a set-top box, or a desktop computer to provide a few examples.

[0079] The content creator 12 may represent a movie studio or other entity that may generate multi-channel audio content for consumption by content consumers, such as the content consumer 14. In some examples, the content creator 12 may represent an individual user who would like to compress HOA coefficients 11. Often, this content creator generates audio content in conjunction with video content. The content consumer 14 represents an individual that owns or has access to an audio playback system, which may refer to any form of audio playback system capable of rendering SHC for play back as multi-channel audio content. In the example of FIG. 3, the content consumer 14 includes an audio playback system 16.

[0080] The content creator 12 includes an audio editing system 18. The content creator 12 obtain live recordings 7 in various formats (including directly as HOA coefficients) and audio objects 9, which the content creator 12 may edit using audio editing system 18. The content creator may, during the editing process, render HOA coefficients 11 from audio objects 9, listening to the rendered speaker feeds in an attempt to identify various aspects of the soundfield that require further editing. The content creator 12 may then edit HOA coefficients 11 (potentially indirectly through manipulation of different ones of the audio objects 9 from which the source HOA coefficients may be derived in the manner described above). The content creator 12 may employ the audio editing system 18 to generate the HOA coefficients 11. The audio editing system 18 represents any system capable of editing audio data and outputting this audio data as one or more source spherical harmonic coefficients.

[0081] When the editing process is complete, the content creator 12 may generate a bitstream 21 based on the HOA coefficients 11. That is, the content creator 12 includes an audio encoding device 20 that represents a device configured to encode or otherwise compress HOA coefficients 11 in accordance with various aspects of the techniques described in this disclosure to generate the bitstream 21. The audio encoding device 20 may generate the bitstream 21 for transmission, as one example, across a transmission channel, which may be a wired or wireless channel, a data storage device, or the like. The bitstream 21 may represent an encoded version of the HOA coefficients 11 and may include a primary bitstream and another side bitstream, which may be referred to as side channel information.

[0082] Although described in more detail below, the audio encoding device 20 may be configured to encode the HOA coefficients 11 based on a vector-based synthesis or a directional-based synthesis. To determine whether to perform the vector-based synthesis methodology or a directional-based synthesis methodology, the audio encoding device 20 may determine, based at least in part on the HOA coefficients 11, whether the HOA coefficients 11 were generated via a natural recording of a soundfield (e.g., live recording 7) or produced artificially (i.e., synthetically) from, as one example, audio objects 9, such as a PCM object. When the HOA coefficients 11 were generated form the audio objects 9, the audio encoding device 20 may encode the HOA coefficients 11 using the directional-based synthesis methodology. When the HOA coefficients 11 were captured live using, for example, an eigenmike, the audio encoding device 20 may encode the HOA coefficients 11 based on the vector-based synthesis methodology. The above distinction represents one example of where vector-based or directional-based synthesis methodology may be deployed. There may be other cases where either or both may be useful for natural recordings, artificially generated content or a mixture of the two (hybrid content). Furthermore, it is also possible to use both methodologies simultaneously for coding a single time-frame of HOA coefficients.

[0083] Assuming for purposes of illustration that the audio encoding device 20 determines that the HOA coefficients 11 were captured live or otherwise represent live recordings, such as the live recording 7, the audio encoding device 20 may be configured to encode the HOA coefficients 11 using a vector-based synthesis methodology involving application of a linear invertible transform (LIT). One example of the linear invertible transform is referred to as a "singular value decomposition" (or "SVD"). In this example, the audio encoding device 20 may apply SVD to the HOA coefficients 11 to determine a decomposed version of the HOA coefficients 11. The audio encoding device 20 may then analyze the decomposed version of the HOA coefficients 11 to identify various parameters, which may facilitate reordering of the decomposed version of the HOA coefficients 11. The audio encoding device 20 may then reorder the decomposed version of the HOA coefficients 11 based on the identified parameters, where such reordering, as described in further detail below, may improve coding efficiency given that the transformation may reorder the HOA coefficients across frames of the HOA coefficients (where a frame commonly includes M samples of the HOA coefficients 11 and M is, in some examples, set to 1024). After reordering the decomposed version of the HOA coefficients 11, the audio encoding device 20 may select those of the decomposed version of the HOA coefficients 11 representative of foreground (or, in other words, distinct, predominant or salient) components of the soundfield. The audio encoding device 20 may specify the decomposed version of the HOA coefficients 11 representative of the foreground components as an audio object and associated directional information.

[0084] The audio encoding device 20 may also perform a soundfield analysis with respect to the HOA coefficients 11 in order, at least in part, to identify those of the HOA coefficients 11 representative of one or more background (or, in other words, ambient) components of the soundfield. The audio encoding device 20 may perform energy compensation with respect to the background components given that, in some examples, the background components may only include a subset of any given sample of the HOA coefficients 11 (e.g., such as those corresponding to zero and first order spherical basis functions and not those corresponding to second or higher order spherical basis functions). When order-reduction is performed, in other words, the audio encoding device 20 may augment (e.g., add / subtract energy to / from) the remaining background HOA coefficients of the HOA coefficients 11 to compensate for the change in overall energy that results from performing the order reduction.

[0085] The audio encoding device 20 may next perform a form of psychoacoustic encoding (such as MPEG surround, MPEG-AAC, MPEG-USAC or other known forms of psychoacoustic encoding) with respect to each of the HOA coefficients 11 representative of background components and each of the foreground audio objects. The audio encoding device 20 may perform a form of interpolation with respect to the foreground directional information and then perform an order reduction with respect to the interpolated foreground directional information to generate order reduced foreground directional information. The audio encoding device 20 may further perform, in some examples, a quantization with respect to the order reduced foreground directional information, outputting coded foreground directional information. In some instances, this quantization may comprise a scalar / entropy quantization. The audio encoding device 20 may then form the bitstream 21 to include the encoded background components, the encoded foreground audio objects, and the quantized directional information. The audio encoding device 20 may then transmit or otherwise output the bitstream 21 to the content consumer 14.

[0086] While shown in FIG. 3 as being directly transmitted to the content consumer 14, the content creator 12 may output the bitstream 21 to an intermediate device positioned between the content creator 12 and the content consumer 14. This intermediate device may store the bitstream 21 for later delivery to the content consumer 14, which may request this bitstream. The intermediate device may comprise a file server, a web server, a desktop computer, a laptop computer, a tablet computer, a mobile phone, a smart phone, or any other device capable of storing the bitstream 21 for later retrieval by an audio decoder. This intermediate device may reside in a content delivery network capable of streaming the bitstream 21 (and possibly in conjunction with transmitting a corresponding video data bitstream) to subscribers, such as the content consumer 14, requesting the bitstream 21.

[0087] Alternatively, the content creator 12 may store the bitstream 21 to a storage medium, such as a compact disc, a digital video disc, a high definition video disc or other storage media, most of which are capable of being read by a computer and therefore may be referred to as computer-readable storage media or non-transitory computer-readable storage media. In this context, the transmission channel may refer to those channels by which content stored to these mediums are transmitted (and may include retail stores and other store-based delivery mechanism). In any event, the techniques of this disclosure should not therefore be limited in this respect to the example of FIG. 3.

[0088] As further shown in the example of FIG. 3, the content consumer 14 includes the audio playback system 16. The audio playback system 16 may represent any audio playback system capable of playing back multi-channel audio data. The audio playback system 16 may include a number of different renderers 22. The renderers 22 may each provide for a different form of rendering, where the different forms of rendering may include one or more of the various ways of performing vector-base amplitude panning (VBAP), and / or one or more of the various ways of performing soundfield synthesis. As used herein, "A and / or B" means "A or B", or both "A and B".

[0089] The audio playback system 16 may further include an audio decoding device 24. The audio decoding device 24 may represent a device configured to decode HOA coefficients 11' from the bitstream 21, where the HOA coefficients 11' may be similar to the HOA coefficients 11 but differ due to lossy operations (e.g., quantization) and / or transmission via the transmission channel. That is, the audio decoding device 24 may dequantize the foreground directional information specified in the bitstream 21, while also performing psychoacoustic decoding with respect to the foreground audio objects specified in the bitstream 21 and the encoded HOA coefficients representative of background components. The audio decoding device 24 may further perform interpolation with respect to the decoded foreground directional information and then determine the HOA coefficients representative of the foreground components based on the decoded foreground audio objects and the interpolated foreground directional information. The audio decoding device 24 may then determine the HOA coefficients 11' based on the determined HOA coefficients representative of the foreground components and the decoded HOA coefficients representative of the background components.

[0090] The audio playback system 16 may, after decoding the bitstream 21 to obtain the HOA coefficients 11' and render the HOA coefficients 11' to output loudspeaker feeds 25. The loudspeaker feeds 25 may drive one or more loudspeakers (which are not shown in the example of FIG. 3 for ease of illustration purposes).

[0091] To select the appropriate renderer or, in some instances, generate an appropriate renderer, the audio playback system 16 may obtain loudspeaker information 13 indicative of a number of loudspeakers and / or a spatial geometry of the loudspeakers. In some instances, the audio playback system 16 may obtain the loudspeaker information 13 using a reference microphone and driving the loudspeakers in such a manner as to dynamically determine the loudspeaker information 13. In other instances or in conjunction with the dynamic determination of the loudspeaker information 13, the audio playback system 16 may prompt a user to interface with the audio playback system 16 and input the loudspeaker information 16.

[0092] The audio playback system 16 may then select one of the audio renderers 22 based on the loudspeaker information 13. In some instances, the audio playback system 16 may, when none of the audio renderers 22 are within some threshold similarity measure (loudspeaker geometry wise) to that specified in the loudspeaker information 13, the audio playback system 16 may generate the one of audio renderers 22 based on the loudspeaker information 13. The audio playback system 16 may, in some instances, generate the one of audio renderers 22 based on the loudspeaker information 13 without first attempting to select an existing one of the audio renderers 22.

[0093] FIG. 4 is a block diagram illustrating, in more detail, one example of the audio encoding device 20 shown in the example of FIG. 3 that may perform various aspects of the techniques described in this disclosure. The audio encoding device 20 includes a content analysis unit 26, a vector-based synthesis methodology unit 27 and a directional-based synthesis methodology unit 28.

[0094] The content analysis unit 26 represents a unit configured to analyze the content of the HOA coefficients 11 to identify whether the HOA coefficients 11 represent content generated from a live recording or an audio object. The content analysis unit 26 may determine whether the HOA coefficients 11 were generated from a recording of an actual soundfield or from an artificial audio object. The content analysis unit 26 may make this determination in various ways. For example, the content analysis unit 26 may code (N+1) 2< -1 channels and predict the last remaining channel (which may be represented as a vector). The content analysis unit 26 may apply scalars to at least some of the (N+1) 2< -1 channels and add the resulting values to determine the last remaining channel. Furthermore, in this example, the content analysis unit 26 may determine an accuracy of the predicted channel. In this example, if the accuracy of the predicted channel is relatively high (e.g., the accuracy exceeds a particular threshold), the HOA coefficients 11 are likely to be generated from a synthetic audio object. In contrast, if the accuracy of the predicted channel is relatively low (e.g., the accuracy is below the particular threshold), the HOA coefficients 11 are more likely to represent a recorded soundfield. For instance, in this example, if a signal-to-noise ratio (SNR) of the predicted channel is over 100 decibels (dbs), the HOA coefficients 11 are more likely to represent a soundfield generated from a synthetic audio object. In contrast, the SNR of a soundfield recorded using an eigen microphone may be 5 to 20 dbs. Thus, there may be an apparent demarcation in SNR ratios between soundfield represented by the HOA coefficients 11 generated from an actual direct recording and from a synthetic audio object.

[0095] More specifically, the content analysis unit 26 may, when determining whether the HOA coefficients 11 representative of a soundfield are generated from a synthetic audio object, obtain a framed of HOA coefficients, which may be of size 25 by 1024 for a fourth order representation (i.e., N = 4). After obtaining the framed HOA coefficients (which may also be denoted herein as a framed SHC matrix 11 and subsequent framed SHC matrices may be denoted as framed SHC matrices 27B, 27C, etc.). The content analysis unit 26 may then exclude the first vector of the framed HOA coefficients 11 to generate a reduced framed HOA coefficients. In some examples, this first vector excluded from the framed HOA coefficients 11 may correspond to those of the HOA coefficients 11 associated with the zero-order, zero-sub-order spherical harmonic basis function.

[0096] The content analysis unit 26 may then predicted the first non-zero vector of the reduced framed HOA coefficients from remaining vectors of the reduced framed HOA coefficients. The first non-zero vector may refer to a first vector going from the first-order (and considering each of the order-dependent sub-orders) to the fourth-order (and considering each of the order-dependent sub-orders) that has values other than zero. In some examples, the first non-zero vector of the reduced framed HOA coefficients refers to those of HOA coefficients 11 associated with the first order, zero-sub-order spherical harmonic basis function. While described with respect to the first non-zero vector, the techniques may predict other vectors of the reduced framed HOA coefficients from the remaining vectors of the reduced framed HOA coefficients. For example, the content analysis unit 26 may predict those of the reduced framed HOA coefficients associated with a first-order, first-sub-order spherical harmonic basis function or a first-order, negative-first-order spherical harmonic basis function. As yet other examples, the content analysis unit 26 may predict those of the reduced framed HOA coefficients associated with a second-order, zero-order spherical harmonic basis function.

[0097] To predict the first non-zero vector, the content analysis unit 26 may operate in accordance with the following equation: ∑ i α i ν i , where i is from 1 to (N + 1) 2< -2, which is 23 for a fourth order representation, α i denotes some constant for the i-th vector, and v i refers to the i-th vector. After predicting the first non-zero vector, the content analysis unit 26 may obtain an error based on the predicted first non-zero vector and the actual non-zero vector. In some examples, the content analysis unit 26 subtracts the predicted first non-zero vector from the actual first non-zero vector to derive the error. The content analysis unit 26 may compute the error as a sum of the absolute value of the differences between each entry in the predicted first non-zero vector and the actual first non-zero vector.

[0098] Once the error is obtained, the content analysis unit 26 may compute a ratio based on an energy of the actual first non-zero vector and the error. The content analysis unit 26 may determine this energy by squaring each entry of the first non-zero vector and adding the squared entries to one another. The content analysis unit 26 may then compare this ratio to a threshold. When the ratio does not exceed the threshold, the content analysis unit 26 may determine that the framed HOA coefficients 11 is generated from a recording and indicate in the bitstream that the corresponding coded representation of the HOA coefficients 11 was generated from a recording. When the ratio exceeds the threshold, the content analysis unit 26 may determine that the framed HOA coefficients 11 is generated from a synthetic audio object and indicate in the bitstream that the corresponding coded representation of the framed HOA coefficients 11 was generated from a synthetic audio object.

[0099] The indication of whether the framed HOA coefficients 11 was generated from a recording or a synthetic audio object may comprise a single bit for each frame. The single bit may indicate that different encodings were used for each frame effectively toggling between different ways by which to encode the corresponding frame. In some instances, when the framed HOA coefficients 11 were generated from a recording, the content analysis unit 26 passes the HOA coefficients 11 to the vector-based synthesis unit 27. In some instances, when the framed HOA coefficients 11 were generated from a synthetic audio object, the content analysis unit 26 passes the HOA coefficients 11 to the directional-based synthesis unit 28. The directional-based synthesis unit 28 may represent a unit configured to perform a directional-based synthesis of the HOA coefficients 11 to generate a directional-based bitstream 21.

[0100] In other words, the techniques are based on coding the HOA coefficients using a front-end classifier. The classifier may work as follows: Start with a framed SH matrix (say 4th order, frame size of 1024, which may also be referred to as framed HOA coefficients or as HOA coefficients) - where a matrix of size 25 x 1024 is obtained.

[0101] Exclude the 1st vector (0th order SH) - so there is a matrix of size 24 x 1024.

[0102] Predict the first non-zero vector in the matrix ( a 1 x 1024 size vector) - from the rest of the of the vectors in the matrix (23 vectors of size 1x1024).

[0103] The prediction is as follows: predicted vector = sum-over-i [alpha-i x vector-I] (where the sum over I is done over 23 indices, i=1...23)

[0104] Then check the error: actual vector - predicted vector = error.

[0105] If the ratio of the energy of the vector / error is large (I.e. The error is small), then the underlying soundfield (at that frame) is sparse / synthetic. Else, the underlying soundfield is a recorded (using say a mic array) soundfield.

[0106] Depending on the recorded vs. synthetic decision, carry out encoding / decoding (which may refer to bandwidth compression) in different ways. The decision is a 1 bit decision, that is sent over the bitstream for each frame.

[0107] As shown in the example of FIG. 4, the vector-based synthesis unit 27 may include a linear invertible transform (LIT) unit 30, a parameter calculation unit 32, a reorder unit 34, a foreground selection unit 36, an energy compensation unit 38, a psychoacoustic audio coder unit 40, a bitstream generation unit 42, a soundfield analysis unit 44, a coefficient reduction unit 46, a background (BG) selection unit 48, a spatio-temporal interpolation unit 50, and a quantization unit 52.

[0108] The linear invertible transform (LIT) unit 30 receives the HOA coefficients 11 in the form of HOA channels, each channel representative of a block or frame of a coefficient associated with a given order, sub-order of the spherical basis functions (which may be denoted as HOA[k], where k may denote the current frame or block of samples). The matrix of HOA coefficients 11 may have dimensions D: M x (N+1) 2< .

[0109] That is, the LIT unit 30 may represent a unit configured to perform a form of analysis referred to as singular value decomposition. While described with respect to SVD, the techniques described in this disclosure may be performed with respect to any similar transformation or decomposition that provides for sets of linearly uncorrelated, energy compacted output. Also, reference to "sets" in this disclosure is generally intended to refer to non-zero sets unless specifically stated to the contrary and is not intended to refer to the classical mathematical definition of sets that includes the so-called "empty set."

[0110] An alternative transformation may comprise a principal component analysis, which is often referred to as "PCA." PCA refers to a mathematical procedure that employs an orthogonal transformation to convert a set of observations of possibly correlated variables into a set of linearly uncorrelated variables referred to as principal components. Linearly uncorrelated variables represent variables that do not have a linear statistical relationship (or dependence) to one another. These principal components may be described as having a small degree of statistical correlation to one another. In any event, the number of so-called principal components is less than or equal to the number of original variables. In some examples, the transformation is defined in such a way that the first principal component has the largest possible variance (or, in other words, accounts for as much of the variability in the data as possible), and each succeeding component in turn has the highest variance possible under the constraint that this successive component be orthogonal to (which may be restated as uncorrelated with) the preceding components. PCA may perform a form of order-reduction, which in terms of the HOA coefficients 11 may result in the compression of the HOA coefficients 11. Depending on the context, PCA may be referred to by a number of different names, such as discrete Karhunen-Loeve transform, the Hotelling transform, proper orthogonal decomposition (POD), and eigenvalue decomposition (EVD) to name a few examples. Properties of such operations that are conducive to the underlying goal of compressing audio data are 'energy compaction' and 'decorrelation' of the multichannel audio data.

[0111] In any event, the LIT unit 30 performs a singular value decomposition (which, again, may be referred to as "SVD") to transform the HOA coefficients 11 into two or more sets of transformed HOA coefficients. These "sets" of transformed HOA coefficients may include vectors of transformed HOA coefficients. In the example of FIG. 4, the LIT unit 30 may perform the SVD with respect to the HOA coefficients 11 to generate a so-called V matrix, an S matrix, and a U matrix. SVD, in linear algebra, may represent a factorization of a y-by-z real or complex matrix X (where X may represent multi-channel audio data, such as the HOA coefficients 11) in the following form: X = USV * U may represent an y-by-y real or complex unitary matrix, where the y columns of U are commonly known as the left-singular vectors of the multi-channel audio data. S may represent an y-by-z rectangular diagonal matrix with non-negative real numbers on the diagonal, where the diagonal values of S are commonly known as the singular values of the multi-channel audio data. V* (which may denote a conjugate transpose of V) may represent an z-by-z real or complex unitary matrix, where the z columns of V* are commonly known as the right-singular vectors of the multi-channel audio data.

[0112] While described in this disclosure as being applied to multi-channel audio data comprising HOA coefficients 11, the techniques may be applied to any form of multi-channel audio data. In this way, the audio encoding device 20 may perform a singular value decomposition with respect to multi-channel audio data representative of at least a portion of soundfield to generate a U matrix representative of left-singular vectors of the multi-channel audio data, an S matrix representative of singular values of the multi-channel audio data and a V matrix representative of right-singular vectors of the multi-channel audio data, and representing the multi-channel audio data as a function of at least a portion of one or more of the U matrix, the S matrix and the V matrix.

[0113] In some examples, the V* matrix in the SVD mathematical expression referenced above is denoted as the conjugate transpose of the V matrix to reflect that SVD may be applied to matrices comprising complex numbers. When applied to matrices comprising only real-numbers, the complex conjugate of the V matrix (or, in other words, the V* matrix) may be considered to be the transpose of the V matrix. Below it is assumed, for ease of illustration purposes, that the HOA coefficients 11 comprise real-numbers with the result that the V matrix is output through SVD rather than the V* matrix. Moreover, while denoted as the V matrix in this disclosure, reference to the V matrix should be understood to refer to the transpose of the V matrix where appropriate. While assumed to be the V matrix, the techniques may be applied in a similar fashion to HOA coefficients 11 having complex coefficients, where the output of the SVD is the V* matrix. Accordingly, the techniques should not be limited in this respect to only provide for application of SVD to generate a V matrix, but may include application of SVD to HOA coefficients 11 having complex components to generate a V* matrix.

[0114] In any event, the LIT unit 30 may perform a block-wise form of SVD with respect to each block (which may refer to a frame) of higher-order ambisonics (HOA) audio data (where this ambisonics audio data includes blocks or samples of the HOA coefficients 11 or any other form of multi-channel audio data). As noted above, a variable M may be used to denote the length of an audio frame in samples. For example, when an audio frame includes 1024 audio samples, M equals 1024. Although described with respect to this typical value for M, the techniques of this disclosure should not be limited to this typical value for M. The LIT unit 30 may therefore perform a block-wise SVD with respect to a block the HOA coefficients 11 having M-by-(N+1) 2< HOA coefficients, where N, again, denotes the order of the HOA audio data. The LIT unit 30 may generate, through performing this SVD, a V matrix, an S matrix, and a U matrix, where each of matrixes may represent the respective V, S and U matrixes described above. In this way, the linear invertible transform unit 30 may perform SVD with respect to the HOA coefficients 11 to output US[k] vectors 33 (which may represent a combined version of the S vectors and the U vectors) having dimensions D: M x (N+1) 2< , and V[k] vectors 35 having dimensions D: (N+1) 2< x (N+1) 2< . Individual vector elements in the US[k] matrix may also be termed X PS (k) while individual vectors of the V[k] matrix may also be termed v(k).

[0115] An analysis of the U, S and V matrices may reveal that these matrices carry or represent spatial and temporal characteristics of the underlying soundfield represented above by X. Each of the N vectors in U (of length M samples) may represent normalized separated audio signals as a function of time (for the time period represented by M samples), that are orthogonal to each other and that have been decoupled from any spatial characteristics (which may also be referred to as directional information). The spatial characteristics, representing spatial shape and position (r, theta, phi) width may instead be represented by individual i th< vectors , v (i)< (k), in the V matrix (each of length (N+1) 2< ). Both the vectors in the U matrix and the V matrix are normalized such that their root-mean-square energies are equal to unity. The energy of the audio signals in U are thus represented by the diagonal elements in S. Multiplying U and S to form US[k] (with individual vector elements X PS (k)), thus represent the audio signal with true energies. The ability of the SVD decomposition to decouple the audio time-signals (in U), their energies (in S) and their spatial characteristics (in V) may support various aspects of the techniques described in this disclosure. Further, this model of synthesizing the underlying HOA[k] coefficients, X, by a vector multiplication of US[k] and V[k] gives rise the term "vector based synthesis methodology," which is used throughout this document.

[0116] Although described as being performed directly with respect to the HOA coefficients 11, the LIT unit 30 may apply the linear invertible transform to derivatives of the HOA coefficients 11. For example, the LIT unit 30 may apply SVD with respect to a power spectral density matrix derived from the HOA coefficients 11. The power spectral density matrix may be denoted as PSD and obtained through matrix multiplication of the transpose of the hoaFrame to the hoaFrame, as outlined in the pseudo-code that follows below. The hoaFrame notation refers to a frame of the HOA coefficients 11.

[0117] The LIT unit 30 may, after applying the SVD (svd) to the PSD, may obtain an S[k] 2< matrix (S_squared) and a V[k] matrix. The S[k] 2< matrix may denote a squared S[k] matrix, whereupon the LIT unit 30 may apply a square root operation to the S[k] 2< matrix to obtain the S[k] matrix. The LIT unit 30 may, in some instances, perform quantization with respect to the V[k] matrix to obtain a quantized V[k] matrix (which may be denoted as V[k]' matrix). The LIT unit 30 may obtain the U[k] matrix by first multiplying the S[k] matrix by the quantized V[k]' matrix to obtain an SV[k]' matrix. The LIT unit 30 may next obtain the pseudo-inverse (pinv) of the SV[k]' matrix and then multiply the HOA coefficients 11 by the pseudo-inverse of the SV[k]' matrix to obtain the U[k] matrix. The foregoing may be represented by the following pseud-code:

[0118] By performing SVD with respect to the power spectral density (PSD) of the HOA coefficients rather than the coefficients themselves, the LIT unit 30 may potentially reduce the computational complexity of performing the SVD in terms of one or more of processor cycles and storage space, while achieving the same source audio encoding efficiency as if the SVD were applied directly to the HOA coefficients. That is, the above described PSD-type SVD may be potentially less computational demanding because the SVD is done on an F*F matrix (with F the number of HOA coefficients). Compared to a M * F matrix with M is the framelength, i.e., 1024 or more samples. The complexity of an SVD may now, through application to the PSD rather than the HOA coefficients 11, be around O(L^3) compared to O(M*L^2) when applied to the HOA coefficients 11 (where O(*) denotes the big-O notation of computation complexity common to the computer-science arts).

[0119] The parameter calculation unit 32 represents unit configured to calculate various parameters, such as a correlation parameter (R), directional properties parameters (θ, φ, r), and an energy property (e). Each of these parameters for the current frame may be denoted as R[k], θ[k], φ[k], r[k] and e[k]. The parameter calculation unit 32 may perform an energy analysis and / or correlation (or so-called cross-correlation) with respect to the US[k] vectors 33 to identify these parameters. The parameter calculation unit 32 may also determine these parameters for the previous frame, where the previous frame parameters may be denoted R[k-1], θ[k-1], φ[k-1], r[k-1] and e[k-1], based on the previous frame of US[k-1] vector and V[k-1] vectors. The parameter calculation unit 32 may output the current parameters 37 and the previous parameters 39 to reorder unit 34.

[0120] That is, the parameter calculation unit 32 may perform an energy analysis with respect to each of the L first US[k] vectors 33 corresponding to a first time and each of the second US[k-1] vectors 33 corresponding to a second time, computing a root mean squared energy for at least a portion of (but often the entire) first audio frame and a portion of (but often the entire) second audio frame and thereby generate 2L energies, one for each of the L first US[k] vectors 33 of the first audio frame and one for each of the second US[k-1] vectors 33 of the second audio frame.

[0121] In other examples, the parameter calculation unit 32 may perform a cross-correlation between some portion of (if not the entire) set of samples for each of the first US[k] vectors 33 and each of the second US[k-1] vectors 33. Cross-correlation may refer to cross-correlation as understood in the signal processing arts. In other words, cross-correlation may refer to a measure of similarity between two waveforms (which in this case is defined as a discrete set of M samples) as a function of a time-lag applied to one of them. In some examples, to perform cross-correlation, the parameter calculation unit 32 compares the last L samples of each the first US[k] vectors 27, turn-wise, to the first L samples of each of the remaining ones of the second US[k-1] vectors 33 to determine a correlation parameter. As used herein, a "turn-wise" operation refers to an element by element operation made with respect to a first set of elements and a second set of elements, where the operation draws one element from each of the first and second sets of elements "in-turn" according to an ordering of the sets.

[0122] The parameter calculation unit 32 may also analyze the V[k] and / or V[k-1] vectors 35 to determine directional property parameters. These directional property parameters may provide an indication of movement and location of the audio object represented by the corresponding US[k] and / or US[k-1] vectors 33. The parameter calculation unit 32 may provide any combination of the foregoing current parameters 37 (determined with respect to the US[k] vectors 33 and / or the V[k] vectors 35) and any combination of the previous parameters 39 (determined with respect to the US[k-1] vectors 33 and / or the V[k-1] vectors 35) to the reorder unit 34.

[0123] The SVD decomposition does not guarantee that the audio signal / object represented by the p-th vector in US[k-1] vectors 33, which may be denoted as the US[k-1][p] vector (or, alternatively, as X PS (p)< (k - 1)), will be the same audio signal / object (progressed in time) represented by the p-th vector in the US[k] vectors 33, which may also be denoted as US[k][p] vectors 33 (or, alternatively as X PS (p)< (k)). The parameters calculated by the parameter calculation unit 32 may be used by the reorder unit 34 to re-order the audio objects to represent their natural evaluation or continuity over time.

[0124] That is, the reorder unit 34 may then compare each of the parameters 37 from the first US[k] vectors 33 turn-wise against each of the parameters 39 for the second US[k-1] vectors 33. The reorder unit 34 may reorder (using, as one example, a Hungarian algorithm) the various vectors within the US[k] matrix 33 and the V[k] matrix 35 based on the current parameters 37 and the previous parameters 39 to output a reordered US[k] matrix 33' (which may be denoted mathematically as US[k]) and a reordered V[k] matrix 35' (which may be denoted mathematically as V[k]) to a foreground sound (or predominant sound - PS) selection unit 36 ("foreground selection unit 36") and an energy compensation unit 38.

[0125] In other words, the reorder unit 34 may represent a unit configured to reorder the vectors within the US[k] matrix 33 to generate reordered US[k] matrix 33'. The reorder unit 34 may reorder the US[k] matrix 33 because the order of the US[k] vectors 33 (where, again, each vector of the US[k] vectors 33, which again may alternatively be denoted as X PS (p)< (k), may represent one or more distinct (or, in other words, predominant) mono-audio object present in the soundfield) may vary from portions of the audio data. That is, given that the audio encoding device 12, in some examples, operates on these portions of the audio data generally referred to as audio frames, the position of vectors corresponding to these distinct mono-audio objects as represented in the US[k] matrix 33 as derived, may vary from audio frame-to-audio frame due to application of SVD to the frames and the varying saliency of each audio object form frame-to-frame.

[0126] Passing vectors within the US[k] matrix 33 directly to the psychoacoustic audio coder unit 40 without reordering the vectors within the US[k] matrix 33 from audio frame-to audio frame may reduce the extent of the compression achievable for some compression schemes, such as legacy compression schemes that perform better when mono-audio objects are continuous (channel-wise, which is defined in this example by the positional order of the vectors within the US[k] matrix 33 relative to one another) across audio frames. Moreover, when not reordered, the encoding of the vectors within the US[k] matrix 33 may reduce the quality of the audio data when decoded. For example, AAC encoders, which may be represented in the example of FIG. 3 by the psychoacoustic audio coder unit 40, may more efficiently compress the reordered one or more vectors within the US[k] matrix 33' from frame-to-frame in comparison to the compression achieved when directly encoding the vectors within the US[k] matrix 33 from frame-to-frame. While described above with respect to AAC encoders, the techniques may be performed with respect to any encoder that provides better compression when mono-audio objects are specified across frames in a specific order or position (channel-wise).

[0127] Various aspects of the techniques may, in this way, enable audio encoding device 12 to reorder one or more vectors (e.g., the vectors within the US[k] matrix 33 to generate reordered one or more vectors within the reordered US[k] matrix 33' and thereby facilitate compression of the vectors within the US[k] matrix 33 by a legacy audio encoder, such as the psychoacoustic audio coder unit 40).

[0128] For example, the reorder unit 34 may reorder one or more vectors within the US[k] matrix 33 from a first audio frame subsequent in time to the second frame to which one or more second vectors within the US[k-1] matrix 33 correspond based on the current parameters 37 and previous parameters 39. While described in the context of a first audio frame being subsequent in time to the second audio frame, the first audio frame may precede in time the second audio frame. Accordingly, the techniques should not be limited to the example described in this disclosure.

[0129] To illustrate consider the following Table 1 where each of the p vectors within the US[k] matrix 33 is denoted as US[k][p], where k denotes whether the corresponding vector is from the k-th frame or the previous (k-1)-th frame and p denotes the row of the vector relative to vectors of the same audio frame (where the US[k] matrix has (N+1) 2< such vectors). As noted above, assuming N is determined to be one, p may denote vectors one (1) through (4). Table 1 Energy Under Consideration Compared To US[k-1][1]US[k][1], US[k][2], US[k][3], US[k][4]US[k-1][2]US[k][1], US[k][2], US[k][3], US[k][4]US[k-1][3]US[k][1], US[k][2], US[k][3], US[k][4]US[k-1][4]US[k][1], US[k][2], US[k][3], US[k][4]

[0130] In the above Table 1, the reorder unit 34 compares the energy computed for US[k-1][1] to the energy computed for each of US[k][1], US[k][2], US[k][3], US[k][4], the energy computed for US[k-1][2] to the energy computed for each of US[k][1], US[k][2], US[k][3], US[k][4], etc. The reorder unit 34 may then discard one or more of the second US[k-1] vectors 33 of the second preceding audio frame (time-wise). To illustrate, consider the following Table 2 showing the remaining second US[k-1] vectors 33: Table 2 Vector Under Consideration Remaining Under Consideration US[k-1][1]US[k][1], US[k][2]US[k-1][2]US[k][1], US[k][2]US[k-1][3]US[k][3], US[k][4]US[k-1][4]US[k][3], US[k][4]

[0131] In the above Table 2, the reorder unit 34 may determine, based on the energy comparison that the energy computed for US[k-1][1] is similar to the energy computed for each of US[k][1] and US[k][2], the energy computed for US[k-1][2] is similar to the energy computed for each of US[k][1] and US[k][2], the energy computed for US[k-1][3] is similar to the energy computed for each of US[k][3] and US[k][4], and the energy computed for US[k-1][4] is similar to the energy computed for each of US[k][3] and US[k][4]. In some examples, the reorder unit 34 may perform further energy analysis to identify a similarity between each of the first vectors of the US[k] matrix 33 and each of the second vectors of the US[k-1] matrix 33.

[0132] In other examples, the reorder unit 32 may reorder the vectors based on the current parameters 37 and the previous parameters 39 relating to cross-correlation. In these examples, referring back to Table 2 above, the reorder unit 34 may determine the following exemplary correlation expressed in Table 3 based on these cross-correlation parameters: Table 3 Vector Under Consideration Correlates To US[k-1][1]US[k][2]US[k-1][2]US[k][1]US[k-1][3]US[k][3]US[k-1][4]US[k][4]

[0133] From the above Table 3, the reorder unit 34 determines, as one example, that US[k-1][1] vector correlates to the differently positioned US[k][2] vector, the US[k-1][2] vector correlates to the differently positioned US[k][1] vector, the US[k-1][3] vector correlates to the similarly positioned US[k][3] vector, and the US[k-1][4] vector correlates to the similarly positioned US[k][4] vector. In other words, the reorder unit 34 determines what may be referred to as reorder information describing how to reorder the first vectors of the US[k] matrix 33 such that the US[k][2] vector is repositioned in the first row of the first vectors of the US[k] matrix 33 and the US[k][1] vector is repositioned in the second row of the first US[k] vectors 33. The reorder unit 34 may then reorder the first vectors of the US[k] matrix 33 based on this reorder information to generate the reordered US[k] matrix 33'.

[0134] Additionally, the reorder unit 34 may, although not shown in the example of FIG. 4, provide this reorder information to the bitstream generation device 42, which may generate the bitstream 21 to include this reorder information so that the audio decoding device, such as the audio decoding device 24 shown in the example of FIGS. 3 and 5, may determine how to reorder the reordered vectors of the US[k] matrix 33' so as to recover the vectors of the US[k] matrix 33.

[0135] While described above as performing a two-step process involving an analysis based first an energy-specific parameters and then cross-correlation parameters, the reorder unit 32 may only perform this analysis only with respect to energy parameters to determine the reorder information, perform this analysis only with respect to cross-correlation parameters to determine the reorder information, or perform the analysis with respect to both the energy parameters and the cross-correlation parameters in the manner described above. Additionally, the techniques may employ other types of processes for determining correlation that do not involve performing one or both of an energy comparison and / or a cross-correlation. Accordingly, the techniques should not be limited in this respect to the examples set forth above. Moreover, other parameters obtained from the parameter calculation unit 32 (such as the spatial position parameters derived from the V vectors or correlation of the vectors in the V[k] and V[k-1]) can also be used (either concurrently / jointly or sequentially) with energy and cross-correlation parameters obtained from US[k] and US[k-1] to determine the correct ordering of the vectors in US.

[0136] As one example of using correlation of the vectors in the V matrix, the parameter calculation unit 34 may determine that the vectors of the V[k] matrix 35 are correlated as specified in the following Table 4: Table 4 Vector Under Consideration Correlates To V[k-1][1]V[k][2]V[k-1][2]V[k][1]V[k-1][3]V[k][3]V[k-1][4]V[k][4] From the above Table 4, the reorder unit 34 determines, as one example, that V[k-1][1] vector correlates to the differently positioned V[k][2] vector, the V[k-1][2] vector correlates to the differently positioned V[k][1] vector, the V[k-1][3] vector correlates to the similarly positioned V[k][3] vector, and the V[k-1][4] vector correlates to the similarly positioned V[k][4] vector. The reorder unit 34 may output the reordered version of the vectors of the V[k] matrix 35 as a reordered V[k] matrix 35'.

[0137] In some examples, the same re-ordering that is applied to the vectors in the US matrix is also applied to the vectors in the V matrix. In other words, any analysis used in reordering the V vectors may be used in conjunction with any analysis used to reorder the US vectors. To illustrate an example in which the reorder information is not solely determined with respect to the energy parameters and / or the cross-correlation parameters with respect to the US[k] vectors 35, the reorder unit 34 may also perform this analysis with respect to the V[k] vectors 35 based on the cross-correlation parameters and the energy parameters in a manner similar to that described above with respect to the V[k] vectors 35. Moreover, while the US[k] vectors 33 do not have any directional properties, the V[k] vectors 35 may provide information relating to the directionality of the corresponding US[k] vectors 33. In this sense, the reorder unit 34 may identify correlations between V[k] vectors 35 and V[k-1] vectors 35 based on an analysis of corresponding directional properties parameters. That is, in some examples, audio object move within a soundfield in a continuous manner when moving or that stays in a relatively stable location. As such, the reorder unit 34 may identify those vectors of the V[k] matrix 35 and the V[k-1] matrix 35 that exhibit some known physically realistic motion or that stay stationary within the soundfield as correlated, reordering the US[k] vectors 33 and the V[k] vectors 35 based on this directional properties correlation. In any event, the reorder unit 34 may output the reordered US[k] vectors 33' and the reordered V[k] vectors 35' to the foreground selection unit 36.

[0138] Additionally, the techniques may employ other types of processes for determining correct order that do not involve performing one or both of an energy comparison and / or a cross-correlation. Accordingly, the techniques should not be limited in this respect to the examples set forth above.

[0139] Although described above as reordering the vectors of the V matrix to mirror the reordering of the vectors of the US matrix, in certain instances, the V vectors may be reordered differently than the US vectors, where separate syntax elements may be generated to indicate the reordering of the US vectors and the reordering of the V vectors. In some instances, the V vectors may not be reordered and only the US vectors may be reordered given that the V vectors may not be psychoacoustically encoded.

[0140] An embodiment where the re-ordering of the vectors of the V matrix and the vectors of US matrix are different are when the intention is to swap audio objects in space - i.e. move them away from the original recorded position (when the underlying soundfield was a natural recording) or the artistically intended position (when the underlying soundfield is an artificial mix of objects). As an example, suppose that there are two audio sources A and B, A may be the sound of a cat "meow" emanating from the "left" part of soundfield and B may be the sound of a dog "woof" emanating from the "right" part of the soundfield. When the re-ordering of the V and US are different, the position of the two sound sources is swapped. After swapping A (the "meow") emanates from the right part of the soundfield, and B ("the woof") emanates from the left part of the soundfield.

[0141] The soundfield analysis unit 44 may represent a unit configured to perform a soundfield analysis with respect to the HOA coefficients 11 so as to potentially achieve a target bitrate 41. The soundfield analysis unit 44 may, based on this analysis and / or on a received target bitrate 41, determine the total number of psychoacoustic coder instantiations (which may be a function of the total number of ambient or background channels (BG TOT ) and the number of foreground channels or, in other words, predominant channels. The total number of psychoacoustic coder instantiations can be denoted as numHOATransportChannels. The soundfield analysis unit 44 may also determine, again to potentially achieve the target bitrate 41, the total number of foreground channels (nFG) 45, the minimum order of the background (or, in other words, ambient) soundfield (N BG or, alternatively, MinAmbHoaOrder), the corresponding number of actual channels representative of the minimum order of background soundfield (nBGa = (MinAmbHoaOrder+1) 2< ), and indices (i) of additional BG HOA channels to send (which may collectively be denoted as background channel information 43 in the example of FIG. 4). The background channel information 42 may also be referred to as ambient channel information 43. Each of the channels that remains from numHOATransportChannels - nBGa, may either be an "additional background / ambient channel", an "active vector based predominant channel", an "active directional based predominant signal" or "completely inactive". In one embodiment, these channel types may be indicated (as a "ChannelType") syntax element by two bits (e.g. 00:additional background channel; 01:vector based predominant signal; 10: inactive signal; 11: directional based signal). The total number of background or ambient signals, nBGa, may be given by (MinAmbHoaOrder +1) 2< + the number of times the index 00 (in the above example) appears as a channel type in the bitstream for that frame.

[0142] In any event, the soundfield analysis unit 44 may select the number of background (or, in other words, ambient) channels and the number of foreground (or, in other words, predominant) channels based on the target bitrate 41, selecting more background and / or foreground channels when the target bitrate 41 is relatively higher (e.g., when the target bitrate 41 equals or is greater than 512 Kbps). In one embodiment, the numHOATransportChannels may be set to 8 while the MinAmbHoaOrder may be set to 1 in the header section of the bitstream (which is described in more detail with respect to FIGS. 10-10O(ii)). In this scenario, at every frame, four channels may be dedicated to represent the background or ambient portion of the soundfield while the other 4 channels can, on a frame-by-frame basis vary on the type of channel - e.g., either used as an additional background / ambient channel or a foreground / predominant channel. The foreground / predominant signals can be one of either vector based or directional based signals, as described above.

[0143] In some instances, the total number of vector based predominant signals for a frame, may be given by the number of times the ChannelType index is 01, in the bitstream of that frame, in the above example. In the above embodiment, for every additional background / ambient channel (e.g., corresponding to a ChannelType of 00), a corresponding information of which of the possible HOA coefficients (beyond the first four) may be represented in that channel. This information, for fourth order HOA content, may be an index to indicate between 5-25 (the first four 1-4 may be sent all the time when minAmbHoaOrder is set to 1, hence only need to indicate one between 5-25). This information could thus be sent using a 5 bits syntax element (for 4 th< order content), which may be denoted as "CodedAmbCoeffldx."

[0144] In a second embodiment, all of the foreground / predominant signals are vector based signals. In this second embodiment, the total number of foreground / predominant signals may be given by nFG = numHOATransportChannels - [(MinAmbHoaOrder +1) 2< + the number of times the index 00].

[0145] The soundfield analysis unit 44 outputs the background channel information 43 and the HOA coefficients 11 to the background (BG) selection unit 46, the background channel information 43 to coefficient reduction unit 46 and the bitstream generation unit 42, and the nFG 45 to a foreground selection unit 36.

[0146] In some examples, the soundfield analysis unit 44 may select, based on an analysis of the vectors of the US[k] matrix 33 and the target bitrate 41, a variable nFG number of these components having the greatest value. In other words, the soundfield analysis unit 44 may determine a value for a variable A (which may be similar or substantially similar to N BG ), which separates two subspaces, by analyzing the slope of the curve created by the descending diagonal values of the vectors of the S[k] matrix 33, where the large singular values represent foreground or distinct sounds and the low singular values represent background components of the soundfield. That is, the variable A may segment the overall soundfield into a foreground subspace and a background subspace.

[0147] In some examples, the soundfield analysis unit 44 may use a first and a second derivative of the singular value curve. The soundfield analysis unit 44 may also limit the value for the variable A to be between one and five. As another example, the soundfield analysis unit 44 may limit the value of the variable A to be between one and (N+1) 2< . Alternatively, the soundfield analysis unit 44 may pre-define the value for the variable A, such as to a value of four. In any event, based on the value of A, the soundfield analysis unit 44 determines the total number of foreground channels (nFG) 45, the order of the background soundfield (N BG ) and the number (nBGa) and the indices (i) of additional BG HOA channels to send.

[0148] Furthermore, the soundfield analysis unit 44 may determine the energy of the vectors in the V[k] matrix 35 on a per vector basis. The soundfield analysis unit 44 may determine the energy for each of the vectors in the V[k] matrix 35 and identify those having a high energy as foreground components.

[0149] Moreover, the soundfield analysis unit 44 may perform various other analyses with respect to the HOA coefficients 11, including a spatial energy analysis, a spatial masking analysis, a diffusion analysis or other forms of auditory analyses. The soundfield analysis unit 44 may perform the spatial energy analysis through transformation of the HOA coefficients 11 into the spatial domain and identifying areas of high energy representative of directional components of the soundfield that should be preserved. The soundfield analysis unit 44 may perform the perceptual spatial masking analysis in a manner similar to that of the spatial energy analysis, except that the soundfield analysis unit 44 may identify spatial areas that are masked by spatially proximate higher energy sounds. The soundfield analysis unit 44 may then, based on perceptually masked areas, identify fewer foreground components in some instances. The soundfield analysis unit 44 may further perform a diffusion analysis with respect to the HOA coefficients 11 to identify areas of diffuse energy that may represent background components of the soundfield.

[0150] The soundfield analysis unit 44 may also represent a unit configured to determine saliency, distinctness or predominance of audio data representing a soundfield, using directionality-based information associated with the audio data. While energy-based determinations may improve rendering of a soundfield decomposed by SVD to identify distinct audio components of the soundfield, energy-based determinations may also cause a device to erroneously identify background audio components as distinct audio components, in cases where the background audio components exhibit a high energy level. That is, a solely energy-based separation of distinct and background audio components may not be robust, as energetic (e.g., louder) background audio components may be incorrectly identified as being distinct audio components. To more robustly distinguish between distinct and background audio components of the soundfield, various aspects of the techniques described in this disclosure may enable the soundfield analysis unit 44 to perform a directionality-based analysis of the HOA coefficients 11 to separate foreground and ambient audio components from decomposed versions of the HOA coefficients 11.

[0151] In this respect, the soundfield analysis unit 44 may represent a unit configured or otherwise operable to identify distinct (or foreground) elements from background elements included in one or more of the vectors in the US[k] matrix 33 and the vectors in the V[k] matrix 35. According to some SVD-based techniques, the most energetic components (e.g., the first few vectors of one or more of the US[k] matrix 33 and the V[k] matrix 35 or vectors derived therefrom) may be treated as distinct components. However, the most energetic components (which are represented by vectors) of one or more of the vectors in the US[k] matrix 33 and the vectors in the V[k] matrix 35 may not, in all scenarios, represent the components / signals that are the most directional.

[0152] The soundfield analysis unit 44 may implement one or more aspects of the techniques described herein to identify foreground / direct / predominant elements based on the directionality of the vectors of one or more of the vectors in the US[k] matrix 33 and the vectors in the V[k] matrix 35 or vectors derived therefrom. In some examples, the soundfield analysis unit 44 may identify or select as distinct audio components (where the components may also be referred to as "objects"), one or more vectors based on both energy and directionality of the vectors. For instance, the soundfield analysis unit 44 may identify those vectors of one or more of the vectors in the US[k] matrix 33 and the vectors in the V[k] matrix 35 (or vectors derived therefrom) that display both high energy and high directionality (e.g., represented as a directionality quotient) as distinct audio components. As a result, if the soundfield analysis unit 44 determines that a particular vector is relatively less directional when compared to other vectors of one or more of the vectors in the US[k] matrix 33 and the vectors in the V[k] matrix 35 (or vectors derived therefrom), then regardless of the energy level associated with the particular vector, the soundfield analysis unit 44 may determine that the particular vector represents background (or ambient) audio components of the soundfield represented by the HOA coefficients 11.

[0153] In some examples, the soundfield analysis unit 44 may identify distinct audio objects (which, as noted above, may also be referred to as "components") based on directionality, by performing the following operations. The soundfield analysis unit 44 may multiply (e.g., using one or more matrix multiplication processes) vectors in the S[k] matrix (which may be derived from the US[k] vectors 33 or, although not shown in the example of FIG. 4 separately output by the LIT unit 30) by the vectors in the V[k] matrix 35. By multiplying the V[k] matrix 35 and the S[k] vectors, the soundfield analysis unit 44 may obtain VS[k] matrix. Additionally, the soundfield analysis unit 44 may square (i.e., exponentiate by a power of two) at least some of the entries of each of the vectors in the VS[k] matrix. In some instances, the soundfield analysis unit 44 may sum those squared entries of each vector that are associated with an order greater than 1.

[0154] As one example, if each vector of the VS[k] matrix, which includes 25 entries, the soundfield analysis unit 44 may, with respect to each vector, square the entries of each vector beginning at the fifth entry and ending at the twenty-fifth entry, summing the squared entries to determine a directionality quotient (or a directionality indicator). Each summing operation may result in a directionality quotient for a corresponding vector. In this example, the soundfield analysis unit 44 may determine that those entries of each row that are associated with an order less than or equal to 1, namely, the first through fourth entries, are more generally directed to the amount of energy and less to the directionality of those entries. That is, the lower order ambisonics associated with an order of zero or one correspond to spherical basis functions that, as illustrated in FIG. 1 and FIG. 2, do not provide much in terms of the direction of the pressure wave, but rather provide some volume (which is representative of energy).

[0155] The operations described in the example above may also be expressed according to the following pseudo-code. The pseudo-code below includes annotations, in the form of comment statements that are included within consecutive instances of the character strings " / *" and "* / " (without quotes). U S V = svd audioframe , ′ ecom ′ ; VS = V * S ;

[0156] / * The next line is directed to analyzing each row independently, and summing the values in the first (as one example) row from the fifth entry to the twenty-fifth entry to determine a directionality quotient or directionality metric for a corresponding vector. Square the entries before summing. The entries in each row that are associated with an order greater than 1 are associated with higher order ambisonics, and are thus more likely to be directional. * / sumVS = sum VS 5 : end , : . ∧ 2 , 1 ;

[0157] / * The next line is directed to sorting the sum of squares for the generated VS matrix, and selecting a set of the largest values (e.g., three or four of the largest values) * / ∼ , idxVS = sort sumVS , ′ descend ′ ; U = U : , idxVS ; V = V : , idxVS ; S = S idxVS idxVS ;

[0158] In other words, according to the above pseudo-code, the soundfield analysis unit 44 may select entries of each vector of the VS[k] matrix decomposed from those of the HOA coefficients 11 corresponding to a spherical basis function having an order greater than one. The soundfield analysis unit 44 may then square these entries for each vector of the VS[k] matrix, summing the squared entries to identify, compute or otherwise determine a directionality metric or quotient for each vector of the VS[k] matrix. Next, the soundfield analysis unit 44 may sort the vectors of the VS[k] matrix based on the respective directionality metrics of each of the vectors. The soundfield analysis unit 44 may sort these vectors in a descending order of directionality metrics, such that those vectors with the highest corresponding directionality are first and those vectors with the lowest corresponding directionality are last. The soundfield analysis unit 44 may then select the a non-zero subset of the vectors having the highest relative directionality metric.

[0159] The soundfield analysis unit 44 may perform any combination of the foregoing analyses to determine the total number of psychoacoustic coder instantiations (which may be a function of the total number of ambient or background channels (BG TOT ) and the number of foreground channels. The soundfield analysis unit 44 may, based on any combination of the foregoing analyses, determine the total number of foreground channels (nFG) 45, the order of the background soundfield (N BG ) and the number (nBGa) and indices (i) of additional BG HOA channels to send (which may collectively be denoted as background channel information 43 in the example of FIG. 4).

[0160] In some examples, the soundfield analysis unit 44 may perform this analysis every M-samples, which may be restated as on a frame-by-frame basis. In this respect, the value for A may vary from frame to frame. An instance of a bitstream where the decision is made every M-samples is shown in FIGS. 10-10O(ii). In other examples, the soundfield analysis unit 44 may perform this analysis more than once per frame, analyzing two or more portions of the frame. Accordingly, the techniques should not be limited in this respect to the examples described in this disclosure.

[0161] The background selection unit 48 may represent a unit configured to determine background or ambient HOA coefficients 47 based on the background channel information (e.g., the background soundfield (N BG ) and the number (nBGa) and the indices (i) of additional BG HOA channels to send). For example, when N BG equals one, the background selection unit 48 may select the HOA coefficients 11 for each sample of the audio frame having an order equal to or less than one. The background selection unit 48 may, in this example, then select the HOA coefficients 11 having an index identified by one of the indices (i) as additional BG HOA coefficients, where the nBGa is provided to the bitstream generation unit 42 to be specified in the bitstream 21 so as to enable the audio decoding device, such as the audio decoding device 24 shown in the example of FIG. 3, to parse the BG HOA coefficients 47 from the bitstream 21. The background selection unit 48 may then output the ambient HOA coefficients 47 to the energy compensation unit 38. The ambient HOA coefficients 47 may have dimensions D: M x [(N BG +1) 2< + nBGa].

[0162] The foreground selection unit 36 may represent a unit configured to select those of the reordered US[k] matrix 33' and the reordered V[k] matrix 35' that represent foreground or distinct components of the soundfield based on nFG 45 (which may represent a one or more indices identifying these foreground vectors). The foreground selection unit 36 may output nFG signals 49 (which may be denoted as a reordered US[k] 1, ..., nFG 49, FG 1 , ..., nfG [k] 49, or X PS 1 .. nFG k 49) to the psychoacoustic audio coder unit 40, where the nFG signals 49 may have dimensions D: M x nFG and each represent mono-audio objects. The foreground selection unit 36 may also output the reordered V[k] matrix 35' (or v (1..nFG)< (k) 35') corresponding to foreground components of the soundfield to the spatio-temporal interpolation unit 50, where those of the reordered V[k] matrix 35' corresponding to the foreground components may be denoted as foreground V[k] matrix 51 k (which may be mathematically denoted as V 1,...,nFG [k]) having dimensions D: (N+1) 2< x nFG.

[0163] The energy compensation unit 38 may represent a unit configured to perform energy compensation with respect to the ambient HOA coefficients 47 to compensate for energy loss due to removal of various ones of the HOA channels by the background selection unit 48. The energy compensation unit 38 may perform an energy analysis with respect to one or more of the reordered US[k] matrix 33', the reordered V[k] matrix 35', the nFG signals 49, the foreground V[k] vectors 51 k and the ambient HOA coefficients 47 and then perform energy compensation based on this energy analysis to generate energy compensated ambient HOA coefficients 47'. The energy compensation unit 38 may output the energy compensated ambient HOA coefficients 47' to the psychoacoustic audio coder unit 40.

[0164] Effectively, the energy compensation unit 38 may be used to compensate for possible reductions in the overall energy of the background sound components of the soundfield caused by reducing the order of the ambient components of the soundfield described by the HOA coefficients 11 to generate the order-reduced ambient HOA coefficients 47 (which, in some examples, have an order less than N in terms of only included coefficients corresponding to spherical basis functions having the following orders / sub-orders: [(N BG +1) 2< - nBGa]). In some examples, the energy compensation unit 38 compensates for this loss of energy by determining a compensation gain in the form of amplification values to apply to each of the [(N BG +1) 2< + nBGa] columns of the ambient HOA coefficients 47 in order to increase the root mean-squared (RMS) energy of the ambient HOA coefficients 47 to equal or at least more nearly approximate the RMS of the HOA coefficients 11 (as determined through aggregate energy analysis of one or more of the reordered US[k] matrix 33', the reordered V[k] matrix 35', the nFG signals 49, the foreground V[k] vectors 51 k and the order-reduced ambient HOA coefficients 47), prior to outputting ambient HOA coefficients 47 to the psychoacoustic audio coder unit 40.

[0165] In some instances, the energy compensation unit 38 may identify the RMS for each row and / or column of on one or more of the reordered US[k] matrix 33' and the reordered V[k] matrix 35'. The energy compensation unit 38 may also identify the RMS for each row and / or column of one or more of the selected foreground channels, which may include the nFG signals 49 and the foreground V[k] vectors 51 k , and the order-reduced ambient HOA coefficients 47. The RMS for each row and / or column of the one or more of the reordered US[k] matrix 33' and the reordered V[k] matrix 35' may be stored to a vector denoted RMS FULL , while the RMS for each row and / or column of one or more of the nFG signals 49, the foreground V[k] vectors 51 k , and the order-reduced ambient HOA coefficients 47 may be stored to a vector denoted RMS REDUCED . The energy compensation unit 38 may then compute an amplification value vector Z, in accordance with the following equation: Z = RMS FULL / RMS REDUCED . The energy compensation unit 38 may then apply this amplification value vector Z or various portions thereof to one or more of the nFG signals 49, the foreground V[k] vectors 51 k , and the order-reduced ambient HOA coefficients 47. In some instances, the amplification value vector Z is applied to only the order-reduced ambient HOA coefficients 47 per the following equation HOA BG-RED ' = HOA BG-RED Z T< , where HOA BG-RED denotes the order-reduced ambient HOA coefficients 47, HOA BG-RED ' denotes the energy compensated, reduced ambient HOA coefficients 47' and Z T< denotes the transpose of the Z vector.

[0166] . In some examples, to determine each RMS of respective rows and / or columns of one or more of the reordered US[k] matrix 33', the reordered V[k] matrix 35', the nFG signals 49, the foreground V[k] vectors 51 k , and the order-reduced ambient HOA coefficients 47, the energy compensation unit 38 may first apply a reference spherical harmonics coefficients (SHC) renderer to the columns. Application of the reference SHC renderer by the energy compensation unit 38 allows for determination of RMS in the SHC domain to determine the energy of the overall soundfield described by each row and / or column of the frame represented by rows and / or columns of one or more of the reordered US[k] matrix 33', the reordered V[k] matrix 35', the nFG signals 49, the foreground V[k] vectors 51 k , and the order-reduced ambient HOA coefficients 47, as described in more detail below.

[0167] The spatio-temporal interpolation unit 50 may represent a unit configured to receive the foreground V[k] vectors 51 k for the k'th frame and the foreground V[k-1] vectors 51 k-1 for the previous frame (hence the k-1 notation) and perform spatio-temporal interpolation to generate interpolated foreground V[k] vectors. The spatio-temporal interpolation unit 50 may recombine the nFG signals 49 with the foreground V[k] vectors 51 k to recover reordered foreground HOA coefficients. The spatio-temporal interpolation unit 50 may then divide the reordered foreground HOA coefficients by the interpolated V[k] vectors to generate interpolated nFG signals 49'. The spatio-temporal interpolation unit 50 may also output those of the foreground V[k] vectors 51 k that were used to generate the interpolated foreground V[k] vectors so that an audio decoding device, such as the audio decoding device 24, may generate the interpolated foreground V[k] vectors and thereby recover the foreground V[k] vectors 51 k . Those of the foreground V[k] vectors 51 k used to generate the interpolated foreground V[k] vectors are denoted as the remaining foreground V[k] vectors 53. In order to ensure that the same V[k] and V[k-1] are used at the encoder and decoder( to create the interpolated vectors V[k]) quantized / dequantized versions of these may be used at the encoder and decoder.

[0168] In this respect, the spatio-temporal interpolation unit 50 may represent a unit that interpolates a first portion of a first audio frame from some other portions of the first audio frame and a second temporally subsequent or preceding audio frame. In some examples, the portions may be denoted as sub-frames, where interpolation as performed with respect to sub-frames is described in more detail below with respect to FIGS. 45-46E. In other examples, the spatio-temporal interpolation unit 50 may operate with respect to some last number of samples of the previous frame and some first number of samples of the subsequent frame, as described in more detail with respect to FIGS. 37-39. The spatio-temporal interpolation unit 50 may, in performing this interpolation, reduce the number of samples of the foreground V[k] vectors 51 k that are required to be specified in the bitstream 21, as only those of the foreground V[k] vectors 51 k that are used to generate the interpolated V[k] vectors represent a subset of the foreground V[k] vectors 51 k . That is, in order to potentially make compression of the HOA coefficients 11 more efficient (by reducing the number of the foreground V[k] vectors 51 k that are specified in the bitstream 21), various aspects of the techniques described in this disclosure may provide for interpolation of one or more portions of the first audio frame, where each of the portions may represent decomposed versions of the HOA coefficients 11.

[0169] The spatio-temporal interpolation may result in a number of benefits. First, the nFG signals 49 may not be continuous from frame to frame due to the block-wise nature in which the SVD or other LIT is performed. In other words, given that the LIT unit 30 applies the SVD on a frame-by-frame basis, certain discontinuities may exist in the resulting transformed HOA coefficients as evidence for example by the unordered nature of the US[k] matrix 33 and V[k] matrix 35. By performing this interpolation, the discontinuity may be reduced given that interpolation may have a smoothing effect that potentially reduces any artifacts introduced due to frame boundaries (or, in other words, segmentation of the HOA coefficients 11 into frames). Using the foreground V[k] vectors 51 k to perform this interpolation and then generating the interpolated nFG signals 49' based on the interpolated foreground V[k] vectors 51 k from the recovered reordered HOA coefficients may smooth at least some effects due to the frame-by-frame operation as well as due to reordering the nFG signals 49.

[0170] In operation, the spatio-temporal interpolation unit 50 may interpolate one or more sub-frames of a first audio frame from a first decomposition, e.g., foreground V[k] vectors 51 k , of a portion of a first plurality of the HOA coefficients 11 included in the first frame and a second decomposition, e.g., foreground V[k] vectors 51 k-1 , of a portion of a second plurality of the HOA coefficients 11 included in a second frame to generate decomposed interpolated spherical harmonic coefficients for the one or more sub-frames.

[0171] In some examples, the first decomposition comprises the first foreground V[k] vectors 51 k representative of right-singular vectors of the portion of the HOA coefficients 11. Likewise, in some examples, the second decomposition comprises the second foreground V[k] vectors 51 k representative of right-singular vectors of the portion of the HOA coefficients 11.

[0172] In other words, spherical harmonics-based 3D audio may be a parametric representation of the 3D pressure field in terms of orthogonal basis functions on a sphere. The higher the order N of the representation, the potentially higher the spatial resolution, and often the larger the number of spherical harmonics (SH) coefficients (for a total of (N+1) 2< coefficients). For many applications, a bandwidth compression of the coefficients may be required for being able to transmit and store the coefficients efficiently. This techniques directed in this disclosure may provide a frame-based, dimensionality reduction process using Singular Value Decomposition (SVD). The SVD analysis may decompose each frame of coefficients into three matrices U, S and V. In some examples, the techniques may handle some of the vectors in US[k] matrix as foreground components of the underlying soundfield. However, when handled in this manner, these vectors (in U S[k] matrix) are discontinuous from frame to frame - even though they represent the same distinct audio component. These discontinuities may lead to significant artifacts when the components are fed through transform-audio-coders.

[0173] The techniques described in this disclosure may address this discontinuity. That is, the techniques may be based on the observation that the V matrix can be interpreted as orthogonal spatial axes in the Spherical Harmonics domain. The U[k] matrix may represent a projection of the Spherical Harmonics (HOA) data in terms of those basis functions, where the discontinuity can be attributed to orthogonal spatial axis (V[k]) that change every frame - and are therefore discontinuous themselves. This is unlike similar decomposition, such as the Fourier Transform, where the basis functions are, in some examples, constant from frame to frame. In these terms, the SVD may be considered of as a matching pursuit algorithm. The techniques described in this disclosure may enable the spatio-temporal interpolation unit 50 to maintain the continuity between the basis functions (V[k]) from frame to frame - by interpolating between them.

[0174] As noted above, the interpolation may be performed with respect to samples. This case is generalized in the above description when the subframes comprise a single set of samples. In both the case of interpolation over samples and over subframes, the interpolation operation may take the form of the following equation: v l ¯ = w l v k + 1 − w l v k − 1 . In this above equation, the interpolation may be performed with respect to the single V-vector v(k) from the single V-vector v(k - 1), which in one embodiment could represent V-vectors from adjacent frames k and k-1. In the above equation, l, represents the resolution over which the interpolation is being carried out, where l may indicate a integer sample and / = 1, ..., T (where T is the length of samples over which the interpolation is being carried out and over which the output interpolated vectors, v(l) are required and also indicates that the output of this process produces l of these vectors). Alternatively, l could indicate subframes consisting of multiple samples. When, for example, a frame is divided into four subframes, l may comprise values of 1, 2, 3 and 4, for each one of the subframes. The value of l may be signaled as a field termed "CodedSpatialInterpolationTime" through a bitstream - so that the interpolation operation may be replicated in the decoder. The w(l) may comprise values of the interpolation weights. When the interpolation is linear, w(l) may vary linearly and monotonically between 0 and 1, as a function of l. In other instances, w(l) may vary between 0 and 1 in a non-linear but monotonic fashion (such as a quarter cycle of a raised cosine) as a function of l. The function, w(l), may be indexed between a few different possibilities of functions and signaled in the bitstream as a field termed "SpatialInterpolationMethod" such that the identical interpolation operation may be replicated by the decoder. When w(l) is a value close to 0, the output, v(l) may be highly weighted or influenced by v(k - 1). Whereas when w(l) is a value close to 1, it ensures that the output, v(l), is highly weighted or influenced by v(k - 1).

[0175] The coefficient reduction unit 46 may represent a unit configured to perform coefficient reduction with respect to the remaining foreground V[k] vectors 53 based on the background channel information 43 to output reduced foreground V[k] vectors 55 to the quantization unit 52. The reduced foreground V[k] vectors 55 may have dimensions D: [(N+1) 2< - (N BG +1) 2< -nBGa] x nFG.

[0176] The coefficient reduction unit 46 may, in this respect, represent a unit configured to reduce the number of coefficients of the remaining foreground V[k] vectors 53. In other words, coefficient reduction unit 46 may represent a unit configured to eliminate those coefficients of the foreground V[k] vectors (that form the remaining foreground V[k] vectors 53) having little to no directional information. As described above, in some examples, those coefficients of the distinct or, in other words, foreground V[k] vectors corresponding to a first and zero order basis functions (which may be denoted as N BG ) provide little directional information and therefore can be removed from the foreground V vectors (through a process that may be referred to as "coefficient reduction"). In this example, greater flexibility may be provided to not only identify these coefficients that correspond N BG but to identify additional HOA channels (which may be denoted by the variable TotalOfAddAmbHOAChan) from the set of [(N BG +1) 2< +1, (N+1) 2< ]. The soundfield analysis unit 44 may analyze the HOA coefficients 11 to determine BG TOT , which may identify not only the (N BG +1) 2< but the TotalOfAddAmbHOAChan, which may collectively be referred to as the background channel information 43. The coefficient reduction unit 46 may then remove those coefficients corresponding to the (N BG +1) 2< and the TotalOfAddAmbHOAChan from the remaining foreground V[k] vectors 53 to generate a smaller dimensional V[k] matrix 55 of size ((N+1) 2< - (BG TOT ) x nFG, which may also be referred to as the reduced foreground V[k] vectors 55.

[0177] The quantization unit 52 may represent a unit configured to perform any form of quantization to compress the reduced foreground V[k] vectors 55 to generate coded foreground V[k] vectors 57, outputting these coded foreground V[k] vectors 57 to the bitstream generation unit 42. In operation, the quantization unit 52 may represent a unit configured to compress a spatial component of the soundfield, i.e., one or more of the reduced foreground V[k] vectors 55 in this example. For purposes of example, the reduced foreground V[k] vectors 55 are assumed to include two row vectors having, as a result of the coefficient reduction, less than 25 elements each (which implies a fourth order HOA representation of the soundfield). Although described with respect to two row vectors, any number of vectors may be included in the reduced foreground V[k] vectors 55 up to (n+1) 2< , where n denotes the order of the HOA representation of the soundfield. Moreover, although described below as performing a scalar and / or entropy quantization, the quantization unit 52 may perform any form of quantization that results in compression of the reduced foreground V[k] vectors 55.

[0178] The quantization unit 52 may receive the reduced foreground V[k] vectors 55 and perform a compression scheme to generate coded foreground V[k] vectors 57. This compression scheme may involve any conceivable compression scheme for compressing elements of a vector or data generally, and should not be limited to the example described below in more detail. The quantization unit 52 may perform, as an example, a compression scheme that includes one or more of transforming floating point representations of each element of the reduced foreground V[k] vectors 55 to integer representations of each element of the reduced foreground V[k] vectors 55, uniform quantization of the integer representations of the reduced foreground V[k] vectors 55 and categorization and coding of the quantized integer representations of the remaining foreground V[k] vectors 55.

[0179] In some examples, various of the one or more processes of this compression scheme may be dynamically controlled by parameters to achieve or nearly achieve, as one example, a target bitrate for the resulting bitstream 21. Given that each of the reduced foreground V[k] vectors 55 are orthonormal to one another, each of the reduced foreground V[k] vectors 55 may be coded independently. In some examples, as described in more detail below, each element of each reduced foreground V[k] vectors 55 may be coded using the same coding mode (defined by various sub-modes).

[0180] In any event, as noted above, this coding scheme may first involve transforming the floating point representations of each element (which is, in some examples, a 32-bit floating point number) of each of the reduced foreground V[k] vectors 55 to a 16-bit integer representation. The quantization unit 52 may perform this floating-point-to-integer-transformation by multiplying each element of a given one of the reduced foreground V[k] vectors 55 by 2 15< , which is, in some examples, performed by a right shift by 15.

[0181] The quantization unit 52 may then perform uniform quantization with respect to all of the elements of the given one of the reduced foreground V[k] vectors 55. The quantization unit 52 may identify a quantization step size based on a value, which may be denoted as an nbits parameter. The quantization unit 52 may dynamically determine this nbits parameter based on the target bitrate 41. The quantization unit 52 may determining the quantization step size as a function of this nbits parameter. As one example, the quantization unit 52 may determine the quantization step size (denoted as "delta" or "Δ" in this disclosure) as equal to 2 16-< nbits< . In this example, if nbits equals six, delta equals 2 10< and there are 2 6< quantization levels. In this respect, for a vector element v, the quantized vector element v q equals [ν / Δ] and -2 nbits-1< < ν q < 2 nbits-1< .

[0182] The quantization unit 52 may then perform categorization and residual coding of the quantized vector elements. As one example, the quantization unit 52 may, for a given quantized vector element ν q identify a category (by determining a category identifier cid) to which this element corresponds using the following equation: cid = 0 , if v q = 0 log 2 v q + 1 , if v q ≠ 0 The quantization unit 52 may then Huffman code this category index cid, while also identifying a sign bit that indicates whether ν q is a positive value or a negative value. The quantization unit 52 may next identify a residual in this category. As one example, the quantization unit 52 may determine this residual in accordance with the following equation: residual = v q − 2 cid − 1 The quantization unit 52 may then block code this residual with cid-1 bits.

[0183] The following example illustrates a simplified example of this categorization and residual coding process. First, assume nbits equals six so that ν q ∈ [-31,31]. Next, assume the following: cid vq Huffman Code for cid 00'1'1-1, 1'01'2-3,-2, 2,3'000'3-7,-6,-5,-4, 4,5,6,7'0010'4-15,-14,...,-8, 8,...,14,15'00110'5-31,-30,...,-16, 16,...,30,31'00111' Also, assume the following: cid Block Code for Residual 0N / A10, 1201,00, 10,113011,010,001,000, 100,101,110,11140111,0110...,0000, 1000,...,1110,1111501111, ... ,00000, 10000, ... ,11111 Thus, for a ν q = [6, -17, 0, 0, 3], the following may be determined: » cid = 3,5,0,0,2 » sign=1,0,x,x,1 » residual = 2,1,x,x,1 » Bits for 6 = '0010' + '1' + '10' » Bits for -17 = '00111' + '0' + '0001' » Bits for 0 = '0' » Bits for 0 = '0' » Bits for 3 = '000' + '1' + '1' » Total bits = 7+10+1+1+5 = 24 » Average bits = 24 / 5 = 4.8

[0184] While not shown in the foregoing simplified example, the quantization unit 52 may select different Huffman code books for different values of nbits when coding the cid. In some examples, the quantization unit 52 may provide a different Huffman coding table for nbits values 6, ..., 15. Moreover, the quantization unit 52 may include five different Huffman code books for each of the different nbits values ranging from 6, ..., 15 for a total of 50 Huffman code books. In this respect, the quantization unit 52 may include a plurality of different Huffman code books to accommodate coding of the cid in a number of different statistical contexts.

[0185] To illustrate, the quantization unit 52 may, for each of the nbits values, include a first Huffman code book for coding vector elements one through four, a second Huffman code book for coding vector elements five through nine, a third Huffman code book for coding vector elements nine and above. These first three Huffman code books may be used when the one of the reduced foreground V[k] vectors 55 to be compressed is not predicted from a temporally subsequent corresponding one of the reduced foreground V[k] vectors 55 and is not representative of spatial information of a synthetic audio object (one defined, for example, originally by a pulse code modulated (PCM) audio object). The quantization unit 52 may additionally include, for each of the nbits values, a fourth Huffman code book for coding the one of the reduced foreground V[k] vectors 55 when this one of the reduced foreground V[k] vectors 55 is predicted from a temporally subsequent corresponding one of the reduced foreground V[k] vectors 55. The quantization unit 52 may also include, for each of the nbits values, a fifth Huffman code book for coding the one of the reduced foreground V[k] vectors 55 when this one of the reduced foreground V[k] vectors 55 is representative of a synthetic audio object. The various Huffman code books may be developed for each of these different statistical contexts, i.e., the non-predicted and non-synthetic context, the predicted context and the synthetic context in this example.

[0186] The following table illustrates the Huffman table selection and the bits to be specified in the bitstream to enable the decompression unit to select the appropriate Huffman table: Pred mode HT info HT table 00HT501HT{1,2,3}10HT411HT5 In the foregoing table, the prediction mode ("Pred mode") indicates whether prediction was performed for the current vector, while the Huffman Table ("HT info") indicates additional Huffman code book (or table) information used to select one of Huffman tables one through five.

[0187] The following table further illustrates this Huffman table selection process given various statistical contexts or scenarios. Recording Synthetic W / O Pred HT{1,2,3}HT5With Pred HT4HT5 In the foregoing table, the "Recording" column indicates the coding context when the vector is representative of an audio object that was recorded while the "Synthetic" column indicates a coding context for when the vector is representative of a synthetic audio object. The "W / O Pred" row indicates the coding context when prediction is not performed with respect to the vector elements, while the "With Pred" row indicates the coding context when prediction is performed with respect to the vector elements. As shown in this table, the quantization unit 52 selects HT{1, 2, 3} when the vector is representative of a recorded audio object and prediction is not performed with respect to the vector elements. The quantization unit 52 selects HT5 when the audio object is representative of a synthetic audio object and prediction is not performed with respect to the vector elements. The quantization unit 52 selects HT4 when the vector is representative of a recorded audio object and prediction is performed with respect to the vector elements. The quantization unit 52 selects HT5 when the audio object is representative of a synthetic audio object and prediction is performed with respect to the vector elements.

[0188] In this respect, the quantization unit 52 may perform the above noted scalar quantization and / or Huffman encoding to compress the reduced foreground V[k] vectors 55, outputting the coded foreground V[k] vectors 57, which may be referred to as side channel information 57. This side channel information 57 may include syntax elements used to code the remaining foreground V[k] vectors 55. The quantization unit 52 may output the side channel information 57 in a manner similar to that shown in the example of one of FIGS. 10B and 10C.

[0189] As noted above, the quantization unit 52 may generate syntax elements for the side channel information 57. For example, the quantization unit 52 may specify a syntax element in a header of an access unit (which may include one or more frames) denoting which of the plurality of configuration modes was selected. Although described as being specified on a per access unit basis, quantization unit 52 may specify this syntax element on a per frame basis or any other periodic basis or non-periodic basis (such as once for the entire bitstream). In any event, this syntax element may comprise two bits indicating which of the four configuration modes were selected for specifying the non-zero set of coefficients of the reduced foreground V[k] vectors 55 to represent the directional aspects of this distinct component. The syntax element may be denoted as "codedVVecLength." In this manner, the quantization unit 52 may signal or otherwise specify in the bitstream which of the four configuration modes were used to specify the coded foreground V[k] vectors 57 in the bitstream. Although described with respect to four configuration modes, the techniques should not be limited to four configuration modes but to any number of configuration modes, including a single configuration mode or a plurality of configuration modes. The scalar / entropy quantization unit 53 may also specify the flag 63 as another syntax element in the side channel information 57.

[0190] The psychoacoustic audio coder unit 40 included within the audio encoding device 20 may represent multiple instances of a psychoacoustic audio coder, each of which is used to encode a different audio object or HOA channel of each of the energy compensated ambient HOA coefficients 47' and the interpolated nFG signals 49' to generate encoded ambient HOA coefficients 59 and encoded nFG signals 61. The psychoacoustic audio coder unit 40 may output the encoded ambient HOA coefficients 59 and the encoded nFG signals 61 to the bitstream generation unit 42.

[0191] In some instances, this psychoacoustic audio coder unit 40 may represent one or more instances of an advanced audio coding (AAC) encoding unit. The psychoacoustic audio coder unit 40 may encode each column or row of the energy compensated ambient HOA coefficients 47' and the interpolated nFG signals 49'. Often, the psychoacoustic audio coder unit 40 may invoke an instance of an AAC encoding unit for each of the order / sub-order combinations remaining in the energy compensated ambient HOA coefficients 47' and the interpolated nFG signals 49'. More information regarding how the background spherical harmonic coefficients 31 may be encoded using an AAC encoding unit can be found in a convention paper by Eric Hellerud, et al., entitled "Encoding Higher Order Ambisonics with AAC," presented at the 124th Convention, 2008 May 17-20 and available at: http: / / ro.uow.edu.au / cgi / viewcontent.cgi?article=8025&context=engpapers. In some instances, the audio encoding unit 14 may audio encode the energy compensated ambient HOA coefficients 47' using a lower target bitrate than that used to encode the interpolated nFG signals 49', thereby potentially compressing the energy compensated ambient HOA coefficients 47' more in comparison to the interpolated nFG signals 49'.

[0192] The bitstream generation unit 42 included within the audio encoding device 20 represents a unit that formats data to conform to a known format (which may refer to a format known by a decoding device), thereby generating the vector-based bitstream 21. The bitstream generation unit 42 may represent a multiplexer in some examples, which may receive the coded foreground V[k] vectors 57, the encoded ambient HOA coefficients 59, the encoded nFG signals 61 and the background channel information 43. The bitstream generation unit 42 may then generate a bitstream 21 based on the coded foreground V[k] vectors 57, the encoded ambient HOA coefficients 59, the encoded nFG signals 61 and the background channel information 43. The bitstream 21 may include a primary or main bitstream and one or more side channel bitstreams.

[0193] Although not shown in the example of FIG. 4, the audio encoding device 20 may also include a bitstream output unit that switches the bitstream output from the audio encoding device 20 (e.g., between the directional-based bitstream 21 and the vector-based bitstream 21) based on whether a current frame is to be encoded using the directional-based synthesis or the vector-based synthesis. This bitstream output unit may perform this switch based on the syntax element output by the content analysis unit 26 indicating whether a directional-based synthesis was performed (as a result of detecting that the HOA coefficients 11 were generated from a synthetic audio object) or a vector-based synthesis was performed (as a result of detecting that the HOA coefficients were recorded). The bitstream output unit may specify the correct header syntax to indicate this switch or current encoding used for the current frame along with the respective one of the bitstreams 21.

[0194] In some instances, various aspects of the techniques may also enable the audio encoding device 20 to determine whether HOA coefficients 11 are generated from a synthetic audio object. These aspects of the techniques may enable the audio encoding device 20 to be configured to obtain an indication of whether spherical harmonic coefficients representative of a sound field are generated from a synthetic audio object.

[0195] In these and other instances, the audio encoding device 20 is further configured to determine whether the spherical harmonic coefficients are generated from the synthetic audio object.

[0196] In these and other instances, the audio encoding device 20 is configured to exclude a first vector from a framed spherical harmonic coefficient matrix storing at least a portion of the spherical harmonic coefficients representative of the sound field to obtain a reduced framed spherical harmonic coefficient matrix.

[0197] In these and other instances, the audio encoding device 20 is configured to exclude a first vector from a framed spherical harmonic coefficient matrix storing at least a portion of the spherical harmonic coefficients representative of the sound field to obtain a reduced framed spherical harmonic coefficient matrix, and predict a vector of the reduced framed spherical harmonic coefficient matrix based on remaining vectors of the reduced framed spherical harmonic coefficient matrix.

[0198] In these and other instances, the audio encoding device 20 is configured to exclude a first vector from a framed spherical harmonic coefficient matrix storing at least a portion of the spherical harmonic coefficients representative of the sound field to obtain a reduced framed spherical harmonic coefficient matrix, and predict a vector of the reduced framed spherical harmonic coefficient matrix based, at least in part, on a sum of remaining vectors of the reduced framed spherical harmonic coefficient matrix.

[0199] In these and other instances, the audio encoding device 20 is configured to predict a vector of a framed spherical harmonic coefficient matrix storing at least a portion of the spherical harmonic coefficients based, at least in part, on a sum of remaining vectors of the framed spherical harmonic coefficient matrix.

[0200] In these and other instances, the audio encoding device 20 is configured to further configured to predict a vector of a framed spherical harmonic coefficient matrix storing at least a portion of the spherical harmonic coefficients based, at least in part, on a sum of remaining vectors of the framed spherical harmonic coefficient matrix, and compute an error based on the predicted vector.

[0201] In these and other instances, the audio encoding device 20 is configured to configured to predict a vector of a framed spherical harmonic coefficient matrix storing at least a portion of the spherical harmonic coefficients based, at least in part, on a sum of remaining vectors of the framed spherical harmonic coefficient matrix, and compute an error based on the predicted vector and the corresponding vector of the framed spherical harmonic coefficient matrix.

[0202] In these and other instances, the audio encoding device 20 is configured to configured to predict a vector of a framed spherical harmonic coefficient matrix storing at least a portion of the spherical harmonic coefficients based, at least in part, on a sum of remaining vectors of the framed spherical harmonic coefficient matrix, and compute an error as a sum of the absolute value of the difference of the predicted vector and the corresponding vector of the framed spherical harmonic coefficient matrix.

[0203] In these and other instances, the audio encoding device 20 is configured to configured to predict a vector of a framed spherical harmonic coefficient matrix storing at least a portion of the spherical harmonic coefficients based, at least in part, on a sum of remaining vectors of the framed spherical harmonic coefficient matrix, compute an error based on the predicted vector and the corresponding vector of the framed spherical harmonic coefficient matrix, compute a ratio based on an energy of the corresponding vector of the framed spherical harmonic coefficient matrix and the error, and compare the ratio to a threshold to determine whether the spherical harmonic coefficients representative of the sound field are generated from the synthetic audio object.

[0204] In these and other instances, the audio encoding device 20 is configured to configured to specify the indication in a bitstream 21 that stores a compressed version of the spherical harmonic coefficients.

[0205] In some instances, the various techniques may enable the audio encoding device 20 to perform a transformation with respect to the HOA coefficients 11. In these and other instances, the audio encoding device 20 may be configured to obtain one or more first vectors describing distinct components of the soundfield and one or more second vectors describing background components of the soundfield, both the one or more first vectors and the one or more second vectors generated at least by performing a transformation with respect to the plurality of spherical harmonic coefficients 11.

[0206] In these and other instances, the audio encoding device 20, wherein the transformation comprises a singular value decomposition that generates a U matrix representative of left-singular vectors of the plurality of spherical harmonic coefficients, an S matrix representative of singular values of the plurality of spherical harmonic coefficients and a V matrix representative of right-singular vectors of the plurality of spherical harmonic coefficients 11.

[0207] In these and other instances, the audio encoding device 20, wherein the one or more first vectors comprise one or more audio encoded U DIST * S DIST vectors that, prior to audio encoding, were generated by multiplying one or more audio encoded U DIST vectors of a U matrix by one or more S DIST vectors of an S matrix, and wherein the U matrix and the S matrix are generated at least by performing the singular value decomposition with respect to the plurality of spherical harmonic coefficients.

[0208] In these and other instances, the audio encoding device 20, wherein the one or more first vectors comprise one or more audio encoded U DIST * S DIST vectors that, prior to audio encoding, were generated by multiplying one or more audio encoded U DIST vectors of a U matrix by one or more S DIST vectors of an S matrix, and one or more V T< DIST vectors of a transpose of a V matrix, and wherein the U matrix and the S matrix and the V matrix are generated at least by performing the singular value decomposition with respect to the plurality of spherical harmonic coefficients 11.

[0209] In these and other instances, the audio encoding device 20, wherein the one or more first vectors comprise one or more U DIST * S DIST vectors that, prior to audio encoding, were generated by multiplying one or more audio encoded U DIST vectors of a U matrix by one or more S DIST vectors of an S matrix, and one or more V T< DIST vectors of a transpose of a V matrix, wherein the U matrix, the S matrix and the V matrix were generated at least by performing the singular value decomposition with respect to the plurality of spherical harmonic coefficients, and wherein the audio encoding device 20 is further configured to obtain a value D indicating the number of vectors to be extracted from a bitstream to form the one or more U DIST * S DIST vectors and the one or more V T< DIST vectors.

[0210] In these and other instances, the audio encoding device 20, wherein the one or more first vectors comprise one or more U DIST * S DIST vectors that, prior to audio encoding, were generated by multiplying one or more audio encoded U DIST vectors of a U matrix by one or more S DIST vectors of an S matrix, and one or more V T< DIST vectors of a transpose of a V matrix, wherein the U matrix, the S matrix and the V matrix were generated at least by performing the singular value decomposition with respect to the plurality of spherical harmonic coefficients, and wherein the audio encoding device 20 is further configured to obtain a value D on an audio-frame-by-audio-frame basis that indicates the number of vectors to be extracted from a bitstream to form the one or more U DIST * S DIST vectors and the one or more V T< DIST vectors.

[0211] In these and other instances, the audio encoding device 20, wherein the transformation comprises a principal component analysis to identify the distinct components of the soundfield and the background components of the soundfield.

[0212] Various aspects of the techniques described in this disclosure may provide for the audio encoding device 20 configured to compensate for quantization error.

[0213] In some instances, the audio encoding device 20 may be configured to quantize one or more first vectors representative of one or more components of a sound field, and compensate for error introduced due to the quantization of the one or more first vectors in one or more second vectors that are also representative of the same one or more components of the sound field.

[0214] In these and other instances, the audio encoding device is configured to quantize one or more vectors from a transpose of a V matrix generated at least in part by performing a singular value decomposition with respect to a plurality of spherical harmonic coefficients that describe the sound field.

[0215] In these and other instances, the audio encoding device is further configured to perform a singular value decomposition with respect to a plurality of spherical harmonic coefficients representative of a sound field to generate a U matrix representative of left-singular vectors of the plurality of spherical harmonic coefficients, an S matrix representative of singular values of the plurality of spherical harmonic coefficients and a V matrix representative of right-singular vectors of the plurality of spherical harmonic coefficients, and configured to quantize one or more vectors from a transpose of the V matrix.

[0216] In these and other instances, the audio encoding device is further configured to perform a singular value decomposition with respect to a plurality of spherical harmonic coefficients representative of a sound field to generate a U matrix representative of left-singular vectors of the plurality of spherical harmonic coefficients, an S matrix representative of singular values of the plurality of spherical harmonic coefficients and a V matrix representative of right-singular vectors of the plurality of spherical harmonic coefficients, configured to quantize one or more vectors from a transpose of the V matrix, and configured to compensate for the error introduced due to the quantization in one or more U * S vectors computed by multiplying one or more U vectors of the U matrix by one or more S vectors of the S matrix.

[0217] In these and other instances, the audio encoding device is further configured to perform a singular value decomposition with respect to a plurality of spherical harmonic coefficients representative of a sound field to generate a U matrix representative of left-singular vectors of the plurality of spherical harmonic coefficients, an S matrix representative of singular values of the plurality of spherical harmonic coefficients and a V matrix representative of right-singular vectors of the plurality of spherical harmonic coefficients, determine one or more U DIST vectors of the U matrix, each of which corresponds to a distinct component of the sound field, determine one or more S DIST vectors of the S matrix, each of which corresponds to the same distinct component of the sound field, and determine one or more V T< DIST vectors of a transpose of the V matrix, each of which corresponds to the same distinct component of the sound field, configured to quantize the one or more V T< DIST vectors to generate one or more V T< Q_DIST vectors, and configured to compensate for the error introduced due to the quantization in one or more U DIST * S DIST vectors computed by multiplying the one or more U DIST vectors of the U matrix by one or more S DIST vectors of the S matrix so as to generate one or more error compensated U DIST * S DIST vectors.

[0218] In these and other instances, the audio encoding device is configured to determine distinct spherical harmonic coefficients based on the one or more U DIST vectors, the one or more S DIST vectors and the one or more V T< DIST vectors, and perform a pseudo inverse with respect to the V T< Q_DIST vectors to divide the distinct spherical harmonic coefficients by the one or more V T< Q_DIST vectors and thereby generate error compensated one or more U C_DIST * S C_DIST vectors that compensate at least in part for the error introduced through the quantization of the V T< DIST vectors.

[0219] In these and other instances, the audio encoding device is further configured to perform a singular value decomposition with respect to a plurality of spherical harmonic coefficients representative of a sound field to generate a U matrix representative of left-singular vectors of the plurality of spherical harmonic coefficients, an S matrix representative of singular values of the plurality of spherical harmonic coefficients and a V matrix representative of right-singular vectors of the plurality of spherical harmonic coefficients, determine one or more U BG vectors of the U matrix that describe one or more background components of the sound field and one or more U DIST vectors of the U matrix that describe one or more distinct components of the sound field, determine one or more S BG vectors of the S matrix that describe the one or more background components of the sound field and one or more S DIST vectors of the S matrix that describe the one or more distinct components of the sound field, and determine one or more V T< DIST vectors and one or more V T< BG vectors of a transpose of the V matrix, wherein the V T< DIST vectors describe the one or more distinct components of the sound field and the V T< BG describe the one or more background components of the sound field, configured to quantize the one or more V T< DIST vectors to generate one or more V T< Q_DIST vectors, and configured to compensate for the error introduced due to the quantization in background spherical harmonic coefficients formed by multiplying the one or more U BG vectors by the one or more S BG vectors and then by the one or more V T< BG vectors so as to generate error compensated background spherical harmonic coefficients.

[0220] In these and other instances, the audio encoding device is configured to determine the error based on the V T< DIST vectors and one or more U DIST * S DIST vectors formed by multiplying the U DIST vectors by the S DIST vectors, and add the determined error to the background spherical harmonic coefficients to generate the error compensated background spherical harmonic coefficients.

[0221] In these and other instances, the audio encoding device is configured to compensate for the error introduced due to the quantization of the one or more first vectors in one or more second vectors that are also representative of the same one or more components of the sound field to generate one or more error compensated second vectors, and further configured to generate a bitstream to include the one or more error compensated second vectors and the quantized one or more first vectors.

[0222] In these and other instances, the audio encoding device is configured to compensate for the error introduced due to the quantization of the one or more first vectors in one or more second vectors that are also representative of the same one or more components of the sound field to generate one or more error compensated second vectors, and further configured to audio encode the one or more error compensated second vectors, and generate a bitstream to include the audio encoded one or more error compensated second vectors and the quantized one or more first vectors.

[0223] The various aspects of the techniques may further enable the audio encoding device 20 to generate reduced spherical harmonic coefficients or decompositions thereof. In some instances, the audio encoding device 20 may be configured to perform, based on a target bitrate, order reduction with respect to a plurality of spherical harmonic coefficients or decompositions thereof to generate reduced spherical harmonic coefficients or the reduced decompositions thereof, wherein the plurality of spherical harmonic coefficients represent a sound field.

[0224] In these and other instances, the audio encoding device 20 is further configured to, prior to performing the order reduction, perform a singular value decomposition with respect to the plurality of spherical harmonic coefficients to identify one or more first vectors that describe distinct components of the sound field and one or more second vectors that identify background components of the sound field, and configured to perform the order reduction with respect to the one or more first vectors, the one or more second vectors or both the one or more first vectors and the one or more second vectors.

[0225] In these and other instances, the audio encoding device 20 is further configured to performing a content analysis with respect to the plurality of spherical harmonic coefficients or the decompositions thereof, and configured to perform, based on the target bitrate and the content analysis, the order reduction with respect to the plurality of spherical harmonic coefficients or the decompositions thereof to generate the reduced spherical harmonic coefficients or the reduced decompositions thereof.

[0226] In these and other instances, the audio encoding device 20 is configured to perform a spatial analysis with respect to the plurality of spherical harmonic coefficients or the decompositions thereof.

[0227] In these and other instances, the audio encoding device 20 is configured to perform a diffusion analysis with respect to the plurality of spherical harmonic coefficients or the decompositions thereof.

[0228] In these and other instances, the audio encoding device 20 is the one or more processors are configured to perform a spatial analysis and a diffusion analysis with respect to the plurality of spherical harmonic coefficients or the decompositions thereof.

[0229] In these and other instances, the audio encoding device 20 is further configured to specify one or more orders and / or one or more sub-orders of spherical basis functions to which those of the reduced spherical harmonic coefficients or the reduced decompositions thereof correspond in a bitstream that includes the reduced spherical harmonic coefficients or the reduced decompositions thereof.

[0230] In these and other instances, the reduced spherical harmonic coefficients or the reduced decompositions thereof have less values than the plurality of spherical harmonic coefficients or the decompositions thereof.

[0231] In these and other instances, the audio encoding device 20 is configured to remove those of the plurality of spherical harmonic coefficients or vectors of the decompositions thereof having a specified order and / or sub-order to generate the reduced spherical harmonic coefficients or the reduced decompositions thereof.

[0232] In these and other instances, the audio encoding device 20 is configured to zero out those of the plurality of spherical harmonic coefficients or those vectors of the decomposition thereof having a specified order and / or sub-order to generate the reduced spherical harmonic coefficients or the reduced decompositions thereof.

[0233] Various aspects of the techniques may also allow for the audio encoding device 20 to be configured to represent distinct components of the soundfield. In these and other instances, the audio encoding device 20 is configured to obtain a first non-zero set of coefficients of a vector to be used to represent a distinct component of a sound field, wherein the vector is decomposed from a plurality of spherical harmonic coefficients describing the sound field.

[0234] In these and other instances, the audio encoding device 20 is configured to determine the first non-zero set of the coefficients of the vector to include all of the coefficients.

[0235] In these and other instances, the audio encoding device 20 is configured to determine the first non-zero set of coefficients as those of the coefficients corresponding to an order greater than an order of a basis function to which one or more of the plurality of spherical harmonic coefficients correspond.

[0236] In these and other instances, the audio encoding device 20 is configured to determine the first non-zero set of coefficients to include those of the coefficients corresponding to an order greater than an order of a basis function to which one or more of the plurality of spherical harmonic coefficients correspond and excluding at least one of the coefficients corresponding to an order greater than the order of the basis function to which the one or more of the plurality of spherical harmonic coefficients correspond.

[0237] In these and other instances, the audio encoding device 20 is configured to determine the first non-zero set of coefficients to include all of the coefficients except for at least one of the coefficients corresponding to an order greater than an order of a basis function to which one or more of the plurality of spherical harmonic coefficients correspond.

[0238] In these and other instances, the audio encoding device 20 is further configured to specify the first non-zero set of the coefficients of the vector in side channel information.

[0239] In these and other instances, the audio encoding device 20 is further configured to specify the first non-zero set of the coefficients of the vector in side channel information without audio encoding the first non-zero set of the coefficients of the vector.

[0240] In these and other instances, the vector comprises a vector decomposed from the plurality of spherical harmonic coefficients using vector based synthesis.

[0241] In these and other instances, the vector based synthesis comprises a singular value decomposition.

[0242] In these and other instances, the vector comprises a V vector decomposed from the plurality of spherical harmonic coefficients using singular value decomposition.

[0243] In these and other instances, the audio encoding device 20 is further configured to select one of a plurality of configuration modes by which to specify the non-zero set of coefficients of the vector, and specify the non-zero set of the coefficients of the vector based on the selected one of the plurality of configuration modes.

[0244] In these and other instances, the one of the plurality of configuration modes indicates that the non-zero set of the coefficients includes all of the coefficients.

[0245] In these and other instances, the one of the plurality of configuration modes indicates that the non-zero set of coefficients include those of the coefficients corresponding to an order greater than an order of a basis function to which one or more of the plurality of spherical harmonic coefficients correspond.

[0246] In these and other instances, the one of the plurality of configuration modes indicates that the non-zero set of the coefficients include those of the coefficients corresponding to an order greater than an order of a basis function to which one or more of the plurality of spherical harmonic coefficients correspond and exclude at least one of the coefficients corresponding to an order greater than the order of the basis function to which the one or more of the plurality of spherical harmonic coefficients correspond,

[0247] In these and other instances, the one of the plurality of configuration modes indicates that the non-zero set of coefficients include all of the coefficients except for at least one of the coefficients.

[0248] In these and other instances, the audio encoding device 20 is further configured to specify the selected one of the plurality of configuration modes in a bitstream.

[0249] Various aspects of the techniques described in this disclosure may also allow for the audio encoding device 20 to be configured to represent that distinct component of the soundfield in various way. In these and other instances, the audio encoding device 20 is configured to obtain a first non-zero set of coefficients of a vector that represent a distinct component of a sound field, the vector having been decomposed from a plurality of spherical harmonic coefficients that describe the sound field.

[0250] In these and other instances, the first non-zero set of the coefficients includes all of the coefficients of the vector.

[0251] In these and other instances, the first non-zero set of coefficients include those of the coefficients corresponding to an order greater than an order of a basis function to which one or more of the plurality of spherical harmonic coefficients correspond.

[0252] In these and other instances, the first non-zero set of the coefficients include those of the coefficients corresponding to an order greater than an order of a basis function to which one or more of the plurality of spherical harmonic coefficients correspond and exclude at least one of the coefficients corresponding to an order greater than the order of the basis function to which the one or more of the plurality of spherical harmonic coefficients correspond.

[0253] In these and other instances, the first non-zero set of coefficients include all of the coefficients except for at least one of the coefficients identified as not have sufficient directional information.

[0254] In these and other instances, the audio encoding device 20 is further configured to extract the first non-zero set of the coefficients as a first portion of the vector.

[0255] In these and other instances, the audio encoding device 20 is further configured to extract the first non-zero set of the vector from side channel information, and obtain a recomposed version of the plurality of spherical harmonic coefficients based on the first non-zero set of the coefficients of the vector.

[0256] In these and other instances, the vector comprises a vector decomposed from the plurality of spherical harmonic coefficients using vector based synthesis.

[0257] In these and other instances, the vector based synthesis comprises singular value decomposition.

[0258] In these and other instances, the audio encoding device 20 is further configured to determine one of a plurality of configuration modes by which to extract the non-zero set of coefficients of the vector in accordance with the one of the plurality of configuration modes, and extract the non-zero set of the coefficients of the vector based on the obtained one of the plurality of configuration modes.

[0259] In these and other instances, the one of the plurality of configuration modes indicates that the non-zero set of the coefficients includes all of the coefficients.

[0260] In these and other instances, the one of the plurality of configuration modes indicates that the non-zero set of coefficients include those of the coefficients corresponding to an order greater than an order of a basis function to which one or more of the plurality of spherical harmonic coefficients correspond.

[0261] In these and other instances, the one of the plurality of configuration modes indicates that the non-zero set of the coefficients include those of the coefficients corresponding to an order greater than an order of a basis function to which one or more of the plurality of spherical harmonic coefficients correspond and exclude at least one of the coefficients corresponding to an order greater than the order of the basis function to which the one or more of the plurality of spherical harmonic coefficients correspond,

[0262] In these and other instances, the one of the plurality of configuration modes indicates that the non-zero set of coefficients include all of the coefficients except for at least one of the coefficients.

[0263] In these and other instances, the audio encoding device 20 is configured to determine the one of the plurality of configuration modes based on a value signaled in a bitstream.

[0264] Various aspects of the techniques may also, in some instances, enable the audio encoding device 20 to identify one or more distinct audio objects (or, in other words, predominant audio objects). In some instances, the audio encoding device 20 may be configured to identify one or more distinct audio objects from one or more spherical harmonic coefficients (SHC) associated with the audio objects based on a directionality determined for one or more of the audio objects.

[0265] In these and other instances, the audio encoding device 20 is further configured to determine the directionality of the one or more audio objects based on the spherical harmonic coefficients associated with the audio objects.

[0266] In these and other instances, the audio encoding device 20 is further configured to perform a singular value decomposition with respect to the spherical harmonic coefficients to generate a U matrix representative of left-singular vectors of the plurality of spherical harmonic coefficients, an S matrix representative of singular values of the plurality of spherical harmonic coefficients and a V matrix representative of right-singular vectors of the plurality of spherical harmonic coefficients, and represent the plurality of spherical harmonic coefficients as a function of at least a portion of one or more of the U matrix, the S matrix and the V matrix, wherein the audio encoding device 20 is configured to determine the respective directionality of the one or more audio objects is based at least in part on the V matrix.

[0267] In these and other instances, the audio encoding device 20 is further configured to reorder one or more vectors of the V matrix such that vectors having a greater directionality quotient are positioned above vectors having a lesser directionality quotient in the reordered V matrix.

[0268] In these and other instances, the audio encoding device 20 is further configured to determine that the vectors having the greater directionality quotient include greater directional information than the vectors having the lesser directionality quotient.

[0269] In these and other instances, the audio encoding device 20 is further configured to multiply the V matrix by the S matrix to generate a VS matrix, the VS matrix including one or more vectors.

[0270] In these and other instances, the audio encoding device 20 is further configured to select entries of each row of the VS matrix that are associated with an order greater than 14, square each of the selected entries to form corresponding squared entries, and for each row of the VS matrix, sum all of the squared entries to determine a directionality quotient for a corresponding vector.

[0271] In these and other instances, the audio encoding device 20 is configured to select the entries of each row of the VS matrix associated with the order greater than 14 comprises selecting all entries beginning at a 18th entry of each row of the VS matrix and ending at a 38th entry of each row of the VS matrix.

[0272] In these and other instances, the audio encoding device 20 is further configured to select a subset of the vectors of the VS matrix to represent the distinct audio objects. In these and other instances, the audio encoding device 20 is configured to select four vectors of the VS matrix, and wherein the selected four vectors have the four greatest directionality quotients of all of the vectors of the VS matrix.

[0273] In these and other instances, the audio encoding device 20 is configured to determine that the selected subset of the vectors represent the distinct audio objects is based on both the directionality and an energy of each vector.

[0274] In these and other instances, the audio encoding device 20 is further configured to perform an energy comparison between one or more first vectors and one or more second vectors representative of the distinct audio objects to determine reordered one or more first vectors, wherein the one or more first vectors describe the distinct audio objects a first portion of audio data and the one or more second vectors describe the distinct audio objects in a second portion of the audio data.

[0275] In these and other instances, the audio encoding device 20 is further configured to perform a cross-correlation between one or more first vectors and one or more second vectors representative of the distinct audio objects to determine reordered one or more first vectors, wherein the one or more first vectors describe the distinct audio objects a first portion of audio data and the one or more second vectors describe the distinct audio objects in a second portion of the audio data.

[0276] Various aspects of the techniques may also, in some instances, enable the audio encoding device 20 to be configured to perform energy compensation with respect to decompositions of the HOA coefficients 11. In these and other instances, the audio encoding device 20 may be configured to perform a vector-based synthesis with respect to a plurality of spherical harmonic coefficients to generate decomposed representations of the plurality of spherical harmonic coefficients representative of one or more audio objects and corresponding directional information, wherein the spherical harmonic coefficients are associated with an order and describe a sound field, determine distinct and background directional information from the directional information, reduce an order of the directional information associated with the background audio objects to generate transformed background directional information, apply compensation to increase values of the transformed directional information to preserve an overall energy of the sound field.

[0277] In these and other instances, the audio encoding device 20 may be configured to perform a singular value decomposition with respect to a plurality of spherical harmonic coefficients to generate a U matrix and an S matrix representative of the audio objects and a V matrix representative of the directional information, determine distinct column vectors of the V matrix and background column vectors of the V matrix, reduce an order of the background column vectors of the V matrix to generate transformed background column vectors of the V matrix, and apply the compensation to increase values of the transformed background column vectors of the V matrix to preserve an overall energy of the sound field.

[0278] In these and other instances, the audio encoding device 20 is further configured to determine a number of salient singular values of the S matrix, wherein a number of the distinct column vectors of the V matrix is the number of salient singular values of the S matrix.

[0279] In these and other instances, the audio encoding device 20 is configured to determine a reduced order for the spherical harmonics coefficients, and zero values for rows of the background column vectors of the V matrix associated with an order that is greater than the reduced order.

[0280] In these and other instances, the audio encoding device 20 is further configured to combine background columns of the U matrix, background columns of the S matrix, and a transpose of the transformed background columns of the V matrix to generate modified spherical harmonic coefficients.

[0281] In these and other instances, the modified spherical harmonic coefficients describe one or more background components of the sound field.

[0282] In these and other instances, the audio encoding device 20 is configured to determine a first energy of a vector of the background column vectors of the V matrix and a second energy of a vector of the transformed background column vectors of the V matrix, and apply an amplification value to each element of the vector of the transformed background column vectors of the V matrix, wherein the amplification value comprises a ratio of the first energy to the second energy.

[0283] In these and other instances, the audio encoding device 20 is configured to determine a first root mean-squared energy of a vector of the background column vectors of the V matrix and a second root mean-squared energy of a vector of the transformed background column vectors of the V matrix, and apply an amplification value to each element of the vector of the transformed background column vectors of the V matrix, wherein the amplification value comprises a ratio of the first energy to the second energy.

[0284] Various aspects of the techniques described in this disclosure may also enable the audio encoding device 20 to perform interpolation with respect to decomposed versions of the HOA coefficients 11. In some instances, the audio encoding device 20 may be configured to obtain decomposed interpolated spherical harmonic coefficients for a time segment by, at least in part, performing an interpolation with respect to a first decomposition of a first plurality of spherical harmonic coefficients and a second decomposition of a second plurality of spherical harmonic coefficients.

[0285] In these and other instances, the first decomposition comprises a first V matrix representative of right-singular vectors of the first plurality of spherical harmonic coefficients.

[0286] In these and other examples, the second decomposition comprises a second V matrix representative of right-singular vectors of the second plurality of spherical harmonic coefficients.

[0287] In these and other instances, the first decomposition comprises a first V matrix representative of right-singular vectors of the first plurality of spherical harmonic coefficients, and the second decomposition comprises a second V matrix representative of right-singular vectors of the second plurality of spherical harmonic coefficients.

[0288] In these and other instances, the time segment comprises a sub-frame of an audio frame.

[0289] In these and other instances, the time segment comprises a time sample of an audio frame.

[0290] In these and other instances, the audio encoding device 20 is configured to obtain an interpolated decomposition of the first decomposition and the second decomposition for a spherical harmonic coefficient of the first plurality of spherical harmonic coefficients.

[0291] In these and other instances, the audio encoding device 20 is configured to obtain interpolated decompositions of the first decomposition for a first portion of the first plurality of spherical harmonic coefficients included in the first frame and the second decomposition for a second portion of the second plurality of spherical harmonic coefficients included in the second frame, and the audio encoding device 20 is further configured to apply the interpolated decompositions to a first time component of the first portion of the first plurality of spherical harmonic coefficients included in the first frame to generate a first artificial time component of the first plurality of spherical harmonic coefficients, and apply the respective interpolated decompositions to a second time component of the second portion of the second plurality of spherical harmonic coefficients included in the second frame to generate a second artificial time component of the second plurality of spherical harmonic coefficients included.

[0292] In these and other instances, the first time component is generated by performing a vector-based synthesis with respect to the first plurality of spherical harmonic coefficients.

[0293] In these and other instances, the second time component is generated by performing a vector-based synthesis with respect to the second plurality of spherical harmonic coefficients.

[0294] In these and other instances, the audio encoding device 20 is further configured to receive the first artificial time component and the second artificial time component, compute interpolated decompositions of the first decomposition for the first portion of the first plurality of spherical harmonic coefficients and the second decomposition for the second portion of the second plurality of spherical harmonic coefficients, and apply inverses of the interpolated decompositions to the first artificial time component to recover the first time component and to the second artificial time component to recover the second time component.

[0295] In these and other instances, the audio encoding device 20 is configured to interpolate a first spatial component of the first plurality of spherical harmonic coefficients and the second spatial component of the second plurality of spherical harmonic coefficients.

[0296] In these and other instances, the first spatial component comprises a first U matrix representative of left-singular vectors of the first plurality of spherical harmonic coefficients.

[0297] In these and other instances, the second spatial component comprises a second U matrix representative of left-singular vectors of the second plurality of spherical harmonic coefficients.

[0298] In these and other instances, the first spatial component is representative of M time segments of spherical harmonic coefficients for the first plurality of spherical harmonic coefficients and the second spatial component is representative of M time segments of spherical harmonic coefficients for the second plurality of spherical harmonic coefficients.

[0299] In these and other instances, the first spatial component is representative of M time segments of spherical harmonic coefficients for the first plurality of spherical harmonic coefficients and the second spatial component is representative of M time segments of spherical harmonic coefficients for the second plurality of spherical harmonic coefficients, and the audio encoding device 20 is configured to interpolate the last N elements of the first spatial component and the first N elements of the second spatial component.

[0300] In these and other instances, the second plurality of spherical harmonic coefficients are subsequent to the first plurality of spherical harmonic coefficients in the time domain.

[0301] In these and other instances, the audio encoding device 20 is further configured to decompose the first plurality of spherical harmonic coefficients to generate the first decomposition of the first plurality of spherical harmonic coefficients.

[0302] In these and other instances, the audio encoding device 20 is further configured to decompose the second plurality of spherical harmonic coefficients to generate the second decomposition of the second plurality of spherical harmonic coefficients.

[0303] In these and other instances, the audio encoding device 20 is further configured to perform a singular value decomposition with respect to the first plurality of spherical harmonic coefficients to generate a U matrix representative of left-singular vectors of the first plurality of spherical harmonic coefficients, an S matrix representative of singular values of the first plurality of spherical harmonic coefficients and a V matrix representative of right-singular vectors of the first plurality of spherical harmonic coefficients.

[0304] In these and other instances, the audio encoding device 20 is further configured to perform a singular value decomposition with respect to the second plurality of spherical harmonic coefficients to generate a U matrix representative of left-singular vectors of the second plurality of spherical harmonic coefficients, an S matrix representative of singular values of the second plurality of spherical harmonic coefficients and a V matrix representative of right-singular vectors of the second plurality of spherical harmonic coefficients.

[0305] In these and other instances, the first and second plurality of spherical harmonic coefficients each represent a planar wave representation of the sound field.

[0306] In these and other instances, the first and second plurality of spherical harmonic coefficients each represent one or more mono-audio objects mixed together.

[0307] In these and other instances, the first and second plurality of spherical harmonic coefficients each comprise respective first and second spherical harmonic coefficients that represent a three dimensional sound field.

[0308] In these and other instances, the first and second plurality of spherical harmonic coefficients are each associated with at least one spherical basis function having an order greater than one.

[0309] In these and other instances, the first and second plurality of spherical harmonic coefficients are each associated with at least one spherical basis function having an order equal to four.

[0310] In these and other instances, the interpolation is a weighted interpolation of the first decomposition and second decomposition, wherein weights of the weighted interpolation applied to the first decomposition are inversely proportional to a time represented by vectors of the first and second decomposition and wherein weights of the weighted interpolation applied to the second decomposition are proportional to a time represented by vectors of the first and second decomposition.

[0311] In these and other instances, the decomposed interpolated spherical harmonic coefficients smooth at least one of spatial components and time components of the first plurality of spherical harmonic coefficients and the second plurality of spherical harmonic coefficients.

[0312] In these and other instances, the audio encoding device 20 is configured to compute Us[n] = HOA(n) * (V _vec[n])-1 to obtain a scalar.

[0313] In these and other instances, the interpolation comprises a linear interpolation. In these and other instances, the interpolation comprises a non-linear interpolation. In these and other instances, the interpolation comprises a cosine interpolation. In these and other instances, the interpolation comprises a weighted cosine interpolation. In these and other instances, the interpolation comprises a cubic interpolation. In these and other instances, the interpolation comprises an Adaptive Spline Interpolation. In these and other instances, the interpolation comprises a minimal curvature interpolation.

[0314] In these and other instances, the audio encoding device 20 is further configured to generate a bitstream that includes a representation of the decomposed interpolated spherical harmonic coefficients for the time segment, and an indication of a type of the interpolation.

[0315] In these and other instances, the indication comprises one or more bits that map to the type of interpolation.

[0316] In this way, various aspects of the techniques described in this disclosure may enable the audio encoding device 20 to be configured to obtain a bitstream that includes a representation of the decomposed interpolated spherical harmonic coefficients for the time segment, and an indication of a type of the interpolation.

[0317] In these and other instances, the indication comprises one or more bits that map to the type of interpolation.

[0318] In this respect, the audio encoding device 20 may represent one embodiment of the techniques in that the audio encoding device 20 may, in some instances, be configured to generate a bitstream comprising a compressed version of a spatial component of a sound field, the spatial component generated by performing a vector based synthesis with respect to a plurality of spherical harmonic coefficients.

[0319] In these and other instances, the audio encoding device 20 is further configured to generate the bitstream to include a field specifying a prediction mode used when compressing the spatial component.

[0320] In these and other instances, the audio encoding device 20 is configured to generate the bitstream to include Huffman table information specifying a Huffman table used when compressing the spatial component.

[0321] In these and other instances, the audio encoding device 20 is configured to generate the bitstream to include a field indicating a value that expresses a quantization step size or a variable thereof used when compressing the spatial component.

[0322] In these and other instances, the value comprises an nbits value.

[0323] In these and other instances, the audio encoding device 20 is configured to generate the bitstream to include a compressed version of a plurality of spatial components of the sound field of which the compressed version of the spatial component is included, where the value expresses the quantization step size or a variable thereof used when compressing the plurality of spatial components.

[0324] In these and other instances, the audio encoding device 20 is further configured to generate the bitstream to include a Huffman code to represent a category identifier that identifies a compression category to which the spatial component corresponds.

[0325] In these and other instances, the audio encoding device 20 is configured to generate the bitstream to include a sign bit identifying whether the spatial component is a positive value or a negative value.

[0326] In these and other instances, the audio encoding device 20 is configured to generate the bitstream to include a Huffman code to represent a residual value of the spatial component.

[0327] In these and other instances, the vector based synthesis comprises a singular value decomposition.

[0328] In this respect, the audio encoding device 20 may further implement various aspects of the techniques in that the audio encoding device 20 may, in some instances, be configured to identify a Huffman codebook to use when compressing a spatial component of a plurality of spatial components based on an order of the spatial component relative to remaining ones of the plurality of spatial components, the spatial component generated by performing a vector based synthesis with respect to a plurality of spherical harmonic coefficients.

[0329] In these and other instances, the audio encoding device 20 is configured to identify the Huffman codebook based on a prediction mode used when compressing the spatial component.

[0330] In these and other instances, a compressed version of the spatial component is represented in a bitstream using, at least in part, Huffman table information identifying the Huffman codebook.

[0331] In these and other instances, a compressed version of the spatial component is represented in a bitstream using, at least in part, a field indicating a value that expresses a quantization step size or a variable thereof used when compressing the spatial component.

[0332] In these and other instances, the value comprises an nbits value.

[0333] In these and other instances, the bitstream comprises a compressed version of a plurality of spatial components of the sound field of which the compressed version of the spatial component is included, and the value expresses the quantization step size or a variable thereof used when compressing the plurality of spatial components.

[0334] In these and other instances, a compressed version of the spatial component is represented in a bitstream using, at least in part, a Huffman code selected form the identified Huffman codebook to represent a category identifier that identifies a compression category to which the spatial component corresponds.

[0335] In these and other instances, a compressed version of the spatial component is represented in a bitstream using, at least in part, a sign bit identifying whether the spatial component is a positive value or a negative value.

[0336] In these and other instances, a compressed version of the spatial component is represented in a bitstream using, at least in part, a Huffman code selected form the identified Huffman codebook to represent a residual value of the spatial component.

[0337] In these and other instances, the audio encoding device 20 is further configured to compress the spatial component based on the identified Huffman codebook to generate a compressed version of the spatial component, and generate the bitstream to include the compressed version of the spatial component.

[0338] Moreover, the audio encoding device 20 may, in some instances, implement various aspects of the techniques in that the audio encoding device 20 may be configured to determine a quantization step size to be used when compressing a spatial component of a sound field, the spatial component generated by performing a vector based synthesis with respect to a plurality of spherical harmonic coefficients.

[0339] In these and other instances, the audio encoding device 20 is further configured to determine the quantization step size based on a target bit rate.

[0340] In these and other instances, the audio encoding device 20 is configured to determine an estimate of a number of bits used to represent the spatial component, and determine the quantization step size based on a difference between the estimate and a target bit rate.

[0341] In these and other instances, the audio encoding device 20 is configured to determine an estimate of a number of bits used to represent the spatial component, determine a difference between the estimate and a target bit rate, and determine the quantization step size by adding the difference to the target bit rate.

[0342] In these and other instances, the audio encoding device 20 is configured to calculate the estimated of the number of bits that are to be generated for the spatial component given a code book corresponding to the target bit rate.

[0343] In these and other instances, the audio encoding device 20 is configured to calculate the estimated of the number of bits that are to be generated for the spatial component given a coding mode used when compressing the spatial component.

[0344] In these and other instances, the audio encoding device 20 is configured to calculate a first estimate of the number of bits that are to be generated for the spatial component given a first coding mode to be used when compressing the spatial component, calculate a second estimate of the number of bits that are to be generated for the spatial component given a second coding mode to be used when compressing the spatial component, select the one of the first estimate and the second estimate having a least number of bits to be used as the determined estimate of the number of bits.

[0345] In these and other instances, the audio encoding device 20 is configured to identify a category identifier identifying a category to which the spatial component corresponds, identify a bit length of a residual value for the spatial component that would result when compressing the spatial component corresponding to the category, and determine the estimate of the number of bits by, at least in part, adding a number of bits used to represent the category identifier to the bit length of the residual value.

[0346] In these and other instances, the audio encoding device 20 is further configured to select one of a plurality of code books to be used when compressing the spatial component.

[0347] In these and other instances, the audio encoding device 20 is further configured to determine an estimate of a number of bits used to represent the spatial component using each of the plurality of code books, and select the one of the plurality of code books that resulted in the determined estimate having the least number of bits.

[0348] In these and other instances, the audio encoding device 20 is further configured to determine an estimate of a number of bits used to represent the spatial component using one or more of the plurality of code books, the one or more of the plurality of code books selected based on an order of elements of the spatial component to be compressed relative to other elements of the spatial component.

[0349] In these and other instances, the audio encoding device 20 is further configured to determine an estimate of a number of bits used to represent the spatial component using one of the plurality of code books designed to be used when the spatial component is not predicted from a subsequent spatial component.

[0350] In these and other instances, the audio encoding device 20 is further configured to determine an estimate of a number of bits used to represent the spatial component using one of the plurality of code books designed to be used when the spatial component is predicted from a subsequent spatial component.

[0351] In these and other instances, the audio encoding device 20 is further configured to determine an estimate of a number of bits used to represent the spatial component using one of the plurality of code books designed to be used when the spatial component is representative of a synthetic audio object in the sound field.

[0352] In these and other instances, the synthetic audio object comprises a pulse code modulated (PCM) audio object.

[0353] In these and other instances, the audio encoding device 20 is further configured to determine an estimate of a number of bits used to represent the spatial component using one of the plurality of code books designed to be used when the spatial component is representative of a recorded audio object in the sound field.

[0354] In each of the various instances described above, it should be understood that the audio encoding device 20 may perform a method or otherwise comprise means to perform each step of the method for which the audio encoding device 20 is configured to perform In some instances, these means may comprise one or more processors. In some instances, the one or more processors may represent a special purpose processor configured by way of instructions stored to a non-transitory computer-readable storage medium. In other words, various aspects of the techniques in each of the sets of encoding examples may provide for a non-transitory computer-readable storage medium having stored thereon instructions that, when executed, cause the one or more processors to perform the method for which the audio encoding device 20 has been configured to perform.

[0355] FIG. 5 is a block diagram illustrating the audio decoding device 24 of FIG. 3 in more detail. As shown in the example of FIG. 5, the audio decoding device 24 may include an extraction unit 72, a directionality-based reconstruction unit 90 and a vector-based reconstruction unit 92.

[0356] The extraction unit 72 may represent a unit configured to receive the bitstream 21 and extract the various encoded versions (e.g., a directional-based encoded version or a vector-based encoded version) of the HOA coefficients 11. The extraction unit 72 may determine from the above noted syntax element (e.g., the ChannelType syntax element shown in the examples of FIGS. 10E and 10H(i)-10O(ii)) whether the HOA coefficients 11 were encoded via the various versions. When a directional-based encoding was performed, the extraction unit 72 may extract the directional-based version of the HOA coefficients 11 and the syntax elements associated with this encoded version (which is denoted as directional-based information 91 in the example of FIG. 5), passing this directional based information 91 to the directional-based reconstruction unit 90. This directional-based reconstruction unit 90 may represent a unit configured to reconstruct the HOA coefficients in the form of HOA coefficients 11' based on the directional-based information 91. The bitstream and the arrangement of syntax elements within the bitstream is described below in more detail with respect to the example of FIGS. 10-10O(ii) and 11.

[0357] When the syntax element indicates that the HOA coefficients 11 were encoded using a vector-based synthesis, the extraction unit 72 may extract the coded foreground V[k] vectors 57, the encoded ambient HOA coefficients 59 and the encoded nFG signals 59. The extraction unit 72 may pass the coded foreground V[k] vectors 57 to the quantization unit 74 and the encoded ambient HOA coefficients 59 along with the encoded nFG signals 61 to the psychoacoustic decoding unit 80.

[0358] To extract the coded foreground V[k] vectors 57, the encoded ambient HOA coefficients 59 and the encoded nFG signals 59, the extraction unit 72 may obtain the side channel information 57, which includes the syntax element denoted codedVVecLength. The extraction unit 72 may parse the codedVVecLength from the side channel information 57. The extraction unit 72 may be configured to operate in any one of the above described configuration modes based on the codedVVecLength syntax element.

[0359] The extraction unit 72 then operates in accordance with any one of configuration modes to parse a compressed form of the reduced foreground V[k] vectors 55 k from the side channel information 57. The extraction unit 72 may operate in accordance with the switch statement presented in the following pseudo-code with the syntax presented in the following syntax table for VVectorData:

[0360] In the foregoing syntax table, the first switch statement with the four cases (case 0-3) provides for a way by which to determine the V T< DIST vector length in terms of the number (VVecLength) and indices of coefficients (VVecCoeffId). The first case, case 0, indicates that all of the coefficients for the V T< DIST vectors (NumOfHoaCoeffs) are specified. The second case, case 1, indicates that only those coefficients of the V T< DIST vector corresponding to the number greater than a MinNumOfCoeffsForAmbHOA are specified, which may denote what is referred to as (N DIST + 1) 2< - (N BG + 1) 2< above. Further those NumOfContAddAmbHoaChan coefficients identified in ContAddAmbHoaChan are substracted. The list ContAddAmbHoaChan specifies additional channels (where "channels" refer to a particular coefficient corresponding to a certain order, sub-order combination) corresponding to an order that exceeds the order MinAmbHoaOrder. The third case, case 2, indicates that those coefficients of the V T< DIST vector corresponding to the number greater than a MinNumOfCoeffsForAmbHOA are specified, which may denote what is referred to as (N DIST + 1) 2< - (N BG + 1) 2< above. The fourth case, case 3, indicates that those coefficients of the V T< DIST vector left after removing coefficients identified by NumOfContAddAmbHoaChan are specified. Both the VVecLength as well as the VVecCoeffId list is valid for all VVectors within on HOAFrame.

[0361] After this switch statement, the decision of whether to perform uniform dequantization may be controlled by NbitsQ (or, as denoted above, nbits), which if equals 5, a uniform 8 bit scalar dequantization is performed. In contrast, an NbitsQ value of greater or equals 6 may result in application of Huffman decoding. The cid value referred to above may be equal to the two least significant bits of the NbitsQ value. The prediction mode discussed above is denoted as the PFlag in the above syntax table, while the HT info bit is denoted as the CbFlag in the above syntax table. The remaining syntax specifies how the decoding occurs in a manner substantially similar to that described above. Various examples of the bitstream 21 that conforms to each of the various cases noted above are described in more detail below with respect to FIGS. 10H(i)-10O(ii).

[0362] The vector-based reconstruction unit 92 represents a unit configured to perform operations reciprocal to those described above with respect to the vector-based synthesis unit 27 so as to reconstruct the HOA coefficients 11'. The vector based reconstruction unit 92 may include a quantization unit 74, a spatio-temporal interpolation unit 76, a foreground formulation unit 78, a psychoacoustic decoding unit 80, a HOA coefficient formulation unit 82 and a reorder unit 84.

[0363] The quantization unit 74 may represent a unit configured to operate in a manner reciprocal to the quantization unit 52 shown in the example of FIG. 4 so as to dequantize the coded foreground V[k] vectors 57 and thereby generate reduced foreground V[k] vectors 55 k . The dequantization unit 74 may, in some examples, perform a form of entropy decoding and scalar dequantization in a manner reciprocal to that described above with respect to the quantization unit 52. The dequantization unit 74 may forward the reduced foreground V[k] vectors 55 k to the reorder unit 84.

[0364] The psychoacoustic decoding unit 80 may operate in a manner reciprocal to the psychoacoustic audio coding unit 40 shown in the example of FIG. 4 so as to decode the encoded ambient HOA coefficients 59 and the encoded nFG signals 61 and thereby generate energy compensated ambient HOA coefficients 47' and the interpolated nFG signals 49' (which may also be referred to as interpolated nFG audio objects 49'). The psychoacoustic decoding unit 80 may pass the energy compensated ambient HOA coefficients 47' to HOA coefficient formulation unit 82 and the nFG signals 49' to the reorder 84.

[0365] The reorder unit 84 may represent a unit configured to operate in a manner similar reciprocal to that described above with respect to the reorder unit 34. The reorder unit 84 may receive syntax elements indicative of the original order of the foreground components of the HOA coefficients 11. The reorder unit 84 may, based on these reorder syntax elements, reorder the interpolated nFG signals 49' and the reduced foreground V[k] vectors 55 k to generate reordered nFG signals 49" and reordered foreground V[k] vectors 55 k '. The reorder unit 84 may output the reordered nFG signals 49" to the foreground formulation unit 78 and the reordered foreground V[k] vectors 55 k ' to the spatio-temporal interpolation unit 76.

[0366] The spatio-temporal interpolation unit 76 may operate in a manner similar to that described above with respect to the spatio-temporal interpolation unit 50. The spatio-temporal interpolation unit 76 may receive the reordered foreground V[k] vectors 55 k ' and perform the spatio-temporal interpolation with respect to the reordered foreground V[k] vectors 55 k ' and reordered foreground V[k-1] vectors 55 k-1 ' to generate interpolated foreground V[k] vectors 55 k ". The spatio-temporal interpolation unit 76 may forward the interpolated foreground V[k] vectors 55 k " to the foreground formulation unit 78.

[0367] The foreground formulation unit 78 may represent a unit configured to perform matrix multiplication with respect to the interpolated foreground V[k] vectors 55 k " and the reordered nFG signals 49" to generate the foreground HOA coefficients 65. The foreground formulation unit 78 may perform a matrix multiplication of the reordered nFG signals 49" by the interpolated foreground V[k] vectors 55 k ".

[0368] The HOA coefficient formulation unit 82 may represent a unit configured to add the foreground HOA coefficients 65 to the ambient HOA channels 47' so as to obtain the HOA coefficients 11', where the prime notation reflects that these HOA coefficients 11' may be similar to but not the same as the HOA coefficients 11. The differences between the HOA coefficients 11 and 11' may result from loss due to transmission over a lossy transmission medium, quantization or other lossy operations.

[0369] In this way, the techniques may enable an audio decoding device, such as the audio decoding device 24, to determine, from a bitstream, quantized directional information, an encoded foreground audio object, and encoded ambient higher order ambisonic (HOA) coefficients, wherein the quantized directional information and the encoded foreground audio object represent foreground HOA coefficients describing a foreground component of a soundfield, and wherein the encoded ambient HOA coefficients describe an ambient component of the soundfield, dequantize the quantized directional information to generate directional information, perform spatio-temporal interpolation with respect to the directional information to generate interpolated directional information, audio decode the encoded foreground audio object to generate a foreground audio object and the encoded ambient HOA coefficients to generate ambient HOA coefficients, determine the foreground HOA coefficients as a function of the interpolated directional information and the foreground audio object, and determine HOA coefficients as a function of the foreground HOA coefficients and the ambient HOA coefficients.

[0370] In this way, various aspects of the techniques may enable a unified audio decoding device 24 to switch between two different decompression schemes. In some instances, the audio decoding device 24 may be configured to select one of a plurality of decompression schemes based on the indication of whether an compressed version of spherical harmonic coefficients representative of a sound field are generated from a synthetic audio object, and decompress the compressed version of the spherical harmonic coefficients using the selected one of the plurality of decompression schemes. In these and other instances, the audio decoding device 24 comprises an integrated decoder.

[0371] In some instances, the audio decoding device 24 may be configured to obtain an indication of whether spherical harmonic coefficients representative of a sound field are generated from a synthetic audio object.

[0372] In these and other instances, the audio decoding device 24 is configured to obtain the indication from a bitstream that stores a compressed version of the spherical harmonic coefficients.

[0373] In this way, various aspects of the techniques may enable the audio decoding device 24 to obtain vectors describing distinct and background components of the soundfield. In some instances, the audio decoding device 24 may be configured to determine one or more first vectors describing distinct components of the soundfield and one or more second vectors describing background components of the soundfield, both the one or more first vectors and the one or more second vectors generated at least by performing a transformation with respect to the plurality of spherical harmonic coefficients.

[0374] In these and other instances, the audio decoding device 24, wherein the transformation comprises a singular value decomposition that generates a U matrix representative of left-singular vectors of the plurality of spherical harmonic coefficients, an S matrix representative of singular values of the plurality of spherical harmonic coefficients and a V matrix representative of right-singular vectors of the plurality of spherical harmonic coefficients.

[0375] In these and other instances, the audio decoding device 24, wherein the one or more first vectors comprise one or more audio encoded U DIST * S DIST vectors that, prior to audio encoding, were generated by multiplying one or more audio encoded U DIST vectors of a U matrix by one or more S DIST vectors of an S matrix, and wherein the U matrix and the S matrix are generated at least by performing the singular value decomposition with respect to the plurality of spherical harmonic coefficients.

[0376] In these and other instances, the audio decoding device 24 is further configured to audio decode the one or more audio encoded U DIST * S DIST vectors to generate an audio decoded version of the one or more audio encoded U DIST * S DIST vectors.

[0377] In these and other instances, the audio decoding device 24, wherein the one or more first vectors comprise one or more audio encoded U DIST * S DIST vectors that, prior to audio encoding, were generated by multiplying one or more audio encoded U DIST vectors of a U matrix by one or more S DIST vectors of an S matrix, and one or more V T< DIST vectors of a transpose of a V matrix, and wherein the U matrix and the S matrix and the V matrix are generated at least by performing the singular value decomposition with respect to the plurality of spherical harmonic coefficients.

[0378] In these and other instances, the audio decoding device 24 is further configured to audio decode the one or more audio encoded U DIST * S DIST vectors to generate an audio decoded version of the one or more audio encoded U DIST * S DIST vectors.

[0379] In these and other instances, the audio decoding device 24 further configured to multiply the U DIST * S DIST vectors by the V T< DIST vectors to recover those of the plurality of spherical harmonics representative of the distinct components of the soundfield.

[0380] In these and other instances, the audio decoding device 24, wherein the one or more second vectors comprise one or more audio encoded U BG * S BG * V T< BG vectors that, prior to audio encoding, were generating by multiplying U BG vectors included within a U matrix by S BG vectors included within an S matrix and then by V T< BG vectors included within a transpose of a V matrix, and wherein the S matrix, the U matrix and the V matrix were each generated at least by performing the singular value decomposition with respect to the plurality of spherical harmonic coefficients.

[0381] In these and other instances, the audio decoding device 24, wherein the one or more second vectors comprise one or more audio encoded U BG * S BG * V T< BG vectors that, prior to audio encoding, were generating by multiplying U BG vectors included within a U matrix by S BG vectors included within an S matrix and then by V T< BG vectors included within a transpose of a V matrix, wherein the S matrix, the U matrix and the V matrix were generated at least by performing the singular value decomposition with respect to the plurality of spherical harmonic coefficients, and wherein the audio decoding device 24 is further configured to audio decode the one or more audio encoded U BG * S BG * V T< BG vectors to generate one or more audio decoded U BG * S BG * V T< BG vectors.

[0382] In these and other instances, the audio decoding device 24, wherein the one or more first vectors comprise one or more audio encoded U DIST * S DIST vectors that, prior to audio encoding, were generated by multiplying one or more audio encoded U DIST vectors of a U matrix by one or more S DIST vectors of an S matrix, and one or more V T< DIST vectors of a transpose of a V matrix, wherein the U matrix, the S matrix and the V matrix were generated at least by performing the singular value decomposition with respect to the plurality of spherical harmonic coefficients, and wherein the audio decoding device 24 is further configured to audio decode the one or more audio encoded U DIST * S DIST vectors to generate the one or more U DIST * S DIST vectors, and multiply the U DIST * S DIST vectors by the V T< DIST vectors to recover those of the plurality of spherical harmonic coefficients that describe the distinct components of the soundfield, wherein the one or more second vectors comprise one or more audio encoded U BG * S BG * V T< BG vectors that, prior to audio encoding, were generating by multiplying U BG vectors included within the U matrix by S BG vectors included within the S matrix and then by V T< BG vectors included within the transpose of the V matrix, and wherein the audio decoding device 24 is further configured to audio decode the one or more audio encoded U BG * S BG * V T< BG vectors to recover at least a portion of the plurality of the spherical harmonic coefficients that describe background components of the soundfield, and add the plurality of spherical harmonic coefficients that describe the distinct components of the soundfield to the at least portion of the plurality of the spherical harmonic coefficients that describe background components of the soundfield to generate a reconstructed version of the plurality of spherical harmonic coefficients.

[0383] In these and other instances, the audio decoding device 24, wherein the one or more first vectors comprise one or more U DIST * S DIST vectors that, prior to audio encoding, were generated by multiplying one or more audio encoded U DIST vectors of a U matrix by one or more S DIST vectors of an S matrix, and one or more V T< DIST vectors of a transpose of a V matrix, wherein the U matrix, the S matrix and the V matrix were generated at least by performing the singular value decomposition with respect to the plurality of spherical harmonic coefficients, and wherein the audio decoding device 20 is further configured to obtain a value D indicating the number of vectors to be extracted from a bitstream to form the one or more U DIST * S DIST vectors and the one or more V T< DIST vectors.

[0384] In these and other instances, the audio decoding device 24, wherein the one or more first vectors comprise one or more U DIST * S DIST vectors that, prior to audio encoding, were generated by multiplying one or more audio encoded U DIST vectors of a U matrix by one or more S DIST vectors of an S matrix, and one or more V T< DIST vectors of a transpose of a V matrix, wherein the U matrix, the S matrix and the V matrix were generated at least by performing the singular value decomposition with respect to the plurality of spherical harmonic coefficients, and wherein the audio decoding device 24 is further configured to obtain a value D on an audio-frame-by-audio-frame basis that indicates the number of vectors to be extracted from a bitstream to form the one or more U DIST * S DIST vectors and the one or more V T< DIST vectors.

[0385] In these and other instances, the audio decoding device 24, wherein the transformation comprises a principal component analysis to identify the distinct components of the soundfield and the background components of the soundfield.

[0386] Various aspects of the techniques described in this disclosure may also enable the audio encoding device 24 to perform interpolation with respect to decomposed versions of the HOA coefficients. In some instances, the audio decoding device 24 may be configured to obtain decomposed interpolated spherical harmonic coefficients for a time segment by, at least in part, performing an interpolation with respect to a first decomposition of a first plurality of spherical harmonic coefficients and a second decomposition of a second plurality of spherical harmonic coefficients.

[0387] In these and other instances, the first decomposition comprises a first V matrix representative of right-singular vectors of the first plurality of spherical harmonic coefficients.

[0388] In these and other examples, the second decomposition comprises a second V matrix representative of right-singular vectors of the second plurality of spherical harmonic coefficients.

[0389] In these and other instances, the first decomposition comprises a first V matrix representative of right-singular vectors of the first plurality of spherical harmonic coefficients, and the second decomposition comprises a second V matrix representative of right-singular vectors of the second plurality of spherical harmonic coefficients.

[0390] In these and other instances, the time segment comprises a sub-frame of an audio frame.

[0391] In these and other instances, the time segment comprises a time sample of an audio frame.

[0392] In these and other instances, the audio decoding device 24 is configured to obtain an interpolated decomposition of the first decomposition and the second decomposition for a spherical harmonic coefficient of the first plurality of spherical harmonic coefficients.

[0393] In these and other instances, the audio decoding device 24 is configured to obtain interpolated decompositions of the first decomposition for a first portion of the first plurality of spherical harmonic coefficients included in the first frame and the second decomposition for a second portion of the second plurality of spherical harmonic coefficients included in the second frame, and the audio decoding device 24 is further configured to apply the interpolated decompositions to a first time component of the first portion of the first plurality of spherical harmonic coefficients included in the first frame to generate a first artificial time component of the first plurality of spherical harmonic coefficients, and apply the respective interpolated decompositions to a second time component of the second portion of the second plurality of spherical harmonic coefficients included in the second frame to generate a second artificial time component of the second plurality of spherical harmonic coefficients included.

[0394] In these and other instances, the first time component is generated by performing a vector-based synthesis with respect to the first plurality of spherical harmonic coefficients.

[0395] In these and other instances, the second time component is generated by performing a vector-based synthesis with respect to the second plurality of spherical harmonic coefficients.

[0396] In these and other instances, the audio decoding device 24 is further configured to receive the first artificial time component and the second artificial time component, compute interpolated decompositions of the first decomposition for the first portion of the first plurality of spherical harmonic coefficients and the second decomposition for the second portion of the second plurality of spherical harmonic coefficients, and apply inverses of the interpolated decompositions to the first artificial time component to recover the first time component and to the second artificial time component to recover the second time component.

[0397] In these and other instances, the audio decoding device 24 is configured to interpolate a first spatial component of the first plurality of spherical harmonic coefficients and the second spatial component of the second plurality of spherical harmonic coefficients.

[0398] In these and other instances, the first spatial component comprises a first U matrix representative of left-singular vectors of the first plurality of spherical harmonic coefficients.

[0399] In these and other instances, the second spatial component comprises a second U matrix representative of left-singular vectors of the second plurality of spherical harmonic coefficients.

[0400] In these and other instances, the first spatial component is representative of M time segments of spherical harmonic coefficients for the first plurality of spherical harmonic coefficients and the second spatial component is representative of M time segments of spherical harmonic coefficients for the second plurality of spherical harmonic coefficients.

[0401] In these and other instances, the first spatial component is representative of M time segments of spherical harmonic coefficients for the first plurality of spherical harmonic coefficients and the second spatial component is representative of M time segments of spherical harmonic coefficients for the second plurality of spherical harmonic coefficients, and the audio decoding device 24 is configured to interpolate the last N elements of the first spatial component and the first N elements of the second spatial component.

[0402] In these and other instances, the second plurality of spherical harmonic coefficients are subsequent to the first plurality of spherical harmonic coefficients in the time domain.

[0403] In these and other instances, the audio decoding device 24 is further configured to decompose the first plurality of spherical harmonic coefficients to generate the first decomposition of the first plurality of spherical harmonic coefficients.

[0404] In these and other instances, the audio decoding device 24 is further configured to decompose the second plurality of spherical harmonic coefficients to generate the second decomposition of the second plurality of spherical harmonic coefficients.

[0405] In these and other instances, the audio decoding device 24 is further configured to perform a singular value decomposition with respect to the first plurality of spherical harmonic coefficients to generate a U matrix representative of left-singular vectors of the first plurality of spherical harmonic coefficients, an S matrix representative of singular values of the first plurality of spherical harmonic coefficients and a V matrix representative of right-singular vectors of the first plurality of spherical harmonic coefficients.

[0406] In these and other instances, the audio decoding device 24 is further configured to perform a singular value decomposition with respect to the second plurality of spherical harmonic coefficients to generate a U matrix representative of left-singular vectors of the second plurality of spherical harmonic coefficients, an S matrix representative of singular values of the second plurality of spherical harmonic coefficients and a V matrix representative of right-singular vectors of the second plurality of spherical harmonic coefficients.

[0407] In these and other instances, the first and second plurality of spherical harmonic coefficients each represent a planar wave representation of the sound field.

[0408] In these and other instances, the first and second plurality of spherical harmonic coefficients each represent one or more mono-audio objects mixed together.

[0409] In these and other instances, the first and second plurality of spherical harmonic coefficients each comprise respective first and second spherical harmonic coefficients that represent a three dimensional sound field.

[0410] In these and other instances, the first and second plurality of spherical harmonic coefficients are each associated with at least one spherical basis function having an order greater than one.

[0411] In these and other instances, the first and second plurality of spherical harmonic coefficients are each associated with at least one spherical basis function having an order equal to four.

[0412] In these and other instances, the interpolation is a weighted interpolation of the first decomposition and second decomposition, wherein weights of the weighted interpolation applied to the first decomposition are inversely proportional to a time represented by vectors of the first and second decomposition and wherein weights of the weighted interpolation applied to the second decomposition are proportional to a time represented by vectors of the first and second decomposition.

[0413] In these and other instances, the decomposed interpolated spherical harmonic coefficients smooth at least one of spatial components and time components of the first plurality of spherical harmonic coefficients and the second plurality of spherical harmonic coefficients.

[0414] In these and other instances, the audio decoding device 24 is configured to compute Us[n] = HOA(n) * (V _vec[n])-1 to obtain a scalar.

[0415] In these and other instances, the interpolation comprises a linear interpolation. In these and other instances, the interpolation comprises a non-linear interpolation. In these and other instances, the interpolation comprises a cosine interpolation. In these and other instances, the interpolation comprises a weighted cosine interpolation. In these and other instances, the interpolation comprises a cubic interpolation. In these and other instances, the interpolation comprises an Adaptive Spline Interpolation. In these and other instances, the interpolation comprises a minimal curvature interpolation.

[0416] In these and other instances, the audio decoding device 24 is further configured to generate a bitstream that includes a representation of the decomposed interpolated spherical harmonic coefficients for the time segment, and an indication of a type of the interpolation.

[0417] In these and other instances, the indication comprises one or more bits that map to the type of interpolation.

[0418] In these and other instances, the audio decoding device 24 is further configured to obtain a bitstream that includes a representation of the decomposed interpolated spherical harmonic coefficients for the time segment, and an indication of a type of the interpolation.

[0419] In these and other instances, the indication comprises one or more bits that map to the type of interpolation.

[0420] Various aspects of the techniques may, in some instances, further enable the audio decoding device 24 to be configured to obtain a bitstream comprising a compressed version of a spatial component of a sound field, the spatial component generated by performing a vector based synthesis with respect to a plurality of spherical harmonic coefficients.

[0421] In these and other instances, the compressed version of the spatial component is represented in the bitstream using, at least in part, a field specifying a prediction mode used when compressing the spatial component.

[0422] In these and other instances, the compressed version of the spatial component is represented in the bitstream using, at least in part, Huffman table information specifying a Huffman table used when compressing the spatial component.

[0423] In these and other instances, the compressed version of the spatial component is represented in the bitstream using, at least in part, a field indicating a value that expresses a quantization step size or a variable thereof used when compressing the spatial component.

[0424] In these and other instances, the value comprises an nbits value.

[0425] In these and other instances, the bitstream comprises a compressed version of a plurality of spatial components of the sound field of which the compressed version of the spatial component is included, and the value expresses the quantization step size or a variable thereof used when compressing the plurality of spatial components.

[0426] In these and other instances, the compressed version of the spatial component is represented in the bitstream using, at least in part, a Huffman code to represent a category identifier that identifies a compression category to which the spatial component corresponds.

[0427] In these and other instances, the compressed version of the spatial component is represented in the bitstream using, at least in part, a sign bit identifying whether the spatial component is a positive value or a negative value.

[0428] In these and other instances, the compressed version of the spatial component is represented in the bitstream using, at least in part, a Huffman code to represent a residual value of the spatial component.

[0429] In these and other instances, the device comprises an audio decoding device.

[0430] Various aspects of the techniques may also enable the audio decoding device 24 to identify a Huffman codebook to use when decompressing a compressed version of a spatial component of a plurality of compressed spatial components based on an order of the compressed version of the spatial component relative to remaining ones of the plurality of compressed spatial components, the spatial component generated by performing a vector based synthesis with respect to a plurality of spherical harmonic coefficients.

[0431] In these and other instances, the audio decoding device 24 is configured to obtain a bitstream comprising the compressed version of a spatial component of a sound field, and decompress the compressed version of the spatial component using, at least in part, the identified Huffman codebook to obtain the spatial component.

[0432] In these and other instances, the compressed version of the spatial component is represented in the bitstream using, at least in part, a field specifying a prediction mode used when compressing the spatial component, and the audio decoding device 24 is configured to decompress the compressed version of the spatial component based, at least in part, on the prediction mode to obtain the spatial component.

[0433] In these and other instances, the compressed version of the spatial component is represented in the bitstream using, at least in part, Huffman table information specifying a Huffman table used when compressing the spatial component, and the audio decoding device 24 is configured to decompress the compressed version of the spatial component based, at least in part, on the Huffman table information.

[0434] In these and other instances, the compressed version of the spatial component is represented in the bitstream using, at least in part, a field indicating a value that expresses a quantization step size or a variable thereof used when compressing the spatial component, and the audio decoding device 24 is configured to decompress the compressed version of the spatial component based, at least in part, on the value.

[0435] In these and other instances, the value comprises an nbits value.

[0436] In these and other instances, the bitstream comprises a compressed version of a plurality of spatial components of the sound field of which the compressed version of the spatial component is included, the value expresses the quantization step size or a variable thereof used when compressing the plurality of spatial components and the audio decoding device 24 is configured to decompress the plurality of compressed version of the spatial component based, at least in part, on the value.

[0437] In these and other instances, the compressed version of the spatial component is represented in the bitstream using, at least in part, a Huffman code to represent a category identifier that identifies a compression category to which the spatial component corresponds and the audio decoding device 24 is configured to decompress the compressed version of the spatial component based, at least in part, on the Huffman code.

[0438] In these and other instances, the compressed version of the spatial component is represented in the bitstream using, at least in part, a sign bit identifying whether the spatial component is a positive value or a negative value, and the audio decoding device 24 is configured to decompress the compressed version of the spatial component based, at least in part, on the sign bit.

[0439] In these and other instances, the compressed version of the spatial component is represented in the bitstream using, at least in part, a Huffman code to represent a residual value of the spatial component and the audio decoding device 24 is configured to decompress the compressed version of the spatial component based, at least in part, on the Huffman code included in the identified Huffman codebook.

[0440] In each of the various instances described above, it should be understood that the audio decoding device 24 may perform a method or otherwise comprise means to perform each step of the method for which the audio decoding device 24 is configured to perform In some instances, these means may comprise one or more processors. In some instances, the one or more processors may represent a special purpose processor configured by way of instructions stored to a non-transitory computer-readable storage medium. In other words, various aspects of the techniques in each of the sets of encoding examples may provide for a non-transitory computer-readable storage medium having stored thereon instructions that, when executed, cause the one or more processors to perform the method for which the audio decoding device 24 has been configured to perform.

[0441] FIG. 6 is a flowchart illustrating exemplary operation of a content analysis unit of an audio encoding device, such as the content analysis unit 26 shown in the example of FIG. 4, in performing various aspects of the techniques described in this disclosure.

[0442] The content analysis unit 26 may, when determining whether the HOA coefficients 11 representative of a soundfield are generated from a synthetic audio object, obtain a framed of HOA coefficients (93), which may be of size 25 by 1024 for a fourth order representation (i.e., N = 4). After obtaining the framed HOA coefficients (which may also be denoted herein as a framed SHC matrix 11 and subsequent framed SHC matrices may be denoted as framed SHC matrices 27B, 27C, etc.), the content analysis unit 26 may then exclude the first vector of the framed HOA coefficients 11 to generate a reduced framed HOA coefficients (94).

[0443] The content analysis unit 26 may then predicted the first non-zero vector of the reduced framed HOA coefficients from remaining vectors of the reduced framed HOA coefficients (95). After predicting the first non-zero vector, the content analysis unit 26 may obtain an error based on the predicted first non-zero vector and the actual non-zero vector (96). Once the error is obtained, the content analysis unit 26 may compute a ratio based on an energy of the actual first non-zero vector and the error (97). The content analysis unit 26 may then compare this ratio to a threshold (98). When the ratio does not exceed the threshold ("NO" 98), the content analysis unit 26 may determine that the framed SHC matrix 11 is generated from a recording and indicate in the bitstream that the corresponding coded representation of the SHC matrix 11 was generated from a recording (100, 101). When the ratio exceeds the threshold ("YES" 98), the content analysis unit 26 may determine that the framed SHC matrix 11 is generated from a synthetic audio object and indicate in the bitstream that the corresponding coded representation of the SHC matrix 11 was generated from a synthetic audio object (102, 103). In some instances, when the framed SHC matrix 11 were generated from a recording, the content analysis unit 26 passes the framed SHC matrix 11 to the vector-based synthesis unit 27 (101). In some instances, when the framed SHC matrix 11 were generated from a synthetic audio object, the content analysis unit 26 passes the framed SHC matrix 11 to the directional-based synthesis unit 28 (104).

[0444] FIG. 7 is a flowchart illustrating exemplary operation of an audio encoding device, such as the audio encoding device 20 shown in the example of FIG. 4, in performing various aspects of the vector-based synthesis techniques described in this disclosure. Initially, the audio encoding device 20 receives the HOA coefficients 11 (106). The audio encoding device 20 may invoke the LIT unit 30, which may apply a LIT with respect to the HOA coefficients to output transformed HOA coefficients (e.g., in the case of SVD, the transformed HOA coefficients may comprise the US[k] vectors 33 and the V[k] vectors 35) (107).

[0445] The audio encoding device 20 may next invoke the parameter calculation unit 32 to perform the above described analysis with respect to any combination of the US[k] vectors 33, US[k-1] vectors 33, the V[k] and / or V[k-1] vectors 35 to identify various parameters in the manner described above. That is, the parameter calculation unit 32 may determine at least one parameter based on an analysis of the transformed HOA coefficients 33 / 35 (108).

[0446] The audio encoding device 20 may then invoke the reorder unit 34, which may reorder the transformed HOA coefficients (which, again in the context of SVD, may refer to the US[k] vectors 33 and the V[k] vectors 35) based on the parameter to generate reordered transformed HOA coefficients 33' / 35' (or, in other words, the US[k] vectors 33' and the V[k] vectors 35'), as described above (109). The audio encoding device 20 may, during any of the foregoing operations or subsequent operations, also invoke the soundfield analysis unit 44. The soundfield analysis unit 44 may, as described above, perform a soundfield analysis with respect to the HOA coefficients 11 and / or the transformed HOA coefficients 33 / 35 to determine the total number of foreground channels (nFG) 45, the order of the background soundfield (N BG ) and the number (nBGa) and indices (i) of additional BG HOA channels to send (which may collectively be denoted as background channel information 43 in the example of FIG. 4) (109).

[0447] The audio encoding device 20 may also invoke the background selection unit 48. The background selection unit 48 may determine background or ambient HOA coefficients 47 based on the background channel information 43 (110). The audio encoding device 20 may further invoke the foreground selection unit 36, which may select those of the reordered US[k] vectors 33' and the reordered V[k] vectors 35' that represent foreground or distinct components of the soundfield based on nFG 45 (which may represent a one or more indices identifying these foreground vectors) (112).

[0448] The audio encoding device 20 may invoke the energy compensation unit 38. The energy compensation unit 38 may perform energy compensation with respect to the ambient HOA coefficients 47 to compensate for energy loss due to removal of various ones of the HOA channels by the background selection unit 48 (114) and thereby generate energy compensated ambient HOA coefficients 47'.

[0449] The audio encoding device 20 also then invoke the spatio-temporal interpolation unit 50. The spatio-temporal interpolation unit 50 may perform spatio-temporal interpolation with respect to the reordered transformed HOA coefficients 33' / 35' to obtain the interpolated foreground signals 49' (which may also be referred to as the "interpolated nFG signals 49‴) and the remaining foreground directional information 53 (which may also be referred to as the "V[k] vectors 53") (116). The audio encoding device 20 may then invoke the coefficient reduction unit 46. The coefficient reduction unit 46 may perform coefficient reduction with respect to the remaining foreground V[k] vectors 53 based on the background channel information 43 to obtain reduced foreground directional information 55 (which may also be referred to as the reduced foreground V[k] vectors 55) (118).

[0450] The audio encoding device 20 may then invoke the quantization unit 52 to compress, in the manner described above, the reduced foreground V[k] vectors 55 and generate coded foreground V[k] vectors 57 (120).

[0451] The audio encoding device 20 may also invoke the psychoacoustic audio coder unit 40. The psychoacoustic audio coder unit 40 may psychoacoustic code each vector of the energy compensated ambient HOA coefficients 47' and the interpolated nFG signals 49' to generate encoded ambient HOA coefficients 59 and encoded nFG signals 61. The audio encoding device may then invoke the bitstream generation unit 42. The bitstream generation unit 42 may generate the bitstream 21 based on the coded foreground directional information 57, the coded ambient HOA coefficients 59, the coded nFG signals 61 and the background channel information 43.

[0452] FIG. 8 is a flow chart illustrating exemplary operation of an audio decoding device, such as the audio decoding device 24 shown in FIG. 5, in performing various aspects of the techniques described in this disclosure. Initially, the audio decoding device 24 may receive the bitstream 21 (130). Upon receiving the bitstream, the audio decoding device 24 may invoke the extraction unit 72. Assuming for purposes of discussion that the bitstream 21 indicates that vector-based reconstruction is to be performed, the extraction device 72 may parse this bitstream to retrieve the above noted information, passing this information to the vector-based reconstruction unit 92.

[0453] In other words, the extraction unit 72 may extract the coded foreground directional information 57 (which, again, may also be referred to as the coded foreground V[k] vectors 57), the coded ambient HOA coefficients 59 and the coded foreground signals (which may also be referred to as the coded foreground nFG signals 59 or the coded foreground audio objects 59) from the bitstream 21 in the manner described above (132).

[0454] The audio decoding device 24 may further invoke the quantization unit 74. The quantization unit 74 may entropy decode and dequantize the coded foreground directional information 57 to obtain reduced foreground directional information 55 k (136). The audio decoding device 24 may also invoke the psychoacoustic decoding unit 80. The psychoacoustic audio coding unit 80 may decode the encoded ambient HOA coefficients 59 and the encoded foreground signals 61 to obtain energy compensated ambient HOA coefficients 47' and the interpolated foreground signals 49' (138). The psychoacoustic decoding unit 80 may pass the energy compensated ambient HOA coefficients 47' to HOA coefficient formulation unit 82 and the nFG signals 49' to the reorder unit 84.

[0455] The reorder unit 84 may receive syntax elements indicative of the original order of the foreground components of the HOA coefficients 11. The reorder unit 84 may, based on these reorder syntax elements, reorder the interpolated nFG signals 49' and the reduced foreground V[k] vectors 55 k to generate reordered nFG signals 49" and reordered foreground V[k] vectors 55 k ' (140). The reorder unit 84 may output the reordered nFG signals 49" to the foreground formulation unit 78 and the reordered foreground V[k] vectors 55 k ' to the spatio-temporal interpolation unit 76.

[0456] The audio decoding device 24 may next invoke the spatio-temporal interpolation unit 76. The spatio-temporal interpolation unit 76 may receive the reordered foreground directional information 55 k ' and perform the spatio-temporal interpolation with respect to the reduced foreground directional information 55 k / 55 k-1 to generate the interpolated foreground directional information 55 k " (142). The spatio-temporal interpolation unit 76 may forward the interpolated foreground V[k] vectors 55 k " to the foreground formulation unit 718.

[0457] The audio decoding device 24 may invoke the foreground formulation unit 78. The foreground formulation unit 78 may perform matrix multiplication the interpolated foreground signals 49" by the interpolated foreground directional information 55 k " to obtain the foreground HOA coefficients 65 (144). The audio decoding device 24 may also invoke the HOA coefficient formulation unit 82. The HOA coefficient formulation unit 82 may add the foreground HOA coefficients 65 to ambient HOA channels 47' so as to obtain the HOA coefficients 11' (146).

[0458] FIGS. 9A-9L are block diagrams illustrating various aspects of the audio encoding device 20 of the example of FIG. 4 in more detail. FIG. 9A is a block diagram illustrating the LIT unit 30 of the audio encoding device 20 in more detail. As shown in the example of FIG. 9A, the LIT unit 30 may include multiple different linear invertible transforms 200-200N. The LIT unit 30 may include, to provide a few examples, a singular value decomposition (SVD) transform 200A ("SVD 200A"), a principle component analysis (PCA) transform 200B ("PCA 200B"), a Karhunen-Loève transform (KLT) 200C ("KLT 200C"), a fast Fourier transform (FFT) 200D ("FFT 200D") and a discrete cosine transform (DCT) 200N ("DCT 200N"). The LIT unit 30 may invoke any one of these linear invertible transforms 200 to apply the respective transform with respect to the HOA coefficients 11 and generate respective transformed HOA coefficients 33 / 35.

[0459] Although described as being performed directly with respect to the HOA coefficients 11, the LIT unit 30 may apply the linear invertible transforms 200 to derivatives of the HOA coefficients 11. For example, the LIT unit 30 may apply the SVD 200 with respect to a power spectral density matrix derived from the HOA coefficients 11. The power spectral density matrix may be denoted as PSD and obtained through matrix multiplication of the transpose of the hoaFrame to the hoaFrame, as outlined in the pseudo-code that follows below. The hoaFrame notation refers to a frame of the HOA coefficients 11.

[0460] The LIT unit 30 may, after applying the SVD 200 (svd) to the PSD, may obtain an S[k] 2< matrix (S_squared) and a V[k] matrix. The S[k] 2< matrix may denote a squared S[k] matrix, whereupon the LIT unit 30 (or, alternatively, the SVD unit 200 as one example) may apply a square root operation to the S[k] 2< matrix to obtain the S[k] matrix. The SVD unit 200 may, in some instances, perform quantization with respect to the V[k] matrix to obtain a quantized V[k] matrix (which may be denoted as V[k]' matrix). The LIT unit 30 may obtain the U[k] matrix by first multiplying the S[k] matrix by the quantized V[k]' matrix to obtain an SV[k]' matrix. The LIT unit 30 may next obtain the pseudo-inverse (pinv) of the SV[k]' matrix and then multiply the HOA coefficients 11 by the pseudo-inverse of the SV[k]' matrix to obtain the U[k] matrix. The foregoing may be represented by the following pseud-code:

[0461] By performing SVD with respect to the power spectral density (PSD) of the HOA coefficients rather than the coefficients themselves, the LIT unit 30 may potentially reduce the computational complexity of performing the SVD in terms of one or more of processor cycles and storage space, while achieving the same source audio encoding efficiency as if the SVD were applied directly to the HOA coefficients. That is, the above described PSD-type SVD may be potentially less computational demanding because the SVD is done on an F*F matrix (with F the number of HOA coefficients). Compared to a M * F matrix with M is the framelength, i.e., 1024 or more samples. The complexity of an SVD may now, through application to the PSD rather than the HOA coefficients 11, be around O(L^3) compared to O(M*L^2) when applied to the HOA coefficients 11 (where O(*) denotes the big-O notation of computation complexity common to the computer-science arts).

[0462] FIG. 9B is a block diagram illustrating the parameter calculation unit 32 of the audio encoding device 20 in more detail. The parameter calculation unit 32 may include an energy analysis unit 202 and a cross-correlation unit 204. The energy analysis unit 202 may perform the above described energy analysis with respect to one or more of the US[k] vectors 33 and the V[k] vectors 35 to generate one or more of the correlation parameter (R), the directional properties parameters (0, φ, r), and the energy property (e) for one or more of the current frame (k) or the previous frame (k-1). Likewise, the cross-correlation unit 204 may perform the above described cross-correlation with respect to one or more of the US[k] vectors 33 and the V[k] vectors 35 to generate one or more of the correlation parameter (R), the directional properties parameters (θ, φ, r), and the energy property (e) for one or more of the current frame (k) or the previous frame (k-1). The parameter calculation unit 32 may output the current frame parameters 37 and the previous frame parameters 39.

[0463] FIG. 9C is a block diagram illustrating the reorder unit 34 of the audio encoding device 20 in more detail. The reorder unit 34 includes a parameter evaluation unit 206 and a vector reorder unit 208. The parameter evaluation unit 206 represents a unit configured to evaluate the previous frame parameters 39 and the current frame parameters 37 in the manner described above to generate reorder indices 205. The reorder indices 205 include indices identifying how the vectors of US[k] vectors 33 and the vectors of the V[k] vectors 35 are to be reordered (e.g., by index pairs with the first index of the pair identifying the index of the current vector location and the second index of the pair identifying the reordered location of the vector). The vector reorder unit 208 represents a unit configured to reorder the US[k] vectors 33 and the V[k] vectors 35 in accordance with the reorder indices 205. The reorder unit 34 may output the reordered US[k] vectors 33' and the reordered V[k] vectors 35', while also passing the reorder indices 205 as one or more syntax elements to the bitstream generation unit 42.

[0464] FIG. 9D is a block diagram illustrating the soundfield analysis unit 44 of the audio encoding device 20 in more detail. As shown in the example of FIG. 9D, the soundfield analysis unit 44 may include a singular value analysis unit 210A, an energy analysis unit 210B, a spatial analysis unit 210C, a spatial masking analysis unit 210D, a diffusion analysis unit 210E and a directional analysis unit 210F. The singular value analysis unit 210A may represent a unit configured to analyze the slope of the curve created by the descending diagonal values of S vectors (forming part of the US[k] vectors 33), where the large singular values represent foreground or distinct sounds and the low singular values represent background components of the soundfield, as described above. The energy analysis unit 210B may represent a unit configured to determine the energy of the V[k] vectors 35 on a per vector basis.

[0465] The spatial analysis unit 210C may represent a unit configured to perform the spatial energy analysis described above through transformation of the HOA coefficients 11 into the spatial domain and identifying areas of high energy representative of directional components of the soundfield that should be preserved. The spatial masking analysis unit 210D may represent a unit configured to perform the spatial masking analysis in a manner similar to that of the spatial energy analysis, except that the spatial masking analysis unit 210D may identify spatial areas that are masked by spatially proximate higher energy sounds. The diffusion analysis unit 210E may represent a unit configured to perform the above described diffusion analysis with respect to the HOA coefficients 11 to identify areas of diffuse energy that may represent background components of the soundfield. The directional analysis unit 210F may represent a unit configured to perform the directional analysis noted above that involves computing the VS[k] vectors, and squaring and summing each entry of each of these VS[k] vectors to identify a directionality quotient. The directional analysis unit 210F may provide this directionality quotient for each of the VS[k] vectors to the background / foreground (BG / FG) identification (ID) unit 212.

[0466] The soundfield analysis unit 44 may also include the BG / FG ID unit 212, which may represent a unit configured to determine the total number of foreground channels (nFG) 45, the order of the background soundfield (N BG ) and the number (nBGa) and indices (i) of additional BG HOA channels to send (which may collectively be denoted as background channel information 43 in the example of FIG. 4) based on any combination of the analysis output by any combination of analysis units 210-210F. The BG / FG ID unit 212 may determine the nFG 45 and the background channel information 43 so as to achieve the target bitrate 41.

[0467] FIG. 9E is a block diagram illustrating the foreground selection unit 36 of the audio encoding device 20 in more detail. The foreground selection unit 36 includes a vector parsing unit 214 that may parse or otherwise extract the foreground US[k] vectors 49 and the foreground V[k] vectors 51 k identified by the nFG syntax element 45 from the reordered US[k] vectors 33' and the reordered V[k] vectors 35'. The vector parsing unit 214 may parse the various vectors representative of the foreground components of the soundfield identified by the soundfield analysis unit 44 and specified by the nFG syntax element 45 (which may also be referred to as foreground channel information 45). As shown in the example of FIG. 9E, the vector parsing unit 214 may select, in some instances, non-consecutive vectors within the foreground US[k] vectors 49 and the foreground V[k] vectors 51 k to represent the foreground components of the soundfield. Moreover, the vector parsing unit 214 may select, in some instances, the same vectors (position-wise) of the foreground US[k] vectors 49 and the foreground V[k] vectors 51 k to represent the foreground components of the soundfield.

[0468] FIG. 9F is a block diagram illustrating the background selection unit 48 of the audio encoding device 20 in more detail. The background selection unit 48 may determine background or ambient HOA coefficients 47 based on the background channel information (e.g., the background soundfield (N BG ) and the number (nBGa) and the indices (i) of additional BG HOA channels to send). For example, when N BG equals one, the background selection unit 48 may select the HOA coefficients 11 for each sample of the audio frame having an order equal to or less than one. The background selection unit 48 may, in this example, then select the HOA coefficients 11 having an index identified by one of the indices (i) as additional BG HOA coefficients, where the nBGa is provided to the bitstream generation unit 42 to be specified in the bitstream 21 so as to enable the audio decoding device, such as the audio decoding device 24 shown in the example of FIG. 5, to parse the BG HOA coefficients 47 from the bitstream 21. The background selection unit 48 may then output the ambient HOA coefficients 47 to the energy compensation unit 38. The ambient HOA coefficients 47 may have dimensions D: M x [(N BG +1) 2< + nBGa].

[0469] FIG. 9G is a block diagram illustrating the energy compensation unit 38 of the audio encoding device 20 in more detail. The energy compensation unit 38 may represent a unit configured to perform energy compensation with respect to the ambient HOA coefficients 47 to compensate for energy loss due to removal of various ones of the HOA channels by the background selection unit 48. The energy compensation unit 38 may include an energy determination unit 218, an energy analysis unit 220 and an energy amplification unit 222.

[0470] The energy determination unit 218 may represent a unit configured to identify the RMS for each row and / or column of on one or more of the reordered US[k] matrix 33' and the reordered V[k] matrix 35'. The energy determination unit 38 may also identify the RMS for each row and / or column of one or more of the selected foreground channels, which may include the nFG signals 49 and the foreground V[k] vectors 51 k , and the order-reduced ambient HOA coefficients 47. The RMS for each row and / or column of the one or more of the reordered US[k] matrix 33' and the reordered V[k] matrix 35' may be stored to a vector denoted RMS FULL , while the RMS for each row and / or column of one or more of the nFG signals 49, the foreground V[k] vectors 51 k , and the order-reduced ambient HOA coefficients 47 may be stored to a vector denoted RMS REDUCED .

[0471] In some examples, to determine each RMS of respective rows and / or columns of one or more of the reordered US[k] matrix 33', the reordered V[k] matrix 35', the nFG signals 49, the foreground V[k] vectors 51 k , and the order-reduced ambient HOA coefficients 47, the energy determination unit 218 may first apply a reference spherical harmonics coefficients (SHC) renderer to the columns. Application of the reference SHC renderer by the energy determination unit 218 allows for determination of RMS in the SHC domain to determine the energy of the overall soundfield described by each row and / or column of the frame represented by rows and / or columns of one or more of the reordered US[k] matrix 33', the reordered V[k] matrix 35', the nFG signals 49, the foreground V[k] vectors 51 k , and the order-reduced ambient HOA coefficients 47. The energy determination unit 38 may pass this RMS FULL and RMS REDUCED vectors to the energy analysis unit 220.

[0472] The energy analysis unit 220 may represent a unit configured to compute an amplification value vector Z, in accordance with the following equation: Z = RMS FULL / RMS REDUCED . The energy analysis unit 220 may then pass this amplification value vector Z to the energy amplification unit 222. The energy amplification unit 222 may represent a unit configured to apply this amplification value vector Z or various portions thereof to one or more of the nFG signals 49, the foreground V[k] vectors 51 k , and the order-reduced ambient HOA coefficients 47. In some instances, the amplification value vector Z is applied to only the order-reduced ambient HOA coefficients 47 per the following equation HOA BG-RED ' = HOA BG-RED Z T< , where HOA BG-RED denotes the order-reduced ambient HOA coefficients 47, HOA BG-RED ' denotes the energy compensated, reduced ambient HOA coefficients 47' and Z T< denotes the transpose of the Z vector.

[0473] FIG. 9H is a block diagram illustrating, in more detail, the spatio-temporal interpolation unit 50 of the audio encoding device 20 shown in the example of FIG. 4. The spatio-temporal interpolation unit 50 may represent a unit configured to receive the foreground V[k] vectors 51 k for the k'th frame and the foreground V[k-1] vectors 51 k-1 for the previous frame (hence the k-1 notation) and perform spatio-temporal interpolation to generate interpolated foreground V[k] vectors. The spatio-temporal interpolation unit 50 may include a V interpolation unit 224 and a foreground adaptation unit 226.

[0474] The V interpolation unit 224 may select a portion of the current foreground V[k] vectors 51 k to interpolate based on the remaining portions of the current foreground V[k] vectors 51 k and the previous foreground V[k-1] vectors 51 k-1 . The V interpolation unit 224 may select the portion to be one or more of the above noted sub-frames or only a single undefined portion that may vary on a frame-by-frame basis. The V interpolation unit 224 may, in some instances, select a single 128 sample portion of the 1024 samples of the current foreground V[k] vectors 51 k to interpolate. The V interpolation unit 224 may then convert each of the vectors in the current foreground V[k] vectors 51 k and the previous foreground V[k-1] vectors 51 k-1 to separate spatial maps by projecting the vectors onto a sphere (using a projection matrix such as a T-design matrix). The V interpolation unit 224 may then interpret the vectors in V as shapes on a sphere. To interpolate the V matrices for the 256 sample portion, the V interpolation unit 224 may then interpolate these spatial shapes - and then transform them back to the spherical harmonic domain vectors via the inverse of the projection matrix. The techniques of this disclosure may, in this manner, provide a smooth transition between V matrices. The V interpolation unit 224 may then generate the remaining V[k] vectors 53, which represent the foreground V[k] vectors 51 k after being modified to remove the interpolated portion of the foreground V[k] vectors 51 k . The V interpolation unit 224 may then pass the interpolated foreground V[k] vectors 51 k ' to the nFG adaptation unit 226.

[0475] When selecting a single portion to interpolation, the V interpolation unit 224 may generate a syntax element denoted CodedSpatialInterpolationTime 254, which identifies the duration or, in other words, time of the interpolation (e.g., in terms of a number of samples). When selecting a single portion of perform the sub-frame interpolation, the V interpolation unit 224 may also generate another syntax element denoted SpatialInterpolationMethod 255, which may identify a type of interpolation performed (or, in some instances, whether interpolation was or was not performed). The spatio-temporal interpolation unit 50 may output these syntax elements 254 and 255 to the bitstream generation unit 42.

[0476] The nFG adaptation unit 226 may represent a unit configured to generated the adapted nFG signals 49'. The nFG adaptation unit 226 may generate the adapted nFG signals 49' by first obtaining the foreground HOA coefficients through multiplication of the nFG signals 49 by the foreground V[k] vectors 51 k . After obtaining the foreground HOA coefficients, the nFG adaptation unit 226 may divide the foreground HOA coefficients by the interpolated foreground V[k] vectors 53 to obtain the adapted nFG signals 49' (which may be referred to as the interpolated nFG signals 49' given that these signals are derived from the interpolated foreground V[k] vectors 51 k ').

[0477] FIG. 9I is a block diagram illustrating, in more detail, the coefficient reduction unit 46 of the audio encoding device 20 shown in the example of FIG. 4. The coefficient reduction unit 46 may represent a unit configured to perform coefficient reduction with respect to the remaining foreground V[k] vectors 53 based on the background channel information 43 to output reduced foreground V[k] vectors 55 to the quantization unit 52. The reduced foreground V[k] vectors 55 may have dimensions D: N + 1 2 − N BG + 1 2 − nBGa × nFG .

[0478] The coefficient reduction unit 46 may include a coefficient minimizing unit 228, which may represent a unit configured to reduce or otherwise minimize the size of each of the remaining foreground V[k] vectors 53 by removing any coefficients that are accounted for in the background HOA coefficients 47 (as identified by the background channel information 43). The coefficient minimizing unit 228 may remove those coefficients identified by the background channel information 43 to obtain the reduced foreground V[k] vectors 55.

[0479] FIG. 9J is a block diagram illustrating, in more detail, the psychoacoustic audio coder unit 40 of the audio encoding device 20 shown in the example of FIG. 4. The psychoacoustic audio coder unit 40 may represent a unit configured to perform psychoacoustic encoding with respect to the energy compensated background HOA coefficients 47'and the interpolated nFG signals 49'. As shown in the example of FIG. 9H, the psychoacoustic audio coder unit 40 may invoke multiple instances of a psychoacoustic audio encoders 40A-40N to audio encode each of the channels of the energy compensated background HOA coefficients 47' (where a channel in this context refers to coefficients for all of the samples in the frame corresponding to a particular order / sub-order spherical basis function) and each signal of the interpolated nFG signals 49'. In some examples, the psychoacoustic audio coder unit 40 instantiates or otherwise includes (when implemented in hardware) audio encoders 40A-40N of sufficient number to separately encode each channel of the energy compensated background HOA coefficients 47' (or nBGa plus the total number of indices (i)) and each signal of the interpolated nFG signals 49' (or nFG) for a total of nBGa plus the total number of indices (i) of additional ambient HOA channels plus nFG. The audio encoders 40A-40N may output the encoded background HOA coefficients 59 and the encoded nFG signals 61.

[0480] FIG. 9K is a block diagram illustrating, in more detail, the quantization unit 52 of the audio encoding device 20 shown in the example of FIG. 4. In the example of FIG. 9K, the quantization unit 52 includes a uniform quantization unit 230, a nbits unit 232, a prediction unit 234, a prediction mode unit 236 ("Pred Mode Unit 236"), a category and residual coding unit 238, and a Huffman table selection unit 240. The uniform quantization unit 230 represents a unit configured to perform the uniform quantization described above with respect to one of the spatial components (which may represent any one of the reduced foreground V[k] vectors 55). The nbits unit 232 represents a unit configured to determine the nbits parameter or value.

[0481] The prediction unit 234 represents a unit configured to perform prediction with respect to the quantized spatial component. The prediction unit 234 may perform prediction by performing an element-wise subtraction of the current one of the reduced foreground V[k] vectors 55 by a temporally subsequent corresponding one of the reduced foreground V[k] vectors 55 (which may be denoted as reduced foreground V[k-1] vectors 55). The result of this prediction may be referred to as a predicted spatial component.

[0482] The prediction mode unit 236 may represent a unit configured to select the prediction mode. The Huffman table selection unit 240 may represent a unit configured to select an appropriate Huffman table for coding of the cid. The prediction mode unit 236 and the Huffman table selection unit 240 may operate, as one example, in accordance with the following pseudo-code:

[0483] For a given nbits, retrieve all the Huffman Tables having nbits

[0484] Category and residual coding unit 238 may represent a unit configured to perform the categorization and residual coding of a predicted spatial component or the quantized spatial component (when prediction is disabled) in the manner described in more detail above.

[0485] As shown in the example of FIG. 9K, the quantization unit 52 may output various parameters or values for inclusion either in the bitstream 21 or side information (which may itself be a bitstream separate from the bitstream 21). Assuming the information is specified in the side channel information, the scalar / entropy quantization unit 50 may output the nbits value as nbits value 233, the prediction mode as prediction mode 237 and the Huffman table information as Huffman table information 241 to bitstream generation unit 42 along with the compressed version of the spatial component (shown as coded foreground V[k] vectors 57 in the example of FIG. 4), which in this example may refer to the Huffman code selected to encode the cid, the sign bit, and the block coded residual. The nbits value may be specified once in the side channel information for all of the coded foreground V[k] vectors 57, while the prediction mode and the Huffman table information may be specified for each one of the coded foreground V[k] vectors 57. The portion of the bitstream that specifies the compressed version of the spatial component is shown in more in the example of FIGS. 10B and / or 10C.

[0486] FIG. 9L is a block diagram illustrating, in more detail, the bitstream generation unit 42 of the audio encoding device 20 shown in the example of FIG. 4. The bitstream generation unit 42 may include a main channel information generation unit 242 and a side channel information generation unit 244. The main channel information generation unit 242 may generate a main bitstream 21 that includes one or more, if not all, of reorder indices 205, the CodedSpatialInterpolationTime syntax element 254, the SpatialInterpolationMethod syntax element 255 the encoded background HOA coefficients 59, and the encoded nFG signals 61. The side channel information generation unit 244 may represent a unit configured to generate a side channel bitstream 21B that may include one or more, if not all, of the nbits value 233, the prediction mode 237, the Huffman table information 241 and the coded foreground V[k] vectors 57. The bitstreams 21 and 21B may be collectively referred to as the bitstream 21. In some contexts, the bitstream 21 may only refer to the main channel bitstream 21, while the bitstream 21B may be referred to as side channel information 21B.

[0487] FIGS. 10A-10O(ii) are diagrams illustrating portions of the bitstream or side channel information that may specify the compressed spatial components in more detail. In the example of FIG. 10A, a portion 250 includes a renderer identifier ("renderer ID") field 251 and a HOADecoderConfig field 252. The renderer ID field 251 may represent a field that stores an ID of the renderer that has been used for the mixing of the HOA content. The HOADecoderConfig field 252 may represent a field configured to store information to initialize the HOA spatial decoder.

[0488] The HOADecoderConfig field 252 further includes a directional information ("direction info") field 253, a CodedSpatialInterpolationTime field 254, a SpatialInterpolationMethod field 255, a CodedVVecLength field 256 and a gain info field 257. The directional information field 253 may represent a field that stores information for configuring the directional-based synthesis decoder. The CodedSpatialInterpolationTime field 254 may represent a field that stores a time of the spatio-temporal interpolation of the vector-based signals. The SpatialInterpolationMethod field 255 may represent a field that stores an indication of the interpolation type applied during the spatio-temporal interpolation of the vector-based signals. The CodedVVecLength field 256 may represent a field that stores a length of the transmitted data vector used to synthesize the vector-based signals. The gain info field 257 represents a field that stores information indicative of a gain correction applied to the signals.

[0489] In the example of FIG. 10B, the portion 258A represents a portion of the side-information channel, where the portion 258A includes a frame header 259 that includes a number of bytes field 260 and an nbits field 261. The number of bytes field 260 may represent a field to express the number of bytes included in the frame for specifying spatial components v1 through vn including the zeros for byte alignment field 264. The nbits field 261 represents a field that may specify the nbits value identified for use in decompressing the spatial components v1-vn.

[0490] As further shown in the example of FIG. 10B, the portion 258A may include sub-bitstreams for v1-vn, each of which includes a prediction mode field 262, a Huffman Table information field 263 and a corresponding one of the compressed spatial components v1-vn. The prediction mode field 262 may represent a field to store an indication of whether prediction was performed with respect to the corresponding one of the compressed spatial components v1-vn. The Huffman table information field 263 represents a field to indicate, at least in part, which Huffman table is to be used to decode various aspects of the corresponding one of the compressed spatial components v1-vn.

[0491] In this respect, the techniques may enable audio encoding device 20 to obtain a bitstream comprising a compressed version of a spatial component of a soundfield, the spatial component generated by performing a vector based synthesis with respect to a plurality of spherical harmonic coefficients.

[0492] FIG. 10C is a diagram illustrating an alternative example of a portion 258B of the side channel information that may specify the compressed spatial components in more detail. In the example of FIG. 10C, the portion 258B includes a frame header 259 that includes an Nbits field 261. The Nbits field 261 represents a field that may specify an nbits value identified for use in decompressing the spatial components v1-vn.

[0493] As further shown in the example of FIG. 10C, the portion 258B may include sub-bitstreams for v1-vn, each of which includes a prediction mode field 262, a Huffman Table information field 263 and a corresponding one of the compressed spatial components v1-vn. The prediction mode field 262 may represent a field to store an indication of whether prediction was performed with respect to the corresponding one of the compressed spatial components v1-vn. The Huffman table information field 263 represents a field to indicate, at least in part, which Huffman table is to be used to decode various aspects of the corresponding one of the compressed spatial components v1-vn.

[0494] Nbits field 261 in the illustrated example includes subfields A 265, B 266, and C 267. In this example, A 265 and B 266 are each 1 bit sub-fields, while C 267 is a 2 bit sub-field. Other examples may include differently-sized sub-fields 265, 266, and 267. The A field 265 and the B field 266 may represent fields that store first and second most significant bits of the Nbits field 261, while the C field 267 may represent a field that stores the least significant bits of the Nbits field 261.

[0495] The portion 258B may also include an AddAmbHoaInfoChannel field 268. The AddAmbHoaInfoChannel field 268 may represent a field that stores information for the additional ambient HOA coefficients. As shown in the example of FIG. 10C, the AddAmbHoaInfoChannel 268 includes a CodedAmbCoeffldx field 246, an AmbCoeffldxTransition field 247. The CodedAmbCoeffIdx field 246 may represent a field that stores an index of an additional ambient HOA coefficient. The AmbCoeffldxTransition field 247 may represent a field configured to store data indicative whether, in this frame, an additional ambient HOA coefficient is either being faded in or faded out.

[0496] FIG. 10C(i) is a diagram illustrating an alternative example of a portion 258B' of the side channel information that may specify the compressed spatial components in more detail. In the example of FIG. 10C(i), the portion 258B' includes a frame header 259 that includes an Nbits field 261. The Nbits field 261 represents a field that may specify an nbits value identified for use in decompressing the spatial components v1-vn.

[0497] As further shown in the example of FIG. 10C(i), the portion 258B' may include sub-bitstreams for v1-vn, each of which includes a Huffman Table information field 263 and a corresponding one of the compressed directional components v1-vn without including the prediction mode field 262. In all other respects, the portion 258B' may be similar to the portion 258B.

[0498] FIG. 10D is a diagram illustrating a portion 258C of the bitstream 21 in more detail. The portion 258C is similar to the portion 258, except that the frame header 259 and the zero byte alignment 264 have been removed, while the Nbits 261 field has been added before each of the bitstreams for v1-vn, as shown in the example of FIG. 10D.

[0499] FIG. 10D(i) is a diagram illustrating a portion 258C' of the bitstream 21 in more detail. The portion 258C' is similar to the portion 258C except that the portion 258C' does not include the prediction mode field 262 for each of the V vectors v1-vn.

[0500] FIG. 10E is a diagram illustrating a portion 258D of the bitstream 21 in more detail. The portion 258D is similar to the portion 258B, except that the frame header 259 and the zero byte alignment 264 have been removed, while the Nbits 261 field has been added before each of the bitstreams for v1-vn, as shown in the example of FIG. 10E.

[0501] FIG. 10E(i) is a diagram illustrating a portion 258D' of the bitstream 21 in more detail. The portion 258D' is similar to the portion 258D except that the portion 258D' does not include the prediction mode field 262 for each of the V vectors v1-vn. In this respect, the audio encoding device 20 may generate a bitstream 21 that does not include the prediction mode field 262 for each compressed V vector, as demonstrated with respect to the examples of FIGS. 10C(i), 10D(i) and 10E(i).

[0502] FIG. 10F is a diagram illustrating, in a different manner, the portion 250 of the bitstream 21 shown in the example of FIG. 10A. The portion 250 shown in the example of FIG. 10D, includes an HOAOrder field (which was not shown in the example of FIG. 10F for ease of illustration purposes), a MinAmbHoaOrder field (which again was not shown in the example of FIG. 10 for ease of illustration purposes), the direction info field 253, the CodedSpatialInterpolationTime field 254, the SpatialInterpolationMethod field 255, the CodedVVecLength field 256 and the gain info field 257. As shown in the example of FIG. 10F, the CodedSpatialInterpolationTime field 254 may comprise a three bit field, the SpatialInterpolationMethod field 255 may comprise a one bit field, and the CodedVVecLength field 256 may comprise two bit field.

[0503] FIG. 10G is a diagram illustrating a portion 248 of the bitstream 21 in more detail. The portion 248 represents a unified speech / audio coder (USAC) three-dimensional (3D) payload including an HOAframe field 249 (which may also be denoted as the sideband information, side channel information, or side channel bitstream). As shown in the example of FIG. 10E, the expanded view of the HOAFrame field 249 may be similar to the portion 258B of the bitstream 21 shown in the example of FIG. 10C. The "ChannelSideInfoData" includes a ChannelType field 269, which was not shown in the example of FIG. 10C for ease of illustration purposes, the A field 265 denoted as "ba" in the example of FIG. 10E, the B field 266 denoted as "bb" in the example of FIG. 10E and the C field 267 denoted as "unitC" in the example of FIG. 10E. The ChannelType field indicates whether the channel is a direction-based signal, a vector-based signal or an additional ambient HOA coefficient. Between different ChannelSideInfoData there is AddAmbHoaInfoChannel fields 268 with the different V vector bitstreams denoted in grey (e.g., "bitstream for v1" and "bitstream for v2").

[0504] FIGS. 10H-10O(ii) are diagrams illustrating another various example portions 248H-248O of the bitstream 21 along with accompanying HOAconfig portions 250H-250O in more detail. FIGS. 10H(i) and 10H(ii) illustrate a first example bitstream 248H and accompanying HOA config portion 250H having been generated to correspond with case 0 in the above pseudo-code. In the example of FIG. 10H(i), the HOAconfig portion 250H includes a CodedVVecLength syntax element 256 set to indicate that all elements of a V vector are coded, e.g., all 16 V vector elements. The HOAconfig portion 250H also includes a SpatialInterpolationMethod syntax element 255 set to indicate that the interpolation function of the spatio-temporal interpolation is a raised cosine. The HOAconfig portion 250H moreover includes a CodedSpatialTnterpolationTime 254 set to indicate an interpolated sample duration of 256. The HOAconfig portion 250H further includes a MinAmbHoaOrder syntax element 150 set to indicate that the MinimumHOA order of the ambient HOA content is one, where the audio decoding device 24 may derive a MinNumofCoeffsForAmbHOA syntax element to be equal to (1+1) 2< or four. The HOAconfig portion 250H includes an HoaOrder syntax element 152 set to indicate the HOA order of the content to be equal to three (or, in other words, N = 3), where the audio decoding device 24 may derive a NumOfHoaCoeffs to be equal to (N + 1) 2< or 16.

[0505] As further shown in the example of FIG. 10H(i), the portion 248H includes a unified speech and audio coding (USAC) three-dimensional (USAC-3D) audio frame in which two HOA frames 249A and 249B are stored in a USAC extension payload given that two audio frames are stored within one USAC-3D frame when spectral band replication (SBR) is enabled. The audio decoding device 24 may derive a number of flexible transport channels as a function of a numHOATransportChannels syntax element and a MinNumOfCoeffsForAmbHOA syntax element. In the following examples, it is assumed that the numHOATransportChannels syntax element is equal to 7 and the MinNumOfCoeffsForAmbHOA syntax element is equal to four, where number of flexible transport channels is equal to the numHOATransportChannels syntax element minus the MinNumOfCoeffsForAmbHOA syntax element (or three).

[0506] FIG. 10H(ii) illustrates the frames 249A and 249B in more detail. As shown in the example of FIG. 10H(ii), frame 249A includes ChannelSideInfoData (CSID) fields 154-154C, an HOAGainCorrectionData (HOAGCD) fields, VVectorData fields 156 and 156B and HOAPredictionInfo fields. The CSID field 154 includes the unitC 267, bb 266 and ba265 along with the ChannelType 269, each of which are set to the corresponding values 01, 1, 0 and 01 shown in the example of FIG. 10H(i). The CSID field 154B includes the unitC 267, bb 266 and ba265 along with the ChannelType 269, each of which are set to the corresponding values 01, 1, 0 and 01 shown in the example of FIG. 10H(ii). The CSID field 154C includes the ChannelType field 269 having a value of 3. Each of the CSID fields 154-154C correspond to the respective one of the transport channels 1, 2 and 3. In effect, each CSID field 154-154C indicates whether the corresponding payload 156 and 156B are direction-based signals (when the corresponding ChannelType is equal to zero), vector-based signals (when the corresponding ChannelType is equal to one), an additional Ambient HOA coefficient (when the corresponding ChannelType is equal to two), or empty (when the ChannelType is equal to three).

[0507] In the example of FIG. 10H(ii), the frame 249A includes two vector-based signals (given the ChannelType 269 equal to 1 in the CSID fields 154 and 154B) and an empty (given the ChannelType 269 equal to 3 in the CSID fields 154C). Given the forgoing HOAconfig portion 250H, the audio decoding device 24 may determine that all 16 V vector elements are encoded. Hence, the VVectorData 156 and 156B each includes all 16 vector elements, each of them uniformly quantized with 8 bits. As noted by the footnote 1, the number and indices of coded VVectorData elements are specified by the parameter CodedVVecLength=0. Moreover, as noted by the single asterisk (*), the coding scheme is signaled by NbitsQ = 5 in the CSID field for the corresponding transport channel.

[0508] In the frame 249B, the CSID field 154 and 154B are the same as that in frame 249, while the CSID field 154C of the frame 249B switched to a ChannelType of one. The CSID field 154C of the frame 249B therefore includes the Cbflag 267, the Pflag 267 (indicating Huffman encoding) and Nbits 261 (equal to twelve). As a result, the frame 249B includes a third VVectorData field 156C that includes 16 V vector elements, each of them uniformly quantized with 12 bits and Huffman coded. As noted above, the number and indices of the coded VVectorData elements are specified by the parameter CodedVVecLength = 0, while the Huffman coding scheme is signaled by the NbitsQ = 12, CbFlag = 0 and Pflag = 0 in the CSID field 154C for this particular transport channel (e.g., transport channel no. 3).

[0509] The example of FIGS. 10I(i) and 10I(ii) illustrate a second example bitstream 248I and accompanying HOA config portion 250I having been generated to correspond with case 0 in the above in the above pseudo-code. In the example of FIG. 10I(i), the HOAconfig portion 250I includes a CodedVVecLength syntax element 256 set to indicate that all elements of a V vector are coded, e.g., all 16 V vector elements. The HOAconfig portion 250I also includes a SpatialInterpolationMethod syntax element 255 set to indicate that the interpolation function of the spatio-temporal interpolation is a raised cosine. The HOAconfig portion 250I moreover includes a CodedSpatialInterpolationTime 254 set to indicate an interpolated sample duration of 256.

[0510] The HOAconfig portion 250I further includes a MinAmbHoaOrder syntax element 150 set to indicate that the MinimumHOA order of the ambient HOA content is one, where the audio decoding device 24 may derive a MinNumofCoeffsForAmbHOA syntax element to be equal to (1+1) 2< or four. The audio decoding device 24 may also derive a MaxNoofAddActiveAmbCoeffs syntax element as set to a difference between the NumOfHoaCoeff syntax element and the MinNumOfCoeffsForAmbHOA, which is assumed in this example to equal 16-4 or 12. The audio decoding device 24 may also derive a AmbAsignmBits syntax element as set to ceil(log2(MaxNoOfAddActiveAmbCoeffs)) = ceil(log2(12)) = 4. The HOAconfig portion 250H includes an HoaOrder syntax element 152 set to indicate the HOA order of the content to be equal to three (or, in other words, N = 3), where the audio decoding device 24 may derive a NumOfHoaCoeffs to be equal to (N + 1) 2< or 16.

[0511] As further shown in the example of FIG. 10I(i), the portion 248H includes a USAC-3D audio frame in which two HOA frames 249C and 249D are stored in a USAC extension payload given that two audio frames are stored within one USAC-3D frame when spectral band replication (SBR) is enabled. The audio decoding device 24 may derive a number of flexible transport channels as a function of a numHOATransportChannels syntax element and a MinNumOfCoeffsForAmbHOA syntax element. In the following examples, it is assumed that the numHOATransportChannels syntax element is equal to 7 and the MinNumOfCoeffsForAmbHOA syntax element is equal to four, where number of flexible transport channels is equal to the numHOATransportChannels syntax element minus the MinNumOfCoeffsForAmbHOA syntax element (or three).

[0512] FIG. 10I(ii) illustrates the frames 249C and 249D in more detail. As shown in the example of FIG. 10I(ii), the frame 249C includes CSID fields 154-154C and VVectorData fields 156. The CSID field 154 includes the CodedAmbCoeffldx 246, the AmbCoeffldxTransition 247 (where the double asterisk (**) indicates that, for flexible transport channel Nr. 1, the decoder's internal state is here assumed to be AmbCoeffldxTransitionState = 2, which results in the CodedAmbCoeffIdx bitfield is signaled or otherwise specified in the bitstream), and the ChannelType 269 (which is equal to two, signaling that the corresponding payload is an additional ambient HOA coefficient). The audio decoding device 24 may derive the AmbCoeffIdx as equal to the CodedAmbCoeffIdx+1+MinNumOfCoeffsForAmbHOA or 5 in this example. The CSID field 154B includes unitC 267, bb 266 and ba265 along with the ChannelType 269, each of which are set to the corresponding values 01, 1, 0 and 01 shown in the example of FIG. 10I(ii). The CSID field 154C includes the ChannelType field 269 having a value of 3.

[0513] In the example of FIG. 10I(ii), the frame 249C includes a single vector-based signal (given the ChannelType 269 equal to 1 in the CSID fields 154B) and an empty (given the ChannelType 269 equal to 3 in the CSID fields 154C). Given the forgoing HOAconfig portion 250I, the audio decoding device 24 may determine that all 16 V vector elements are encoded. Hence, the VVectorData 156 includes all 16 vector elements, each of them uniformly quantized with 8 bits. As noted by the footnote 1, the number and indices of coded VVectorData elements are specified by the parameter CodedVVecLength=0. Moreover, as noted by the footnote 2, the coding scheme is signaled by NbitsQ = 5 in the CSID field for the corresponding transport channel.

[0514] In the frame 249D, the CSID field 154 includes an AmbCoeffldxTransition 247 indicating that no transition has occurred and therefore the CodedAmbCoeffIdx 246 may be implied from the previous frame and need not be signaled or otherwise specified again. The CSID field 154B and 154C of the frame 249D are the same as that for the frame 249C and thus, like the frame 249C, the frame 249D includes a single VVectorData field 156, which includes all 16 vector elements, each of them uniformly quantized with 8 bits.

[0515] FIGS. 10J(i) and 10J(ii) illustrate a first example bitstream 248J and accompanying HOA config portion 250J having been generated to correspond with case 1 in the above pseudo-code. In the example of FIG. 10J(i), the HOAconfig portion 250J includes a CodedVVecLength syntax element 256 set to indicate that all elements of a V vector are coded, except for the elements 1 through a MinNumOfCoeffsForAmbHOA syntax elements and those elements specified in a ContAddAmbHoaChan syntax element (assumed to be zero in this example). The HOAconfig portion 250J also includes a SpatialInterpolationMethod syntax element 255 set to indicate that the interpolation function of the spatio-temporal interpolation is a raised cosine. The HOAconfig portion 250J moreover includes a CodedSpatialInterpolationTime 254 set to indicate an interpolated sample duration of 256. The HOAconfig portion 250J further includes a MinAmbHoaOrder syntax element 150 set to indicate that the MinimumHOA order of the ambient HOA content is one, where the audio decoding device 24 may derive a MinNumofCoeffsForAmbHOA syntax element to be equal to (1+1) 2< or four. The HOAconfig portion 250J includes an HoaOrder syntax element 152 set to indicate the HOA order of the content to be equal to three (or, in other words, N = 3), where the audio decoding device 24 may derive a NumOfHoaCoeffs to be equal to (N + 1) 2< or 16.

[0516] As further shown in the example of FIG. 10J(i), the portion 248J includes a USAC-3D audio frame in which two HOA frames 249E and 249F are stored in a USAC extension payload given that two audio frames are stored within one USAC-3D frame when spectral band replication (SBR) is enabled. The audio decoding device 24 may derive a number of flexible transport channels as a function of a numHOATransportChannels syntax element and a MinNumOfCoeffsForAmbHOA syntax element. In the following examples, it is assumed that the numHOATransportChannels syntax element is equal to 7 and the MinNumOfCoeffsForAmbHOA syntax element is equal to four, where number of flexible transport channels is equal to the numHOATransportChannels syntax element minus the MinNumOfCoeffsForAmbHOA syntax element (or three).

[0517] FIG. 10J(ii) illustrates the frames 249E and 249F in more detail. As shown in the example ofFIG. 10J(ii), frame 249E includes CSID fields 154-154C and VVectorData fields 156 and 156B. The CSID field 154 includes the unitC 267, bb 266 and ba265 along with the ChannelType 269, each of which are set to the corresponding values 01, 1, 0 and 01 shown in the example of FIG. 10J(i). The CSID field 154B includes the unitC 267, bb 266 and ba265 along with the ChannelType 269, each of which are set to the corresponding values 01, 1, 0 and 01 shown i...

Claims

1. A device comprising one or more processors configured to: obtain one or more first vectors describing distinct components of a soundfield and one or more second vectors describing background components of the soundfield, both the one or more first vectors and the one or more second vectors representative of a plurality of spherical harmonic coefficients descriptive of the soundfield and generated by performing a transformation with respect to the plurality of spherical harmonic coefficients, wherein the transformation comprises a principal component analysis to identify the distinct components of the soundfield and the background components of the soundfield; render one or more loudspeaker feeds based on the one or more first vectors and the one or more second vectors; and output the one or more loudspeaker feeds.

2. The device of claim 1, wherein performing the transformation generates a matrix representative of principle components of the spherical harmonic coefficients and a matrix representative of right-singular vectors of the spherical harmonic coefficients.

3. The device of claim 1 or claim 2, wherein the one or more processors are further configured to reconstruct the plurality of spherical harmonic coefficients based on the one or more first vectors and the one or more second vectors, wherein the one or more processors are configured to render, based on the plurality of spherical harmonic coefficients, the loudspeaker feeds.

4. A method performed by an audio decoding device, the method comprising: obtaining one or more first vectors describing distinct components of a soundfield and one or more second vectors describing background components of the soundfield, both the one or more first vectors and the one or more second vectors representative of a plurality of spherical harmonic coefficients descriptive of the soundfield and generated by performing a transformation with respect to the plurality of spherical harmonic coefficients, wherein the transformation comprises a principal component analysis to identify the distinct components of the soundfield and the background components of the soundfield; rendering one or more loudspeaker feeds based on the one or more first vectors and the one or more second vectors; and outputting the one or more loudspeaker feeds.

5. A computer-readable storage medium having stored thereon instructions that, when executed by one or more processors, cause the one or more processors to perform a method according to claim 4.