Patent Publication Number: US-2006013405-A1

Title: Multichannel audio data encoding/decoding method and apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
      This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/587,626, filed on Jul. 14, 2004, in the U.S. Patent and Trademark Office and Korean Patent Application No. 2005-0021840, filed on Mar. 16, 2005, in the Korean Intellectual Property Office, the disclosures of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to audio encoding and decoding, and more particularly, to a multichannel audio data encoding and decoding method and apparatus.  
      2. Description of Related Art  
      As of 2003, terrestrial digital multimedia broadcasting (DMB) has used an audio coder/decoder (codec) MPEG-4 bit sliced arithmetic coding (BSAC). Though only stereo is serviced at present, it is expected that multichannel services will be included in the future. The MPEG-4 BSAC should be able to add compression efficiency and function improving technologies, for example, bandwidth extension and spatial audio.  
      In the conventional BSAC multichannel, center, front left, front right, rear left and rear right channels are coded in one layer alternately.  FIG. 1  illustrates the structure of the conventional BSAC multichannel. The BSAC structure provides a fine grain scalability (FGS) function. That is, all five channels are in one layer and data can be cut off from the last layer. Tool side information on a channel should be defined in a general_header. High performance compression requires individual side information considering the characteristic in each channel.  
       FIG. 2  is a block diagram of functional modules of an audio encoding apparatus using the conventional BASC method. The apparatus includes a psychoacoustic model unit  200 , a time/frequency mapping unit  210 , a temporal noise shaping (TNS) unit  220 , an intensity stereo processing unit  230 , a perceptual noise substitution (PNS) unit  240 , a mid/side (M/S) stereo processing unit  250 , a quantization unit  260 , and a bit packing unit  270 .  
      The time/frequency mapping unit  210  converts an audio signal in the time domain into a signal in the frequency domain since the difference between signals that a human being can perceive is not so big with respect to time. However, in the case of the signals in the frequency domain, the difference between a signal that can be perceived by a human being and a signal that cannot be perceived by a human being is big in each bandwidth with respect to a human psychoacoustic model. Accordingly, by varying the number of bits allocated with respect to each frequency bandwidth, the efficiency of compression can be enhanced.  
      The psychoacoustic unit  200  combines audio signals, which are converted from the time domain into the frequency domain by the time/frequency mapping unit  210 , into signals of appropriate subbands, and by using a masking phenomenon occurring by interactions of each signals, calculates a masking threshold in each subband. The TNS unit  220  is used to control the temporal shape of a quantization noise in each conversion window. The TNS is enabled by applying the filtering process of frequency data. This TNS unit  220  is optionally used in an encoder. The intensity stereo processing unit  230  is a devise for processing a stereo signal more efficiently. In this device, only quantized information on a scalefactor band in relation to one of two channels is encoded and only a scalefactor is transmitted in relation to the remaining channel. The unit  230  is not necessarily used in an encoder. In case of a signal having a strong noise characteristic in a current frame, the PNS unit  240  can reduce the amount of generated bits to be used by encoding the energy value of each of frequency components corresponding to a scalefactor band instead of encoding the value of a frequency coefficient. The PNS unit  240  can determine whether or not to use bits in units of scalefactor bands. The M/S stereo processing unit  230  is also a device processing a stereo signal more efficiently. In this device, the signal of a left channel and the signal of a right channel are converted to an added signal and a subtracted signal, respectively, and then these signals are processed. The M/S stereo processing unit is also not necessarily used in an encoder. The quantization unit  260  performs scalar quantization of the frequency signals of each band so that the size of quantization noise in each band is made to be less than the masking threshold such that a human being does not to sense the noise. The bit packing unit  270  collects information items generated in each mode of the encoding apparatus and forms a bitstream according to a syntax generated appropriate to a scalable codec.  
      However, in the conventional BSAC multichannel structure shown in  FIG. 1 , mid/side (M/S) stereo cannot be used. This is because in the conventional encoding and decoding syntax, when the number of channels is 2 or more, the M/S stereo function cannot be used. Accordingly, the coding efficiency is lowered. Also, since window switching and PNS should use identical side information to all channels, the coding efficiency is lowered. Furthermore, since 5 channels are all interleaved, a memory 5 times larger than that of mono audio is required.  
     BRIEF SUMMARY  
      An aspect of the present invention provides a multichannel audio data encoding method and apparatus complying with MPEG standardization and improving the performance of the conventional multichannel BSAC method.  
      An aspect of the present invention also provides a multichannel audio data decoding method and apparatus complying with MPEG standardization and improving the performance of the conventional multichannel BSAC.  
      According to an aspect of the present invention, there is provided a multichannel audio signal encoding method including: encoding mono and/or stereo audio data; and encoding extended multichannel audio data other than the mono and/or stereo audio data. The mono and/or stereo audio data may have a layered bitrate.  
      The extended multichannel audio data may include type information of the extended channel indicating at least the configuration of an audio channel and be expressed as a channel configuration index. The encoding of the extended multichannel audio data may include: encoding a specified start code (zero_code, syncword) indicating the start of the extended multichannel audio data; and encoding the extended audio data by channel. The start code may include: the zero_code formed with 32 bits of continuous 0&#39;s; and the syncword formed with 8 bits of continuous 1&#39;s.  
      The encoding of the extended data by channel may include: encoding the type of the extended channel indicating the configuration of the audio channel; and encoding the extended channel audio data. The type of the extended channel may be formed with a channel configuration index. The encoding of the extended data by channel may include: encoding the length of the extended data; and encoding side information (bsac header, general header).  
      The encoding of the extended channel audio data may include: encoding a base layer having a lowest bitrate; and encoding an enhancement layer having a higher bitrate than that of the base layer, and if there are a plurality of enhancement layers, increasing a bitrate with the number of the enhancement layers.  
      According to another aspect of the present invention, there is provided a multichannel audio signal encoding apparatus including: a mono/stereo encoding unit encoding mono and/or stereo audio data; and an extended data encoding unit encoding extended multichannel audio data other than the mono and/or stereo audio data. The mono/stereo encoding unit may encode the mono and/or stereo audio data having a layered bitrate.  
      The extended multichannel audio data of the extended data encoding unit may include type information of the extended channel indicating at least the configuration of an audio channel and expressed as a channel configuration index. The extended data encoding unit may include: a start code encoding unit encoding a specified start code (zero_code, syncword) indicating the start of the extended multichannel audio data; and a channel encoding unit encoding the extended audio data by channel.  
      The start code of the start code encoding unit may include: the zero_code formed with 32 bits of continuous 0&#39;s; and the syncword formed with 8 bits of continuous 1&#39;s. The channel encoding unit may include: an extended channel type encoding unit encoding the type of the extended channel indicating the configuration of the audio channel; and an extended audio encoding unit encoding the extended channel audio data. The type of the extended channel may be formed with a channel configuration index. The channel encoding unit may include: an extended data length encoding unit encoding the length of the extended data; and an side information encoding unit encoding side information (bsac header, general header).  
      The extended audio encoding unit may include: a base layer encoding unit encoding a base layer having a lowest bitrate; and an enhancement layer encoding unit encoding an enhancement layer having a higher bitrate than that of the base layer, and if there are a plurality of enhancement layers, increasing a bitrate with the number of the enhancement layers.  
      According to still another aspect of the present invention, there is provided a multichannel audio signal decoding method including: decoding mono and/or stereo audio data; checking whether or not there is extended multichannel audio data to be decoded other than the mono and/or stereo audio data; and if there is extended data to be decoded, decoding the extended multichannel audio data. The mono and/or stereo audio data may have a layered bitrate.  
      The extended multichannel audio data may include type information of the extended channel indicating at least the configuration of an audio channel and expressed as a channel configuration index. In the checking of whether or not extended multichannel audio data exists, the presence of a specified start code (zero_code, syncword) indicating the start of the extended multichannel audio data may be checked and if the start code exists, it may be determined that the extended data exists. The start code may include: the zero_code formed with 32 bits of continuous 0&#39;s; and the syncword formed with 8 bits of continuous 1&#39;s. In the decoding of the extended multichannel audio data, if extended data to be decoded exists, the extended data may be decoded by channel. The decoding of the extended data by channel may include: decoding the type of the extended channel indicating the configuration of the audio channel; and decoding the extended channel audio data. The type of the extended channel may be formed with a channel configuration index.  
      The decoding of the extended data by channel may include: decoding the length of the extended data; and decoding side information (bsac header, general header). The decoding of the extended channel audio data may include: decoding a base layer having a lowest bitrate; and decoding an enhancement layer having a higher bitrate than that of the base layer, and if there are a plurality of enhancement layers, increasing a bitrate with the number of the enhancement layers.  
      According to yet still another aspect of the present invention, there is provided a multichannel audio signal decoding apparatus including: a mono/stereo decoding unit decoding mono and/or stereo audio data; an extended data checking unit checking whether or not there is extended multichannel audio data to be decoded other than the mono and/or stereo audio data; and an extended data decoding unit, decoding the extended multichannel audio data if data to be decoded exists. The mono and/or stereo audio data may have a layered bitrate. The extended data checking unit may check by the presence of a specified start code (zero_code, syncword) indicating the start of the extended multichannel audio data, and if the start code exists, determine that the extended data exists. The start code may include: the zero_code formed with 32 bits of continuous 0&#39;s; and the syncword formed with 8 bits of continuous 1&#39;s. If data to be decoded exists, the extended data decoding unit may decode the extended data by channel. The extended data decoding unit may include: an extended channel type decoding unit decoding the type of the extended channel indicating the configuration of the audio channel; and an extended channel audio decoding unit decoding the extended channel audio data. The type of the extended channel may be formed with a channel configuration index.  
      The extended data decoding unit may include: an extended data length decoding unit decoding the length of the extended data; and an side information decoding unit decoding side information (bsac header, general header). The extended channel audio decoding unit may include: a base layer decoding unit decoding a base layer having a lowest bitrate; and an enhancement layer decoding unit decoding an enhancement layer having a higher bitrate than that of the base layer, and if there are a plurality of enhancement layers, increasing a bitrate with the number of the enhancement layers.  
      According to a further aspect of the present invention, there is provided a multichannel audio signal encoding method comprising: encoding a base layer of mono/stereo audio data; encoding an enhancement layer of mono/stereo audio data; encoding specified start codes (zero_code, syncword) indicating the start of extended multichannel audio data; and encoding a base layer for at least one channel data that constitutes the extended multichannel audio data and encoding an enhancement layer for the at least one channel data.  
      The encoding of the base layer for the at least one channel data may include: encoding a length of the channel data; encoding a channel configuration index (channel_configuration_index) indicating the type of the channel; encoding side information (bsac header, general header); and encoding audio data of the base layer.  
      According to a further aspect of the present invention, there is provided a multichannel audio signal decoding method comprising: decoding a base layer of mono/stereo audio data; decoding an enhancement layer of mono/stereo audio data; checking if there is extended multichannel audio data to be decoded other than the mono/stereo audio data; if there is extended multichannel audio data to be decoded, decoding specified start codes (zero_code, syncword) indicating the start of the extended multichannel audio data; and decoding a base layer for at least one channel data that constitutes the extended multichannel audio data and decoding an enhancement layer for the at least one channel data.  
      The decoding of the base layer for the at least one channel data may include: decoding a length of the channel data; decoding a channel configuration index (channel_configuration_index) indicating the type of the channel; decoding side information (bsac header, general header); and decoding audio data of the base layer.  
      According to additional aspects of the present invention, there are provided computer readable recording media encoded with processing instructions for causing a processor to execute multichannel audio data encoding and decoding methods according to aspects of the present invention.  
      Additional and/or other aspects and advantages of the present invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and/or other aspects and advantages of the present invention will become apparent and more readily appreciated from the following detailed description, taken in conjunction with the accompanying drawings of which:  
       FIG. 1  illustrates the structure of the conventional bit sliced arithmetic coding (BSAC) multichannel;  
       FIG. 2  is a block diagram of functional modules of an audio encoding apparatus using the conventional BSAC method;  
       FIG. 3  is a block diagram of the structure of a multichannel audio data encoding apparatus according to an embodiment of the present invention;  
       FIG. 4  is a detailed block diagram of the extended data encoding unit of  FIG. 3 ;  
       FIG. 5  is a detailed block diagram of the extended audio encoding unit of  FIG. 4 ;  
       FIG. 6  illustrates a basic data structure for multichannel audio data encoding according to an embodiment of the present invention;  
       FIG. 7  is a flowchart of the operations performed in a multichannel audio data encoding method according to an embodiment of the present invention;  
       FIG. 8  is a detailed flowchart of the audio data encoding for an extended channel operation of  FIG. 7 ;  
       FIG. 9  is a block diagram of the structure of a multichannel audio decoding apparatus according to an embodiment of the present invention;  
       FIG. 10  is a block diagram of the extended data decoding unit of  FIG. 9 ;  
       FIG. 11  is a block diagram of the extended channel audio decoding unit of  FIG. 9 ;  
       FIG. 12  is a flowchart of operations of a multichannel audio decoding method according to an embodiment of the present invention;  
       FIG. 13  is a detailed flowchart of the audio data decoding for an extended channel operation of  FIG. 12 ;  
       FIG. 14  illustrates the syntax of Bsac_raw_data_block( ) showing an example of operations  1200  through  1240  of  FIG. 13 ;  
       FIG. 15  illustrates the syntax of extended_bsac_raw_data_block( ) showing an example of each extended audio channel decoding;  
       FIG. 16  illustrates the syntax for an example of extended_bsac_base_element( ) of the enhancement layer decoding operation of  FIG. 11 ; and  
       FIG. 17  illustrates the test result of measuring sound quality by using a multichannel audio signal encoding and/or decoding method and apparatus according to an embodiment of the present invention.  
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS  
      Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present invention by referring to the figures  
      A multichannel audio encoding and/or decoding apparatus and method according to an embodiment of the present invention will now be described.  
       FIG. 3  is a block diagram of the structure of a multichannel audio data encoding apparatus according to an embodiment of the present invention. The apparatus includes a mono/stereo encoding unit  300  and an extended data encoding unit  350 .  
      The mono/stereo encoding unit  300  encodes mono or stereo audio data. The mono/stereo encoding unit  300  may encode mono or stereo audio data having layered bitrates. In particular, the mono or stereo audio data may be encoded in a bit sliced arithmetic coding (BSAC) method according to ISO/IEC 14496-3. Because the audio encoding of the BSAC method is a known technology, the explanation thereof will be omitted here.  
      The extended data encoding unit  350  encodes extended multichannel audio data in addition to the mono or stereo audio data.  
      The extended multichannel audio data may include at least type information of an extended channel indicating the configuration of an audio channel, and the extended channel type information is expressed as a channel configuration index (channel_configuration_index). The channel configuration index may have a 3-bit field indicating the audio output channel configuration as shown in Table 1. Thus the channel configuration index indicates the characteristic of each speaker corresponding to a channel.  
                       TABLE 1                               Number of       Index   Channel to speaker mapping   channels (nch)                  0   center front speaker   1       1   left, right front speaker   2       2   rear surround speakers   1       3   left surround, right surround rear speakers   2       4   front low frequency effects speaker   1       5   left, right outside front speakers   2       6-7   reserved   —                  
 
       FIG. 4  is a detailed block diagram of the extended data encoding unit  350  of  FIG. 3  including a start code encoding unit  400  and a channel encoding unit  450 . The start code encoding unit  400  encodes a specified start code indicating the start of extended multichannel audio data. The start code is formed with a zero_code and a syncword. The zero_code is formed by 32 bits of continuous 0&#39;s indicating completion of arithmetic decoding of stereo audio data. The syncword is formed by 8 bits of continuous 1&#39;s indicating the start of extended multichannel audio data. The bit string is 1111 1111.  
      The channel encoding unit  450  encodes extended audio data in each channel, and is formed with an extended channel length encoding unit  452 , an extended channel type encoding unit  454 , an side information encoding unit  456 , and an extended audio encoding unit  458 .  
      The extended channel length encoding unit  452  encodes the length of extended data. The extended data length information is used when arithmetic decoding is performed.  
      The extended channel type encoding unit  454  encodes the type of an extended channel indicating the configuration of an audio channel. The side information encoding unit encodes side information (bsac_header, general_header). The side information (bsac_header, general_header) is the same as the side information used when the mono or stereo audio data is encoded in the BSAC method. The extended audio encoding unit  458  encodes extended channel audio data.  
       FIG. 5  is a detailed block diagram of the extended audio encoding unit  458  of  FIG. 4 . The extended audio encoding unit  458  includes a base layer encoding unit  500  and an enhancement layer encoding unit  550 . The base layer encoding unit  500  encodes a base layer having a lowest bitrate. The enhancement layer encoding unit  550  encodes an enhancement layer which has a higher bitrate than that of the base layer, and if there are a plurality of layers, increases the bitrate with the number of the layers.  
      The present embodiment uses a method of extending channels in the conventional stereo bitstream. A channel configuration index is assigned to each channel element and the possibility of modifying side information on each available tool when audio is encoded is indicated. Since there is a general header in each channel element of window, M/S, and PNS information, all tools requiring modification can be modified.  
       FIG. 6  illustrates a basic data structure for multichannel audio data encoding according to an embodiment of the present invention.  FIG. 7  is a flowchart of operations of a multichannel audio data encoding method according to an embodiment of the present invention. Referring to  FIGS. 3-7 , the operations of a multichannel audio encoding method and apparatus according to an embodiment of the present invention will now be explained.  
      First, mono or stereo audio data is encoded in the mono/stereo encoding unit  300  in operation  700 . Then, extended multichannel audio data other than the mono or stereo audio is encoded in the extended data encoding unit  350 . The mono or stereo data may have layered bitrates as described above. Also, the extended multichannel audio data includes the type information of the extended channel described above, indicating at least the configuration of an audio channel and expressed as a channel configuration index.  
      Encoding of the extended multichannel audio data will now be explained in more detail. Mono or stereo audio data is encoded and then it is checked whether data to be encoded exists in operation  710 . If data to be encoded exists, a specified start code (zero_code, syncword) indicating the start of extended multichannel audio data is encoded in the start code encoding unit  400  in operation  720 . The start code is the same as in the encoding apparatus described above. Then, extended audio data for each channel is encoded through the channel encoding unit  450 . Here, extended audio data for one channel is first encoded in operation  730  and when the encoding of the channel is completed, it is checked whether or not audio data to be encoded for another channel exists in operation  740 . If audio data to be encoded for another channel exists, the audio data for the channel is encoded. This process is performed for all extended channels.  
       FIG. 8  is a detailed flowchart of the audio data encoding for an extended channel in the operation  730 . Referring to  FIGS. 4 and 8 , the length of the extended data is encoded in the extended data length encoding unit  452  in operation  800 . Also, the type of the extended channel indicating the configuration of the audio channel is encoded in the extended channel type encoding unit  454  in operation  820 . Side information (bsac header, general header) is encoded in the side information encoding unit  456  in operation  840 . Then, the extended channel audio data is encoded in the extended audio encoding unit  458  in operation  860 .  
      Referring to  FIGS. 5 and 8 , the encoding of the extended channel audio data in operation  860 , first, the audio data in the base layer having a lowest bitrate is encoded in the base layer encoding unit  500 , and then the audio data of an enhancement layer is encoded in the enhancement layer encoding unit  550 . The enhancement layer has a bitrate higher than that of the base layer. When a plurality of enhancement layers exists, a bitrate is increasing with the number of the enhancement layers.  
      Meanwhile, a multichannel audio decoding apparatus and method according to an embodiment of the present invention will now be explained. The multichannel audio decoding is generally performed in the reverse order of the encoding operations.  
       FIG. 9  is a block diagram of the structure of a multichannel audio decoding apparatus. The apparatus includes a mono/stereo decoding unit  900 , an extended data checking unit  920 , and an extended data decoding unit  940 .  
      The mono/stereo decoding unit  900  decodes mono or stereo audio data. The mono or stereo audio data may have a layered bitrate and is decoded in the BSAC method according to the ISO/IEC 14496-3.  
      The extended data checking unit  920  checks whether or not there is extended multichannel audio data to be decoded in addition to the mono or stereo audio data. The extended data checking unit  920  checks the presence of a specified start code (zero_code, syncword) indicating the start of extended multichannel audio data, and if there is the start code, determines that there is extended data. The start code is formed with a zero_code and a syncword. The zero_code is formed by 32 bits of continuous 0&#39;s indicating completion of arithmetic decoding of stereo audio data. The syncword is formed by 8 bits of continuous 1&#39;s indicating the start of extended multichannel audio data. The bit string is 1111 1111.  
      The extended data decoding unit  940  decodes extended multichannel audio data if the extended data to be decoded exists. Also, the extended data decoding unit  940  may decode extended data by channel when decoding is performed.  
       FIG. 10  is a block diagram of the extended data decoding unit  940  of  FIG. 9 , which is formed with an extended data length decoding unit  1000 , an extended channel type decoding unit  1020 , a side information decoding unit  1040 , and an extended channel audio decoding unit  1060 .  
      The extended data length decoding unit  1000  decodes the length information of the extended data. The extended channel type decoding unit  1020  decodes the type of the extended channel indicating the configuration of the audio channel. The extended channel type information may be expressed as a channel configuration index (channel_configuration_index). The channel configuration index defines the number of the channels when the channels are mapped to a speaker, and has a 3-bit field indicating the audio output channel configuration as shown in the table 1.  
      The side information decoding unit  1040  decodes side information. The side information is required for decoding audio data and is information other than the audio data, such as a bsac header and a general header. The side information (bsac_header, general_header) is the same as the side information required for decoding mono or stereo audio data in the BSAC method.  
      The extended channel audio decoding unit  1060  decodes extended audio data.  FIG. 11  is a block diagram of the extended channel audio decoding unit  1060  of  FIG. 9 , including a base layer decoding unit  1100  and an enhancement layer decoding unit  1150 . Referring to  FIGS. 9 and 11 , the base layer decoding unit  1100  decodes the base layer having a lowest bitrate. The enhancement layer decoding unit  1150  decodes an enhancement layer which has a higher bitrate than that of the base layer, and if there are a plurality of layers, increases the bitrate increasing with the increasing number of the layers  
       FIG. 12  is a flowchart of the operations performed by a multichannel audio decoding method according to an embodiment of the present invention. Referring to  FIGS. 9 and 12 , the operations of the multichannel audio data decoding method and apparatus according to the present embodiment will now be explained.  
      First, mono or stereo audio data is decoded through the mono/stereo decoding unit  900  in operation  1200 . Then, it is checked by the extended data checking unit  920  whether or not there is extended multichannel audio data in addition to the mono/stereo audio data in operation  1210 . The presence of the extended multichannel audio data is determined by decoding a specified start code (zero_code, syncword) indicating the start of extended multichannel audio data, and checking the presence of the start code in operation  1220 . If there is the start code, it is determined that the extended data exists. That is, if there is the zero_code, it indicates that decoding of the mono or stereo audio data is completed and if there is the syncword after that, it indicates that there is multichannel audio data to be decoded.  
      If it is determined through the start code that there is extended data to be decoded, the extended multichannel audio data is decoded through the extended data decoding unit  940  in operation  1230 .  
      An embodiment of the operations  1200  through  1230  is expressed in syntax (Bsac_raw_data_block( )) as shown in  FIG. 14 .  
      Referring to  FIG. 14 , Bsac_raw_data_block( ) is a raw data block containing encoded audio data, related information and other data, and is basically formed with a bsac_base_element( ) and several bsac_layer_element( )s. Bsac_raw_data_block( ) is a module for determining whether or not a bsac bitstream has an extended part. The mono or stereo data may have layered bitrates as described above. Also, the extended multichannel audio data includes the type information described above of the extended channel, indicating at least the configuration of an audio channel and expressed as a channel configuration index.  
      After the extended audio data in relation to one channel is decoded in operation  1230 , it is checked whether or not there is audio data for another channel to be decoded in operation  1240 . If there is audio data for another channel to be decoded, the audio data for the other channel is decoded. By performing this process for all extended channels, all the extended channel audio data are decoded.  
      Syntax (extended_bsac_raw_data_block( ) ) showing an example of the decoding of each audio channel is shown in  FIG. 15 .  
      Referring to  FIG. 15 , the extended_bsac_raw_data_block( ) is a raw data block including encoded audio data corresponding to multichannel extended data, and information related to the audio data. The extended_bsac_raw_data_block( ) is basically formed with an extended_bsca_base_element( ) and several bsac_layer_element( )s.  
       FIG. 13  is a detailed flowchart of the operation of audio data decoding for an extended channel. Referring to  FIG. 13 , in the extended data length decoding unit  1000 , the length of the extended data is decoded in operation  1300 . Also, in the extended channel type decoding unit  1020 , the type of the extended channel indicating the configuration of the audio channel is decoded in operation  1320 . In the side information decoding unit  1040 , the side information (bsac header, general header) is decoded in operation  1340 . The performing order of the decoding operations  1300  through  1340  does not matter. Then, the extended channel audio data is decoded in the extended channel audio decoding unit  1060  in operation  1360 .  
      In the decoding of the extended channel audio data in the operation  1360 , the audio data of the base layer having a lowest bitrate is first decoded in the base layer decoding unit  1100 , and then, the audio data of the enhancement layer is decoded in the enhancement layer decoding unit  1150 . The enhancement layer has a higher bitrate than that of the base layer and, if there are a plurality of enhancement layers, increases a bitrate with the number of the enhancement layers. An example of syntax(extended_bsac_raw_data_block( )) for operation  1230  of  FIGS. 12 and 13  is shown in  FIG. 16 .  
      Referring to  FIG. 16 , the extended_bsac_base_element( ) is a syntactic element of a base layer bitstream, containing the encoded audio data corresponding to a BSAC extended part and information related to the audio data.  
      Embodiments of present invention can also be embodied as computer (including all apparatuses having an information processing function) readable codes on a computer readable recording medium. A computer readable recording medium is any data storage device that can store data which can be thereafter read by a computer system. Examples of the computer readable recording medium include read-only memory (ROM), random-access memory (RAM), CD-ROMs, magnetic tapes, floppy disks, and optical data storage devices.  
      According to the above-described embodiments, in a multichannel audio encoding and/or decoding apparatus and method of the present invention, the memory requirement for multichannel data interleaving is about 20% less than that of the memory requirement using the conventional BSAC method. This is because when the multichannel method according to the present invention is used, channel elements being added are sequentially processed and therefore the amount of the simultaneous memory usage is relatively small, while in the conventional multichannel method, all the data of the entire multichannel should be loaded on the memory.  
      The result of measuring sound quality by using the multichannel audio signal encoding and/or decoding method and apparatus according to the present invention is shown in  FIG. 14 .  
      The listening experiment conditions were as follows. A window switching &amp; M/S stereo tool was used and bitrates were controlled in each of the front and rear channel elements. Four audio experts participated in the experiment, and the relative sound quality (−2-+2) in relation to the conventional BSAC was measured. For the test items, a total member of 46 items used for MPEG-2 NBC were selected.  
      According to the multichannel audio encoding and/or decoding method and apparatus of above-described embodiments of the present invention, with only one bitstream, mono, stereo, and multichannel audio can be provided according to a user environment. Also in multichannel audio, an FGS function is provided according to the states of a user terminal and a network. Furthermore, enhancement of the performance of multichannel BSAC, for example, a high sound quality, low complexity, and scalability, is enabled. In particular, a variety of requirements for MPEG standardization (compatibility with conventional BSAC, maintaining the FGS function, and minimum modification) can be satisfied. Also, the method and apparatus can be employed in more lifelike digital multimedia broadcasting and mobile- and home-theater-based services.  
      Although a few embodiments of the present invention have been shown and described, the present invention is not limited to the described embodiments. Instead, it would be appreciated by those skilled in the art that changes may be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the claims and their equivalents.