Backward compatible integration of harmonic converter for high frequency reconstruction of audio signal

Abstract

To disclose a method of decoding an encoded audio bitstream.SOLUTION: The method includes receiving an encoded audio bitstream and decoding audio data to generate a decoded lowband audio signal. The method further includes extracting high frequency reconstruction metadata and filtering the decoded lowband audio signal with a decomposition filter bank to generate a filtered lowband audio signal. The method also includes extracting a flag indicating whether either spectral translation or harmonic conversion is to be performed on the audio data, and regenerating a highband portion of the audio signal using the filtered lowband audio signal and the high frequency reconstruction metadata in accordance with the flag.SELECTED DRAWING: Figure 5

Description

Technical Field

[0001] Embodiments relate to audio signal processing, and more particularly to the encoding, decoding or transcoding of an audio bitstream having control data specifying that either a basic form of high frequency reconstruction (HFR) or an improved form of HFR should be performed on audio data.

Background Art

[0002] A typical audio bitstream includes both audio data (e.g., encoded audio data) representing one or more channels of audio content, and metadata representing at least one characteristic of the audio data or audio content. One well-known format for generating an encoded audio bitstream is the MPEG-4 Advanced Audio Coding (AAC) format described in the MPEG standard ISO / IEC 14496-3:2009. In the MPEG-4 standard, AAC represents "advanced audio coding", and HE-AAC represents "high-efficiency advanced audio coding".

[0003] The MPEG-4 AAC standard defines several audio profiles, which determine which objects and encoding tools are present in a conforming encoder or decoder. Three of these audio profiles are: (1) the AAC profile, (2) the HE-AAC profile, and (3) the HE-AAC v2 profile. The AAC profile includes the AAC low complexity (or "AAC-LC") object type. The AAC-LC object corresponds to the MPEG-2 AAC low complexity profile with some adjustments, and does not include the spectral band replication ("SBR") object type nor the parametric stereo ("PS") object type. The HE-AAC profile is a superset of the AAC profile and additionally includes the SBR object type. The HE-AAC v2 profile is a superset of the HE-AAC profile and additionally includes the PS object type.

[0004] The SBR object type includes a spectral band replication tool. This is an important high-frequency reconstruction ("HFR") coding tool that significantly improves the compression efficiency of perceptual audio coders. SBR reconstructs the high-frequency components of an audio signal at the receiver side (e.g., in a decoder). Therefore, the encoder only needs to encode and transmit the low-frequency components, allowing for much higher audio quality at low data rates. SBR is based on replicating sequences of harmonics that were previously truncated to reduce the data rate from the available bandwidth-limited signal and control data obtained from the encoder. The ratio between tonal-like components and noise-like components is maintained by adaptive inverse filtering as well as optional addition of noise and sine waves. In the MPEG-4 AAC standard, the SBR tool performs spectral patching (also called linear translation or spectral translation), where several consecutive quadrature mirror filter (QMF) subbands are copied (or "patched") from the transmitted low-frequency portion of the audio signal to the high-frequency portion of the audio signal generated at the decoder.

Prior Art Documents

Non-Patent Documents

[0005]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] Spectral patching or linear translation may not be ideal for certain types of audio, such as music content with a relatively low crossover frequency. Therefore, techniques for improving spectral band replication are needed.

Means for Solving the Problems

[0007] The first class of embodiments relates to a method of decoding an encoded audio bitstream. The method includes receiving the encoded audio bitstream, decoding the audio data to produce a decoded low-band audio signal. The method further includes extracting high-frequency reconstruction metadata, filtering the decoded low-band audio signal with a decomposition filter bank to produce a filtered low-band audio signal. The method further includes extracting a flag indicating whether spectral translation or harmonic conversion should be performed on the audio data, and regenerating a high-band portion of the audio signal using the filtered low-band audio signal and the high-frequency reconstruction metadata according to the flag. Finally, the method includes combining the filtered low-band audio signal and the regenerated high-band portion to form a wide-band audio signal.

[0008] A second class of embodiments relates to an audio decoder for decoding an encoded audio bit stream. The decoder includes an input interface for receiving the encoded audio bit stream, where the encoded audio bit stream includes audio data representing a low-frequency portion of an audio signal, and a core decoder for decoding the audio data to generate a decoded low-frequency audio signal. The decoder also includes a demultiplexer for extracting high-frequency reconstruction metadata from the encoded audio bit stream, where the high-frequency reconstruction metadata includes operating parameters for a high-frequency reconstruction process that linearly translates several consecutive subbands from the low-frequency portion of the audio signal to the high-frequency portion of the audio signal, and a decomposition filter bank for filtering the decoded low-frequency audio signal to generate a filtered low-frequency audio signal. The decoder further includes a demultiplexer for extracting a flag from the encoded audio bit stream indicating whether linear translation or harmonic conversion should be performed on the audio data, and a high-frequency regenerator for regenerating the high-frequency portion of the audio signal using the filtered low-frequency audio signal and the high-frequency reconstruction metadata according to the flag. Finally, the decoder includes a synthesis filter bank for combining the filtered low-frequency audio signal and the regenerated high-frequency portion to form a wideband audio signal.

[0009] Another class of embodiments relates to encoding and transcoding an audio bit stream that includes metadata for identifying whether an enhanced spectral band replication (eSBR) process should be performed. BRIEF DESCRIPTION OF THE DRAWINGS

[0010]

Figure 1

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DETAILED DESCRIPTION OF THE INVENTION

[0011] Throughout this disclosure including the claims, the expression of performing an operation “on” a signal or data (e.g., filtering, scaling, converting, or applying a gain to the signal or data) is used in a broad sense to represent performing the operation directly on the signal or data or on a processed version of the signal or data (e.g., on a version of the signal that has received preliminary filtering or preprocessing prior to the performance of the operation).

[0012] Throughout the present disclosure, including the claims, the expressions "audio processing unit" or "audio processor" are used in a broad sense to represent a system, device, or apparatus configured to process audio data. Examples of audio processing units include, but are not limited to, encoders, transcoders, decoders, codecs, pre-processing systems, post-processing systems, and bitstream processing systems (sometimes referred to as bitstream processing tools). Virtually any consumer electronic device, such as a mobile phone, television, laptop, and tablet computer, includes an audio processing unit or an audio processor.

[0013] Throughout the present disclosure, including the claims, the terms "couple" or "coupled" are used in a broad sense to mean a direct or indirect connection. Thus, when a first device is coupled to a second device, that connection may be through a direct connection or through an indirect connection via other devices and connections. Further, components integrated within or together with other components are also coupled to each other.

[0014] 〈Detailed Description of Embodiments of the Present Invention〉 The MPEG-4 AAC standard contemplates including metadata that indicates each type of high-frequency reconstruction (HFR) processing to be applied (if any) by a decoder to decode the audio content of the bitstream of an encoded MPEG-4 AAC bitstream and / or controls such HFR processing and / or indicates at least one characteristic or parameter of at least one HFR tool to be used to decode the audio content of the bitstream. Here, the expression "SBR metadata" is used to represent this type of metadata described or referred to in the MPEG-4 AAC standard for use with spectral band replication (SBR). As will be understood by those skilled in the art, SBR is one form of HFR.

[0015] SBR is preferably used as a dual-rate system. While SBR operates at the original sampling rate, the underlying codec operates at half the original sampling rate. The SBR encoder functions in parallel with the underlying core codec, but at a higher sampling rate. SBR is mainly a post-process in the decoder, but important parameters are extracted in the encoder to ensure the most accurate high-frequency reconstruction in the decoder. The encoder estimates the spectral envelope of the SBR range for the time and frequency range / resolution suitable for the current input signal segment characteristics. The spectral envelope is estimated by complex QMF decomposition and subsequent energy calculation. The time and frequency resolution of the spectral envelope can be selected with a high degree of freedom to ensure the most suitable time-frequency resolution for a given input segment. The envelope estimation needs to take into account that the original transient components (e.g., hi-hat), which are mainly located in the high-frequency region, will be present to a small extent in the SBR generated in the high-frequency region before envelope adjustment. This is because the high-frequency region in the decoder is based on the low-frequency region where the transient components are much less prominent than in the high-frequency region. This aspect imposes different requirements on the time-frequency resolution of the spectral envelope data compared to the normal spectral envelope estimation used in other audio coding algorithms.

[0016] Apart from the spectral envelope, several additional parameters are extracted that represent the spectral characteristics of the input signal for different time and frequency regions. Since the encoder naturally has access to the original signal and information on how the SBR unit in the decoder generates the high frequencies, given a specific set of control parameters, the system can handle situations where the low frequencies form a strong harmonic series and the regenerated high frequencies mainly consist of random signal components, and situations where strong tone-like components exist in the original high frequencies but there is no corresponding component in the low frequencies underlying the high frequency region. Further, the SBR encoder functions in close relation to the underlying core codec in order to evaluate which frequency ranges should be covered by SBR at a given time. The SBR data is efficiently encoded prior to transmission by exploiting entropy coding and, in the case of stereo signals, the channel dependency of the control data.

[0017] The control parameter extraction algorithm typically needs to be carefully tuned to the underlying codec for a given bitrate and a given sampling rate. This is because lower bitrates generally imply a larger SBR range compared to higher bitrates, and different sampling rates correspond to different temporal resolutions of the SBR frames.

[0018] The SBR decoder typically comprises several different parts. The SBR decoder has a bitstream decoding module, a high frequency reconstruction (HFR) module, an additional high frequency component module and an envelope regulator module. The system is based on per complex-valued QMF filterbank. In the bitstream extraction module, control data is read from and decoded from the bitstream. Prior to reading the envelope data from the bitstream, the time-frequency lattice for the current frame is obtained. The underlying core decoder decodes the audio signal of the current frame (albeit at the lower sampling rate as mentioned above) to generate a time-domain audio signal. The resulting frame of audio data is used for high frequency reconstruction by the HFR module. The decoded low-band signal is then decomposed using a QMF filterbank. High frequency reconstruction and envelope adjustment are then performed on the subband samples of the QMF filterbank. The high frequencies are reconstructed from the low band in a flexible manner based on the given control parameters. Further, the reconstructed high band is adaptively filtered for each subband / channel according to the control data to ensure the appropriate spectral characteristics in the given time / frequency domain.

[0019] The top level of the MPEG-4 AAC bitstream is a sequence of data blocks ("raw_data_block" elements), where each data block is a segment of data (referred to herein as a "block") containing audio data (typically over a time period of 1024 or 960 samples) and related information and / or other data. Here, the term "block" is used to represent a segment of the MPEG-4 AAC bitstream containing audio data (and corresponding metadata and optionally other related data) that determines or indicates one (and no more than one) "raw_data_block" element.

[0020] Each block of an MPEG-4 AAC bitstream can contain several syntax elements (each of which is also embodied as a segment of data in the bitstream). Seven types of such syntax elements are defined in the MPEG-4 AAC standard. Each syntax element is identified by a different value of the data element "id_syn_ele". Examples of syntax elements include "single_channel_element()", "channel_pair_element()", and "fill_element()". A single channel element is a container that contains audio data for a single audio channel (monophonic audio signal). A channel pair element contains audio data for two audio channels (i.e., a stereo audio signal).

[0021] A fill element is a container of information that contains an identifier (e.g., the value of the above element "id_syn_ele") and the subsequent data called "fill data". Fill elements have historically been used to adjust the instantaneous bitrate of a bitstream that is to be transmitted through a constant rate channel. By adding an appropriate amount of fill data to each block, a constant data rate can be achieved.

[0022] According to embodiments of the present invention, the fill data can include one or more extended payloads that extend the type of data (e.g., metadata) that can be transmitted in the bitstream. A decoder that receives a bitstream with fill data containing a new type of data can optionally be used by the device (e.g., decoder) that receives the bitstream to extend the functionality of the device. Thus, as will be understood by those skilled in the art, a fill element is a special type of data structure that is different from the data structures typically used to transmit audio data (e.g., an audio payload containing channel data).

[0023] In some embodiments of the present invention, the identifier used to identify a padding element may consist of a three-bit unsigned integer transmitted most significant bit first (uimsbf) having a value of 0x6. In one block, several instances of the same type of syntactic element (e.g., several padding elements) may occur.

[0024] Another standard for encoding an audio bitstream is the MPEG Unified Speech and Audio Coding (USAC) standard (ISO / IEC 23003-3:2012). The MPEG USAC standard describes encoding and decoding audio content using spectral band replication processing (including the SBR processing described in the MPEG-4 AAC standard and other improved forms of spectral band replication processing). This process applies an extended and improved version of the spectral band replication tool (sometimes referred to herein as the "enhanced SBR tool" or "eSBR tool") of the set of SBR tools described in the MPEG-4 AAC standard. Thus, eSBR (defined in the USAC standard) is an improvement over SBR (defined in the MPEG-4 AAC standard).

[0025] In this document, the expression "enhanced SBR processing" (or "eSBR processing") is used to represent spectral band replication processing that uses at least one eSBR tool not described or referred to in MPEG-4 AAC (e.g., at least one eSBR tool described or referred to in the MPEG USAC standard). Examples of such eSBR tools are harmonic transposition, QMF patching additional preprocessing or "pre-flattening".

[0026] The harmonic converter of integer order T maps a sine wave of frequency ω to a sine wave of frequency Tω while preserving the signal duration. Typically, three orders T = 2, 3, 4 are used sequentially to generate each part of the desired output frequency range using the smallest possible transposition order. If an output above the fourth transposition range is required, it may be generated by a frequency shift. When possible, to minimize the computational load, a nearly critically sampled baseband time domain is generated for said processing.

[0027] A bitstream generated according to the MPEG USAC standard (sometimes referred to herein as the "USAC bitstream") contains encoded audio content and typically includes metadata indicating each type of spectral band replication process to be applied by a decoder to decode the audio content of the USAC bitstream and / or metadata indicating at least one characteristic or parameter of at least one SBR tool and / or eSBR tool to be used to control such spectral band replication process and / or to decode the audio content of the USAC bitstream.

[0028] Here, the expression "enhanced SBR metadata" (or "eSBR metadata") refers to metadata that represents each type of spectral band replication process to be applied by a decoder to decode the audio content of an encoded audio bitstream (e.g., a USAC bitstream) and / or controls such spectral band replication process and / or indicates at least one characteristic or parameter of at least one SBR tool and / or eSBR tool to be used to decode such audio content, and is used to represent what is not described or referred to in the MPEG-4 AAC standard. An example of eSBR metadata is metadata that is described or referred to in the MPEG USAC standard but is neither described nor referred to in the MPEG-4 AAC standard (for indicating or controlling the spectral band replication process). Thus, the eSBR metadata in this document represents metadata that is not SBR metadata, and the SBR metadata in this document represents metadata that is not eSBR metadata.

[0029] The USAC bitstream may contain both SBR metadata and eSBR metadata. More specifically, the USAC bitstream may contain eSBR metadata that controls the execution of eSBR processing by the decoder and SBR metadata that controls the execution of SBR processing by the decoder. According to an exemplary embodiment of the present invention, eSBR metadata (e.g., configuration setting data specific to eSBR) is included in the MPEG-4 AAC bitstream (in accordance with the present invention) (e.g., in the sbr_extension() container at the end of the SBR payload).

[0030] During the decoding of an encoded bitstream using a set of eSBR tools (including at least one eSBR tool), the execution of eSBR processing by a decoder regenerates the high-frequency band of an audio signal based on the replication of a sequence of harmonics truncated during encoding. Such eSBR processing typically adjusts the spectral envelope of the generated high-frequency band, applies inverse filtering, and adds noise and sine wave components to reproduce the spectral characteristics of the original audio signal.

[0031] According to an exemplary embodiment of the present invention, eSBR metadata (such as a few control bits that are eSBR metadata) is included in one or more of the metadata segments of an encoded audio bitstream (such as an MPEG-4 AAC bitstream). The encoded audio bitstream also includes encoded audio data in other segments (audio data segments). Typically, at least one such metadata segment of each block of the bitstream is a stuffing element (including an identifier indicating the start of the stuffing element) (or includes a stuffing element), and the eSBR metadata is included in the stuffing element after the identifier.

[0032] FIG. 1 is a block diagram of an exemplary audio processing chain (audio data processing system), and one or more of the elements of the system may be configured according to an embodiment of the present invention. The system includes the following elements coupled together as shown: an encoder 1, a delivery subsystem 2, a decoder 3, and a post-processing unit 4. In variations of the illustrated system, one or more of the elements are omitted, or additional audio data processing units are included.

[0033] In some implementations, the encoder 1 (which optionally includes a preprocessing unit) is configured to receive as input PCM (time domain) samples including audio content and output an encoded audio bitstream (in a format compliant with the MPEG-4 AAC standard) representing the audio content. The data of the bitstream representing the audio content is sometimes referred to herein as "audio data" or "encoded audio data". When the encoder is configured according to a typical embodiment of the present invention, the audio bitstream output from the encoder includes eSBR metadata (typically other metadata as well) in addition to the audio data.

[0034] One or more encoded audio bitstreams output from the encoder 1 may be presented to the encoded audio delivery subsystem 2. The subsystem 2 is configured to store and / or deliver each encoded bitstream output from the encoder 1. The encoded audio bitstream output from the encoder 1 may be stored by the subsystem 2 (e.g., in the form of a DVD or Blu-ray disc), or may be transmitted by the subsystem 2 (which may implement a transmission link or network), or may be stored and transmitted by the subsystem 2.

[0035] Decoder 3 is configured to decode the encoded MPEG-4 AAC audio bitstream received via subsystem 2 (generated by encoder 1). In some embodiments, decoder 3 extracts eSBR metadata from each block of the bitstream and decodes the bitstream (including by performing eSBR processing using the extracted eSBR metadata) to generate decoded audio data (e.g., a stream of decoded PCM audio samples). In some embodiments, decoder 3 extracts SBR metadata from the bitstream (but ignores eSBR metadata included in the bitstream) and decodes the bitstream (including by performing SBR processing using the extracted SBR metadata) to generate decoded audio data (e.g., a stream of decoded PCM audio samples). Typically, decoder 3 includes a buffer that stores segments of the encoded audio bitstream received from subsystem 2 (e.g., in a non-transitory manner).

[0036] The post-processing unit 4 of FIG. 1 is configured to receive a stream of decoded audio data (e.g., decoded PCM audio samples) from decoder 3 and perform post-processing thereon. The post-processing unit may be configured to render the post-processed audio content (or the decoded audio received from decoder 3) for playback by one or more speakers.

[0037] Figure 2 is a block diagram of an encoder (100) which is an embodiment of the audio processing unit of the present invention. Any of the components or elements of encoder 100 may be implemented in hardware, software, or a combination of hardware and software, as one or more processes and / or one or more circuits (e.g., ASIC, FPGA, or other integrated circuit). Encoder 100 includes an encoder 105, a stuffing / formatter stage 107, a metadata generation stage 106, and a buffer memory 109, connected as shown in the figure. Typically, encoder 100 also includes other processing elements (not shown). Encoder 100 is configured to convert an input audio bitstream into an encoded output MPEG-4 AAC bitstream.

[0038] The metadata generator 106 is coupled and configured to generate (and / or pass through to stage 107) metadata (including eSBR metadata and SBR metadata) to be included by stage 107 in the encoded bitstream to be output from encoder 100.

[0039] The encoder 105 is coupled and configured to encode the input audio data (e.g., by performing compression thereon) and present the resulting encoded audio to stage 107 for inclusion in the encoded bitstream to be output from stage 107.

[0040] Stage 107 is configured to multiplex the encoded audio from encoder 105 and the metadata (including eSBR metadata and SBR metadata) from generator 106 to generate the encoded bitstream to be output from stage 107. Preferably, the encoded bitstream has a format defined by one of the embodiments of the present invention.

[0041] Buffer memory 109 is configured to store (e.g., in a non-transitory manner) at least one block of the encoded audio bitstream output from stage 107. Thereafter, a sequence of blocks of the encoded audio bitstream is presented from buffer memory 109 to the delivery system as the output from encoder 100.

[0042] FIG. 3 is a block diagram of a system including a decoder (200) which is an embodiment of the audio processing unit of the present invention and optionally also including a post-processor (300) coupled thereto. Any of the components or elements of decoder 200 may be implemented in hardware, software, or a combination of hardware and software as one or more processes and / or one or more circuits (e.g., ASIC, FPGA or other integrated circuit). Decoder 200 has a buffer memory 201, a bitstream payload de-formatter (parser) 205, an audio decode subsystem 202 (sometimes referred to as the “core” decode stage or “core” decode subsystem), an eSBR processing stage 203, and a control bit generation stage 204, connected as shown in the figure. Typically, decoder 200 also includes other processing elements (not shown).

[0043] Buffer memory (buffer) 201 stores at least one block of the encoded MPEG-4 AAC audio bitstream received by decoder 200 (e.g., in a non-transitory manner). In the operation of decoder 200, a sequence of blocks of the bitstream is presented from buffer 201 to de-formatter 205.

[0044] In a variation of the embodiment of FIG. 3 (or the embodiment of FIG. 4 described below), an APU that is not a decoder (e.g., the APU 500 of FIG. 6) includes a buffer memory (e.g., the same buffer memory as buffer 201) that stores (e.g., in a non-transitory manner) at least one block of an encoded audio bitstream of the same type as that received by buffer 201 of FIG. 3 or FIG. 4 (e.g., an MPEG-4 AAC audio bitstream), i.e., an encoded audio bitstream that includes eSBR metadata.

[0045] Referring again to FIG. 3, the demultiplexer 205 demultiplexes each block of the bitstream and then extracts SBR metadata (including quantized envelope data) and eSBR metadata (typically other metadata as well), and is coupled and configured to present at least the eSBR metadata and the SBR metadata to the eSBR processing stage 203 and typically also present other extracted metadata to the decode subsystem 202 (optionally also to the control bit generator 204). The demultiplexer 205 is also coupled and configured to extract audio data from each block of the bitstream and present the extracted audio data to the decode subsystem (decode stage) 202.

[0046] The system of FIG. 3 optionally also includes a post-processor 300. The post-processor 300 includes a buffer memory (buffer) 301 and other processing elements (not shown) including at least one processing element coupled to the buffer 301. The buffer 301 stores (e.g., in a non-transitory manner) at least one block (or frame) of the decoded audio data received by the post-processor 300 from the decoder 200. The processing elements of the post-processor 300 are coupled and configured to adaptively process a sequence of blocks (or frames) of the decoded audio output from the buffer 301 using metadata output from the decode subsystem 202 (and / or the demultiplexer 205) and / or control bits output from stage 204 of the decoder 200.

[0047] The audio decoding subsystem 202 of decoder 200 decodes the audio data extracted by parser 205 (such decoding may be referred to as "core" decoding operation), generates the decoded audio data, and is configured to present the decoded audio data to eSBR processing stage 203. The decoding is performed in the frequency domain and typically includes inverse quantization followed by spectral processing. Typically, the final stage of processing in subsystem 202 applies a conversion from the frequency domain to the time domain to the decoded frequency domain audio data, so the output of the subsystem is the decoded audio data in the time domain. Stage 203 is configured to apply the SBR tools and eSBR tools indicated by the SBR metadata and eSBR metadata (extracted by parser 205) to the decoded audio data (i.e., perform SBR and eSBR processing on the output of decoding subsystem 202 using the SBR and eSBR metadata) to generate the fully decoded audio data output from decoder 200 (e.g., to post-processor 300). Typically, decoder 200 includes a memory (accessible by subsystem 202 and stage 203) that stores the demultiplexed audio data and metadata output from demultiplexer 205, and stage 203 is configured to access the audio data and metadata (including SBR metadata and eSBR metadata) as needed during SBR and eSBR processing. The SBR processing and eSBR processing in stage 203 may be considered as post-processing on the output of core decoding subsystem 202. Optionally, decoder 200 also includes a final upmixing subsystem (which can apply the parametric stereo ("PS") tools defined in the MPEG-4 AAC standard using the PS metadata extracted by demultiplexer 205 and / or the control bits generated in subsystem 204).The upmix subsystem is coupled and configured to perform an upmix on the output of stage 203 to produce fully decoded, upmixed audio output from decoder 200. Alternatively, post-processor 300 may be configured to perform an upmix on the output of decoder 200 (e.g., using PS metadata extracted by demultiplexer 205 and / or control bits generated in subsystem 204).

[0048] In response to metadata extracted by demultiplexer 205, control bit generator 204 may generate control data. The control data may be used within decoder 200 (e.g., in a final upmix subsystem), and / or presented as an output of decoder 200 (e.g., to post-processor 300 for use in post-processing). In response to metadata extracted from the input bitstream (optionally in response to control data as well), stage 204 may generate (and present to post-processor 300) control bits indicating that the decoded audio data output from eSBR processing stage 203 should undergo a particular type of post-processing. In some implementations, decoder 200 is configured to present to post-processor 300 metadata extracted by demultiplexer 205 from the input bitstream, and post-processor 300 is configured to perform post-processing on the decoded audio data output from decoder 200 using the metadata.

[0049] Figure 4 is a block diagram of an audio processing unit ("APU") (210) which is another embodiment of the audio processing unit of the present invention. APU 210 is a legacy decoder not configured to perform eSBR processing. Any of the components or elements of APU 210 may be implemented as one or more processes and / or one or more circuits (e.g., ASIC, FPGA or other integrated circuit) in hardware, software or a combination of hardware and software. APU 210 has a buffer memory 201, a bitstream payload format disassembler (parser) 215, an audio decode subsystem 202 (sometimes referred to as the "core" decode stage or "core" decode subsystem) and an SBR processing stage 213, connected as shown in the figure. Typically, APU 210 also includes other processing elements (not shown). APU 210 may represent, for example, an audio encoder, decoder or transcoder.

[0050] Elements 201 and 202 of APU 210 are identical to the identically numbered elements of decoder 200 (of FIG. 3) and the above description thereof will not be repeated. In the operation of APU 210, a sequence of blocks of the encoded audio bitstream (MPEG-4 AAC bitstream) received by APU 210 is presented from buffer 201 to disassembler 215.

[0051] The format stripper 215 is coupled and configured to multiplex-separate each block of the bitstream and then extract SBR metadata (including quantized envelope data), typically other metadata as well, but to ignore eSBR which may be included in the bitstream according to any embodiment of the present invention. The format stripper 215 is configured to present at least said SBR metadata to the SBR processing stage 213. The format stripper 215 is also coupled and configured to extract audio data from each block of the bitstream and present the extracted audio data to a decode subsystem (decode stage) 202.

[0052] The audio decoding subsystem 202 of decoder 200 decodes the audio data extracted by the demultiplexer 215 (such decoding may be referred to as the "core" decoding operation), generates decoded audio data, and is configured to present the decoded audio data to the SBR processing stage 213. The decoding is performed in the frequency domain. Typically, the final stage of processing in subsystem 202 applies an inverse transform from the frequency domain to the time domain to the decoded frequency domain audio data, so the output of the subsystem is time domain decoded audio data. Stage 213 is configured to apply the SBR tools indicated by the SBR metadata (extracted by the demultiplexer 215) to the decoded audio data (but not the eSBR tools) (i.e., perform SBR processing on the output of the decoding subsystem 202 using the SBR metadata) to generate fully decoded audio data output from the APU 210 (e.g., to the post-processor 300). Typically, the APU 210 includes a memory (accessible by the subsystem 202 and stage 213) that stores the demultiplexed audio data and metadata output from the demultiplexer 215, and stage 213 is configured to access the audio data and metadata (including SBR metadata) as needed during SBR processing. The SBR processing in stage 213 may be considered a post-processing of the output of the core decoding subsystem 202. Optionally, the APU 210 also includes a final upmixing subsystem (which may apply parametric stereo ("PS") tools defined in the MPEG-4 AAC standard using the PS metadata extracted by the demultiplexer 215). The upmixing subsystem is coupled and configured to perform an upmix on the output of stage 213 to generate fully decoded, upmixed audio output from the APU 210.Alternatively, the post-processor is configured to perform upmixing on the output of the APU 210 (e.g., using the PS metadata extracted by the demultiplexer 215 and / or the control bits generated in the APU 210).

[0053] Various implementations of the encoder 100, decoder 200, and APU 210 are configured to perform different embodiments of the method of the present invention.

[0054] According to some embodiments, a legacy decoder (configured not to parse eSBR metadata or use any eSBR tools related to eSBR metadata) ignores the eSBR metadata, but still decodes the bitstream as much as possible without using the eSBR metadata or any eSBR tools related to the eSBR metadata, typically without any significant penalty in the decoded audio quality. The eSBR metadata (e.g., a few control bits that are the eSBR metadata) is included in the encoded audio bitstream (e.g., an MPEG-4 AAC bitstream). However, an eSBR decoder configured to parse the bitstream to identify the eSBR metadata and use at least one eSBR tool in response to the eSBR metadata enjoys the benefits of using at least one such eSBR tool. Accordingly, embodiments of the present invention provide a means for efficiently transmitting enhanced spectral band replication (eSBR) control data or metadata in a backward-compatible manner.

[0055] Typically, the eSBR metadata in the bitstream indicates one or more of the following eSBR tools (e.g., indicates at least one characteristic or parameter of one or more of the following eSBR tools), which are described in the MPEG USAC standard and may or may not be applied by the encoder during generation of the bitstream): · Harmonic conversion; and ·Preprocessing for QMF patching (pre - flattening).

[0056] For example, the eSBR metadata included in the bitstream may indicate the values of the parameters: sbrPatchingMode[ch], sbrOversamplingFlag[ch], sbrPitchInBins[ch], sbrPitchInBins[ch], and bs_sbr_preprocessing (as described in the MPEG USAC standard and this disclosure).

[0057] Here, the notation X[ch], where X is some parameter, indicates that the parameter relates to the channel ("ch") of the audio content of the encoded bitstream to be decoded. For simplicity, sometimes the expression [ch] is omitted, assuming that the relevant parameter relates to a certain channel of the audio content.

[0058] Here, the notation X[ch][env], where X is some parameter, indicates that the parameter relates to the SBR envelope ("env") of the channel ("ch") of the audio content of the encoded bitstream to be decoded. For simplicity, sometimes the expressions [env] and [ch] are omitted, assuming that the relevant parameter relates to a certain SBR envelope of a certain channel of the audio content.

[0059] During the decoding of the encoded bitstream, the execution of the harmonic conversion during the decoding eSBR processing stage (for each channel "ch" of the audio content indicated by the bitstream) is controlled by the following eSBR metadata parameters: sbrPatchingMode[ch]; sbrOversamplingFlag[ch]; sbrPitchInBinsFlag[ch] and sbrPitchInBins[ch].

[0060] The value of sbrPatchingMode[ch] indicates the type of transposer used in eSBR. sbrPatchingMode[ch]=1 indicates non-harmonic patching as described in section 4.6.18.6.3 of the MPEG-4 AAC standard; sbrPatchingMode[ch]=0 indicates harmonic SBR patching as described in sections 7.5.3 or 7.5.4 of the MPEG USAC standard.

[0061] The value of sbrOversamplingFlag[ch] indicates the use of signal-adaptive frequency-domain oversampling in eSBR combined with the DFT-based harmonic SBR patching described in section 7.5.3 of the MPEG USAC standard. This flag controls the size of the DFT used in the transposer. 1 indicates signal-adaptive frequency-domain oversampling enabled as described in section 7.5.3.1 of the MPEG USAC standard; 0 indicates signal-adaptive frequency-domain oversampling disabled as described in section 7.5.3.1 of the MPEG USAC standard.

[0062] The value of sbrPitchInBinsFlag[ch] controls the interpretation of the sbrPitchInBins[ch] parameter. 1 indicates that the value in sbrPitchInBins[ch] is valid and greater than 0; 0 indicates that the value of sbrPitchInBins[ch] is set to 0.

[0063] The value of sbrPitchInBins[ch] controls the addition of the cross-product term in the SBR harmonic transposer. The value sbrPitchInBins[ch] is an integer value in the range [0,127] and represents the distance measured in the frequency bins of a 1536-line DFT operating on the sampling frequency of the core coder.

[0064] When an MPEG-4 AAC bitstream indicates SBR channel pairs that are not combined across channels (rather than a single SBR channel), the bitstream indicates two instances of the above syntax (for harmonic or non-harmonic conversion). One instance for each channel of the sbr_channel_pair_element().

[0065] Harmonic conversion of the eSBR tool typically improves the quality of the decoded music signal at relatively low crossover frequencies. Non-harmonic conversion (i.e., legacy spectral patching) typically improves the speech signal. Thus, the starting point in determining which type of conversion is preferred for encoding a particular audio content is to select the conversion method depending on speech / music detection. Here, harmonic conversion is used for music content and spectral patching is used for speech content.

[0066] The execution of pre-emphasis during eSBR processing is controlled by the value of a one-bit eSBR metadata parameter known as bs_sbr_preprocessing. It is in the sense that pre-emphasis is either executed or not depending on this single-bit value. When the SBR QMF patching algorithm described in section 4.6.18.6.3 of the MPEG-4 AAC standard is used, the pre-emphasis stage may be executed (when indicated by the bs_sbr_preprocessing parameter) in order to avoid discontinuities in the shape of the spectral envelope of the high-frequency signal from being input to a subsequent envelope adjuster (which performs another stage of the eSBR processing). Pre-emphasis typically improves the operation of the subsequent envelope adjustment stage and, as a result, the perceived high-frequency signal becomes more stable.

[0067] The overall bitrate requirements for including eSBR metadata indicating the eSBR tools (harmonic conversion and pre-flattening) described above in an MPEG-4 AAC bitstream are expected to be on the order of several hundred bits per second. This is because, according to some embodiments of the present invention, only the differential control data required to perform eSBR processing is transmitted. Since this information is included in a backward-compatible manner (as will be described later), legacy decoders can ignore this information. Therefore, the adverse effects on the bitrate associated with including eSBR metadata can be ignored for several reasons, including: · The bitrate penalty (due to including eSBR metadata) is a very small percentage of the total bitrate since only the differential control data required to perform eSBR processing is transmitted (not simulcasting of SBR control data); and · Tuning of the control information related to SBR typically does not depend on the details of the conversion.

[0068] Thus, embodiments of the present invention provide means for efficiently transmitting improved spectral band replication (eSBR) control data or metadata in a backward-compatible manner. This efficient transmission of eSBR control data reduces the memory requirements in decoders, encoders, and transcoders that use aspects of the present invention without a significant adverse effect on the bitrate. Furthermore, the complexity and processing requirements associated with performing eSBR according to embodiments of the present invention are also reduced. This is because the SBR data only needs to be processed once and does not need to be simulcast as would be the case if eSBR were treated as a completely separate object type in MPEG-4 AAC rather than being integrated into the MPEG-4 AAC codec in a backward-compatible manner.

[0069] Next, referring to FIG. 7, elements of a block (raw_data_block) of an MPEG-4 AAC bitstream that includes eSBR metadata are described according to some embodiments of the present invention. FIG. 7 is a diagram of a block (raw_data_block) of an MPEG-4 AAC bitstream, showing some of its segments.

[0070] A block of an MPEG-4 AAC bitstream may include at least one single_channel_element() (e.g., the single channel element shown in FIG. 7) and / or at least one channel_pair_element() (not specifically shown in FIG. 7 but which may exist) that includes audio data for an audio program. The block may also include some fill_element()s (e.g., fill element 1 and / or fill element 2 in FIG. 7) that include data related to the program (e.g., metadata). Each single_channel_element() includes an identifier (e.g., "ID1" in FIG. 7) indicating the start of the single channel element and can include audio data indicating different channels of a multi-channel audio program. Each channel_pair_element includes an identifier (not shown in FIG. 7) indicating the start of the channel pair element and can include audio data indicating two channels of the program.

[0071] The fill_element (referred to as the filling element in this paper) of the MPEG-4 AAC bitstream contains an identifier (such as "ID2" in Figure 7) indicating the start of the filling element, followed by filling data after the identifier. The identifier ID2 may consist of a three-bit unsigned integer ( "uimsbf") with the value of 0x6, where the most significant bit is transmitted first. The filling data can include an extension_payload() element (sometimes referred to as the extended payload in this paper). Its syntax is shown in Table 4.57 of the MPEG-4 AAC standard. There are several types of extended payloads, which are identified through the extension_type parameter. This parameter is a four-bit unsigned integer ( "uimsbf") where the most significant bit is transmitted first.

[0072] The filling data (such as its extended payload) can include a header or identifier (such as "Header 1" in Figure 7) indicating a segment of the filling data representing the SBR object (i.e., the header initializes the "SBR object" type referred to as sbr_extension_data() in the MPEG-4 AAC standard). For example, the spectral band replication (SBR) extended payload is identified by having the value "1101" or "1110" for the extension_type field in the header. The identifier "1101" identifies the extended payload using SBR data, and "1110" identifies the extended payload using SBR data with a cyclic redundancy check (CRC) for verifying the correctness of the SBR data.

[0073] When the header (e.g., the extension_type field) initializes an SBR object type, SBR metadata (sometimes referred to in this document as "spectral band replication data" and as sbr_data() in the MPEG-4 AAC standard) follows the header, and at least one spectral band replication extension element (e.g., the "SBR extension element" of padding element 1 in FIG. 7) can follow the SBR metadata. Such a spectral band replication extension element (a segment of the bitstream) is referred to as an sbr_extension() container in the MPEG-4 AAC standard. The spectral band replication extension element optionally includes a header (e.g., the "SBR extension header" of padding element 1 in FIG. 7).

[0074] The MPEG-4 AAC standard contemplates that the spectral band replication extension element can include PS (parametric stereo) data for the program's audio data. The MPEG-4 AAC standard contemplates that when the header of a padding element (e.g., its extended payload) initializes an SBR object type and the spectral band replication extension element of the padding element includes PS data, the padding element (e.g., its extended payload) includes a spectral band replication data bs_extension_id parameter. The value of this parameter (i.e., bs_extension_id = 2) indicates that PS data is included in the spectral band replication extension element of the padding element.

[0075] According to some embodiments of the present invention, eSBR metadata (e.g., a flag indicating whether enhanced spectral band replication (eSBR) processing is performed on the audio content of that block) is included in the spectral band replication extension element of the padding element. For example, such a flag is shown in the padding element 1 of FIG. 7, and the flag appears after the header of the "SBR extension element" of the padding element 1 (the "SBR extension header" of the padding element 1). Optionally, such a flag and additional eSBR metadata are included in the spectral band replication extension element after the header of the spectral band replication extension element (e.g., in the SBR extension element of the padding element 1 in FIG. 7, after the SBR extension header). According to some embodiments of the present invention, the padding element including eSBR metadata also includes a bs_extension_id parameter. The value of the parameter (e.g., bs_extension_id = 3) indicates that the padding element includes eSBR metadata and that eSBR processing should be performed on the audio content of the block.

[0076] According to some embodiments of the present invention, eSBR metadata is included in padding elements of an MPEG-4 AAC bitstream other than the spectral band replication extension element (SBR extension element) of the padding element (e.g., padding element 2 in FIG. 7). This is because padding elements containing SBR data or extension_payload() with SBR data having CRC do not contain any other extension payloads of any other extension types. Therefore, in embodiments where eSBR metadata is stored in its own extension payload, a separate padding element is used to store the eSBR metadata. Such a padding element includes an identifier indicating the start of the padding element (e.g., "ID2" in FIG. 7), followed by padding data after the identifier. The padding data can include an extension_payload() element (sometimes referred to as an extension payload in this document). Its syntax is shown in Table 4.57 of the MPEG-4 AAC standard. The padding data (e.g., its extension payload) can include a header indicating an eSBR object (e.g., "header 2" of padding element 2 in FIG. 7) (i.e., the header initializes the enhanced spectral band replication (eSBR) object type), and the padding data (e.g., its extension payload) includes eSBR metadata after the header. For example, padding element 2 in FIG. 7 includes such a header ("header 2"), and after the header, it also includes eSBR metadata (i.e., a "flag" within padding element 2 indicating whether enhanced spectral band replication (eSBR) processing is performed on the audio content of that block). Optionally, additional eSBR metadata is also included in the padding data of padding element 2 in FIG. 7 after header 2. In the embodiments described in this paragraph, the header (e.g., header 2 in FIG. 7) has an identification information value indicating an eSBR extension payload instead of one of the normal values specified in Table 4.57 of the MPEG-4 AAC standard (thus, the extension_type field of the header indicates that the padding data contains eSBR metadata).

[0077] In a first class of embodiments, the present invention is an audio processing unit (e.g., a decoder) that: A memory (e.g., buffer 201 of FIG. 3 or FIG. 4) configured to store at least one block of an encoded audio bitstream (e.g., at least one block of an MPEG-4 AAC bitstream); A bitstream payload format demultiplexer (e.g., element 205 of FIG. 3 or element 215 of FIG. 4) coupled to the memory and configured to demultiplex at least a portion of the block of the bitstream; A decode subsystem (e.g., elements 202 and 203 of FIG. 3 or elements 202 and 213 of FIG. 4) coupled and configured to decode at least one portion of the audio content of the block of the bitstream, wherein the block Includes padding elements, an identifier indicating the start of the padding elements (e.g., an id_syn_ele identifier having a value of 0x6 in Table 4.85 of the MPEG-4 AAC standard), and padding data after the identifier, and the padding data: Includes at least one flag for identifying whether an enhanced spectral band replication (eSBR) process should be performed on the audio content of the block (e.g., using spectral band replication data and eSBR metadata included in the block); Is an audio processing unit.

[0078] The flag is eSBR metadata, an example of the flag is the sbrPatchingMode flag. Another example of the flag is the harmonicSBR flag. All of these flags indicate whether basic form spectral band replication or enhanced form spectral replication should be performed on the audio data of the block. Basic form spectral replication is spectral patching, and enhanced form spectral band replication is harmonic conversion.

[0079] In some embodiments, the padding data also includes additional eSBR metadata (i.e., eSBR metadata other than the flag).

[0080] The memory may be a buffer memory (e.g., the implementation of buffer 201 in FIG. 4) that stores (e.g., in a non-transitory manner) the at least one block of the encoded audio bitstream.

[0081] The complexity of the execution of eSBR processing (using eSBR harmonic conversion and pre-emphasis flattening) by an eSBR decoder during the decoding of an MPEG-4 AAC bitstream including eSBR metadata (wherein the eSBR metadata indicates these eSBR tools) is estimated to be as follows for a typical decoding using the parameters shown: ● Harmonic conversion (16 kbps, 14400 / 28800 Hz) ○ DFT-based: 3.68 WMOPS (weighted million operations per second); ○ QMF-based: 0.98 WMOPS; ● QMF patching pre-processing (pre-emphasis flattening): 0.1 WMOPS For transients, it has been found that DFT-based conversion typically exhibits better performance than QMF-based conversion.

[0082] According to some embodiments of the present invention, a stuffing element (of an encoded audio bitstream) containing eSBR metadata includes a parameter (e.g., bs_extension_id parameter) having a value (e.g., bs_extension_id = 3) indicating that the eSBR metadata is included in the stuffing element and that eSBR processing should be performed on the audio content of the block, and / or a parameter (e.g., the same bs_extension_id parameter) having a value (e.g., bs_extension_id = 2) indicating that the sbr_extension() container of the stuffing element contains PS data. For example, as shown in Table 1 below, such a parameter having the value bs_extension_id = 2 may indicate that the sbr_extension() container of the stuffing element contains PS data, and such a parameter having the value bs_extension_id = 3 may indicate that the sbr_extension() container of the stuffing element contains eSBR metadata.

Table 1

Table 2

[0083] For example, in some embodiments, esbr_data() may have the syntax shown in Table 3 to indicate these metadata parameters. [Table 3-1] [Table 3-2] The above syntax enables an efficient implementation as an extension to a legacy decoder of an improved form of spectral band replication, such as harmonic conversion. Specifically, the eSBR data in Table 3 includes only those parameters that are required to perform an improved form of spectral band replication and that are not already supported in the bitstream or that cannot be directly derived from parameters already supported in the bitstream. All other parameters and processing data required to perform an improved form of spectral band replication are extracted from existing parameters at positions already defined in the bitstream.

[0084] For example, an MPEG-4 HE-AAC or HE-AAC-v2 compliant decoder may be extended to include an improved form of spectral band replication, such as harmonic conversion. This improved form of spectral band replication is in addition to the basic form of spectral band replication already supported by the decoder. In the context of an MPEG-4 HE-AAC or HE-AAC-v2 compliant decoder, this basic form of spectral band replication is the QMF spectral patching SBR tool defined in Section 4.6.18 of the MPEG-4 AAC standard.

[0085] When performing upward spectral band replication, the extended HE-AAC decoder can reuse many of the bitstream parameters already included in the SBR extension payload of the bitstream. Specific parameters that can be reused include, for example, various parameters that determine the master frequency band table. These parameters include bs_start_freq (a parameter that determines the start of the master frequency table parameters), bs_stop_freq (a parameter that determines the end of the master frequency table), bs_freq_scale (a parameter that determines the number of frequency bands per octave), and bs_alter_scale (a parameter that changes the scale of the frequency bands). The parameters that can be reused also include the parameter (bs_noise_bands) that determines the noise band table and the limiter band table parameters (bs_limiter_bands). Thus, in various embodiments, at least some of the equivalent parameters specified in the USAC standard are omitted from the bitstream, thereby reducing the control overhead in the bitstream. Typically, when the parameters specified in the AAC standard have equivalent parameters specified in the USAC standard, the equivalent parameters specified in the USAC standard have the same name as the parameters specified in the AAC standard. For example, the envelope scale factor E OrigMapped . However, the equivalent parameters specified in the USAC standard typically have different values that are "tuned" for the enhanced SBR processing defined in the USAC standard rather than for the SBR processing defined in the AAC standard.

[0086] In addition to the many parameters described above, other data elements may also be reused by an enhanced HE-AAC decoder when performing an improved form of spectral band replication in accordance with embodiments of the present invention. For example, envelope data and noise floor data may be extracted from bs_data_env (envelope scale factor) and bs_noise_env (noise floor scale factor) data and used during the improved form of spectral band replication.

[0087] Essentially, these embodiments utilize configuration parameters and envelope data already supported by a legacy HE-AAC or HE-AAC v2 decoder in the SBR extension payload. This is to enable an improved form of spectral band replication with minimal additional transmission data required. The metadata was originally tuned for basic form HFR (e.g., spectral patching of SBR), but based on the embodiments, is used for an improved form of HFR (e.g., harmonic conversion of eSBR). As discussed earlier, the metadata is generally tuned and intended to be used with basic form HFR (e.g., linear translation) and represents the intended operating parameters (e.g., envelope scale factor, noise floor scale factor, time / frequency lattice parameters, sinusoid addition information, variable crossover frequency / band, inverse filtering mode, envelope resolution, smoothing mode, frequency interpolation mode). However, this metadata may be combined with additional metadata parameters specific to the improved form of HFR (e.g., harmonic conversion) and used to efficiently and effectively process audio data using the improved form of HFR.

[0088] Therefore, an extended decoder that supports an improved form of spectral band replication can be generated in a very efficient manner by relying on already defined bitstream elements (such as those within the SBR extension payload) and only adding (within the padding element extension payload) the parameters that are required to support the improved form of spectral band replication. This data reduction feature, in combination with placing the newly added parameters in a reserved data field such as an extension container, ensures that the bitstream is backward compatible with legacy decoders that do not support the improved form of spectral band replication, substantially reducing the barriers to creating decoders that support the improved form of spectral band replication.

[0089] In Table 3, the numbers in the right column indicate the number of bits of the corresponding parameter in the left column.

[0090] In some embodiments, the SBR object type defined in MPEG-4 AAC is updated to include aspects of SBR-Tool or enhanced SBR (eSBR) such that it is signaled in the SBR extension element (bs_extension_id == EXTENSION_ID_ESBR).

[0091] In some embodiments, the present invention is a method that includes encoding audio data to generate an encoded bitstream (e.g., an MPEG-4 AAC bitstream). The generation includes including eSBR metadata in at least one segment of at least one block of the encoded bitstream and including the audio data in at least one other segment of the at least one block. In typical embodiments, the method includes multiplexing the audio data with the eSBR metadata in each block of the encoded bitstream. In a typical decoding of the encoded bitstream in an eSBR decoder, the decoder extracts the eSBR metadata from the bitstream (which includes parsing and demultiplexing the eSBR metadata and the audio data), and uses the eSBR metadata to process the audio data to generate a stream of decoded audio data.

[0092] Another aspect of the present invention is an eSBR decoder configured to perform eSBR processing (e.g., using at least one of the eSBR tools known as harmonic conversion or pre-emphasis flattening) during the decoding of an encoded audio bitstream (e.g., an MPEG-4 AAC bitstream) that does not include eSBR metadata. An example of such a decoder is described with reference to FIG. 5.

[0093] The eSBR decoder (400) of FIG. 5 includes a buffer memory 201 (which is the same as the memory 201 of FIGS. 3 and 4), a bitstream payload formatter 215 (which is the same as the formatter 215 of FIG. 4), an audio decoding subsystem 202 (sometimes referred to as the "core" decoding stage or "core" decoding subsystem and the same as the core decoding subsystem 202 of FIG. 3), an eSBR control data generation subsystem 401, and an eSBR processing stage 203 (which is the same as the stage 203 of FIG. 3), connected as shown in the figure. Typically, the decoder 400 also includes other processing elements (not shown).

[0094] In the operation of the decoder 400, a sequence of blocks of an encoded audio bitstream (MPEG-4 AAC bitstream) received by the decoder 400 is presented from the buffer 201 to the de-formatter 215.

[0095] The de-formatter 215 is combined and configured to demultiplex each block of the bitstream and then extract SBR metadata (including quantized envelope data), typically other metadata as well. The de-formatter 215 is configured to present at least the SBR metadata to the eSBR processing stage 203. The de-formatter 215 is also combined and configured to extract audio data from each block of the bitstream and present the extracted audio data to the decode subsystem (decode stage) 202.

[0096] The audio decoding subsystem 202 of decoder 400 decodes the audio data extracted by the demultiplexer 215 (such decoding may be referred to as "core" decoding operation), generates decoded audio data, and is configured to present the decoded audio data to the eSBR processing stage 203. The decoding is performed in the frequency domain. Typically, the final stage of processing in subsystem 202 applies an inverse transform from the frequency domain to the time domain to the decoded frequency domain audio data, so that the output of the subsystem is time domain decoded audio data. Stage 203 is configured to apply the SBR (and eSBR) tools indicated by the SBR metadata (extracted by the demultiplexer 215) and the eSBR metadata generated in subsystem 401 to the decoded audio data (i.e., perform SBR and eSBR processing on the output of decoding subsystem 202 using the SBR and eSBR metadata) to generate fully decoded audio data output from decoder 400. Typically, decoder 400 includes a memory (accessible by subsystem 202 and stage 203) that stores the demultiplexed audio data and metadata output from the demultiplexer 215 (and optionally subsystem 401), and stage 203 is configured to access the audio data and metadata as needed during SBR and eSBR processing. The SBR processing in stage 203 may be considered a post-processing on the output of the core decoding subsystem 202. Optionally, decoder 400 also includes a final upmixing subsystem (which can apply parametric stereo ("PS") tools defined in the MPEG-4 AAC standard using the PS metadata extracted by the demultiplexer 215). The upmixing subsystem is coupled and configured to perform upmixing on the output of stage 203 to generate fully decoded, upmixed audio output from APU 210.

[0097] The control data generation subsystem 401 of FIG. 5 detects at least one attribute of an encoded audio bitstream to be decoded and is coupled and configured to generate eSBR control data (which, according to other embodiments of the present invention, may be or include eSBR metadata of any type among the types included in the encoded audio bitstream) in response to at least one result of the detection stage. The eSBR control data is presented at stage 203 to trigger and / or control the application of individual eSBR tools or combinations of eSBR tools when a particular attribute (or combination of attributes) of the bitstream is detected. For example, to control the execution of eSBR processing using harmonic conversion, some embodiments of the control data generation subsystem 401 include: a music detector (e.g., a simplified version of a normal music detector) for setting the sbrPatchingMode[ch] parameter (and presenting the set parameter at stage 203) in response to detecting whether the bitstream indicates music; a transient detector for setting the sbrOversamplingFlag[ch] parameter (and presenting the set parameter at stage 203) in response to detecting the presence or absence of transient components in the audio content indicated by the bitstream; and / or a pitch detector for setting the sbrPitchInBinsFlag[ch] and sbrPitchInBins[ch] parameters (and presenting the set parameters at stage 203) in response to detecting the pitch of the audio content indicated by the bitstream. Another aspect of the present invention is an audio bitstream decoding method performed by any embodiment of the decoder of the present invention described in this and the previous paragraph.

[0098] Aspects of the present invention include an encoding or decoding method of a type configured (e.g., programmed) to be executed by an embodiment of an APU, system, or device of the present invention. Other aspects of the present invention include a system or device configured (e.g., programmed) to execute an embodiment of any of the methods of the present invention and a computer-readable medium (e.g., a disk) storing code (e.g., in a non-transitory manner) for implementing an embodiment or stage of any of the methods of the present invention. For example, a system of the present invention may be or include a programmable general-purpose processor, a digital signal processor, or a microprocessor programmed (and / or otherwise configured) using software or firmware to perform any of a variety of operations including an embodiment or stage of a method of the present invention on data. Such a general-purpose processor may be a computer system including an input device, memory, and processing circuitry programmed (and / or otherwise configured) to perform an embodiment (or stage thereof) of a method of the present invention in response to data presented thereto, or may include the same.

[0099] Embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof (e.g., as a programmable logic array). Unless otherwise specified, algorithms or processes included as part of the present invention are not inherently related to any particular computer or other apparatus. In particular, various general-purpose machines may be used with programs written in accordance with the teachings herein, or it may be more convenient to construct a more specialized apparatus (e.g., an integrated circuit) to perform the required method steps. Thus, the present invention may be implemented in one or more computer programs executed on one or more programmable computer systems (e.g., any implementation of the elements of FIG. 1 or the encoder 100 (or certain elements thereof) of FIG. 2 or the decoder 200 (or certain elements thereof) of FIG. 3 or the decoder 210 (or certain elements thereof) of FIG. 4 or the decoder 400 (or certain elements thereof) of FIG. 5). Each computer system has at least one processor, at least one data storage system (including volatile and non-volatile memory and / or storage elements), at least one input device or port, and at least one output device or port. Program code is applied to input data to perform the functions described herein and generate output information. The output information is applied to one or more output devices in a known manner.

[0100] Each such program may be implemented in any desired computer language (including machine language, assembly, or high-level procedural, logical, or object-oriented programming languages) to communicate with the computer system. In any case, the language may be a compiled or interpreted language.

[0101] For example, when implemented by a computer software instruction sequence, the various functions and steps of the embodiments of the present invention may be implemented by a multi-threaded software instruction sequence running on suitable digital signal processing hardware. In that case, the various devices, steps, and functions of the embodiments may correspond to portions of the software instructions.

[0102] Each such computer system is preferably stored or downloaded on a storage medium or device readable by a general-purpose or special-purpose programmable computer (such as a semiconductor memory or media or magnetic or optical media). This is for configuring and operating the computer to execute the procedures described in this document when the storage medium or device is read by the computer system. The system of the present invention may be implemented as a computer-readable storage medium configured with a computer program (i.e., storing a computer program). Here, the storage medium configured in this way causes the computer system to operate in a specific predefined manner so as to execute the functions described in this document.

[0103] Some embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications can be made without departing from the spirit and scope of the present invention. Numerous modifications and variations of the present invention are possible in light of the above teachings. For example, a phase shift may be used in combination with a complex QMF decomposition and synthesis filter bank to facilitate an efficient implementation. The decomposition filter bank is responsible for filtering the time-domain low-frequency signal generated by the core decoder into a plurality of subbands (e.g., QMF subbands). The synthesis filter bank is responsible for combining the reproduced high-frequency generated by the selected HFR technique (indicated by the received sbrPatchingMode parameter) with the decoded low-frequency to generate a broadband output audio signal. However, a given filter bank implementation operating in certain sample rate modes, such as normal dual-rate operation or downsampled SBR mode, should not have a phase shift that is bitstream-dependent. The QMF bank used in SBR is a complex-exponential extension of the theory of cosine-modulated filter banks. It has been shown that when a cosine-modulated filter bank is extended using complex-exponential modulation, the alias cancellation constraint becomes unnecessary. Thus, for the SBR QMF bank, the decomposition filter h k (n) and the synthesis filter f k (n) can both be defined by the following equation:

Equation

[0104] The coefficients of the prototype filter p0(n) may be defined with a length L of 640 as shown in Table 4 below.

Table 4-1

Table 4-2

Table 4-3

Table 4-4

Table 4-5

Table 4-6

[0105] It is understood that within the scope of the appended claims, the invention may be practiced in ways other than those specifically described in this specification. Even if reference numerals are included in the claims, they are for illustrative purposes only and should not be used to interpret or limit the claims in any way.

[0106] Some aspects are described below. [Aspect 1] A method for decoding an encoded audio bitstream, the method comprising: receiving an encoded audio bitstream, the encoded audio bitstream including audio data representing a low-frequency portion of an audio signal; decoding the audio data to generate a decoded low-frequency audio signal; extracting high-frequency reconstruction metadata from the encoded audio bitstream, the high-frequency reconstruction metadata including operation parameters for a high-frequency reconstruction process that linearly translates several consecutive subbands from the low-frequency portion of the audio signal to the high-frequency portion of the audio signal; filtering the decoded low-frequency audio signal with an analysis filter bank to generate a filtered low-frequency audio signal; extracting a flag from the encoded audio bitstream indicating whether linear translation or harmonic conversion should be performed on the audio data; regenerating a high-frequency portion of the audio signal using the filtered low-frequency audio signal and the high-frequency reconstruction metadata according to the flag; Combining the filtered low - frequency audio signal and the regenerated high - frequency portion to form a wide - band audio signal. Method. [Aspect 2] The encoded audio bitstream further includes padding elements, the padding elements having an identifier indicating the start of the padding element and padding data after the identifier, the padding data including the flag, the method according to aspect 1. [Aspect 3] The method according to aspect 2, wherein the identifier is a three - bit unsigned integer with a value of 0x6, and the most significant bit is transmitted first. [Aspect 4] The padding data includes an extended payload, the extended payload includes spectral band replication extension data, the extended payload is identified using a four - bit unsigned integer with a value of "1101" or "1110" and the most significant bit is transmitted first, and optionally, The spectral band replication extension data is: An optional spectral band replication header, Spectral band replication data after the header, Including spectral band replication extension elements after the spectral band replication data, the flag being included in the spectral band replication extension elements, the method according to aspect 2. The method according to aspect 2. [Aspect 5] The method according to any one of aspects 1 to 4, wherein the high - frequency reconstruction metadata includes operating parameters selected from the group consisting of an envelope scale factor, a noise floor scale factor, sine - wave addition information, time / frequency lattice information, a crossover frequency, and an inverse filtering mode. [Aspect 6] The decomposition filter bank is

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Claims

1. 1. A method for decoding an encoded audio bitstream, the method comprising: receiving an encoded audio bitstream, the encoded audio bitstream including audio data representing a low-frequency portion of an audio signal, the encoded audio bitstream further including filler elements, the filler elements having an identifier indicating the beginning of the filler element and filler data following the identifier, the identifier being a three-bit unsigned integer having a value of 0x6, most significant bit transmitted first; decoding the audio data to generate a decoded low-band audio signal; extracting high-frequency reconstruction metadata from the encoded audio bitstream, the high-frequency reconstruction metadata including linear translation operation parameters tuned for a high-frequency reconstruction process that linearly translates several consecutive subbands from a low-frequency portion of the audio signal to a high-frequency portion of the audio signal, the linear translation operation parameters including sinusoidal additional information; filtering the decoded lowband audio signal with a analysis filter bank to generate a filtered lowband audio signal; extracting from the encoded audio bitstream a flag indicating whether a linear translation or a harmonic transformation should be performed on the audio data; If the flag indicates that harmonic transformation should be performed on the audio data: regenerating the high-frequency portion of the audio signal by performing a harmonic transformation using the filtered low-frequency audio signal and the high-frequency reconstruction metadata including the sinusoidal additional information, the sinusoidal additional information being reused for the harmonic transformation despite being encoded for linear translation processing; A method comprising:

2. 1. A decoder for decoding an encoded audio bitstream, the decoder comprising: an input interface for receiving an encoded audio bitstream, the encoded audio bitstream comprising audio data representing a low-frequency portion of an audio signal, the encoded audio bitstream further comprising filler elements having an identifier indicating the beginning of the filler element and filler data following the identifier, the identifier being a three-bit unsigned integer having a value of 0x6, transmitted most significant bit first; a core decoder for decoding the audio data to generate a decoded low-band audio signal; a deformatter for extracting high-frequency reconstruction metadata from the encoded audio bitstream, the high-frequency reconstruction metadata including linear translation operation parameters tuned for a high-frequency reconstruction process that linearly translates several consecutive subbands from a low-frequency portion of the audio signal to a high-frequency portion of the audio signal, the linear translation operation parameters including sinusoidal additional information; an analysis filter bank for filtering the decoded lowband audio signal to generate a filtered lowband audio signal; a deformatter that extracts from the encoded audio bitstream a flag that indicates whether a linear translation or a harmonic transformation should be performed on the audio data; a high frequency regenerator for regenerating a high frequency portion of the audio signal by performing a harmonic transformation using the filtered low frequency audio signal and the high frequency reconstruction metadata including the sinusoidal additional information if the flag indicates that a harmonic transformation should be performed on the audio data, the high frequency regenerator reusing the sinusoidal additional information for the harmonic transformation despite being encoded for linear translation processing; a decoder having:

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