Abstract:
Provided is a stereo signal encoding device that enables a lower bitrate without decreasing quality when applying an intermittent transmission technique to a stereo signal. A stereo encoding unit ( 103 ) generates first stereo encoded data by encoding the stereo signal when the stereo signal of the current frame is an audio section A stereo DTX encoding unit ( 104 ) is a means for encoding the stereo signal when the stereo signal of the current frame is a non-audio section, and generates second stereo encoded data by encoding each of: a monaural signal spectral parameter that is a spectral parameter of a monaural signal generated using the first channel signal and the second channel signal; first channel signal information relating to the first channel signal; and second channel signal information relating to the second channel signal.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a stereo signal encoding apparatus, a stereo signal decoding apparatus, a stereo signal encoding method, and a stereo signal decoding method. 
       BACKGROUND ART 
       [0002]    In mobile communication systems, in order to make effective use of radio spectrum resources and the like, there is a need to compress a speech signal to a low bit rate for transmission thereof. There is also a desire for a telephone service with improved speech quality and a good feeling of naturalness, and the achievement thereof makes desirable the high-quality encoding of not only monaural signals, but also multichannel audio signals, and in particular stereo audio signals. 
         [0003]    A known method for encoding a stereo audio signal at low bit rate is the intensity stereo method. In the intensity stereo method, a monaural signal is multiplied by scaling coefficients to generate an L-channel signal (left-channel signal) and an R-channel signal (right-channel signal). A method such as this is called amplitude panning. 
         [0004]    The most basic method of amplitude panning is that of multiplying a monaural signal in the time domain by gain coefficients for amplitude panning (panning gain coefficient) to determine the L-channel signal and the R-channel signal (refer, for example, to the Non-Patent Literature 1). Another method is that of multiplying a monaural signal by panning gain coefficients for each frequency component (or each frequency group) in the frequency domain to determine the L-channel signal and the R-channel signal (refer to, for example, Non-Patent Literature 2). 
         [0005]    If panning gain coefficients are used as encoding parameters of parametric stereo, scalable encoding (monaural-stereo scalable encoding) of a stereo signal can be done (refer to, for example, Patent Literatures 1 and 2). The panning gain coefficients are described in Patent Literature 1 as balance parameters and are described in Patent Literature 2 as ILDs (level differences). 
         [0006]    In a mobile communication system, in order to make effective use of radio spectrum resources, a technique exists as intermittent transmission (DTX: discontinuous transmission) exists (refer to, for example, Non-Patent Literature 3). The DTX technique is a technique that, when speech is not emitted, information representing background noise is intermittently transmitted at an ultra-low bit rate. This enables reduction of the average bit rate during a conversation, and also accommodation of more mobile terminals with the same frequency band. 
         [0007]    For example, in Non-Patent Literature 3, at a rate of one time every eight frames in a frame that is judged to be a non-speech section (inactive speech section, background noise section), LPC (linear prediction coding) coefficients are quantized by 29 bits (for example, by converting LPC coefficients to LSF (line spectral frequency) coefficients, and the frame energy is quantized by 6 bits, making a total of 35 bits (bit rate: 1.75 kbits/s). In the decoding section, ten pulses per frame generated based on random numbers are multiplied by the decoded frame energy, and the result is passed through a synthesis filter constituted by the decoded LPC coefficients to generate a decoded signal. This decoding processing is performed, while updating the LPC coefficients and the frame energy every eight frames. 
       CITATION LIST 
     Patent Literature 
     PTL 1 
       [0000]    
       
         Japanese Translation of a PCT Application Laid-Open No. 2004-535145 
       
     
       PTL 2 
       [0000]    
       
         Japanese Translation of a PCT Application Laid-Open No. 2005-533271 
       
     
       Non-Patent Literature 
     NPL 1 
       [0010]    V. Pulkki and M. Karjalainen, “Localization of amplitude-panned virtual sources I: Stereophonic panning,” Journal of the Audio Engineering Society, Vol. 49, No. 9, September 2001, pp. 739-752. 
       NPL 2 
       [0000]    
       
         B. Cheng, C. Ritz and I. Burnett, “Principles and analysis of the squeezing approach to low bit rate spatial audio coding,” proc. IEEE ICASSP2007, pp. 1-13-1-16, April 2007. 
       
     
       NPL 3 
       [0000]    
       
         3GPP TS 26.092 V4.0.0, “AMR Speech Codec: Comfort noise aspects (Release 4),” May 2001. 
       
     
       SUMMARY OF INVENTION 
     Technical Problem 
       [0013]    Consider the case of applying an intermittent transmission technique to a stereo signal. In the above-noted conventional art, when panning coefficients are used with respect to the spectral profile of a background noise signal, because sub-hands are multiplied by panning coefficients, there is a problem that energy steps occurring in the spectra between sub-bands reduce the quality. This problem becomes prominent with a simple background noise signal, compared with a speech spectral profile. Although narrowing the width of the sub-bands to suppress the occurrence of energy steps can be envisioned as a method of solving this problem, the number of panning coefficients that must be transmitted from the encoder side to the decoder side increases, resulting in an increase in the bit rate. 
         [0014]    In contrast, if the spectral profile of the background noise signal is represented by LPC coefficients, the above-noted energy steps do not occur in the spectrum. However, it is necessary to encode the LPC coefficients for both the L channel and the R channel, this resulting in the problem of an increased bit rate. 
         [0015]    An object of the present invention is to provide a stereo signal encoding apparatus, a stereo signal decoding apparatus, a stereo signal encoding method, and a stereo signal decoding method that enable a reduction of the bit rate, without reducing the quality when an intermittent transmission technique is applied to a stereo signal. 
       Solution to Problem 
       [0016]    A stereo signal encoding apparatus according to an embodiment of the present invention encodes a stereo signal having a first channel signal and a second channel signal; the stereo signal encoding apparatus adapts a constitution of comprising: a first encoding section that generates first encoded stereo data by encoding the stereo signal when the stereo signal of the current frame is a speech part; a second encoding section that encodes the stereo signal when the stereo signal of the current frame is a non-speech part and that generates second encoded stereo data by encoding each of: monaural signal spectral parameters that are spectral parameters of a monaural signal generated using the first channel signal and the second channel signal; first channel signal information regarding the amount of variation between the spectral parameters of the monaural signal and the spectral parameters of the first channel signal; and second channel signal information regarding the amount of variation between the spectral parameters of the monaural signal and the spectral parameters of the second channel signal; and a transmitting section that transmits the first encoded stereo data or the second encoded stereo data. 
         [0017]    A stereo signal decoding apparatus adapts a constitution of comprising: a receiving section that obtains first encoded stereo data to be generated when a stereo signal having a first channel signal and a second channel signal is a speech part in an encoding apparatus or second encoded stereo data to be generated when the stereo signal is a non-speech part in the encoding apparatus; a first decoding section that obtains a first decoded stereo signal by decoding the first encoded stereo data; and a second decoding section that decodes the second encoded stereo data, obtaining a second decoded stereo signal having a first decoded channel signal and a second decoded channel signal, using monaural signal spectral parameters that are spectral parameters of a monaural signal obtained from encoded data generated using the first channel signal and the second channel signal, the first channel signal and the second channel signal being obtained from encoded data included in the second encoded stereo data, first channel signal information regarding the amount of variation between the spectral parameters of the monaural signal and the spectral parameters of the first channel signal, and second channel signal information regarding the amount of variation between the spectral parameters of the monaural signal and the spectral parameters of the second channel signal. 
         [0018]    A stereo signal encoding method according to an embodiment of the present invention encodes a stereo signal having a first channel signal and a second channel signal; the stereo signal encoding method has a first encoding step of generating first encoded stereo data by encoding the stereo signal when the stereo signal of the current frame is a speech part; a second encoding step of encoding the stereo signal when the stereo signal of the current frame is a non-speech part and of generating second encoded stereo data by encoding each of: monaural signal spectral parameters that are spectral parameters of a monaural signal generated using the first channel signal and the second channel signal; first channel signal information regarding the amount of variation between the spectral parameters of the monaural signal and the spectral parameters of the first channel signal; and second channel signal information regarding the amount of variation between the spectral parameters of the monaural signal and the spectral parameters of the second channel signal; and a transmitting step of transmitting the first encoded stereo data or the second encoded stereo data. 
         [0019]    A stereo signal decoding method according to an embodiment of the present invention has a receiving step of obtaining first encoded stereo data to be generated when a stereo signal having a first channel signal and a second channel signal is a speech part in an encoding apparatus or second encoded stereo data to be generated when the stereo signal is a non-speech part in the encoding apparatus; a first decoding step of obtaining a first decoded stereo signal by decoding the first encoded stereo data; and a second decoding step of decoding the second encoded stereo data. obtaining a second decoded stereo signal having a first decoded channel signal and a second decoded channel signal, using monaural signal spectral parameters that are spectral parameters of a monaural signal generated using the first channel signal and the second channel signal, the first channel signal and the second channel signal being obtained from encoded data included in the second encoded stereo data, first channel signal information regarding the amount of variation between the spectral parameters of the monaural signal and the spectral parameters of the first channel signal, and second channel signal information regarding the amount of variation between the spectral parameters of the monaural signal and the spectral parameters of the second channel signal. 
       Advantageous Effects of Invention 
       [0020]    According to the present invention, in applying an intermittent transmission technique to a stereo signal, the bit rate can be reduced, without reducing the quality. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0021]      FIG. 1  is a block diagram showing the constitution of a stereo signal encoding apparatus according to Embodiment 1 of the present invention; 
           [0022]      FIG. 2  is a block diagram showing the constitution of a stereo signal decoding apparatus according to Embodiment 1 of the present invention; 
           [0023]      FIG. 3  is a block diagram showing the internal constitution of a stereo DTX encoding section according to Embodiment 1 of the present invention; 
           [0024]      FIG. 4  is a block diagram showing the internal constitution of a stereo DTX decoding section according to Embodiment 1 of the present invention; 
           [0025]      FIG. 5  is a block diagram showing the constitution of a stereo DTX encoding section according to Embodiment 2 of the present invention; 
           [0026]      FIG. 6  is a block diagram showing the constitution of a stereo DTX decoding section according to Embodiment 2 of the present invention; 
           [0027]      FIG. 7  is a drawing showing the relationship of correspondence of the frame energy difference between the channels and deformation coefficients for each channel according to Embodiment 2 of the present invention; 
           [0028]      FIG. 8  is a block diagram showing the constitution of a stereo DTX encoding section according to Embodiment 3 of the present invention; and 
           [0029]      FIG. 9  is a block diagram showing the constitution of a stereo DTX decoding section according to Embodiment 3 of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0030]    Embodiments of the present invention will now be described in detail, with reference to the accompanying drawings. 
       Embodiment 1 
       [0031]      FIG. 1  is a block diagram showing the constitution of stereo signal encoding apparatus  100  according to Embodiment 1 of the present invention. 
         [0032]    Stereo signal encoding apparatus  100  is mainly constituted by VAD (voice active detector) section  101 , switching sections  102  and  105 , stereo encoding section  103 , stereo DTX encoding section  104 , and multiplexing section  106 . Stereo signal encoding apparatus  100  forms frames of a stereo signal at a prescribed time interval (for example, 20 ms), and encodes the stereo signal in units of the frames. Each of the constituent elements will be described in detail below. 
         [0033]    VAD section  101  analyzes an input signal (a stereo signal formed by an L-channel signal and an R-channel signal) and judges whether the input signal of the current frame is a speech part or a non-speech part. A non-speech part corresponds to an inactive speech part, which, because the signal amplitude value is extremely small, is sensed as inactive speech by the sense of hearing, a background noise part or the like, which is typified by environmental sounds that are perceived in everyday life (operation sounds of ducts or the traveling sounds of vehicles), or the like. In the following, a background noise part will be described as a typical non-speech part. In this analysis, at least the signal energy is used. As a result of the analysis, if VAD section  101  judges the input signal of the current frame to be a speech part, it generates VAD data indicating that the input signal of the current frame is a speech part, and if VAD section  101  judges the input signal of the current frame to be a background noise part, it generates VAD data indicating that the input signal of the current frame is a background noise part. VAD section  101  outputs the generated VAD data to switching sections  102  and  105  and to multiplexing section  106 . 
         [0034]    Switching section  102 , in accordance with the VAD data input from VAD section  101 , switches the output destination of the input signal (stereo signal) between stereo signal encoding section  103  and stereo DTX encoding section  104 . Specifically, if the VAD data indicates a speech part, switching section  102  switches the output destination to stereo encoding section  103  and outputs the input signal to stereo encoding section  103 . If, however, the VAD data indicates a background noise part, switching section  102  switches the output destination to stereo DTX encoding section  104  and outputs the input signal to stereo DTX encoding section  104 . 
         [0035]    Stereo encoding section  103  encodes the input signal (speech part) input from switching section  102 . Specifically, stereo encoding section  103  uses the correlation between the L-channel signal and the R-channel signal that constitute the stereo signal to encode the stereo signal. The method indicated in Non-Patent Literature 1, for example, is used as the method of encoding the above-noted stereo signal. Stereo encoding section  103  outputs the encoded stereo data generated by encoding processing to switching section  105 . 
         [0036]    Stereo DTX encoding section  104  encodes the input signal (background noise part) input from switching section  102 . For example, stereo DTX encoding section  104  performs encoding processing one time for each prescribed number of frames (for example, eight frames). This is because it is assumed that there is little time variation of the characteristics of background noise. As a result, the bit rate can be further reduced. Stereo DTX encoding section  104  outputs the encoded stereo data generated by encoding processing to multiplexing section  106 , via switching section  105 . For frames for which encoding processing does not operate, stereo DTX encoding section  104  outputs to switching section  105  an SID that is a specific code (for example, silence description) indicating that encoding processing has not been done as encoded stereo data. The encoding processing in stereo DTX encoding section  104  will be described later in detail. 
         [0037]    Switching section  105 , similar to switching section  102 , in accordance with the VAD data input from VAD section  101 , switches the input source of the encoded stereo data between stereo encoding section  103  and stereo DTX encoding section  104 . Specifically, if the VAD data indicates a speech part, switching section  105  switches the input source to stereo encoding section  103 , and outputs the encoded stereo data generated by the stereo encoding section  103  to multiplexing section  106 . If, however, the VAD data indicates a background noise part, switching section  105  switches the input source to stereo DTX encoding section  104 , and outputs the encoded stereo data generated by the stereo DTX encoding section  104  to multiplexing section  106 . 
         [0038]    Multiplexing section  106  multplexes the VAD data input from VAD section  101  and the encoded stereo data input from switching section  105  to generate multiplexed data. By doing this, the multiplexed data is transmitted to the stereo signal decoding apparatus. 
         [0039]    The above completes the description of the constitution of stereo signal encoding apparatus  100 . 
         [0040]    Next, a stereo signal decoding apparatus  200  according to the present embodiment will be described, using  FIG. 2 , which is a block diagram showing the constitution of stereo signal decoding apparatus  200 . 
         [0041]    Stereo signal decoding apparatus  200  is mainly constituted by demultiplexing section  201 , switching sections  202  and  205 , stereo decoding section  203 , and stereo DTX decoding section  204 . Each of the constituent elements will be described in detail below. 
         [0042]    Demultplexing section  201  receives the input multiplexed data, and demultiplexes it into VAD data and encoded stereo data. Demultipexing section  201  outputs the VAD data to switching sections  202  and  205  and outputs the encoded stereo data to switching section  202 . 
         [0043]    In accordance with the VAD data (data indicating that the input signal of the current frame is either a speech part or a background noise part) input from demultipexing section  201 , switching section  202  switches the output destination of the encoded stereo data between stereo decoding section  203  and stereo DTX decoding section  204 . Specifically, if the VAD data indicates a speech part, switching section  202  switches the output destination to stereo decoding section  203  and outputs the encoded stereo data to stereo decoding section  203 . If, however, the VAD data indicates a background noise part, switching section  202  switches the output destination to stereo DTX decoding section  204  and outputs the encoded stereo data to stereo DTX decoding section  204 . 
         [0044]    Stereo decoding section  203  decodes the encoded stereo data input from switching section  202  (that is, the encoded stereo data generated in stereo signal encoding apparatus  100  when the stereo signal is a speech part) to generate a decoded stereo signal (decoded L-channel signal and decoded R-channel signal). Stereo decoding section  203  then outputs the generated decoded stereo signal to switching section  205 . 
         [0045]    Stereo DTX decoding section  204  decodes the encoded stereo data input from switching section  202  (that is, the encoded stereo data generated in stereo signal encoding apparatus  100  when the stereo signal is a background noise part) to generate a decoded stereo signal (decoded L-channel signal and decoded R-channel signal). Stereo DTX decoding section  204  then outputs the generated decoded stereo signal to switching section  205 . As described above, because stereo DTX encoding section  104  ( FIG. 1 ) performs encoding processing at a rate of one time each prescribed number of frames (for example, eight frames), stereo DTX decoding section  204  receives the encoded stereo data at a rate of one time every prescribed number of frames (for example, eight frames), and receives SID (silence description) for other frames, that is, frames for which the encoding processing did not operate. Upon receiving the SID, stereo DTX decoding section  204  uses the recently received encoded stereo data to perform decoding processing to generate a decoded stereo signal. That is, stereo DTX decoding section  204  uses the received encoded stereo data continuously for a prescribed number of frames (for example, eight frames). The decoding processing in stereo DTX decoding section  204  will be described later in detail. 
         [0046]    Switching section  205 , similar to switching section  202 , in accordance with the VAD data input from demultipexing section  201 , switches the input source of the decoded stereo signal between stereo decoding section  203  and stereo DTX decoding section  204 . Specifically, if the VAD data indicates a speech part, switching section  205  switches the input source to stereo decoding section  203  and outputs the decoded stereo signal generated by the stereo decoding section  203 . If, however, the VAD data indicates a background noise part, switching section  205  switches the input source to stereo DTX decoding section  204  and outputs the decoded stereo signal generated by stereo DTX decoding section  204 . 
         [0047]    The above completes the description of the constitution of stereo signal decoding apparatus  200 . 
         [0048]    Next, the constitution of stereo DTX encoding section  104  in stereo signal encoding apparatus  100  will be described, using  FIG. 3 . In the following description, LSP (line spectral pair) parameters are assumed to be used as the spectral parameters for each signal. For example, the LSP parameters of the signals are determined by converting the LPC coefficients obtained by LPC analysis of the signals. However, the spectral parameters that are used are not restricted to being the LSP parameter, and may be LSF (line spectral frequency) parameters, ISF (immitance spectral frequency) parameters, or the like. 
         [0049]      FIG. 3  is a block diagram showing the internal constitution of stereo DTX encoding section  104 . 
         [0050]    Stereo DTX encoding section  104  is mainly constituted by frame energy encoding sections  301  and  302 , spectral parameter analysis sections  303  and  304 , average spectrum parameter calculation section  305 , average spectral parameter quantization section  306 , average spectral parameter decoding section  307 , error spectral parameter calculation sections  308  and  309 , error spectral parameter quantization sections  310  and  311 , and multiplexing section  312 . Each of the constituent elements will be described in detail below. 
         [0051]    Frame energy encoding section  301  determines the frame energy of the input L-channel signal and generates quantized L-channel signal frame energy information by performing scalar quantization (encoding) of the frame energy. Frame energy encoding section  301  then outputs the quantized L-channel signal frame energy information to multiplexing section  312 . 
         [0052]    Frame energy encoding section  302  determines the frame energy of the input R-channel signal and generates quantized R-channel signal frame energy information by performing scalar quantization (encoding) of the frame energy. Frame energy encoding section  302  then outputs the quantized R-channel signal frame energy information to multiplexing section  312 . 
         [0053]    Spectral parameter analysis section  303  performs LPC analysis of the input L-channel signal to generate LSP parameters indicating the spectral characteristics of the L-channel signal. Spectral parameter analysis section  303  then outputs the L-channel signal LSP parameters to average spectral parameter calculation section  305  and error spectral parameter calculation section  308 . 
         [0054]    Spectral parameter analysis section  304 , similar to spectral parameter analysis section  303 , performs LPC analysis of the input R-channel signal to generate LSP parameters indicating the spectral characteristics of the R-channel signal. Spectral parameter analysis section  304  then outputs the R-channel signal LSP parameters to average spectral parameter calculation section  305  and error spectral parameter calculation section  309 . 
         [0055]    Average spectral parameter calculation section  305  calculates the average spectral parameters, using the L-channel signal LSP parameters and the R-channel signal LSP parameters. Average spectral parameter calculation section  305  then outputs the average spectral parameters to average spectral parameter quantization section  306 . 
         [0056]    For example, average spectral parameter calculation section  305  calculates the average spectral parameters LSP m (i) in accordance with the following Equation (1). 
         [0000]    
       
         
           
             
               
                 
                   
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         [0057]    In the above, LSP L (i) indicates the LSP parameters of the L-channel signal, LSP R (i) indicates the LSP parameters of the R-channel signal, and N LSP  indicates the order of the LSP parameters. 
         [0058]    Average spectral parameter calculation section  305  may calculate the average spectral parameters based on the L-channel signal energy and the R-channel signal energy, as shown in the following Equation (2). 
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         [0059]    In the above, w indicates weighting that is determined based on the L-channel signal energy E L  and the R-channel signal energy E R , and set with respect to the calculated average spectral parameters LSP m (i) so that the influence of LSP parameters for the channel having a large energy becomes large. For example, w is calculated by the following Equation (3). 
         [0000]      [Eq. 3] 
         [0000]        w=E   L /( E   L   +E   R )  (3)
 
         [0060]    Stated differently, average spectral parameter calculation section  305  calculates the average of the L-channel signal LSP parameters and the R-channel signal LSP parameters as the LSP parameters of a monaural signal generated from the L-channel signal and the R-channel signal. Average spectral parameter calculation section  305  may down-mix the L-channel signal and the R-channel signal to generate a monaural signal and take the LSP parameters calculated from this monaural signal (monaural signal LSP parameters) as the average spectral parameters. 
         [0061]    Average spectral parameter quantization section  306 , based on vector quantization, scalar quantization, or a quantization method that is a combination thereof, quantizes (encodes) the average spectral parameters. Average spectral parameter quantization section  306  outputs the quantized average spectral parameter information determined by quantization processing to average spectral parameter decoding section  307  and multiplexing section  312 . 
         [0062]    Average spectral parameter decoding section  307  decodes the quantized average spectral parameter information (that is, the encoded data of the average spectral parameters) to generate decoded average spectral parameters. Average spectral parameter decoding section  307  then outputs the decoded average spectral parameters to error spectral parameter calculation sections  308  and  309 . 
         [0063]    Error spectral parameter calculation section  308  subtracts the decoded average spectral parameters from the L-channel signal LSP parameters to calculate the L-channel signal error spectral parameters. Error spectral parameter calculation section  308  then outputs the L-channel signal error spectral parameters to error spectral parameter quantization section  310 . 
         [0064]    Error spectral parameter calculation section  309  subtracts the decoded average spectral parameters from the R-channel signal LSP parameters to calculate the R-channel signal error spectral parameters. Error spectral parameter calculation section  309  then outputs the R-channel signal error spectral parameters to error spectral parameter quantization section  311 . 
         [0065]    Error spectral parameter quantization section  310 , based on vector quantization, scalar quantization, or a quantization method that is a combination thereof, quantizes (encodes) the L-channel signal error spectral parameters. Error spectral parameter quantization section  310  then outputs the quantized L-channel signal error spectral parameter information to multiplexing section  312 . 
         [0066]    Error spectral parameter quantization section  311 , similar to the error spectral parameter quantization section  310 , quantizes (encodes) the R-channel signal error spectral parameters. Error spectral parameter quantization section  311  then outputs the quantized R-channel signal error spectral parameter information to multiplexing section  312 . 
         [0067]    Multiplexing section  312  multiplexes the quantized L-channel signal frame energy information, the quantized R-channel signal frame energy information, the quantized average spectral parameter information, the quantized L-channel signal error spectral parameter information, and the quantized R-channel signal error spectral parameter information to generate encoded stereo data. Multiplexing section  312  then outputs the encoded stereo data to switching section  105  ( FIG. 1 ). In stereo DTX encoding section  104 , the multiplexing section  312  is not an essential constituent element. For example, quantized L-channel signal frame energy information, the quantized R-channel signal frame energy information, the quantized average spectral parameter information, the quantized L-channel signal error spectral parameter information, and the quantized R-channel signal error spectral parameter information may be directly output as encoded stereo data to switching section  105  ( FIG. 1 ) from the constituent elements that generate each of the data. 
         [0068]    The above completes the description of the constitution of stereo DTX encoding section  104 . 
         [0069]    Next, the constitution of stereo DTX decoding section  204  in stereo signal decoding apparatus  200  will be described, using  FIG. 4 , which is a block diagram showing the internal constitution of stereo DTX decoding section  204 . 
         [0070]    Stereo DTX decoding section  204  is mainly constituted by demultiplexing section  401 , frame gain decoding sections  402  and  403 , average spectral parameter decoding section  404 , error spectral parameters decoding sections  405  and  406 , spectral parameter generation sections  407  and  408 , excitation generation sections  409  and  412 , multiplication sections  410  and  413 , and synthesis filter sections  411  and  414 . Each of the constituent elements will be described in detail below. 
         [0071]    Demultiplexing section  401  demultiplexer the encoded stereo data input from switching section  202  ( FIG. 2 ) into the quantized L-channel signal frame energy information, the quantized R-channel signal frame energy information, the quantized average spectral parameter information, the quantized L-channel signal error spectral parameter information, and the quantized R-channel signal error spectral parameter information. Demultiplexing section  401  then outputs the quantized L-channel signal frame energy information to frame gain encoding section  402 , the quantized R-channel signal frame information to frame gain encoding section  403 , the quantized average spectral parameter information to average spectral parameter decoding section  404 , the quantized L-channel signal error spectral parameter information to error spectral parameter decoding section  405 , and the quantized R-channel signal error spectral parameter information to error spectral parameter decoding section  406 . 
         [0072]    In stereo DTX decoding section  204 , demultiplexing section  401  is not an essential constituent element. For example, by the demultiplexing processing in demultiplexing section  201  shown in  FIG. 2 , the quantized L-channel signal frame energy information, the quantized R-channel signal frame energy information, the quantized average spectral parameter information, the quantized L-channel signal error spectral parameter information, and the quantized R-channel signal error spectral parameter information may be obtained and each of these data may be directly output to frame gain decoding section  402  and  403 , average spectral parameter decoding section  404 , and error spectral parameter decoding section  405  and  406 , respectively. 
         [0073]    Frame gain decoding section  402  decodes the quantized L-channel signal frame energy information and outputs the obtained decoded L-channel signal frame energy to multiplication section  410 . 
         [0074]    Frame gain decoding section  403  decodes the quantized R-channel signal frame energy information and outputs the obtained decoded R-channel signal frame energy to multiplication section  413 . 
         [0075]    Average spectral parameter decoding section  404  decodes the quantized average spectral parameter information and outputs the obtained decoded average spectral parameters to spectral parameter generation sections  407  and  408 . 
         [0076]    Error spectral parameter decoding section  405  decodes the quantized L-channel signal error spectral parameter information and outputs the obtained decoded L-channel signal error spectral parameters to spectral parameter generation section  407 . 
         [0077]    Error spectral parameter decoding section  406  decodes the quantized R-channel signal error spectral parameter information and outputs the obtained decoded R-channel signal error spectral parameters to spectral parameter generation section  408 . 
         [0078]    Spectral parameter generation section  407  uses the decoded average spectral parameters and the decoded L-channel signal error spectral parameters to generate the decoded L-channel signal spectral parameters. Spectral parameter generation section  407  then converts the generated decoded L-channel signal spectral parameters to decoded L-channel signal LPC coefficients and outputs the obtained decoded L-channel signal LPC coefficients to synthesis filter section  411 . 
         [0079]    For example, spectral parameter generation section  407 , in accordance with the following Equation (4), uses the decoded average spectral parameters LSP qm (i) and the decoded L-channel signal error spectral parameters ELSP qL (i) to generate the decoded L-channel signal spectral parameters LSP qL (i). 
         [0000]      [Eq. 4] 
         [0000]        LSPq   L ( i )= LSPq   m ( i )+ ELSPq   L ( i ) i= 0 , . . . ,N   LSP −1  (4)
 
         [0080]    Spectral parameter generation section  408  uses the decoded average spectral parameters and the decoded R-channel signal error spectral parameters to generate the decoded R-channel signal spectral parameters. Spectral parameter generation section  408  then converts the generated decoded R-channel signal spectral parameters to decoded R-channel signal LPC coefficients and outputs the obtained decoded R-channel signal LPC coefficients to synthesis filter section  414 . 
         [0081]    For example, spectral parameter generation section  408 , in accordance with the following Equation (5), uses the decoded average spectral parameters LSP qm (i) and the decoded R-channel signal error spectral parameters ELSP qR (i) to generate the decoded R-channel signal spectral parameters LSP qR (i). 
         [0000]      [Eq. 5] 
         [0000]        LSPq   R ( i )= LSPq   m ( i )+ ELSPq   R ( i ) i= 0 , . . . ,N   LSP −1  (5)
 
         [0082]    Excitation generation section  409 , multiplication section  410 , and synthesis filter  411  are constituent elements corresponding to the L-channel signal. 
         [0083]    Excitation generation section  409  generates an excitation signal represented by a random signal or a limited number of pulses and outputs the excitation signal to multiplication section  410 . Normalization is done so that the frame energy of the excitation signal is 1. 
         [0084]    Multiplication section  410  multiplies the excitation signal by the decoded L-channel signal frame energy and outputs the multiplication result to synthesis filter section  411 . 
         [0085]    Synthesis filter section  411  has a synthesis filter constituted by the decoded L-channel signal LPC coefficients input from spectral parameter generation section  407  and passes the multiplication result input from the multiplication section  410  (the excitation signal multiplied by the decoded L-channel signal frame energy) through the synthesis filter to generate a decoded L-channel signal. This decoded L-channel signal is output as the output signal. 
         [0086]    Excitation generation section  412 , multiplication section  413 , and synthesis filter  414  are constituent elements corresponding to the R-channel signal. 
         [0087]    Excitation generation section  412  generates an excitation signal represented by a random signal or a limited number of pulses and outputs the excitation signal to multiplication section  413 . Normalization is done so that the frame energy of the excitation signal is 1. 
         [0088]    Multiplication section  413  multiplies the excitation signal by the decoded R-channel signal frame energy and outputs the multiplication result to synthesis filter section  414 . 
         [0089]    Synthesis filter section  414  has a synthesis filter constituted by the decoded R-channel signal LPC coefficients input from spectral parameter generation section  408  and passes the multiplication result input from the multiplication section  413  (the excitation signal multiplied by the decoded R-channel signal frame energy) through the synthesis filter to generate a decoded R-channel signal. This decoded R-channel signal is output as the output signal. 
         [0090]    In this manner, when the stereo signal of the current frame is a background noise part, stereo signal encoding apparatus  100  generates, as encoded stereo data, encoded average spectral data, which is the average of spectral parameters of the L-channel signal and the spectral parameters of the R-channel signal (that corresponds to the encoded data of the LPC coefficients of a monaural signal); encoded data of the varying component (error) between the average spectral parameters and the LSP parameters of the L-channel signal; and encoded data of the varying component (error) between the average spectral parameters and the LSP parameters of the R-channel signal. 
         [0091]    That is, even if the spectral profile of the background noise signal is represented by LPC coefficients, rather than encoding the LPC coefficients of the L-channel signal and the LPC coefficients of the R-channel signal, in addition to the encoded data of the LPC coefficients of a monaural signal, stereo signal encoding apparatus  100  adds, as information added to the LPC coefficients of the monaural signal, the difference (amount of variation) between the LSP parameters of the monaural signal and the LSP parameters of the L-channel signal (information regarding the L-channel signal) and the difference (amount of variation) between the LSP parameters of the monaural signal and the LSP parameters of the R-channel signal (information regarding the R-channel signal). Stated differently, stereo signal encoding apparatus  100  uses the correlation between the LPC coefficients of the monaural signal and the LPC coefficients of the L-channel signal and the correlation between the LPC coefficients of the monaural signal and the LPC coefficients of the R-channel signal to encode the stereo signal. 
         [0092]    Because it is sufficient to encode only the LPC coefficients of the monaural signal and added information regarding the monaural signal and each channel signal, the bit rate can be reduced, compared to the case of encoding LPC coefficients for two channels (L channel and R channel). 
         [0093]    Also, when the stereo signal of the current frame is a background noise part, stereo signal decoding apparatus  200  obtains a decoded stereo signal that is made up of a decoded L-channel signal and a decoded R-channel signal, using encoded data of the average spectral parameters (that corresponds to the encoded data of the LPC coefficients of a monaural signal); encoded data of the varying component (error) between the average spectral parameters and the LSP parameters of the L-channel signal; and encoded data of the varying component (error) between the average spectral parameters and the LSP parameters of the R-channel signal, which are included in the encoded stereo data. 
         [0094]    As a result, using the LPC coefficients of the monaural signal and the information added to the LPC coefficients of the monaural signal (varying component of LSP parameters of the monaural signal and the LSP parameters of each channel signal), the LPC coefficients of the L-channel signal and the LPC coefficients of the R-channel signal are obtained. This enables the achievement of the same quality as the case of receiving the LPC coefficients for two channels (L channel and R channel). 
         [0095]    Thus, according to the present embodiment, in applying an intermittent transmission technique to a stereo signal, the bit rate can be reduced, without reducing the quality. 
       Embodiment 2 
       [0096]      FIG. 5  is a block diagram showing the internal constitution of stereo DTX encoding section  104  of stereo signal encoding apparatus  100  ( FIG. 1 ) according to Embodiment 2 of the present invention. 
         [0097]    Stereo DTX encoding section  104  shown in  FIG. 5  is mainly constituted by frame energy encoding sections  301  and  302 , monaural signal generation section  501 , spectral parameter analysis section  502 , spectral parameter quantization section  503 , and multiplexing section  312 . Each of the constituent elements will be described below in detail. In  FIG. 5 , parts having the same constitution as in  FIG. 3  are assigned the same reference signs, and the description thereof will be omitted. 
         [0098]    Monaural signal generation section  501  down-mixes the L-channel signal and the R-channel signal making up a stereo signal to generate a monaural signal. Monaural signal generation section  501  then outputs the generated monaural signal to spectral parameter analysis section  502 . 
         [0099]    Spectral parameter analysis section  502  performs LPC analysis of the monaural signal to generate LSP parameters that indicate the spectral characteristics of the monaural signal. The LSP parameters of a monaural signal can be determined, for example, by converting the LPC coefficients obtained by analysis with respect to the monaural signal. Spectral parameter analysis section  502  then outputs the LSP parameters of the monaural signal to spectral parameter quantization section  503 . 
         [0100]    Spectral parameter quantization section  503 , based on vector quantization, scalar quantization, or a quantization method that is a combination thereof, quantizes (encodes) the LSP parameters of the monaural signal. Spectral parameter quantization section  503  outputs the quantized monaural signal spectral parameter information determined by quantization processing to multiplexing section  312 . 
         [0101]    The above completes the description of the constitution of stereo DTX encoding section  104 . 
         [0102]    Next, the constitution of stereo DTX decoding section  204  of stereo signal decoding apparatus  200  ( FIG. 2 ) of Embodiment 2 of the present invention will be described, using  FIG. 6 , which is a block diagram of the internal constitution of stereo DTX decoding section  204  according to Embodiment 2 of the present invention. 
         [0103]    Stereo DTX decoding section  204  shown in  FIG. 6  is mainly constituted by demultiplexing section  401 , frame gain decoding sections  402  and  403 , spectral parameter decoding section  601 , frame gain comparison  602 , spectral parameter generation sections  603  and  604 , excitation generation sections  409  and  412 , multiplication sections  410  and  413 , and synthesis filter sections  411  and  414 . Each of the constituent elements will be described below in detail. In  FIG. 6 , parts having the same constitution as in  FIG. 4  are assigned the same reference signs, and the description thereof will be omitted. 
         [0104]    Spectral parameter decoding section  601  decodes the quantized monaural signal spectral parameter information to obtain the monaural signal spectral parameters, and outputs the monaural signal spectral parameters to spectral parameter generation sections  603  and  604 . 
         [0105]    Frame gain comparison section  602  compares the decoded L-channel signal frame energy and the decoded R-channel signal frame energy and, in according to the comparison result, determines deformation coefficients for deforming at least one of the decoded L-channel signal LPC coefficients and the decoded R-channel signal LPC coefficients. 
         [0106]    Spectral parameter generation section  603  converts the monaural signal spectral parameters to monaural signal LPC coefficients and calculates the decoded L-channel signal LPC coefficients (deformed LPC coefficients) to be used in the synthesis filter section  411 , using the monaural signal LPC coefficients and the deformation coefficients corresponding to the L-channel signal. 
         [0107]    Similar to spectral parameter generation section  603 , spectral parameter generation section  604  converts the monaural signal spectral parameters to monaural signal LPC coefficients, and calculates the decoded R-channel signal LPC coefficients (deformed LPC coefficients) to be used in synthesis filter section  414 , using the monaural signal LPC coefficients and the deformation coefficients corresponding to the R-channel signal. 
         [0108]    In this manner, spectral parameter generation sections  603  and  604  calculate the decoded L-channel signal LPC coefficients and the decoded R-channel signal LPC coefficients to be used, respectively, in the synthesis filter sections  411  and  414 , using the deformation coefficients obtained based on the comparison result at frame gain comparison section  602  and the monaural signal spectral parameters. 
         [0109]    In this case, the description has been for the case in which it is the frame gain comparison section  602  that determines the deformation coefficients in accordance with the comparison result. This is not a restriction, however; for example, spectral parameter generation sections  603  and  604  may determine the deformation coefficients in accordance with the comparison result input from the frame gain comparison section  602 . 
         [0110]    For example, let the deformation coefficients for deforming the decoded L-channel signal LPC coefficients LPC L (i) be α L  and let the deformation coefficients for deforming the decoded R-channel signal LPC coefficients LPC R (i) be α R . In this case, it is assumed that 0.0≦α L ≦1.0 and 0.0≦α R ≦1.0. In this case, the synthesis filters H L (Z) and H R (Z) that correspond, respectively, to the L-channel signal and the R-channel signal are represented by the following Equation (6) and Equation (7). 
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         [0111]    In the above, N LPC  is the order of the LPC coefficients. That is, the LPC coefficients of the signals of each channel are deformed by the deformation coefficients α, as shown in Equations (6) and (7). 
         [0112]    The deformation coefficients α L  and α R  may be formed, for example, by the method of using the following Equations (8). 
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         [0113]    The intention of this is to make the LPC coefficients of the channel having the smaller frame energy approach (flatten to) white noise. 
         [0114]    Specifically, if the decoded L-channel signal frame energy E L  is 10 dB larger than the decoded R-channel signal frame energy E R  (upper line in Equation (8)), the decoded L-channel signal LPC coefficients LPC L (i) are not deformed (α L =1.0), and the decoded R-channel signal LPC coefficients LPC R (i) are made smaller (α R =0.8). That is, deformation is applied in the direction that increases the degree of making the decoded R-channel signal LPC coefficients LPC R (i) white. 
         [0115]    If, however, the decoded R-channel signal frame energy E R  is 10 dB larger than the decoded L-channel signal frame energy E L  (lower line in Equation (8)), the decoded R-channel signal LPC coefficients LPC R (i) are not deformed (α R =1.0), and the decoded L-channel signal LPC coefficients LPC L (i) are made smaller (α L =0.8). That is, deformation is applied in the direction that increases the degree of making the decoded L-channel signal LPC coefficients LPC L (i) white. 
         [0116]    That is, if the difference between the decoded L-channel signal frame energy and the decoded R-channel signal frame energy exceeds a threshold (in this case, 10 dB), stereo DTX decoding section  204  applies deformation to the LPC coefficients of the channel signal having the smaller frame energy between the decoded L-channel signal LPC coefficients and the decoded R-channel signal coefficients in the direction that increases the degree of making those LPC coefficients white. 
         [0117]    In cases other than the above (that is, if the energy difference is within 10 dB, shown by the middle line in Equation (8)), the LPC coefficients of neither channel signal are deformed (α L =α R =1.0). 
         [0118]    The method of determining the above-noted deformation coefficients α L  and α R  is based on the following idea. 
         [0119]    It is possible to judge that, compared to the channel having a large frame energy, the channel having a small frame energy is farther away from the source of the background noise. When the distance from the source of background noise becomes large, there is a tendency to be influenced by external perturbation (for example, reflection from a wall or other noise) from the source up until reaching the microphone, so that the spectrum approaches white noise. Thus, even if added information representing the L-channel signal LPC coefficients and the R-channel signal LPC coefficients is not encoded at the encoder side, by making the LPC coefficients of the channel having small frame energy (the channel that is distant from the source of the background noise) approach white (flatten), high-quality background noise can be generated. 
         [0120]    Finer setting can be made of this correspondence between the frame energy and the LPC coefficients (deformation coefficients).  FIG. 7  shows an example of the correspondence between the frame energy and the LPC coefficients (deformation coefficients). In  FIG. 7 , the broken line shows the value of the deformation coefficients α L  (the range from 0.0 to 1.0) and the solid line shows the value of the deformation coefficients α R  (the range from 0.0 to 1.0). 
         [0121]    As shown in  FIG. 7 , the larger the decoded L-channel signal frame energy E L  is with respect to the decoded R-channel signal frame energy E R  (the larger log 10  (E L /E R ) is), the greater is the deformation that increases making the decoded R-channel signal LPC coefficients white (that is, the smaller the deformation coefficients α R  are made). 
         [0122]    In contrast, the larger the decoded R-channel signal frame energy E R  is with respect to the decoded L-channel signal frame energy E L  (the smaller log 10  (E L /E R ) is), the greater is the deformation that increases making the decoded L-channel signal LPC coefficients white (that is, the smaller the deformation coefficients α L  are made). 
         [0123]    That is, the larger is the difference between the decoded L-channel signal frame energy and the decoded R-channel signal frame energy, the stereo DTX decoding section  204  applies greater deformation to the LPC coefficients of the channel signal having the smaller frame energy between the decoded L-channel signal LPC coefficients and the decoded R-channel signal LPC coefficients, in the direction that increases the degree of making those LPC coefficients white. 
         [0124]    Further, if the difference between the decoded L-channel signal frame energy E L  and the decoded R-channel signal frame energy E R  exceeds 50 dB, the LPC coefficients of the channel signal with the smaller frame energy becomes completely flat. 
         [0125]    In this manner, in the present embodiment, stereo signal encoding apparatus  100  encodes the monaural signal LPC coefficients, the L-channel signal frame energy, and the R-channel signal frame energy. Then, based on the relationship between the frame energies of the received L-channel signal and R-channel signal, stereo signal decoding apparatus  200  deforms the LPC coefficients of the monaural signal so as to generate the decoded L-channel signal LPC coefficients and the decoded R-channel signal LPC coefficients. 
         [0126]    That is, even if the spectral profile of the background noise signal is represented by LPC coefficients, rather than encoding the LPC coefficients of the L-channel signal and the LPC coefficients of the R-channel signal, in addition to the encoded data of the LPC coefficients of a monaural signal, stereo signal encoding apparatus  100  adds, as information added to the LPC coefficients of the monaural signal, the frame energy of the L-channel signal (information regarding the L-channel signal) and the frame energy of the R-channel signal (information regarding the R-channel signal). 
         [0127]    If the present embodiment is compared to Embodiment 1, the encoded data of the frame energies of each channel signal are transmitted from the encoder side to the decoder in both embodiments. In the present embodiment, however, the encoded data of the frame energies of each channel signal is further used as information added to the monaural signal LPC coefficients. As a result, in the stereo signal decoding apparatus  100 , it is not necessary to encode added information that is required to express the LPC coefficients of the channel signals (in Embodiment 1, varying components between the monaural signal LPC coefficients and LPC coefficients of each of the channel signals). 
         [0128]    Stereo signal encoding apparatus  200  applies deformation to the LPC coefficients of the channel signal having the smaller frame energy between the channel signals constituting the stereo signal, in the direction that increases the degree of making those coefficients white. This enables generation of high-quality background noise, even if only the LPC coefficients of the monaural signal are received. 
         [0129]    Thus, in the present embodiment, even when only the LPC coefficients of a monaural signal are transmitted, high-quality background noise can be generated, and also the bit rate can be reduced further, relative to Embodiment 1. 
       Embodiment 3 
       [0130]      FIG. 8  is a block diagram showing the internal constitution of stereo DTX encoding section  104  of stereo signal encoding apparatus  100  ( FIG. 1 ) according to Embodiment 3 of the present invention. 
         [0131]    Stereo DTX encoding section  104  shown in  FIG. 8  is mainly constituted by frame energy encoding sections  301  and  302 , monaural signal generating section  501 , spectral parameter analysis section  502 , spectral parameter quantization section  503 , spectral parameter analysis sections  701  and  702 , spectral parameter decoding section  703 , frame gain decoding sections  704  and  705 , frame gain comparison section  706 , spectral parameter estimation section  707 , error spectral parameter calculation sections  708  and  709 , error spectral parameter quantization sections  710  and  711 , and multiplexing section  312 . Each of the constituent elements will be described in detail below. In  FIG. 8 , parts having the same constitution as in  FIG. 5  are assigned the same reference signs, and the description thereof will be omitted. 
         [0132]    Spectral parameter analysis section  701  performs LPC analysis of the input L-channel signal, generates and outputs to error spectral parameter calculation section  708  LSP parameters indicating the spectral characteristics of the L-channel signal. 
         [0133]    Spectral parameter analysis section  702  performs LPC analysis of the input R-channel signal, generates and outputs to error spectral parameter calculation section  709  LSP parameters indicating the spectral characteristics of the R-channel signal. 
         [0134]    Spectral parameter decoding section  703  decodes the quantized monaural signal spectral parameter information input from spectral parameter quantization section  503 , generates the monaural signal spectral parameters, and outputs the monaural signal spectral parameters to spectral parameter estimation section  707 . 
         [0135]    Frame gain decoding section  704  decodes the quantized L-channel signal frame energy information input from frame energy encoding section  301  and outputs the obtained decoded L-channel signal frame energy to frame gain comparison section  706 . 
         [0136]    Frame gain decoding section  705  decodes the quantized R-channel signal frame energy information input from frame energy encoding section  302  and outputs the obtained decoded R-channel signal frame energy to frame gain comparison section  706 . 
         [0137]    Frame gain comparison section  706  compares the decoded L-channel signal frame energy and the decoded R-channel signal frame energy. Then, frame gain comparison section  706 , in accordance with the comparison result, determines the deformation coefficients for deforming at least one of the decoded L-channel signal LPC coefficients and the decoded R-channel signal LPC coefficients. Frame gain comparison section  706  outputs the determined deformation coefficients to spectral parameter estimation section  707 . Because the method of determining the deformation coefficients has been described in Embodiment 2, the description thereof will be omitted. 
         [0138]    Spectral parameter estimation section  707 , using the monaural signal spectral parameters and the deformation coefficients, calculates the estimated L-channel signal spectral parameter and the estimated R-channel signal spectral parameters. Spectral parameter estimation section  707  outputs the calculated estimated L-channel signal spectral parameters to error spectral parameter calculation section  708  and outputs the estimated R-channel signal spectral parameters to error spectral parameter calculation section  709 . 
         [0139]    Spectral parameter estimation section  707  calculates the estimated L-channel signal spectral parameters and the estimated R-channel signal spectral parameters as indicated, for example, below. 
         [0140]    First, spectral parameter estimation section  707  converts the monaural signal spectral parameters to determine monaural signal LPC coefficients. Then, spectral parameter estimation section  707  imparts deformation to the monaural signal LPC coefficients, using the L-channel deformation coefficients, to determine the deformed L-channel LPC coefficients. Because the method of deformation has already been described in Embodiment 2, the description thereof will be omitted. Spectral parameter estimation section  707  converts the deformed L-channel LPC coefficients determined in this manner to spectral parameters such as LSP parameters or LSF parameters, and outputs these as the estimated L-channel signal spectral parameters to error spectral parameter calculation section  708 . 
         [0141]    Spectral parameter estimation section  707  performs the same type of processing as the L channel with respect to the R channel as well. That is, spectral parameter estimation section  707  imparts deformation to the monaural signal LPC coefficients using the deformation coefficients for the R channel to determine the deformed R-channel LPC coefficients. Spectral parameter estimation section  707  converts the R-channel LPC coefficients to determine and output to error spectral parameter calculation section  709  the estimated R-channel signal spectral parameters. 
         [0142]    Error spectral parameter calculation section  708  subtracts the estimated L-channel signal spectral parameters from the spectral parameters of the L-channel signal (LSP parameters of a L-channel signal) to calculate and output to error spectral parameter quantization section  710  the L-channel signal error spectral parameters. 
         [0143]    Error spectral parameter calculation section  709  subtracts the estimated R-channel signal spectral parameters from the spectral parameters of the R-channel signal (LSP parameters of a R-channel signal) to calculate and output to error spectral parameter quantization section  711  the R-channel signal error spectral parameters. 
         [0144]    Error spectral parameter quantization section  710 , based on vector quantization, scalar quantization, or a quantization method that is a combination thereof, quantizes (encodes) the L-channel signal error spectral parameters. Error spectral parameter quantization section  710  outputs the quantized L-channel signal error spectral parameter information determined by quantization processing to multiplexing section  312 . 
         [0145]    Error spectral parameter quantization section  711 , based on vector quantization, scalar quantization, or a quantization method that is a combination thereof, quantizes (encodes) the R-channel signal error spectral parameters. Error spectral parameter quantization section  711  outputs the quantized R-channel signal error spectral parameter information determined by quantization processing to multiplexing section  312 . 
         [0146]      FIG. 9  is a block diagram showing the internal constitution of stereo DTX decoding section  204  of stereo signal decoding apparatus  200  ( FIG. 2 ) according to Embodiment 3 of the present invention. 
         [0147]    Stereo DTX decoding section  204  shown in  FIG. 9  is mainly constituted by demultiplexing section  401 , frame gain decoding sections  402  and  403 , spectral parameter decoding section  601 , error spectral parameter decoding sections  801  and  802 , frame gain comparison section  602 , spectral parameter generation sections  803  and  804 , excitation generation sections  409  and  412 , multiplication sections  410  and  413 , and synthesis filter sections  411  and  414 . Each of the constituent elements will be described in detail below. In  FIG. 9 , parts having the same constitution as in  FIG. 6  are assigned the same reference signs, and the description thereof will be omitted. 
         [0148]    Error spectral parameter decoding section  801  decodes the quantized L-channel signal error spectral parameter information and outputs the obtained decoded L-channel signal error spectral parameters to spectral parameter generation section  803 . 
         [0149]    Error spectral parameter decoding section  802  decodes the quantized R-channel signal error spectral parameter information and outputs the obtained decoded R-channel signal error spectral parameters to spectral parameter generation section  804 . 
         [0150]    Spectral parameter generation section  803  converts the monaural signal spectral parameters to monaural signal LPC coefficients and uses the deformation coefficients for the L channel with respect to the monaural signal LPC coefficients, to determine the deformed L-channel LPC coefficients. Because the method of the deformation has been described in Embodiment 2, the description thereof will be omitted. After conversion of the deformed L-channel LPC coefficients to spectral parameters, the decoded L-channel signal error spectral parameters are added and conversion is done again to LPC coefficients. Spectral parameter generation section  803  outputs the LPC coefficients to synthesis filter section  411  as the decoded L-channel LPC coefficients. 
         [0151]    Spectral parameter generation section  804  converts the monaural signal spectral parameters to monaural signal LPC coefficients and uses the deformation coefficients for the R channel with respect to the monaural signal LPC coefficients, to determine the deformed R-channel LPC coefficients. Because the method of deformation has been described in Embodiment 2, the description thereof will be omitted. After conversion of the deformed R-channel LPC coefficients to spectral parameters, the decoded R-channel signal error spectral parameters are added and conversion is done again to LPC coefficients. Spectral parameter generation section  804  outputs the LPC coefficients to synthesis filter section  414  as the decoded R-channel LPC coefficients. 
         [0152]    In this manner, in the present embodiment, stereo signal encoding apparatus  100 , similar to Embodiment 2, estimates the L-channel signal LPC coefficients and the R-channel signal LPC coefficients from the relationship between the L-channel signal frame energy and the R-channel signal frame energy, and then encodes the error signal between these estimated values and the original signals (in this case, the L-channel signal LPC coefficients and the R-channel signal LPC coefficients). Stereo signal decoding apparatus  200  compares the frame energy of the L-channel signal with the frame energy of the R-channel signal and, using the comparison result, the monaural signal spectral parameters, the decoded L-channel signal error spectral parameters, and the decoded R-channel signal error spectral parameters, calculates the decoded L-channel signal LPC coefficients and the decoded R-channel signal LPC coefficients. 
         [0153]    That is, if spectral profile of the background noise signal is represented by LPC coefficients, similar to Embodiment 2, in addition to the encoded data of the LPC coefficients of a monaural signal, stereo signal encoding apparatus  100  adds, as information added to the LPC coefficients of the monaural signal, the frame energies of each of the L-channel signal and the R-channel signal (information regarding the L-channel signal and the R-channel signal). Additionally, in the present embodiment, stereo encoding apparatus  100  adds the difference between the L-channel signal spectral parameters (L-channel signal LPC coefficients) and the estimated L-channel signal spectral parameters (deformed L-channel LPC coefficients) (information regarding the L-channel signal) and the difference between the R-channel signal spectral parameters (R-channel signal LPC coefficients) and the estimated R-channel signal spectral parameters (deformed R-channel LPC coefficients) (information regarding the R-channel signal). 
         [0154]    In this manner, by encoding the error components of the LPC coefficients after estimation, stereo signal encoding apparatus  100  encodes efficiently with a small number of bits, and can reduce the bit rate. 
         [0155]    Stereo signal encoding apparatus  100  deforms the LPC coefficients of the channel signal having the smaller frame energy between the channel signals constituting the stereo signal, in the direction that increases the degree of making those coefficients white. As a result, even if stereo signal decoding apparatus  200  receives only the LPC coefficients for a monaural signal, high-quality background noise can be generated. 
         [0156]    Thus, in the present embodiment, even when only the LPC coefficients of a monaural signal are transmitted, high-quality background noise can be generated, and also the bit rate can be reduced. 
         [0157]    The above completes the description of the embodiments of the present invention. 
         [0158]    The present invention may be applied regardless of whether a speech signal or an audio signal is used as the input signal. 
         [0159]    The above-noted embodiments have been described for the case in which VAD data indicates a background noise part, with the switching section connecting to the stereo DTX encoding section in the stereo signal encoding apparatus and connecting to the stereo DTX decoding section in the stereo signal decoding apparatus. However, even if the VAD data indicates a non-speech part other than a background noise part (for example, an inactive speech part or the like), it is obvious that the same type of operation and effect can be exhibited. 
         [0160]    The present invention is not restricted to the above-noted embodiments, and can be subjected to various modifications. 
         [0161]    The stereo signal decoding apparatus in the above-noted embodiments performs processing using encoded data transmitted from the stereo signal encoding apparatus in the above-noted embodiments. The present invention is, however, not restricted in this manner, and as long as the encoded data includes the required parameters and data, processing is possible even if the data is not the encoded data from the stereo signal encoding apparatus in the above-noted embodiments. 
         [0162]    Also, even for the case in which a signal processing program for operation is recorded by writing it into a machine-readable recording medium such as a memory, a disk, a tape, a CD, a DVD, or the like, the present invention can be applied, and the same operation and effect as the present embodiments can be obtained. 
         [0163]    Although in the above-noted embodiments the description has been for the case of constituting the present invention with hardware, the present invention may be implemented by software in concert with hardware. 
         [0164]    Each of the functional blocks used in the descriptions of the above-noted embodiments is typically implemented by an LSI device, which is an integrated circuit. These may be made into a single separate chip, and one chip may be made to include a part or all thereof. In this case, although an LSI device is cited, depending upon the level of integration, this may be called an integrated circuit, a system LSI device, a super LSI device, or an ultra LSI device. 
         [0165]    The method of integrated circuit implementation is not restricted to large-scale integration, and implementation may be done by dedicated circuitry or a general-purpose processor. A programmable FPGA (field programmable gate array) or a reconfigurable processor, in which circuit cell connections or settings within an LSI device can be reconfigured after manufacture of an LSI device, may be used. 
         [0166]    Additionally, in the event of the appearance of integrated circuit technology taking the place of large-scale integration, either by advances in semiconductor technology or other, derivative technology, the functional blocks may be, of course, integrated using that technology. Biotechnology may also be applied. 
         [0167]    The disclosure of Japanese Patent Application No. 2010-256915, filed on Nov. 17, 2010, including the specification, drawings and abstract, is incorporated herein by reference in its entirety. 
       INDUSTRIAL APPLICABILITY 
       [0168]    The present invention is particularly suitable for use in an encoding apparatus that encodes a speech signal or an audio signal that is made up of a L-channel signal and a R-channel signal, and in a decoding apparatus that decodes the encoded signal. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           100  Stereo signal encoding apparatus 
           101  VAD section 
           102 ,  105 ,  202 ,  205  Switching section 
           103  Stereo encoding section 
           104  Stereo DTX encoding section 
           106  Multiplexing section 
           200  Stereo signal decoding apparatus 
           201 ,  401  Demultiplexing section 
           203  Stereo decoding section 
           204  Stereo DTX decoding section 
           301 ,  302  Frame energy encoding section 
           303 ,  304 ,  502 ,  701 ,  702  Spectral parameter analysis section 
           305  Average spectral parameter calculation section 
           306  Average spectral parameter quantization section 
           307  Average spectral parameter decoding section 
           308 ,  209 ,  708 ,  709  Error spectral parameter calculation section 
           310 ,  311 ,  710 ,  711  Error spectral parameter quantization section 
           312  Multiplexing section 
           402 ,  403 ,  704 ,  705  Frame gain decoding section 
           404  Average spectral parameter decoding section 
           405 ,  406 ,  801 ,  802  Error spectral parameter decoding section 
           407 ,  408 ,  603 ,  604 ,  803 ,  804  Spectral parameter generation section 
           409 ,  412  Excitation generation section 
           410 ,  413  Multiplication section 
           411 ,  414  Synthesis filter section 
           501  Monaural signal generation section 
           503  Spectral parameter quantization section 
           601 ,  703  Spectral parameter decoding section 
           602 ,  706  Frame gain comparison section 
           707  Spectral parameter estimation section