Time alignment of qmf-based processing data

Abstract

To provide time alignment of encoded data of an audio encoder with associated metadata, such as spectral band replication (SBR) metadata.SOLUTION: An audio decoder (100, 300) configured to determine reconstructed frames of an audio signal (237) from access units (110) of a received data stream is described. The access unit (110) includes waveform data (111) and metadata (112), where the waveform data (111) and the metadata (112) are associated with a same reconstructed frame of the audio signal (127). An audio decoder (100, 300) comprises a waveform processing path (101, 102, 103, 104, 105) configured to generate a plurality of waveform subband signals (123) from waveform data (111), and a metadata processing path (108, 109) configured to generate decoded metadata (128) from metadata (111).SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Provisional Patent Application No. 61 / 877,194, filed September 12, 2013, and U.S. Provisional Patent Application No. 61 / 909,593, filed November 27, 2013, the contents of each of which are incorporated herein by reference in their entirety.

[0002] Technical Field This paper concerns the time alignment of encoded data in audio encoders with associated metadata such as Spectral Band Replication (SBR), especially High Efficiency (HE) Advanced Audio Coding (AAC) metadata. Summary of the Invention [Problem to be solved by the invention]

[0003] One technical challenge in the context of audio coding is to provide an audio encoding and decoding system that exhibits low latency to allow real-time applications such as live broadcast. Furthermore, it is desirable to provide an audio encoding and decoding system that exchanges encoded bitstreams that can be spliced ​​with other bitstreams. Furthermore, to allow cost-effective implementation of the system, a computationally efficient audio encoding and decoding system should be provided. This paper addresses the technical challenge of providing encoded bitstreams that can be spliced ​​in an efficient manner while maintaining latency at a level appropriate for live broadcast. This paper describes an audio encoding and decoding system that allows bitstream splicing with a reasonable degree of coding delay, thereby enabling applications such as live broadcast, where the broadcast bitstream may be generated from multiple source bitstreams. [Means for solving the problem]

[0004] According to one aspect, an audio decoder configured to determine reconstructed frames of an audio signal from access units of a received data stream is described. Typically, the data stream includes a sequence of access units for determining a respective sequence of reconstructed frames of the audio signal. A frame of the audio signal typically includes a predetermined number N of time-domain samples of the audio signal, where N is greater than 1. A sequence of access units may correspondingly describe a sequence of frames of the audio signal.

[0005] An access unit includes waveform data and metadata, where the waveform data and the metadata are associated with the same reconstructed frame of the audio signal. In other words, the waveform data and the metadata for determining a reconstructed frame of the audio signal are included in the same access unit. Each access unit in a sequence of access units may include the waveform data and the metadata for generating a respective reconstructed frame of the sequence of reconstructed frames of the audio signal. In particular, the access unit for a particular frame may include (e.g., all) data necessary to determine a reconstructed frame for that particular frame.

[0006] In one example, an access unit for a particular frame may include (e.g., all) data necessary to perform a high frequency reconstruction (HFR) scheme to generate a high frequency signal for that particular frame based on the low frequency signal for that particular frame (contained within the waveform data of the access unit) and based on decoded metadata.

[0007] Alternatively or additionally, the access unit for a particular frame may contain (e.g., all) data necessary to perform dynamic range expansion of that particular frame. In particular, expansion or widening of the low-frequency signal of that particular frame may be performed based on the decoded metadata. For this purpose, the decoded metadata may contain one or more expansion parameters. The one or more expansion parameters may indicate one or more of: whether compression / expansion is applied to that particular frame; whether compression / expansion is applied uniformly for all channels of the multi-channel audio signal (i.e., whether the same expansion gain(s) are applied for all channels of the multi-channel audio signal or whether different expansion gain(s) are applied for different channels of the multi-channel audio signal); and / or the temporal resolution of the expansion gain.

[0008] Providing a sequence of access units, each of which contains the data necessary to generate a corresponding reconstructed frame of the audio signal independently of the preceding or following access unit, is beneficial for splicing applications because it allows the data stream to be spliced ​​between two adjacent access units without affecting the perceptual quality of the reconstructed frame of the audio signal at the splice point (e.g., immediately after the splice point).

[0009] In one example, a reconstructed frame of an audio signal includes a low-pass signal and a high-pass signal, wherein the waveform data is indicative of the low-pass signal. The metadata is indicative of a spectral envelope of the high-pass signal. The low-pass signal may correspond to a component of the audio signal covering a relatively low frequency range (e.g., including frequencies below a predetermined crossover frequency). The high-pass signal may correspond to a component of the audio signal covering a relatively high frequency range (e.g., including frequencies above the predetermined crossover frequency). The low-pass signal and the high-pass signal may be complementary in terms of the frequency ranges covered by the low-pass signal and the high-pass signal. The audio decoder may be configured to perform high-frequency reconstruction (HFR), such as spectral band replication (SBR), of the high-pass signal using the metadata and waveform data. Thus, the metadata may include HFR or SBR metadata indicative of a spectral envelope of the high-pass signal.

[0010] The audio decoder may include a waveform processing path configured to generate a plurality of waveform subband signals from the waveform data. The plurality of waveform subband signals may correspond to a representation of a time-domain waveform signal in a subband domain (e.g., in a QMF domain). The time-domain waveform signal may correspond to the above-mentioned low-pass signal, and the plurality of waveform subband signals may correspond to a plurality of low-pass subband signals. Furthermore, the audio decoder may include a metadata processing path configured to generate decoded metadata from the metadata.

[0011] Furthermore, the audio decoder may have a metadata application and synthesis unit configured to generate the reconstructed frame of the audio signal from the plurality of waveform subband signals and from the decoded metadata. In particular, the metadata application and synthesis unit may be configured to perform an HFR and / or SBR scheme to generate a plurality of (e.g., scaled) higher-band subband signals from the plurality of waveform subband signals (i.e., from the plurality of low-band subband signals in that case) and from the decoded metadata. The reconstructed frame of the audio signal may then be determined based on the plurality of (e.g., scaled) higher-band subband signals and based on the plurality of low-band signals.

[0012] Alternatively or additionally, the audio decoder may comprise an expansion unit configured to expand or perform the expansion of the plurality of waveform subband signals using at least a part of the decoded metadata, in particular using the one or more expansion parameters included in the decoded metadata. To this end, the expansion unit may be configured to apply one or more expansion gains to the plurality of waveform subband signals. The expansion unit may be configured to determine the one or more expansion gains based on the plurality of waveform subband signals, based on one or more predetermined compression / expansion rules or functions and / or based on the one or more expansion parameters.

[0013] The waveform processing path and / or the metadata processing path may comprise at least one delay unit configured to time-align the plurality of waveform subband signals and the decoded metadata. In particular, the at least one delay unit may be configured to align the plurality of waveform subband signals and the decoded metadata and / or to insert at least one delay in the waveform processing path and / or the metadata processing path such that an overall delay of the waveform processing path corresponds to an overall delay of the metadata processing path. Alternatively or additionally, the at least one delay unit may be configured to time-align the plurality of waveform subband signals and the decoded metadata such that the plurality of waveform subband signals and the decoded metadata are provided to the metadata application and combination unit in time for processing performed by the metadata application and combination unit. In particular, the plurality of waveform subband signals and the decoded metadata may be provided to the metadata application and synthesis unit such that there is no need to buffer the plurality of waveform subband signals and / or the decoded metadata prior to performing processing (e.g. HFR or SBR processing) on ​​the plurality of waveform subband signals and / or the decoded metadata.

[0014] In other words, the audio decoder may be configured to delay providing the decoded metadata and / or the plurality of waveform subband signals to the metadata application and synthesis unit, which may be configured to perform an HFR scheme, so that the decoded metadata and / or the plurality of waveform subband signals are provided when needed for processing. The inserted delay may be selected to reduce (e.g., minimize) the overall delay of the audio codec (including the audio decoder and corresponding audio encoder) while enabling splicing of bitstreams comprising a sequence of access units. Thus, the audio decoder may be configured to handle time-aligned access units comprising the waveform data and the metadata to determine a particular reconstructed frame of the audio signal with minimal impact on the overall delay of the audio codec. Furthermore, the audio decoder may be configured to handle time-aligned access units without the need to resample metadata. In this way, the audio decoder is configured to determine a particular reconstructed frame of the audio signal in a computationally efficient manner and without degrading audio quality. Thus, an audio decoder can be configured to allow splicing applications in a computationally efficient manner while maintaining high audio quality and low overall delay.

[0015] Furthermore, the use of at least one delay unit configured to time-align the plurality of subband signals and the decoded metadata may ensure precise and consistent alignment of the plurality of waveform subband signals and the decoded metadata in the subband domain (the domain in which the processing of the plurality of waveform subband signals and the decoded metadata is typically performed).

[0016] The metadata processing path may include a metadata delay unit configured to delay the decoded metadata by an integer multiple greater than 0 of a frame length N of the reconstructed frame of the audio signal. The additional delay introduced by the metadata delay unit may be referred to as a metadata delay. The frame length N may correspond to the number N of time-domain samples included in the reconstructed frame of the audio signal. The integer multiple may be such that the delay introduced by the metadata delay unit is greater than the delay introduced by the processing in the waveform processing path (e.g., without considering the additional waveform delay introduced in the waveform processing path). The metadata delay may depend on the frame length N of the reconstructed frame of the audio signal. This may be due to the fact that the delay caused by the processing in the waveform processing path depends on the frame length N. In particular, the integer multiple may be 1 for frame lengths N greater than 960 and / or the integer multiple may be 2 for frame lengths N less than or equal to 960.

[0017] As described above, the metadata application and synthesis unit may be configured to process the decoded metadata and the plurality of waveform subband signals in the subband domain (e.g., in the QMF domain). Furthermore, the decoded metadata may indicate metadata in the subband domain (e.g., indicative of spectral coefficients describing the spectral envelope of a high-band signal). Furthermore, the metadata delay unit may be configured to delay the decoded metadata. Using a metadata delay that is an integer multiple of a frame length N greater than zero may be beneficial because it ensures consistent alignment of the plurality of waveform subband signals and the decoded metadata in the subband domain (e.g., for processing within the metadata application and synthesis unit). In particular, this ensures that the decoded metadata can be applied to the correct frame of the waveform signal (i.e., to the correct frame of the plurality of waveform subband signals) without the need to resample the metadata.

[0018] The waveform processing path may comprise a waveform delay unit configured to delay the plurality of waveform subband signals such that an overall delay of the waveform processing path corresponds to an integer multiple of a frame length N of a reconstructed frame of the audio signal that is greater than zero. The additional delay introduced by the waveform delay unit may be referred to as a waveform delay. The integer multiple of the waveform processing path may correspond to the integer multiple of the metadata processing path.

[0019] The waveform delay unit and / or the metadata delay unit may be implemented as a buffer configured to store the plurality of waveform subband signals and / or the decoded metadata for an amount of time corresponding to the waveform delay and / or for an amount of time corresponding to the metadata delay. The waveform delay unit may be located anywhere in the waveform processing path upstream of the metadata application and combination unit. Thus, the waveform delay unit may be configured to delay the waveform data and / or the plurality of waveform subband signals (and / or any intermediate data or signals in the waveform processing path). In one example, the waveform delay units may be distributed along the waveform processing path, with each distributed delay unit providing a portion of the overall waveform delay. Distribution of waveform delay units may be beneficial for cost-effective implementation of waveform delay units. Similar to waveform delay units, metadata delay units may be located anywhere in the metadata processing path upstream of the metadata application and combination unit. Furthermore, the waveform delay units may be distributed along the metadata processing path.

[0020] The waveform processing path may include a decoding and dequantization unit configured to decode and dequantize the waveform data to provide a plurality of frequency coefficients indicative of the waveform signal. Thus, the waveform data may include or be indicative of the plurality of frequency coefficients. This allows for the generation of the waveform signal of the reconstructed frame of the audio signal. Furthermore, the waveform processing path may include a waveform synthesis unit configured to generate the waveform signal from the plurality of frequency coefficients. The waveform synthesis unit may be configured to perform a frequency-domain to time-domain transformation. In particular, the waveform synthesis unit may be configured to perform an inverse modified discrete cosine transform (MDCT). The waveform synthesis unit or the processing of the waveform synthesis unit may introduce a delay that depends on the frame length N of the reconstructed frame of the audio signal. In particular, the delay introduced by the waveform synthesis unit may correspond to half the frame length N.

[0021] After reconstructing the waveform signal from the waveform data, the waveform signal may be processed in conjunction with the decoded metadata. In one example, the waveform signal may be used in the context of an HFR or SBR scheme for determining the high-band signal using the decoded metadata. To this end, the waveform processing path may include a decomposition unit configured to generate the plurality of waveform subband signals from the waveform signal. The decomposition unit may be configured to perform a transformation from the time domain to the subband domain, for example, by applying a quadrature mirror filter (QMF) bank. Typically, the frequency resolution of the transformation performed by the waveform synthesis unit is higher (e.g., at least 5 or 10 times) than the frequency resolution of the transformation performed by the decomposition unit. This may be indicated by the terms "frequency domain" and "subband domain." Here, the frequency domain may be associated with a higher frequency resolution than the subband domain. The decomposition unit may introduce a fixed delay that is independent of the frame length N of the reconstructed frames of the audio signal. The fixed delay introduced by the decomposition unit may depend on the length of the filters of the filter bank used by the decomposition unit. As an example, the fixed delay introduced by the decomposition unit may correspond to 320 samples of the audio signal.

[0022] The overall delay of the waveform processing path may further depend on a predetermined look-ahead between metadata and waveform data. Such look-ahead may be beneficial for increasing continuity between adjacent reconstructed frames of the audio signal. The predetermined look-ahead and / or the associated look-ahead delay may correspond to 192 or 384 samples of the audio samples. The look-ahead delay may be in the context of determining HFR or SBR metadata indicative of the spectral envelope of a high-frequency signal. In particular, the look-ahead may allow a corresponding audio encoder to determine the HFR or SBR metadata for the particular frame of the audio signal based on a predetermined number of samples from a immediately succeeding frame of the audio signal. This may be beneficial if the particular frame contains an acoustic transient. The look-ahead delay may be applied by a look-ahead delay unit included in the waveform processing path.

[0023] Thus, the overall delay of the waveform processing path, i.e., the waveform delay, may depend on the various processes performed within the waveform processing path. Furthermore, the waveform delay may depend on the metadata delay introduced by the metadata processing path. The waveform delay may correspond to any multiple of the samples of the audio signal. For this reason, it may be beneficial to utilize a waveform delay unit configured to delay the waveform signal, where the waveform signal is represented in the time domain. In other words, it may be beneficial to apply a waveform delay to the waveform signal. In this way, precise and consistent application of a waveform delay corresponding to any multiple of the samples of the audio signal may be ensured.

[0024] An exemplary decoder may include a metadata delay unit configured to apply a metadata delay to the metadata, which may be represented in the subband domain, and a waveform delay unit configured to apply a waveform delay to a waveform signal, which may be represented in the time domain. The metadata delay unit may apply a metadata delay corresponding to an integer multiple of a frame length N, and the waveform delay unit may apply a waveform delay corresponding to an integer multiple of samples of the audio signal. As a result, precise and consistent alignment of the waveform subband signals and the decoded metadata for processing within the metadata application and synthesis unit may be ensured. The processing of the waveform subband signals and the decoded metadata may occur in the subband domain. The alignment of the waveform subband signals and the decoded metadata may be achieved without resampling of the decoded metadata, thereby providing a computationally efficient and quality-preserving alignment means.

[0025] As outlined above, the audio decoder may be configured to perform an HFR or SBR technique. The metadata application and synthesis unit may include a metadata application unit configured to perform high-frequency reconstruction (e.g., SBR) using the plurality of low-frequency subband signals and using the decoded metadata. In particular, the metadata application unit may be configured to transform one or more of the plurality of low-frequency subband signals to generate a plurality of high-frequency subband signals. Furthermore, the metadata application unit may be configured to apply the decoded metadata to the plurality of high-frequency subband signals to provide a plurality of scaled high-frequency subband signals. The plurality of scaled high-frequency subband signals may represent the high-frequency signals of the reconstructed frame of the audio signal. To generate the reconstructed frame of the audio signal, the metadata application and synthesis unit may further include a synthesis unit configured to generate the reconstructed frame of the audio signal from the plurality of low-frequency subband signals and from the plurality of scaled high-frequency subband signals. The synthesis unit may be configured to perform an inverse transform of the transform performed by the decomposition unit, for example by applying an inverse QMF bank. The number of filters included in the filter bank of the synthesis unit may be greater than the number of filters included in the filter bank of the decomposition unit (e.g., to take into account the extended frequency range resulting from the multiple scaled high-pass subband signals).

[0026] As described above, the audio decoder may include an expanding unit. The expanding unit may be configured to modify (e.g., increase) the dynamic range of the plurality of waveform subband signals. The expanding unit may be located upstream of the metadata application and synthesis unit. In particular, the plurality of expanded waveform subband signals may be used to implement an HFR or SBR scheme. In other words, the plurality of low-pass subband signals used to implement an HFR or SBR scheme may correspond to the plurality of expanded waveform subband signals at the output of the expanding unit.

[0027] The expansion unit is preferably located downstream of the look-ahead delay unit. In particular, the expansion unit may be located between the look-ahead delay unit and the metadata application and synthesis unit. By locating the expansion unit downstream of the look-ahead delay unit, i.e., by applying a look-ahead delay to the waveform data before expanding the plurality of waveform subband signals, it is ensured that the one or more expansion parameters included in the metadata are applied to the correct waveform data. In other words, performing expansion on waveform data that has already been delayed by the look-ahead delay ensures that the one or more expansion parameters from the metadata are synchronized with the waveform data.

[0028] Thus, the decoded metadata may include one or more extension parameters, and the audio decoder may have an extension unit configured to generate a plurality of extended waveform subband signals based on the plurality of waveform subband signals using the one or more extension parameters. In particular, the extension unit may be configured to generate the plurality of extended waveform subband signals using an inverse of a predetermined compression function. The one or more extension parameters may indicate the inverse of the predetermined compression function. The reconstructed frames of the audio signal may be determined from the plurality of extended waveform subband signals.

[0029] As described above, the audio decoder may include a look-ahead delay unit configured to delay the plurality of waveform subband signals according to the predetermined look-ahead to produce a plurality of delayed waveform subband signals. An extension unit may be configured to generate the plurality of extended waveform subband signals by extending the plurality of delayed waveform subband signals. In other words, the extension unit may be located downstream of the look-ahead unit. This ensures synchronization between the one or more extension parameters and the plurality of waveform subband signals to which the one or more extension parameters are applicable.

[0030] The metadata application and synthesis unit may be configured to generate the reconstructed frame of the audio signal by using the decoded metadata for a temporal portion of the plurality of waveform subband signals (in particular by using SBR / HFR related metadata). The temporal portion may correspond to several time slots of the plurality of waveform subband signals. The temporal length of the temporal portion may be variable. That is, the temporal length of the plurality of waveform subband signals to which the decoded metadata is applied may vary from one frame to the next. In other words, the framing of the decoded metadata may vary. The variation in the temporal length of the temporal portion may be limited to a predetermined limit. The predetermined range may correspond to the frame length minus the look-ahead delay and the frame length plus the look-ahead delay. Applying the decoded waveform data (or a portion thereof) for temporal portions of various durations may be useful for handling transient audio signals.

[0031] The extension unit may be configured to generate the plurality of extended waveform subband signals by using the one or more extension parameters for the same temporal portion of the plurality of waveform subband signals. In other words, the framing of the one or more extension parameters may be the same as the framing for the decoded metadata used by the metadata application and synthesis unit (e.g., the framing for SBR / HFR metadata). By doing so, consistency between the SBR scheme and the companding scheme can be ensured, and the perceptual quality of the coding system can be improved.

[0032] According to a further aspect, an audio encoder configured to encode frames of an audio signal into access units of a data stream is described. The audio encoder may be configured to perform corresponding processing tasks related to those performed by an audio decoder. In particular, the audio encoder may be configured to determine waveform data and metadata from a frame of the audio signal and insert the waveform data and metadata into an access unit. The waveform data and metadata may indicate a reconstructed version of that frame of the audio signal. In other words, the waveform data and metadata enable a corresponding audio decoder to determine a reconstructed version of an original frame of the audio signal. The frame of the audio signal may include a low-band signal and a high-band signal. The waveform data may indicate the low-band signal, and the metadata may indicate a spectral envelope of the high-band signal.

[0033] The audio encoder may have a waveform processing path configured to generate the waveform data from the frames of the audio signal, e.g., from the low-band signal (e.g., using an audio core decoder such as Advanced Audio Coder AAC). Further, the audio encoder has a metadata processing path configured to generate the metadata from the frames of the audio signal, e.g., from the high-band signal and the low-band signal. By way of example, the audio encoder may be configured to perform High Efficiency (HE) AAC, and a corresponding audio decoder may be configured to decode the received data stream in accordance with HE AAC.

[0034] The waveform processing path and / or the metadata processing path may comprise at least one delay unit configured to time-align the waveform data and the metadata such that an access unit for the frame of the audio signal includes the waveform data and the metadata for the same frame of the audio signal. The at least one delay unit may be configured to time-align the waveform data and the metadata such that an overall delay of the waveform processing path corresponds to an overall delay of the metadata processing path. In particular, the at least one delay unit may be a waveform delay unit configured to insert an additional delay into the waveform processing path such that an overall delay of the waveform processing path corresponds to an overall delay of the metadata processing path. Alternatively or additionally, the at least one delay unit may be configured to time-align the waveform data and the metadata such that the waveform data and the metadata are provided to an access unit generation unit of an audio encoder just in time for generating a single access unit from the waveform data and the metadata. In particular, the waveform data and the metadata may be provided such that the single access unit can be generated without the need for a buffer for buffering the waveform data and / or the metadata.

[0035] The audio encoder may include a decomposition unit configured to generate a plurality of subband signals from the frames of the audio signal, where the plurality of subband signals may include a plurality of low-band signals indicative of the low-band signal. The audio encoder may include a compression unit configured to compress the plurality of low-band signals using a compression function to provide a plurality of compressed low-band signals. The waveform data may be indicative of the plurality of compressed low-band signals, and the metadata may be indicative of the compression function used by the compression unit. The metadata indicative of the spectral envelope of the high-band signal may be applicable to the same portion of the audio signal as the metadata indicative of the compression function. In other words, the metadata indicative of the spectral envelope of the high-band signal may be synchronized with the metadata indicative of the compression function.

[0036] According to a further aspect, a data stream is described that includes a sequence of access units corresponding to a sequence of frames of an audio signal. Each access unit from the sequence of access units has waveform data and metadata. The waveform data and metadata are associated with the same particular frame of the sequence of frames of the audio signal. The waveform data and metadata may indicate a reconstructed frame of the particular frame. In one example, the particular frame of the audio signal includes a low-frequency signal and a high-frequency signal, where the waveform data indicates the low-frequency signal and the metadata indicates a spectral envelope of the high-frequency signal. The metadata may enable an audio decoder to generate the high-frequency signal from the low-frequency signal using an HFR scheme. Alternatively or additionally, the metadata may indicate a compression function applied to the low-frequency signal. Thus, the metadata may enable an audio decoder to perform a dynamic range expansion (using the inverse of the compression function) of the received low-frequency signal.

[0037] According to a further aspect, a method for determining a reconstructed frame of an audio signal from an access unit of a received data stream is described. The access unit includes waveform data and metadata, where the waveform data and the metadata are associated with the same reconstructed frame of the audio signal. In one example, the reconstructed frame of the audio signal includes a low-band signal and a high-band signal, where the waveform data indicates the low-band signal (e.g., frequency coefficients describing the low-band signal) and the metadata indicates a spectral envelope of the high-band signal (e.g., scale factors for multiple scale factor bands of the high-band signal). The method includes generating multiple waveform subband signals from the waveform data and generating decoded metadata from the metadata. The method further includes time-aligning the multiple waveform subband signals and the decoded metadata as described herein. The method further includes generating the reconstructed frame of the audio signal from the time-aligned multiple waveform subband signals and the decoded metadata.

[0038] According to another aspect, a method for encoding frames of an audio signal into access units of a data stream is described. The frames of the audio signal are encoded such that the access units include waveform data and metadata. The waveform data and the metadata indicate a reconstructed frame of the frame of the audio signal. In one example, the frames of the audio signal include a low-frequency signal and a high-frequency signal, and the frames are encoded such that the waveform data indicates the low-frequency signal and the metadata indicates a spectral envelope of the high-frequency signal. The method includes generating the waveform data from the frames of the audio signal, e.g., from the low-frequency signal, and generating the metadata from the frames of the audio signal, e.g., from the high-frequency signal and the low-frequency signal (e.g., according to an HFR scheme). The method further includes time-aligning the waveform data and the metadata such that the access unit for a frame of the audio signal includes the waveform data and the metadata for the same frame of the audio signal.

[0039] According to a further aspect, a software program is described, the software program may be adapted for execution on a processor to perform the method steps outlined herein when executed on the processor.

[0040] According to another aspect, a storage medium (e.g., a non-transitory storage medium) is described that may have a software program adapted for execution on a processor, the software program being adapted to perform the method steps outlined herein when executed on the processor.

[0041] According to a further aspect, a computer program product is described, which may include executable instructions for performing the method steps outlined herein when executed on a computer.

[0042] It should be noted that the methods and systems, including the preferred embodiments, outlined in this patent application may be used alone or in combination with other methods and systems disclosed herein. Furthermore, all aspects of the methods and systems outlined in this patent application may be combined in any manner. In particular, the features of the claims may be combined with each other in any manner. [Brief explanation of the drawings]

[0043] The invention is described below, by way of example, with reference to the accompanying drawings, in which: [Figure 1] 1 shows a block diagram of an exemplary audio decoder. [Figure 2a] 10 shows a block diagram of another exemplary audio decoder. [Figure 2b] 1 shows a block diagram of an exemplary audio encoder. [Figure 3a] FIG. 1 is a block diagram of an exemplary audio decoder configured to perform audio enhancements. [Figure 3b] FIG. 1 is a block diagram of an example audio encoder configured to perform audio compression. [Figure 4] 2 illustrates an exemplary frame structure of a sequence of frames of an audio signal; DETAILED DESCRIPTION OF THE INVENTION

[0044] As mentioned above, this paper concerns metadata alignment. In the following, metadata alignment is outlined in the context of the MPGE HE (High Efficiency) AAC (Advanced Audio Coding) system; however, the principles of metadata alignment described in this paper are applicable to other audio encoding / decoding systems. In particular, the metadata alignment method described in this paper is applicable to audio encoding / decoding systems that utilize HFR (High Frequency Reconstruction) and / or SBR (Spectral Bandwidth Replication) and transmit HFR / SBR metadata from an audio encoder to a corresponding audio decoder. Furthermore, the metadata alignment method described in this paper is applicable to audio encoding / decoding systems that utilize applications in the subband (particularly QMF) domain. An example of such an application is SBR. Other examples are A-coupling, post-processing, etc. In the following, the metadata alignment method is described in the context of SBR metadata alignment. However, it should be noted that the metadata alignment method is applicable to other types of metadata, particularly in the subband domain.

[0045] An MPEG HE-AAC data stream contains SBR metadata (also referred to as A-SPX metadata). The SBR metadata for a particular encoded frame of the data stream (also referred to as an access unit (AU) of the data stream) typically relates to past waveform (W) data. In other words, the SBR metadata and waveform data contained within an AU of the data stream typically do not correspond to the same frame of the original audio signal. This is due to the fact that after the waveform data is decoded, it undergoes several processing stages (e.g., IMDCT (inverse modified discrete cosine transform) and QMF (quadrature mirror filter) decomposition), which introduce signal delays. At the time the SBR metadata is applied to the waveform data, it is synchronized with the processed waveform data. Therefore, the SBR metadata and waveform data are inserted into the MPEG HE-AAC data stream so that the SBR metadata arrives at the audio decoder when it is needed for SBR processing. This type of metadata delivery is sometimes referred to as "just-in-time" (JIT) metadata delivery. This is because the SBR metadata is inserted into the data stream so that it can be applied directly to the signal or processing chain of an audio decoder.

[0046] JIT metadata delivery can be beneficial for a typical encode-transmit-decode processing chain to reduce overall coding delay and memory requirements at the audio decoder. However, splicing of a data stream along the transmission path can lead to a mismatch between the waveform data and the corresponding SBR metadata. Such a mismatch can lead to audible artifacts at the splicing point because incorrect SBR metadata is used for spectral band replication at the audio decoder.

[0047] In view of the above, it would be desirable to provide an audio encoding / decoding system that allows for splicing of data streams while maintaining a low overall coding delay.

[0048] FIG. 1 shows a block diagram of an exemplary audio decoder 100 that addresses the above-mentioned technical challenges. Specifically, the audio decoder 100 of FIG. 1 allows decoding of a data stream having AUs 110 that include waveform data 111 for a particular segment (e.g., a frame) of an audio signal and that include corresponding metadata 112 for the particular segment of the audio signal. By providing an audio decoder 100 that decodes a data stream that includes AUs 110 with time-aligned waveform data 111 and corresponding metadata 112, consistent splicing of data streams is enabled. In particular, it is ensured that data streams can be spliced ​​in a manner that maintains corresponding pairs of waveform data 111 and metadata 112.

[0049] The audio decoder 100 includes a delay unit 105 in the processing chain of the waveform data 111. The delay unit 105 may be located after or downstream of the MDCT synthesis unit 102 and before or upstream of the QMF synthesis unit 107 in the audio decoder 100. In particular, the delay unit 105 may be located before or upstream of a metadata application unit 106 (e.g., SBR unit 106) configured to apply decoded metadata 128 to the processed waveform data. The delay unit 105 (also referred to as waveform delay unit 105) is configured to apply a delay (also referred to as waveform delay) to the processed waveform data. The waveform delay is preferably chosen so that the overall processing delay of the waveform processing chain or waveform processing path (e.g., from the MDCT synthesis unit 102 to the application of the metadata in the metadata application unit 106) sums to exactly one frame (or an integer multiple thereof). By doing so, the parametric control data can be delayed by one frame (or a multiple thereof) to achieve alignment within the AU 110.

[0050] FIG. 1 shows components of an exemplary audio decoder 100. Waveform data 111 taken from an AU 110 is decoded and dequantized in a waveform decoding and dequantization unit 101 to provide a plurality of frequency coefficients 121 (in the frequency domain). The plurality of frequency coefficients 121 are synthesized into a low-pass signal 122 (in the time domain) using a frequency-to-time domain transform (e.g., an inverse MDCT (Modified Discrete Cosine Transform)) applied in a low-pass synthesis unit 102 (e.g., an MDCT synthesis unit). The low-pass signal 122 is then transformed into a plurality of low-pass sub-band signals 123 using a decomposition unit 103. The decomposition unit 103 may be configured to apply a quadrature mirror filter (QMF) bank to the low-pass signal 122 to provide the plurality of low-pass sub-band signals 123. Metadata 112 is typically applied to the plurality of low-pass sub-band signals 123 (or to a translated version thereof).

[0051] The metadata 112 from the AUs 110 is decoded and dequantized in a metadata decoding and dequantization unit 108 to provide decoded metadata 128. Furthermore, the audio decoder 100 may comprise a further delay unit 109 (also referred to as a metadata delay unit 109) configured to apply a delay (also referred to as a metadata delay) to the decoded metadata 128. The metadata delay may correspond to an integer multiple of the frame length N, e.g., D1=N, where D1 is the metadata delay. Thus, the overall delay of the metadata processing chain corresponds to D1, e.g., D1=N.

[0052] To ensure that the processed waveform data (i.e., the delayed lowband subband signals 123) and the processed metadata (i.e., the delayed decoded metadata 128) arrive at the metadata application unit 106 simultaneously, the overall delay of the waveform processing chain (or path) should correspond to the overall delay of the metadata processing chain (or path) (i.e., D1). Within the waveform processing chain, the lowband synthesis unit 102 typically inserts a delay of N / 2 (i.e., half the frame length). The synthesis unit 103 typically inserts a fixed delay (e.g., 320 samples). Furthermore, a look-ahead (i.e., a fixed offset between the metadata and the waveform data) may need to be taken into account. In the case of MPEG HE-AAC, such an SBR look-ahead may correspond to 384 samples (represented by the look-ahead unit 104). The lookahead unit 104 (sometimes referred to as a lookahead delay unit 104) may be configured to delay the waveform data 111 (e.g., delay the plurality of low-frequency sub-band signals 123) by a fixed SBR lookahead delay, which enables a corresponding audio encoder to determine SBR metadata based on subsequent frames of the audio signal.

[0053] To provide an overall delay in the metadata processing chain that corresponds to the overall delay in the waveform processing chain, the waveform delay D2 is D1=320+384+D2+N / 2 That is, D2=N / 2-320-384 (when D1=N).

[0054] Table 1 shows the waveform delay D2 for several different frame lengths N. It can be seen that the maximum waveform delay D2 for various frame lengths N of HE-AAC is 928 samples, resulting in an overall maximum decoder latency of 2177 samples. In other words, the alignment of the waveform data 111 and the corresponding metadata 112 within a single AU 110 results in an additional PCM delay of up to 928 samples. For blocks with frame size N = 1920 / 1536, the metadata is delayed by one frame, and for frame sizes N = 960 / 768 / 512 / 384, the metadata is delayed by two frames. In other words, the playback delay in the audio decoder 100 is increased depending on the block size N, and the overall encoding delay is increased by one or two complete frames. The maximum PCM delay in the corresponding audio encoder is 1664 samples (corresponding to the intrinsic latency of the audio decoder 100).

[0055] [Table 1] Therefore, this paper proposes to address the shortcomings of JIT metadata by using signal-aligned-metadata (SAM) 112, which is aligned with the corresponding waveform data 111 in a single AU 110. Specifically, it is proposed to introduce one or more additional delay units into the audio decoder 100 and / or the corresponding audio encoder, so that every encoded frame (or AU) carries metadata (e.g., A-SPX) for use in later processing stages, e.g., when the metadata is applied to the underlying waveform data.

[0056] It should be noted that, in principle, it is conceivable to apply a metadata delay D1 corresponding to a fraction of the frame length N. By doing so, the overall coding delay can potentially be reduced. However, as shown, for example, in FIG. 1 , the metadata delay D1 is applied in the QMF domain (i.e., in the subband domain). In light of this, and in light of the fact that the metadata 112 is typically defined only once per frame, i.e., the metadata 112 typically includes one dedicated set of parameters per frame, inserting a metadata delay D1 corresponding to a fraction of the frame length N may lead to synchronization problems with the waveform data 111. On the other hand, the waveform delay D2 (as shown in FIG. 1 ) is applied in the time domain, and in this case, a delay corresponding to a fraction of a frame can be implemented in a precise manner (e.g., by delaying the time-domain signal by the number of samples corresponding to the waveform delay D2). Therefore, it is beneficial to delay the metadata 112 by an integer number of frames (where a frame corresponds to the lowest time resolution for which the metadata 112 is defined) and delay the waveform data 111 by the waveform delay D2, which can take any value. A metadata delay D1 corresponding to an integer multiple of the frame length N can be implemented in a precise manner in the subband domain, and a waveform delay D2 corresponding to any multiple of samples can be implemented in a precise manner in the time domain. As a result, the combination of the metadata delay D1 and the waveform delay D2 allows precise synchronization of the metadata 112 and the waveform data 111.

[0057] Applying a metadata delay D1 corresponding to a fraction of the frame length N can be implemented by resampling the metadata 112 according to the metadata delay D1. However, resampling the metadata 112 involves substantial computational costs. Furthermore, resampling the metadata 112 may lead to distortion of the metadata 112, thereby affecting the quality of the reconstructed frames of the audio signal. In view of this, it is beneficial, both in terms of computational efficiency and in terms of audio quality, to limit the metadata delay D1 to an integer multiple of the frame length N.

[0058] 1 illustrates further processing of the delayed metadata 128 and the delayed plurality of low-band sub-band signals 123. The metadata application unit 106 is configured to generate a plurality of (e.g., scaled) high-band sub-band signals 126 based on the plurality of low-band sub-band signals 123 and based on the metadata 128. To this end, the metadata application unit 106 may be configured to transpose one or more of the plurality of low-band sub-band signals 123 to generate a plurality of high-band sub-band signals. The transposition may include a copy-up process of the plurality of low-band sub-band signals 123 onto the one or more of the plurality of low-band sub-band signals 123. Furthermore, the metadata application unit 106 may be configured to apply the metadata 128 (e.g., scale factors included in the metadata 128) to the plurality of high-band sub-band signals to generate the plurality of scaled high-band sub-band signals 126. The plurality of scaled high-frequency subband signals 126 are typically scaled using the scale factor so that the spectral envelope of the plurality of high-frequency subband signals 126 mimics the spectral envelope of the high-frequency signal of the original frame of the audio signal (which corresponds to a reconstructed frame of an audio signal 127 generated from the plurality of scaled high-frequency subband signals 126 based on the plurality of low-frequency subband signals 123).

[0059] Furthermore, the audio decoder 100 comprises a synthesis unit 107 configured to generate the reconstructed frame of the audio signal 127 from the plurality of low-frequency subband signals 123 and from the plurality of scaled high-frequency subband signals 126 (e.g., using an inverse QMF bank).

[0060] 2a shows a block diagram of another exemplary audio decoder 100. The audio decoder 100 of FIG. 2a has the same components as the audio decoder 100 of FIG. 1. Furthermore, exemplary components 210 for multi-channel audio processing are shown. It can be seen that in the example of FIG. 2a, the waveform delay unit 105 is located immediately after the inverse MDCT unit 102. The determination of the reconstructed frame of the audio signal 127 may be performed for each channel of the multi-channel audio signal (e.g., of a 5.1 or 7.1 multi-channel audio signal).

[0061] 2b shows a block diagram of an exemplary audio encoder 250 corresponding to the audio decoder 100 of FIG. 2a. The audio encoder 250 is configured to generate a data stream including AUs carrying corresponding pairs of waveform data 111 and metadata 112. The audio encoder 250 includes metadata processing chains 256, 257, 258, 259, and 260 for determining the metadata. The metadata processing chains may include a metadata delay unit 256 for aligning the metadata with the corresponding waveform data. In the illustrated example, the metadata delay unit 256 of the audio encoder 250 does not introduce any additional delay (because the delay introduced by the metadata processing chain is greater than the delay introduced by the waveform processing chain).

[0062] Furthermore, the audio encoder 250 comprises waveform processing chains 251, 252, 253, 254, 255 configured to determine the waveform data from the original audio signal at the input of the audio encoder 250. The waveform processing chains comprise a waveform delay unit 252 configured to introduce an additional delay into the waveform processing chain in order to align the waveform data with the corresponding metadata. The delay introduced by the waveform delay unit 252 may be such that the overall delay of the metadata processing chain (including the waveform delay inserted by the waveform delay unit 252) corresponds to the overall delay of the waveform processing chain. For a frame length N=2048, the delay of the waveform delay unit 252 may be 2048−320=1728 samples.

[0063] Figure 3a shows an excerpt of an audio decoder 300 comprising an extension unit 301. The audio decoder 300 of Figure 3a may correspond to the audio decoder 100 of Figure 1 and / or Figure 2a and further comprises the extension unit 301 configured to determine a plurality of extended low-band signals from the plurality of low-band signals 123 using one or more extension parameters 310 taken from the decoded metadata 128 of an access unit 110. Typically, the one or more extension parameters 310 are combined with SBR (e.g. A-SPX) metadata included in the access unit 110. In other words, the one or more extension parameters 310 are typically applicable to the same excerpt or part of the audio signal as the SBR metadata.

[0064] As outlined above, the metadata 112 of an access unit 110 is typically associated with waveform data 111 of a frame of an audio signal, where the frame has a predetermined number N of samples. The SBR metadata is typically determined based on a plurality of low-pass signals (also referred to as a plurality of waveform subband signals), where the plurality of low-pass signals may be determined using QMF analysis. QMF analysis provides a time-frequency representation of a frame of an audio signal. In particular, the N samples of a frame of an audio signal can be represented by Q (e.g., Q=64) low-pass signals, each having N / Q time slots or slots. For a frame with N=2048 samples, for Q=64, each low-pass signal has N / Q=32 slots.

[0065] In the case of transient signals within a particular frame, it may be beneficial to determine the SBR metadata based on samples of the immediately following frame. This feature is called SBR look-ahead. In particular, the SBR metadata may be determined based on a predetermined number of slots from the immediately following frame. As an example, up to six slots of the immediately following frame may be taken into account (i.e., Q*6=384 samples).

[0066] The use of SBR lookahead is illustrated in FIG. 4, which shows a sequence of frames 401, 402, 403 of an audio signal using different frame structures 400, 430 for SBR or HFR schemes. In the case of frame structure 400, the SBR / HFR scheme does not take advantage of the flexibility offered by SBR lookahead. Nevertheless, to enable the use of SBR lookahead, a fixed offset, i.e., a fixed SBR lookahead delay 480, is used. In the illustrated example, the fixed offset corresponds to six time slots. As a result of this fixed offset 480, the metadata 112 of a particular access unit 110 in a particular frame 402 is partially applicable to the time slots of waveform data 111 contained in the access unit 110 preceding that particular access unit 110 (and associated with the immediately preceding frame 401). This is indicated by the offset between the SBR metadata 411, 412, 413 and the frames 401, 402, 403. Thus, the SBR metadata 411, 412, 413 contained within the access unit 110 may be applicable to the waveform data 111 offset by the SBR lookahead delay 480. The SBR metadata 411, 412, 413 are applied to the waveform data 111 to provide reconstructed frames 421, 422, 423.

[0067] Frame structure 430 utilizes SBR lookahead. For example, due to the occurrence of transients within frame 401, it can be seen that SBR metadata 431 is applicable to more than 32 time slots of waveform data 111. On the other hand, subsequent SBR metadata 432 is applicable to fewer than 32 time slots of waveform data 111. SBR metadata 433 is again applicable to 32 time slots. Thus, SBR lookahead allows flexibility regarding the time resolution of the SBR metadata. Regardless of the use of SBR lookahead and regardless of the applicability of SBR metadata 431, 432, and 433, reconstructed frames 421, 422, and 423 are generated using a fixed offset 480 relative to frames 401, 402, and 403.

[0068] The audio encoder may be configured to determine the SBR metadata and the one or more extension parameters using the same excerpt or portion of the audio signal. Thus, if SBR metadata is determined using an SBR lookahead, the one or more extension parameters may be determined and applicable for the same SBR lookahead. In particular, the one or more extension parameters may be applicable for the same number of time slots as the corresponding SBR metadata 431, 432, 433.

[0069] The expansion unit 301 may be configured to apply one or more expansion gains to the plurality of lowband signals 123, where the one or more expansion gains typically depend on the one or more expansion parameters 310. In particular, the one or more expansion parameters 310 may have an influence on one or more compression / expansion rules used to determine the one or more expansion gains. In other words, the one or more expansion parameters 310 may indicate a compression function used by a compression unit of a corresponding audio encoder. The one or more expansion parameters 310 may enable an audio decoder to determine the inverse of this compression function.

[0070] The one or more extension parameters 310 may comprise a first extension parameter indicating whether a corresponding audio encoder has compressed the plurality of low-band signals. If no compression has been applied, no extension will be applied by the audio decoder. Thus, the first extension parameter may be used to turn on or off a companding function.

[0071] Alternatively or additionally, the one or more expansion parameters 310 may comprise a second expansion parameter indicating whether the same one or more expansion gains should be applied to all of the channels of the multi-channel audio signal, thus allowing switching between a per-channel or per-multi-channel application of a companding function.

[0072] Alternatively or additionally, the one or more expansion parameters 310 may comprise a third expansion parameter indicating whether the same one or more expansion gains should be applied to all time slots of a frame, which may then be used to control the time resolution of the companding function.

[0073] Using the one or more extension parameters 310, the extension unit 301 may determine the plurality of extended low-band signals by applying an inverse of a compression function applied in a corresponding audio encoder, which compression function is signaled to the audio decoder 300 using the one or more extension parameters 310.

[0074] The expansion unit 301 may be located downstream of the look-ahead delay unit 104. This ensures that the one or more expansion parameters 310 are applied to the correct parts of the plurality of lowband signals 123. In particular, this ensures that the one or more expansion parameters 310 are applied (in the SBR application unit 106) to the same parts of the plurality of lowband signals as the SBR parameters. This ensures that the expansion operates on the same time frame structure 400, 430 as the SBR scheme. Due to SBR look-ahead, the frame structure 400, 430 may have a variable number of time slots, and as a result, the expansion may operate on a variable number of time slots (as outlined in the context of FIG. 4). By locating the expansion unit 301 downstream of the look-ahead delay unit 104, it is ensured that the correct frame structure 400, 430 is applied to the one or more expansion parameters. As a result, a high-quality audio signal can be guaranteed even after the splice point.

[0075] 3b shows an excerpt of an audio encoder 350 comprising a compression unit 351. The audio encoder 350 may comprise components of the audio encoder 250 of FIG. 2b. The compression unit 351 may be configured to compress the plurality of low-band signals (e.g., reduce their dynamic range) using a compression function. Furthermore, the compression unit 351 may be configured to determine one or more expansion parameters 310 indicative of the compression function used by the compression unit 351, so that a corresponding expansion unit 301 of the audio decoder 300 can apply the inverse of the compression function.

[0076] Compression of the plurality of low-band signals may be performed downstream of the SBR lookahead 258. Furthermore, the audio encoder 350 may comprise an SBR frame construction unit 353 configured to ensure that SBR metadata is determined for the same part of the audio signal as the one or more extension parameters 310. In other words, the SBR frame construction unit 353 may ensure that the SBR scheme operates on the same frame structure 400, 430 as the companding scheme. In view of the fact that the SBR scheme may operate on extended frames (e.g. in transient cases), the companding scheme may also operate on extended frames (with additional time slots).

[0077] This paper describes an audio encoder and corresponding audio decoder that allow encoding an audio signal into a sequence of time-aligned AUs that contain waveform data and metadata associated with a sequence of segments of the audio signal. The use of time-aligned AUs enables splicing of data streams with reduced artifacts at the splice points. Furthermore, the audio encoder and audio decoder are designed such that splicable data streams are processed in a computationally efficient manner and the overall coding delay remains low.

[0078] The methods and systems described herein may be implemented as software, firmware, and / or hardware. Certain components may be implemented, for example, as software running on a digital signal processor or microprocessor. Other components may be implemented, for example, as hardware and / or application-specific integrated circuits. Signals encountered in the described methods and systems may be stored on media such as random access memory or optical storage media. Such signals may be transmitted over networks such as radio, satellite, wireless, or wired networks, e.g., the Internet. Typical devices utilizing the methods and systems described herein are portable electronic devices or other consumer devices used to store and / or render audio signals.

[0079] Various aspects of the present invention can be understood from the following enumerated example embodiments (EEE). [EEE1] 1. An audio decoder (100, 300) configured to determine a reconstructed frame of an audio signal from an access unit of a received data stream, the access unit including waveform data and metadata, the waveform data and the metadata being associated with the same reconstructed frame of the audio signal, the audio decoder comprising: a waveform processing path (101, 102, 103, 104, 105) configured to generate a plurality of waveform subband signals from the waveform data; a metadata processing path (108, 109) configured to generate decoded metadata from the metadata; a metadata application and synthesis unit (106, 107) configured to generate the reconstructed frames of the audio signal from the plurality of waveform subband signals and from the decoded metadata, the waveform processing path and / or the metadata processing path comprises at least one delay unit (105, 109) configured to time-align the plurality of waveform subband signals and the decoded metadata; Audio decoder. [EEE2] The audio decoder of EEE1, wherein the at least one delay unit is configured to time-align the plurality of waveform subband signals and the decoded metadata such that an overall delay of the waveform processing path corresponds to an overall delay of a metadata processing path. [EEE3] 3. The audio decoder of claim 8, wherein the at least one delay unit is configured to time-align the plurality of waveform subband signals and the decoded metadata so that the plurality of waveform subband signals and the decoded metadata are provided to the metadata application and synthesis unit in time for processing performed by the metadata application and synthesis unit. [EEE4] 4. The audio decoder of any one of EEE1 to EEE3, wherein the metadata processing path comprises a metadata delay unit (109) configured to delay the decoded metadata by an integer multiple of a frame length N of the reconstructed frames of the audio signal, the integer multiple being greater than 0. [EEE5] The audio decoder according to EEE4, wherein the integer multiple is such that the delay introduced by the metadata delay unit is greater than the delay introduced by processing in the waveform processing path. [EEE6] The audio decoder according to EEE4 or 5, wherein the integer multiple is 1 for frame lengths N greater than 960, and the integer multiple is 2 for frame lengths N less than or equal to 960. [EEE7] 7. An audio decoder according to any one of EEE1 to EEE6, wherein the waveform processing path comprises a waveform delay unit (105) configured to delay the plurality of waveform subband signals such that an overall delay of the waveform processing path corresponds to an integer multiple of a frame length N of the reconstructed frames of the audio signal that is greater than zero. [EEE8] The waveform processing path includes: a decoding and dequantization unit (101) configured to decode and dequantize the waveform data (111) to provide a plurality of frequency coefficients (121) indicative of the waveform signal; a waveform synthesis unit (102) configured to generate the waveform signal (122) from the plurality of frequency coefficients; a decomposition unit (103) configured to generate the plurality of waveform subband signals from the waveform signal, 8. An audio decoder according to any one of claims EEE1 to EEE7. [EEE9] the waveform synthesis unit is configured to perform a frequency domain to time domain transformation; the decomposition unit is configured to perform a transformation from a time domain to a subband domain; the frequency resolution of the transformation performed by the waveform synthesis unit is higher than the frequency resolution of the transformation performed by the decomposition unit; EEE8 audio decoder. [EEE10] the waveform synthesis unit is configured to perform an inverse modified discrete cosine transform; the decomposition unit is configured to apply a quadrature mirror filter bank; EEE9 audio decoder. [EEE11] the waveform synthesis unit introduces a delay that depends on the frame length N of the reconstructed frames of the audio signal; and / or the decomposition unit introduces a fixed delay that is independent of the frame length N of the reconstructed frames of the audio signal, 8. An audio decoder according to any one of claims 8 to 10. [EEE12] the delay introduced by said waveform synthesis unit corresponds to half the frame length N; and / or the fixed delay introduced by the decomposition unit corresponds to 320 samples of the audio signal; Audio decoder according to EEE11. [EEE13] 13. An audio decoder according to any one of claims 8 to 12, wherein the overall delay of the waveform processing path depends on a predetermined look-ahead between metadata and waveform data. [EEE14] The audio decoder according to EEE13, wherein the predetermined lookahead corresponds to 192 or 384 samples of the audio samples. [EEE15] the decoded metadata includes one or more extension parameters; the audio decoder comprises an extension unit configured to generate a plurality of extended waveform subband signals based on the plurality of waveform subband signals using the one or more extension parameters; the reconstructed frames of the audio signal are determined from the plurality of extended waveform subband signals; 8. An audio decoder according to any one of claims EEE1 to EEE14. [EEE16] the audio decoder having a look-ahead delay unit configured to delay the plurality of waveform subband signals according to a predetermined look-ahead to produce a plurality of delayed waveform subband signals; the extending unit is configured to generate the plurality of extended waveform subband signals by extending the plurality of delayed waveform subband signals; Audio decoder according to EEE15. [EEE17] the expansion unit is configured to generate the plurality of expanded waveform subband signals using an inverse of a predetermined compression function; the one or more expansion parameters represent an inverse of the predetermined compression function; Audio decoder according to EEE15 or 16. [EEE18] the metadata application and synthesis unit is configured to generate the reconstructed frame of the audio signal by using the decoded metadata for a temporal portion of the plurality of waveform subband signals; the extension unit is configured to generate the plurality of extended waveform subband signals by using the one or more extension parameters for the same temporal portion of the plurality of waveform subband signals; 8. An audio decoder according to any one of claims EEE15 to 17. [EEE19] 8. The audio decoder of claim 7, wherein the temporal portions of the plurality of waveform subband signals have a variable time length. [EEE20] 20. The audio decoder of any one of EEE8 to 19, wherein the waveform delay unit is configured to delay the waveform signal, the waveform signal being expressed in the time domain. [EEE21] 21. The audio decoder of any one of EEE1 to EEE20, wherein the metadata application and synthesis unit is configured to process the decoded metadata and the plurality of waveform subband signals in the subband domain. [EEE22] the reconstructed frames of the audio signal include a low-band signal and a high-band signal; the plurality of waveform subband signals represent the low-pass signal; the metadata is indicative of a spectral envelope of the highband signal; the metadata application and synthesis unit comprises a metadata application unit configured to perform high frequency reconstruction using the plurality of waveform subband signals and the decoded metadata; 8. An audio decoder according to any one of claims EEE1 to EEE21. [EEE23] The metadata application unit: transforming one or more of the plurality of waveform subband signals to generate a plurality of high-pass subband signals; configured to apply the decoded metadata to the plurality of higher frequency sub-band signals to provide a plurality of scaled higher frequency sub-band signals; the plurality of scaled highband subband signals represent the highband signals of the reconstructed frames of the audio signal. Audio decoder according to EEE22. [EEE24] 8. The audio decoder according to claim 6, wherein the metadata application and synthesis unit further comprises a synthesis unit (107) configured to generate the reconstructed frame of the audio signal from the plurality of waveform subband signals and from the plurality of scaled highband subband signals. [EEE25] 8. The audio decoder according to claim 82, wherein the synthesis unit is configured to perform an inverse transformation relative to the transformation performed by the decomposition unit. [EEE26] 1. An audio encoder (250, 350) configured to encode frames of an audio signal into access units of a data stream, the access units including waveform data and metadata, the waveform data and the metadata indicating reconstructed frames of the frames of the audio signal, the audio encoder comprising: a waveform processing path (251, 252, 253, 254, 255) configured to generate the waveform data from the frames of the audio signal; a metadata processing path (256, 257, 258, 259, 260) configured to generate the metadata from the frames of the audio signal, the waveform processing path and / or the metadata processing path comprises at least one delay unit configured to time-align the waveform data and the metadata such that the access unit for the frame of the audio signal comprises the waveform data and the metadata for the same frame of the audio signal. Audio encoder. [EEE27] 8. The audio encoder of claim 6, wherein the at least one delay unit (252, 256) is configured to time-align the waveform data and the metadata such that an overall delay of the waveform processing path corresponds to an overall delay of a metadata processing path. [EEE28] 28. The audio encoder according to EEE26 or 27, wherein the at least one delay unit is configured to time-align the waveform data and the metadata so that the waveform data and the metadata are provided to an access unit generation unit of the audio encoder in time for generating a single access unit from the waveform data and the metadata. [EEE29] 29. The audio encoder of any one of EEE26 to EEE28, wherein the waveform processing path comprises a waveform delay unit (252) configured to insert at least one delay in the waveform processing path. [EEE30] the frames of the audio signal include a low-frequency signal and a high-frequency signal; the waveform data represents the low-pass signal; the metadata is indicative of a spectral envelope of the highband signal; the waveform processing path is configured to generate the waveform data from the low-pass signal; the metadata processing path is configured to generate the metadata from the low-band signal and the high-band signal; 30. An audio encoder according to any one of claims EEE26 to 29. [EEE31] the audio encoder comprising a decomposition unit configured to generate a plurality of subband signals from the frames of the audio signal; the plurality of subband signals include a plurality of low-band signals indicative of the low-band signal; the audio encoder comprising a compression unit configured to compress the plurality of low-pass signals using a compression function to provide a plurality of compressed low-pass signals; the waveform data representing the plurality of compressed low-pass signals; The metadata indicates the compression function used by the compression unit, EEE30 audio encoder. [EEE32] 8. An audio encoder as claimed in claim 81, wherein the metadata indicative of the spectral envelope of the highband signal is applicable to the same part of the audio signal as the metadata indicative of the compression function. [EEE33] A data stream comprising a sequence of access units, each access unit for a sequence of frames of an audio signal, wherein an access unit from the sequence of access units comprises waveform data and metadata, the waveform data and the metadata relating to the same particular frame of the sequence of frames of the audio signal, and the waveform data and the metadata indicating a reconstructed version of that particular frame. [EEE34] 8. The data stream according to claim 3, wherein the particular frame of the audio signal includes a low-band signal and a high-band signal, the waveform data being indicative of the low-band signal, and the metadata being indicative of a spectral envelope of the high-band signal. [EEE35] 34. The data stream according to claim 33, wherein the metadata indicates a compression function applied to the low-pass signal. [EEE36] 1. A method for determining a reconstructed frame of an audio signal from an access unit of a received data stream, the access unit including waveform data and metadata, the waveform data and the metadata being associated with the same reconstructed frame of the audio signal, the method comprising: generating a plurality of waveform subband signals from the waveform data; generating decoded metadata from said metadata; time-aligning the plurality of waveform subband signals and the decoded metadata; generating the reconstructed frame of the audio signal from a plurality of time-aligned waveform subband signals and decoded metadata; method. [EEE37] 1. A method for encoding frames of an audio signal into access units of a data stream, the access units including waveform data and metadata, the waveform data and the metadata indicating reconstructed frames of the frames of the audio signal, the method comprising: generating the waveform data from the frames of the audio signal; generating the metadata from the frames of the audio signal; time-aligning the waveform data and the metadata such that the access unit for the frame of the audio signal includes the waveform data and the metadata for the same frame of the audio signal, method.

Claims

1. 1. An audio decoder apparatus for decoding an audio signal, the apparatus comprising: a processor for processing a waveform processing path, the processor configured to generate at least one waveform signal from waveform data obtained from an access unit of the audio signal; a metadata processor for processing a metadata processing path configured to generate decoded metadata from metadata obtained from the access units, the metadata processing path comprising a metadata delay unit configured to delay the decoded metadata by a delay, the delay having a value greater than 0, the value of the delay being a first integer, and the delay being equal to a frame length; a metadata application and synthesis unit configured to generate reconstructed frames of the audio signal from the at least one waveform signal and from the decoded metadata; An apparatus wherein at least one waveform signal and the decoded metadata are time-aligned such that an overall delay of the waveform processing path corresponds to an overall delay of a metadata processing path.

2. The apparatus of claim 1 , wherein the frame length is greater than 960 time-domain samples.

3. 1. A method of decoding an audio signal, comprising: generating, using a waveform processing path, at least one waveform signal from waveform data obtained from an access unit of the audio signal; generating decoded metadata from metadata obtained from the access units using a metadata processing path, the metadata processing path having a metadata delay unit configured to delay the decoded metadata by a delay, the delay having a value greater than 0, the value of the delay being a first integer, and the delay equal to a frame length; generating, using a metadata application and synthesis unit, a reconstructed frame of the audio signal from the at least one waveform signal and from the decoded metadata; The method, wherein the at least one waveform signal and the decoded metadata are time-aligned such that an overall delay of the waveform processing path corresponds to an overall delay of a metadata processing path.

4. The method of claim 3 , wherein the frame length is greater than 960 time-domain samples.

5. A non-transitory storage medium storing a program for causing a processor to execute the method of claim 3.

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