Patent Publication Number: US-9905232-B2

Title: Device and method for encoding and decoding of an audio signal

Description:
TECHNICAL FIELD 
     The present technology relates to an encoding device and method, a decoding device and method, and a program therefor, and more particularly to an encoding device and method, a decoding device and method, and a program therefor capable of improving audio signal transmission efficiency. 
     BACKGROUND ART 
     Multichannel encoding based on MPEG (Moving Picture Experts Group)-2 AAC (Advanced Audio Coding) or MPEG-4 AAC, which are international standards, for example, is known as a method for encoding audio signals (refer to Non-patent Document 1, for example). 
     CITATION LIST 
     Non-Patent Document 
     
         
         Non-Patent Document 1: INTERNATIONAL STANDARD ISO/IEC 14496-3 Fourth edition 2009-09-01 Information technology-coding of audio-visual objects—part 3: Audio 
       
    
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     For reproduction giving higher realistic sensation than conventional 5.1-channel surround reproduction and for transmission of multiple sound materials (objects), a coding technology using more audio channels is required. 
     For encoding 31 channels at 256 kbps, for example, an average number of bits that can be used per one channel and per one audio frame in coding according to the MPEG AAC standard is about 176 bits. With such a number of bits, however, the sound quality is likely to be significantly deteriorated in encoding of a high bandwidth of 16 kHz or higher using a typical scalar encoding. 
     In addition, in exiting audio encoding, since an encoding process is also performed on signals that are silent or that can be regarded as being silent, not a small number of bits are required for encoding. 
     In multichannel low bit-rate encoding, it is important to allocate as many bits as possible for use in encoding channels; while in encoding according to the MPEG AAC standard, the number of bits for encoding a silent frame is 30 to 40 bits per element of each frame. Thus, as the number of silent channels in one frame is larger, the number of bits required or encoding silent data becomes less negligible. 
     As described above, with the technologies mentioned above, even when signals that need not necessarily be encoded, such as audio signals that are silent or that can be regarded as being silent, are present, the audio signals cannot be transmitted efficiently. 
     The present technology is achieved in view of the aforementioned circumstances and allows improvement in audio signal transmission efficiency. 
     Solutions to Problems 
     An encoding device according to a first aspect of the present technology includes: an encoding unit configured to encode an audio signal when identification information indicating whether or not the audio signal is to be encoded is information indicating that encoding is to be performed, and not to encode the audio signal when the identification information is information indicating that encoding is not to be performed; and a packing unit configured to generate a bit stream containing a first bit stream element in which the identification information is stored, and multiple second bit stream elements in which audio signals of one channel encoded according to the identification information are stored or at least one third bit stream element in which audio signals of two channels encoded according to the identification information are stored. 
     The encoding device can further be provided with an identification information generation unit configured to generate the identification information according to the audio signal. 
     When the audio signal is a silent signal, the identification information generation unit can generate the identification information indicating that encoding is not to be performed. 
     When the audio signal is a signal capable of being regarded as a silent signal, the identification information generation unit can generate the identification information indicating that encoding is not to be performed. 
     The identification information generation unit can determine whether or not the audio signal is a signal capable of being regarded as a silent signal according to a distance between a sound source position of the audio signal and a sound source position of another audio signal, a level of the audio signal and a level of the another audio signal. 
     An encoding method or program according to the first aspect of the present technology includes the steps of: encoding an audio signal when identification information indicating whether or not the audio signal is to be encoded is information indicating that encoding is to be performed, and not encoding the audio signal when the identification information is information indicating that encoding is not to be performed; and generating a bit stream containing a first bit stream element in which the identification information is stored, and multiple second bit stream elements in which audio signals of one channel encoded according to the identification information are stored or at least one third bit stream element in which audio signals of two channels encoded according to the identification information are stored. 
     In the first aspect of the present technology, an audio signal is encoded when identification information indicating whether or not the audio signal is to be encoded is information indicating that encoding is to be performed, and the audio signal is not encoded when the identification information is information indicating that encoding is not to be performed; and a bit stream containing a first bit stream element in which the identification information is stored, and multiple second bit stream elements in which audio signals of one channel encoded according to the identification information are stored or at least one third bit stream element in which audio signals of two channels encoded according to the identification information are stored is generated. 
     A decoding device according to a second aspect of the present technology includes: an acquisition unit configured to acquire a bit stream containing a first bit stream element in which identification information indicating whether or not to encode an audio signal is stored, and multiple second bit stream elements in which audio signals of one channel encoded according to the identification information indicating that encoding is to be performed are stored or at least one third bit stream element in which audio signals of two channels encoded according to the identification information indicating that encoding is to be performed are stored; an extraction unit configured to extract the identification information and the audio signal from the bit stream; and a decoding unit configured to decode the audio signal extracted from the bit stream and decode the audio signal with the identification information indicating that encoding is not to be performed as a silent signal. 
     For decoding the audio signal as a silent signal, the decoding unit can set a MDCT coefficient to 0 and perform an IMDCT process to generate the audio signal. 
     A decoding method or program according to the second aspect of the present technology includes the steps of: acquiring a bit stream containing a first bit stream element in which identification information indicating whether or not to encode an audio signal is stored, and multiple second bit stream elements in which audio signals of one channel encoded according to the identification information indicating that encoding is to be performed are stored or at least one third bit stream element in which audio signals of two channels encoded according to the identification information indicating that encoding is to be performed are stored; extracting the identification information and the audio signal from the bit stream; and decoding the audio signal extracted from the bit stream and decoding the audio signal with the identification information indicating that encoding is not to be performed as a silent signal. 
     In the second aspect of the present technology, a bit stream containing a first bit stream element in which identification information indicating whether or not to encode an audio signal is stored, and multiple second bit stream elements in which audio signals of one channel encoded according to the identification information indicating that encoding is to be performed are stored or at least one third bit stream element in which audio signals of two channels encoded according to the identification information indicating that encoding is to be performed are stored is acquired; the identification information and the audio signal are extracted from the bit stream; and the audio signal extracted from the bit stream is decoded and the audio signal with the identification information indicating that encoding is not to be performed is decoded as a silent signal. 
     Effects of the Invention 
     According to the first aspect and the second aspect of the present technology, audio signal transmission efficiency can be improved. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram explaining a bit stream. 
         FIG. 2  is a diagram explaining whether or not encoding is required. 
         FIG. 3  is a table explaining a status of encoding of each frame for each channel. 
         FIG. 4  is a table explaining structures of bit streams. 
         FIG. 5  is a table explaining identification information. 
         FIG. 6  is a diagram explaining a DSE. 
         FIG. 7  is a diagram explaining a DSE. 
         FIG. 8  is a diagram illustrating an example configuration of an encoder. 
         FIG. 9  is a flowchart explaining an identification information generation process. 
         FIG. 10  is a flowchart explaining an encoding process. 
         FIG. 11  is a diagram illustrating an example configuration of a decoder. 
         FIG. 12  is a flowchart explaining a decoding process. 
         FIG. 13  is a diagram illustrating an example configuration of a computer. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     Embodiments to which the present technology is applied will be described below with reference to the drawings. 
     First Embodiment 
     &lt;Outline of the Present Technology&gt; 
     The present technology improves audio signal transmission efficiency in such a manner that encoded data of multichannel audio signals in units of frames that meet a condition under which the signals can be regarded as being silent or equivalent thereto and thus need not be transmitted are not transmitted. In this case, identification information indicating whether or not to encode audio signals of each channel in units of frames is transmitted to a decoder side, which allows encoded data transmitted to the decoder side to be allocated to right channels. 
     While a case in which multichannel audio signals are encoded according to the AAC standard will be described in the following, similar processes will be performed in cases in which audio signals are encoded according to other systems. 
     In the case in which multichannel audio signals are encoded according to the AAC standard and then transmitted, for example, the audio signals of the respective channels are encoded and transmitted in units of frames. 
     Specifically, as illustrated in  FIG. 1 , encoded audio signals and information necessary for decoding and the like of the audio signals are stored in multiple elements (bit stream elements) and bit streams each constituted by such elements are transmitted. 
     In this example, a bit stream of a frame includes n elements EL 1  to ELn arranged in this order from the head, and an identifier TERM arranged at the end and indicating an end position of information of the frame. 
     The element EL 1  arranged at the head, for example, is an ancillary data area called a DSE (Data Stream Element), in which information on multiple channels such as information on downmixing of audio signals and identification information is written. 
     In the elements EL 2  to ELn following the element EL 1 , encoded audio signals are stored. In particular, an element in which an audio signal of a single channel is stored is called a SCE, and an element in which audio signals of two channels that constitute a pair are stored is called a CPE. 
     In the present technology, audio signals of channels that are silent or that can be regarded as being silent are not encoded, and such audio signals of channels for which encoding is not performed are not stored in bit streams. 
     When audio signals of one or more channels are not stored in bit streams, however, it is difficult to identify which channel an audio signal contained in a bit stream belongs to. Thus, in the present technology, identification information indicating whether or not to encode an audio signal of each channel is generated and stored in a DSE. 
     Assume, for example, that audio signals of successive frames F 11  to F 13  as illustrated in  FIG. 2  are to be encoded. 
     In such a case, an encoder determines whether or not to encode an audio signal of each of the frames. For example, the encoder determines whether or not an audio signal is a silent signal on the basis of an amplitude of the audio signal. If the audio signal is a silent signal or can be regarded as being a silent signal, the audio signal of the frame is then determined not to be encoded. 
     In the example of  FIG. 2 , since the audio signals of the frames F 11  and F 13  are not silent, the audio signals are determined to be encoded; and since the audio signal of the frame F 12  is a silent signal, the audio signal is determined not to be encoded. 
     In this manner, the encoder determines whether or not an audio signal of each frame is to be encoded for each channel before encoding audio signals. 
     More specifically, when two channels, such as an R channel and an L channel, are paired, it is determined whether or not to perform encoding for one pair. Assume, for example, that an R channel and an L channel are paired and that audio signals of these channels are encoded and stored in one CPE (element). 
     In such a case, when audio signals of both the R channel and the L channel are silent signals or can be regarded as being silent signals, encoding of these audio signals is not to be performed. In other words, when at least one of audio signals of two channels is not silent, encoding of these two audio signals is to be performed. 
     When encoding of audio signals of respective channels is performed while determination on whether or not encoding is to be performed is made for each channel, or more specifically for each element in this manner, only audible audio signals that are not silent are to be encoded as illustrated in  FIG. 3 . 
     In  FIG. 3 , the vertical direction in the drawing represents channels and the horizontal direction therein represents time, that is, frames. In this example, in the first frame, for example, all of the audio signals of eight channels CH 1  to CH 8  are encoded. 
     In the second frame, the audio signals of five channels CH 1 , CH 2 , CH 5 , CH 7 , and CH 8  are encoded and the audio signals of the other channels are not encoded. 
     Furthermore, in the sixth frame, only the audio signal of the channel CH 1  is encoded and the audio signals of the other channels are not encoded. 
     In a case where encoding of audio signals as illustrated in  FIG. 3  is performed, only the encoded audio signals are arranged in order and packed as illustrated in  FIG. 4 , and transmitted to the decoder. In this example, particularly in the sixth frame, since only the audio signal of the channel CH 1  is transmitted, the amount of data in a bit stream can be significantly reduced, and as a result, the transmission efficiency can be improved. 
     In addition, the encoder generates identification information indicating whether or not each frame of each channel, or more specifically each element, is encoded as illustrated in  FIG. 5 , and transmits the identification information with the encoded audio signal to the decoder. 
     In  FIG. 5 , a number “0” entered in each box represents identification information indicating that encoding has been performed, which a number “1” entered in each box represents identification information indicating that encoding has not been performed. Identification information of one frame for one channel (element) generated by the can be written in one bit. Such identification information of each channel (element) is written for each frame in a DSE. 
     As a result of determining whether or not to encode an audio signal for each element and writing and transmitting an audio signal encoded where necessary and identification information indicating whether or not encoding of each element has been performed in a bit stream as described above, the transmission efficiency of audio signals can be improved. Furthermore, the number of bits of audio signals that have not been transmitted, that is, the reduced amount of data can be allocated as a code amount for other frames or other audio signals of the current frame to be transmitted. In this manner, the quality of sound of audio signals to be encoded can be improved. 
     Since the example in which encoding is performed according to the AAC is described herein, identification information is generated for each bit stream element, but identification information may be generated for each channel where necessary according to another system. 
     When identification information and the like described above are written in a DSE, information shown in  FIGS. 6 and 7  is written in a DSE, for example. 
       FIG. 6  shows syntax of “3da_fragmented_header” contained in a DSE. In this information, “num_of_audio_element” is written as information indicating the number of audio elements contained in a bit stream, that is, the number of elements such as SCEs and CPEs in which encoded audio signals are contained. 
     After “num_of_audio_element,” “element_is_cpe[i]” is written as information indicating whether each element is an element of a single channel or an element of a channel pair, that is, an SCE or a CPE. 
     Furthermore,  FIG. 7  shows syntax of “3da_fragmented_data” contained in a DSE. 
     In this information, “3da_fragmented_header_flag” that is a flag indicating whether or not “3da_fragmented_header” shown in  FIG. 6  is contained in a DSE is written. 
     Furthermore, when the value of “3da_fragmented_header_flag” is “1” that is a value indicating that “3da_fragmented_header” shown in  FIG. 6  is written in a DSE, “3da_fragmented_header” is placed after “3da_fragmented_header_flag.” 
     Furthermore, in “3da_fragmented_data,” “fragment_element_flag[i]” that is identification information is written, the number of “fragment_element_flag[i]” corresponding to the number of elements in which audio signals are stored. 
     &lt;Example Configuration of Encoder&gt; 
     Next, a specific embodiment of an encoder to which the present technology is applied will be described. 
       FIG. 8  is a diagram illustrating an example configuration of the encoder to which the present technology is applied. 
     The encoder  11  includes an identification information generation unit  21 , an encoding unit  22 , a packing unit  23 , and an output unit  24 . 
     The identification information generation unit  21  determines whether or not an audio signal of each element is to be encoded on the basis of an audio signal supplied from outside, and generates identification information indicating the determination result. The identification information generation unit  21  supplies the generated identification information to the encoding unit  22  and the packing unit  23 . 
     The encoding unit  22  refers to the identification information supplied from the identification information generation unit  21 , encodes the audio signal supplied from outside where necessary, and supplies the encoded audio signal (hereinafter also referred to as encoded data) to the packing unit  23 . The encoding unit  22  also includes a time-frequency conversion unit  31  that performs time-frequency conversion of an audio signal. 
     The packing unit  23  packs the identification information supplied from the identification information generation unit  21  and the encoded data supplied from the encoding unit  22  to generate a bit stream, and supplies the bit stream to the output unit  24 . The output unit  24  outputs the bit stream supplied from the packing unit  23  to the decoder. 
     &lt;Explanation of Identification Information Generation Process&gt; 
     Subsequently, operation of the encoder  11  will be described. 
     First, with reference to a flowchart of  FIG. 9 , an identification information generation process that is a process in which the encoder  11  generates identification information will be described. 
     In step S 11 , the identification information generation unit  21  determines whether or not input data are present. If audio signal of elements of one frame are newly supplied from outside, for example, it is determined that input data are present. 
     If it is determined in step S 11  that input data are present, the identification information generation unit  21  determines whether or not a counter i&lt;the number of elements is satisfied in step S 12 . 
     The identification information generation unit  21  holds the counter i indicating what number of element is the current element, for example, and at a time point when encoding of an audio signal for a new frame is started, the value of the counter i is 0. 
     If it is determined that the counter i&lt;the number of elements in step S 12 , that is, if not all of the elements have not been processed for the current frame, the process proceeds to step S 13 . 
     In step S 13 , the identification information generation unit  21  determines whether or not the i-th element that is the current element is an element that need not be encoded. 
     If the amplitudes of the audio signal of the current element at some times are not larger than a predetermined threshold, for example, the identification information generation unit  21  determines that the audio signal of the element is silent or can be regarded as being silent and that the element thus need not be encoded. 
     In this case, when audio signals constituting the element are audio signals of two channels, it is determined that the element need not be encoded if both of the two audio signals are silent or can be regarded as being silent. 
     If the amplitude of an audio signal is larger than the threshold only at a certain time and the amplitude part at that time is noise, for example, the audio signal may be regarded as being silent. 
     Furthermore, if the amplitude (sound volume) of an audio signal is much smaller than that of an audio signal of the same frame in another channel and if a sound source position of the audio signal is close to that of the another audio signal of the another channel, for example, the audio signal may be regarded as being silent and may not be encoded. In other words, if a sound source that outputs sound louder than the audio signal of a low volume is close to the sound source of the audio signal, the audio signal from the sound source may be regarded as being a silent signal. 
     In such a case, it is determined whether or not the audio signal is a signal that can be regarded as being silent on the basis of the distance between the sound source position of the audio signal and the sound source position of the another audio signal and on the levels (amplitudes) of the audio signal and the another audio signal. 
     If it is determined in step S 13  that the current element is an element that need not be encoded, the identification information generation unit  21  sets the value of the identification information ZeroChan[i] of the element to “1” and supplies the value to the encoding unit  22  and the packing unit  23  in step S 14 . Thus, identification information having a value “1” is generated. 
     After the identification information is generated for the current element, the counter i is incremented by 1, the process then returns to step S 12 , and the processing as described above is repeated. 
     If it is determined in step S 13  that the current element is not an element that need not be encoded, the identification information generation unit  21  sets the value of the identification information ZeroChan[i] of the element to “0” and supplies the value to the encoding unit  22  and the packing unit  23  in step S 15 . Thus, identification information having a value “0” is generated. 
     After the identification information is generated for the current element, the counter i is incremented by 1, the process then returns to step S 12 , and the processing as described above is repeated. 
     If it is determined in step S 12  that the counter i&lt;the number of elements is not satisfied, the process returns to step S 11 , and the processing as described above is repeated. 
     Furthermore, if it is determined in step S 11  that no input data are present, that is, if identification information of the element has been generated for each of all the frames, the identification information generation process is terminated. 
     As described above, the encoder  11  determines whether or not an audio signal of each element needs to be encoded on the basis of the audio signal, and generates identification information of each element. As a result of generating identification information for each element in this manner, the amount of data of bit streams to be transmitted can be reduced and the transmission efficiency can be improved. 
     &lt;Explanation of Encoding Process&gt; 
     Furthermore, an encoding process in which the encoder  11  encodes an audio signal will be described with reference to  FIG. 10 . This encoding process is performed at the same time as the identification information generation process described with reference to  FIG. 9 . 
     In step S 41 , the packing unit  23  encodes identification information supplied from the identification information generation unit  21 . 
     Specifically, the packing unit  23  encodes the identification information by generating a DSE in which “3da_fragmented_header” shown in  FIG. 6  and “3da_fragmented_data” shown in  FIG. 7  are contained as necessary on the basis of identification information of elements of one frame. 
     In step S 42 , the encoding unit  22  determines whether or not input data are present. If an audio signal of an element of a frame that has not been processed is present, for example, it is determined that input data are present. 
     If it is determined in step S 42  that input data are present, the encoding unit  22  determines whether or not the counter i&lt;the number of elements is satisfied in step S 43 . 
     The encoding unit  22  holds the counter i indicating what number of element is the current element, for example, and at a time point when encoding of an audio signal for a new frame is started, the value of the counter i is 0. 
     If it is determined in step S 43  that the counter i&lt;the number of elements is satisfied, the encoding unit  22  determines whether or not the value of the identification information ZeroChan[i] of the i-th element supplied from the identification information generation unit  21  is “0” in step S 44 . 
     If it is determined in step S 44  that the value of the identification information ZeroChan[i] is “0,” that is, if the i-th element needs to be encoded, the process proceeds to step S 45 . 
     In step S 45 , the encoding unit  22  encodes an audio signal of the i-th element supplied from outside. 
     Specifically, the time-frequency conversion unit  31  performs MDCT (Modified Discrete Cosine Transform) on the audio signal to convert the audio signal from a time signal to a frequency signal. 
     The encoding unit  22  also encodes a MDCT coefficient obtained by the MDCT on the audio signal, and obtains a scale factor, side information, and quantized spectra. The encoding unit  22  then supplies the obtained scale factor, side information and quantized spectra as encoded data resulting from encoding the audio signal to the packing unit  23 . 
     After the audio signal is encoded, the process proceeds to step S 46 . 
     If it is determined in step S 44  that the value of the identification information ZeroChan[i] is “1,” that is, if the i-th element need not be encoded, the process skips the processing in step S 45  and proceeds to step S 46 . In this case, the encoding unit  22  does not encode the audio signal. 
     If it is determined in step S 45  that the audio signal has been encoded or if it is determined in step S 44  that the value of the identification information ZeroChan[i] “1,” the encoding unit  22  increments the value of the counter i by 1 in step S 46 . 
     After the counter i is updated, the process returns to step S 43  and the processing described above is repeated. 
     If it is determined in step S 43  that the counter i&lt;the number of elements is not satisfied, that is if encoding has been performed on all the elements of the current frame, the process proceeds to step S 47 . 
     In step S 47 , the packing unit  23  packs the DSE obtained by encoding the identification information and the encoded data supplied from the encoding unit  22  to generate a bit stream. 
     Specifically, the packing unit  23  generates a bit stream that contains SCEs and CPEs in which encoded data are stored, a DSE, and the like for the current frame, and supplies the bit stream to the output unit  24 . In addition, the output unit  24  outputs the bit stream supplied from the packing unit  23  to the decoder. 
     After the bit stream of one frame is output, the process returns to step S 42  and the processing described above is repeated. 
     Furthermore, if it is determined in step S 42  that no input data are present, that is, if bit streams are generated and output for all the frames, the encoding process is terminated. 
     As described above, the encoder  11  encodes an audio signal according to the identification information and generates a bit stream containing the identification information and encoded data. As a result of generating bit streams containing identification information of respective elements and encoded data of encoded elements among multiple elements in this manner, the amount of data of bit streams to be transmitted can be reduced. Consequently, the transmission efficiency can be improved. Note that the example in which identification information of multiple channels, that is, multiple identification information data are stored in a DSE in a bit stream of one frame has been described. However, in such cases where audio signals are not multichannel signals, for example, identification information of one channel, that is, one piece of identification information may be stored in a DSE in a bit stream of one frame. 
     &lt;Example Configuration of Decoder&gt; 
     Next, a decoder that receives bit streams output from the encoder  11  and decodes audio signals will be described. 
       FIG. 11  is a diagram illustrating an example configuration of the decoder to which the present technology is applied. 
     The decoder  51  of  FIG. 11  includes an acquisition unit  61 , an extraction unit  62 , a decoding unit  63 , and an output unit  64 . 
     The acquisition unit  61  acquires a bit stream from the encoder  11  and supplies the bit stream to the extraction unit  62 . The extraction unit  62  extracts identification information from the bit stream supplied from the acquisition unit  61 , sets a MDCT coefficient and supplies the MDCT coefficient to the decoding unit  63  where necessary, extracts encoded data from the bit stream and supplies the encoded data to the decoding unit  63 . 
     The decoding unit  63  decodes the encoded data supplied from the extraction unit  62 . Furthermore, the decoding unit  63  includes a frequency-time conversion unit  71 . The frequency-time conversion unit  71  performs IMDCT (Inverse Modified Discrete Cosine Transform) on the basis of a MDCT coefficient obtained as a result of decoding of the encoded data by the decoding unit  63  or a MDCT coefficient supplied from the extraction unit  62 . The decoding unit  63  supplies an audio signal obtained by the IMDCT to the output unit  64 . 
     The output unit  64  outputs an audio signals of each frame in each channel supplied from the decoding unit  63  to a subsequent reproduction device or the like. 
     &lt;Explanation of Decoding Process&gt; 
     Subsequently, operation of the decoder  51  will be described. 
     When a bit stream is transmitted from the encoder  11 , the decoder  51  starts a decoding process of receiving and decoding the bit stream. 
     Hereinafter, the decoding process performed by the decoder  51  will be described with reference to the flowchart of  FIG. 12 . 
     In step S 71 , the acquisition unit  61  receives a bit stream transmitted from the encoder  11  and supplies the bit stream to the extraction unit  62 . In other words, a bit stream is acquired. 
     In step S 72 , the extraction unit  62  acquires identification information from a DSE of the bit stream supplied from the acquisition unit  61 . In other words, the identification information is decoded. 
     In step S 73 , the extraction unit  62  determines whether or not input data are present. If a frame that has not been processed is present, for example, it is determined that input data are present. 
     If it is determined in step S 73  that input data are present, the extraction unit  62  determines whether or not the counter i&lt;the number of elements is satisfied in step S 74 . 
     The extraction unit  62  holds the counter i indicating what number of element is the current element, for example, and at a time point when decoding of an audio signal for a new frame is started, the value of the counter i is 0. 
     If it is determined in step S 74  that the counter i&lt;the number of elements is satisfied, the extraction unit  62  determines whether or not the value of the identification information ZeroChan[i] of the i-th element that is the current element is “0” in step S 75 . 
     If it is determined in step S 75  that the value of the identification information ZeroChan[i] is “0,” that is, if the audio signal has been encoded, the process proceeds to step S 76 . 
     In step S 76 , the extraction unit  62  unpacks the audio signal, that is, the encoded data of the i-th element that is the current element. 
     Specifically, the extraction unit  62  reads encoded data of a SCE or a CPE that is the current element of a bit stream from the element, and supplies the encoded data to the decoding unit  63 . 
     In step S 77 , the decoding unit  63  decodes the encoded data supplied from the extraction unit  62  to obtain a MDCT coefficient, and supplies the MDCT coefficient to the frequency-time conversion unit  71 . Specifically, the decoding unit  63  calculates the MDCT coefficient on the basis of a scale factor, side information, and quantized spectra supplied as the encoded data. 
     After the MDCT coefficient is calculated, the process proceeds to step S 79 . 
     If it is determined in step S 75  that the value of the identification information ZeroChan[i] is “1,” that is, if the audio signal has not been encoded, the process proceeds to step S 78 . 
     In step S 78 , the extraction unit  62  assigns “0” to the MDCT coefficient array of the current element, and supplies the MDCT coefficient array to the frequency-time conversion unit  71  of the decoding unit  63 . In other words, each MDCT coefficient of the current element is set to “0.” In this case, the audio signal is decoded on the assumption that the audio signal is a silent signal. 
     After the MDCT coefficient is supplied to the frequency-time conversion unit  71 , the process proceeds to step S 79 . 
     After the MDCT coefficient is supplied to the frequency-time conversion unit  71  in step S 77  or in step S 78 , the frequency-time conversion unit  71  performs an IMDCT process on the basis of the MDCT coefficient supplied from the extraction unit  62  or the decoding unit  63  in step S 79 . Specifically, frequency-time conversion of the audio signal is performed, and an audio signal that is a time signal is obtained. 
     The frequency-time conversion unit  71  supplies the audio signal obtained by the IMDCT process to the output unit  64 . The output unit  64  outputs the audio signal supplied from the frequency-time conversion unit  71  to a subsequent component. 
     When the audio signal obtained by decoding is output, the extraction unit  62  increments the counter i held by the extraction unit  62  by 1, and the process returns to step S 74 . 
     If it is determined in step S 74  that the counter i&lt;the number of elements is not satisfied, the process returns to step S 73 , and the processing as described above is repeated. 
     Furthermore, if it is determined in step S 73  that no input data are present, that is, if audio signals of all the frames have been decoded, the decoding process is terminated. 
     As described above, the decoder  51  extracts identification information from a bit stream, and decodes an audio signal according to the identification information. As a result of performing decoding using identification information in this manner, unnecessary data need not be stored in a bit stream, and the amount of data of transmitted bit streams can be reduced. Consequently, the transmission efficiency can be improved. 
     The series of processes described above can be performed either by hardware or by software. When the series of processes described above is performed by software, programs constituting the software are installed in a computer. Note that examples of the computer include a computer embedded in dedicated hardware and a general-purpose computer capable of executing various functions by installing various programs therein. 
       FIG. 13  is a block diagram showing an example structure of the hardware of a computer that performs the above described series of processes in accordance with programs. 
     In the computer, a CPU  501 , a ROM  502 , and a RAM  503  are connected to one another via a bus  504 . 
     An input/output interface  505  is further connected to the bus  504 . An input unit  506 , an output unit  507 , a recording unit  508 , a communication unit  509 , and a drive  510  are connected to the input/output interface  505 . 
     The input unit  506  includes a keyboard, a mouse, a microphone, an image sensor, and the like. The output unit  507  includes a display, a speaker, and the like. The recording unit  508  is a hard disk, a nonvolatile memory, or the like. The communication unit  509  is a network interface or the like. The drive  510  drives a removable medium  511  such as a magnetic disk, an optical disk, a magnetooptical disk, or a semiconductor memory. 
     In the computer having the above described structure, the CPU  501  loads a program recorded in the recording unit  508  into the RAM  503  via the input/output interface  505  and the bus  504  and executes the program, for example, so that the above described series of processes are performed. 
     Programs to be executed by the computer (CPU  501 ) may be recorded on a removable medium  511  that is a package medium or the like and provided therefrom, for example. Alternatively, the programs can be provided via a wired or wireless transmission medium such as a local area network, the Internet, or digital satellite broadcasting. 
     In the computer, the programs can be installed in the recording unit  508  via the input/output interface  505  by mounting the removable medium  511  on the drive  510 . Alternatively, the programs can be received by the communication unit  509  via a wired or wireless transmission medium and installed in the recording unit  508 . Still alternatively, the programs can be installed in advance in the ROM  502  or the recording unit  508 . 
     Programs to be executed by the computer may be programs for carrying out processes in chronological order in accordance with the sequence described in this specification, or programs for carrying out processes in parallel or at necessary timing such as in response to a call. 
     Furthermore, embodiments of the present technology are not limited to the embodiments described above, but various modifications may be made thereto without departing from the scope of the technology. 
     For example, the present technology can be configured as cloud computing in which one function is shared by multiple devices via a network and processed in cooperation. 
     In addition, the steps explained in the above flowcharts can be performed by one device and can also be shared among multiple devices. 
     Furthermore, when multiple processes are included in one step, the processes included in the step can be performed by one device and can also be shared among multiple devices. 
     Furthermore, the present technology can have the following configurations. 
     [1] 
     An encoding device including: 
     an encoding unit configured to encode an audio signal when identification information indicating whether or not the audio signal is to be encoded is information indicating that encoding is to be performed, and not to encode the audio signal when the identification information is information indicating that encoding is not to be performed; and 
     a packing unit configured to generate a bit stream containing a first bit stream element in which the identification information is stored, and multiple second bit stream elements in which audio signals of one channel encoded according to the identification information are stored or at least one third bit stream element in which audio signals of two channels encoded according to the identification information are stored. 
     [2] 
     The encoding device described in [1], further including an identification information generation unit configured to generate the identification information according to the audio signal. 
     [3] 
     The encoding device described in [2], wherein when the audio signal is a silent signal, the identification information generation unit generates the identification information indicating that encoding is not to be performed 
     [4] 
     The encoding device described in [2], wherein when the audio signal is a signal capable of being regarded as a silent signal, the identification information generation unit generates the identification information indicating that encoding is not to be performed. 
     [5] 
     The encoding device described in [4], wherein the identification information generation unit determines whether or not the audio signal is a signal capable of being regarded as a silent signal according to a distance between a sound source position of the audio signal and a sound source position of another audio signal, a level of the audio signal and a level of the another audio signal. 
     [6] 
     An encoding method including the steps of: encoding an audio signal when identification information indicating whether or not the audio signal is to be encoded is information indicating that encoding is to be performed, and not encoding the audio signal when the identification information is information indicating that encoding is not to be performed; and 
     generating a bit stream containing a first bit stream element in which the identification information is stored, and multiple second bit stream elements in which audio signals of one channel encoded according to the identification information are stored or at least one third bit stream element in which audio signals of two channels encoded according to the identification information are stored. 
     [7] 
     A program causing a computer to execute a process including the steps of: encoding an audio signal when identification information indicating whether or not the audio signal is to be encoded is information indicating that encoding is to be performed, and not encoding the audio signal when the identification information is information indicating that encoding is not to be performed; and 
     generating a bit stream containing a first bit stream element in which the identification information is stored, and multiple second bit stream elements in which audio signals of one channel encoded according to the identification information are stored or at least one third bit stream element in which audio signals of two channels encoded according to the identification information are stored. 
     [8] 
     A decoding device including: 
     an acquisition unit configured to acquire a bit stream containing a first bit stream element in which identification information indicating whether or not to encode an audio signal is stored, and multiple second bit stream elements in which audio signals of one channel encoded according to the identification information indicating that encoding is to be performed are stored or at least one third bit stream element in which audio signals of two channels encoded according to the identification information indicating that encoding is to be performed are stored; 
     an extraction unit configured to extract the identification information and the audio signal from the bit stream; and 
     a decoding unit configured to decode the audio signal extracted from the bit stream and decode the audio signal with the identification information indicating that encoding is not to be performed as a silent signal. 
     [9] 
     The decoding device described in [8], wherein for decoding the audio signal as a silent signal, the decoding unit sets a MDCT coefficient to 0 and performs an IMDCT process to generate the audio signal. 
     [10] 
     A decoding method including the steps of: 
     acquiring a bit stream containing a first bit stream element in which identification information indicating whether or not to encode an audio signal is stored, and multiple second bit stream elements in which audio signals of one channel encoded according to the identification information indicating that encoding is to be performed are stored or at least one third bit stream element in which audio signals of two channels encoded according to the identification information indicating that encoding is to be performed are stored; 
     extracting the identification information and the audio signal from the bit stream; and 
     decoding the audio signal extracted from the bit stream and decoding the audio signal with the identification information indicating that encoding is not to be performed as a silent signal. 
     [11] 
     A program causing a computer to execute a process including the steps of: 
     acquiring a bit stream containing a first bit stream element in which identification information indicating whether or not to encode an audio signal is stored, and multiple second bit stream elements in which audio signals of one channel encoded according to the identification information indicating that encoding is to be performed are stored or at least one third bit stream element in which audio signals of two channels encoded according to the identification information indicating that encoding is to be performed are stored; 
     extracting the identification information and the audio signal from the bit stream; and 
     decoding the audio signal extracted from the bit stream and decoding the audio signal with the identification information indicating that encoding is not to be performed as a silent signal. 
     REFERENCE SIGNS LIST 
     
         
           11  Encoder 
           21  Identification information generation unit 
           22  Encoding unit 
           23  Packing unit 
           24  Output unit 
           31  Time-frequency conversion unit 
           51  Decoder 
           61  Acquisition unit 
           62  Extraction unit 
           63  Decoding unit 
           64  Output unit 
           71  Frequency-time conversion unit