Patent Publication Number: US-2010121633-A1

Title: Stereo audio encoding device and stereo audio encoding method

Description:
TECHNICAL FIELD 
     The present invention relates to a stereo speech coding apparatus that encodes stereo speech signals and a stereo speech coding method supporting this apparatus. 
     BACKGROUND ART 
     Communication in a monophonic scheme (i.e. monophonic communication) such as a telephone call by mobile telephones is presently the mainstream in speech communication in a mobile communication system. However, if the transmission bit rate becomes higher in the future, such as with fourth-generation mobile communication systems, it is possible to secure a band to transmit a plurality of channels, so that communication in a stereophonic scheme (i.e. stereophonic communication) is expected to become widespread in speech communication. 
     For example, taking into account the current situation in which a growing number of users record music in a portable audio player with a built-in HDD (Hard Disk Drive) and enjoy stereo music by plugging stereo earphones or headphones in this player, a future lifestyle can be predicted in which a mobile telephone and music player are combined and in which it is common practice to perform stereo speech communication using equipment such as stereo earphones or headphones. 
     Even if stereo communication becomes widespread, monophonic communication will still be performed. Because monophonic communication has a lower bit rate and is therefore expected to offer lower communication costs, while mobile telephones supporting only monophonic communication has the smaller circuit scale and is therefore less expensive, and therefore users not requiring high-quality speech communication will probably purchase mobile phones supporting only monophonic communication. That is, in one communication system, mobile phones supporting stereo communication and mobile phones supporting monophonic communication exist separately, and, consequently, the communication system needs to support both stereo communication and monophonic communication. Furthermore, in a mobile communication system, depending on the propagation environment, part of communication data may be lost because communication data is exchanged by radio signals. Thus, even if part of communication data is lost, when a mobile phone is provided with a function of reconstructing the original communication data from remaining received data, it is extremely useful. As a function to support both stereo communication and monophonic communication and allow reconstruction of original communication data from receive data remaining after some communication data is lost, there is scalable coding, which supports both stereo signals and monaural signals. 
     In this scalable coding, techniques for synthesizing stereo signals from monaural signals include, for example, ISC (Intensity Stereo Coding) used in MPEG-2/4 AAC (Moving Picture Experts Group 2/4 Advanced Audio Coding), disclosed in Non-Patent Document 1, MPEG 4-enhanced AAC, disclosed in Non-Patent Document 2, and BCC (Binaural Cue Coding) used in MPEG surround, disclosed in Non-Patent Document 3. In these kinds of coding, when the left channel signal and right channel signal of a stereo signal are reconstructed from a monaural signal, the energy of the monaural signal is distributed between the right and left channel signals to be decoded, such that the energy ratio between the decoded right and left channel signals is equal to the energy ratio between the original left and right channel signals encoded in the coding side. Further, to enhance the sound width in these kinds of coding, reverberation components are added to reconstructed signals using a decorrelator. 
     Also, as another method of reconstructing a stereo signal such as the left channel signal and right channel signal from a monaural signal, there is ICP (Inter-Channel Prediction), whereby the right and left channel signals of a stereo signal are reconstructed by applying FIR (Finite Impulse Response) filtering processing to a monaural signal. Filter coefficients of a FIR filter used in ICP coding are determined based on the least mean squared error (“MSE”) such that the least mean squared error between the monaural signal and stereo signal is minimum. This stereo coding of an ICP scheme is suitable for encoding a signal with energy concentrated in lower frequencies, such as a speech signal. 
     Non-Patent Document 1: General Audio Coding AAC, TwinVQ, BSAC, ISO/IEC, 14496-3: part 3, subpart 4, 2005 
     Non-Patent Document 2: Parametric Coding for High Quality Audio, ISO/IEC, 14496-3, 2004 
     Non-Patent Document 3: MPEG Surround, ISO/IEC, 23003-1, 2006 
     DISCLOSURE OF INVENTION 
     Problems to be Solved by the Invention 
     However, stereo coding based on the ICP scheme uses the unique correlation between channels, as the information to use to predict the left channel and right channel, and, consequently, if coding of the ICP scheme is applied to a speech signal having low inter-channel correlation, there is a problem that the sound quality of decoded speech degrades. Especially, it is difficult to apply ICP to a signal in which transition of signal waveforms in the time domain is not smooth, such as a residual signal of the voiced speech signal characterized by regular pitch spikes on a noise floor. 
     The right and left channel signals acquired by receiving the same source signal at different positions, have different distances from the source, and therefore one channel signal is a delayed copy of the other channel signal. This delay between the right and left channels causes misalignment between pitch spikes. This alignment of pitch spikes causes the correlation between the right and left channel signals to decrease, and causes an ICP prediction not to be performed adequately. Here, ICP is not performed adequately, which causes discontinuity between frames of decoded speech and instability of stereo sound image of decoded speech. 
     To solve these problems, a method of increasing the ICP prediction order is suggested. However, to suppress the discontinuity between frames of decoded speech and instability of stereo sound image to an extent that does not give feeling of discomfort to listeners, the ICP order needs to increase approximately to the frame size, meaning that the bit rate increases significantly. 
     It is therefore an object of the present invention to provide a stereo speech coding apparatus and stereo speech coding method that can improve the ICP performance of stereo signals having low inter-channel correlation while suppressing the bit rate. 
     Means for Solving the Problem 
     The stereo speech coding apparatus of the present invention employs a configuration having: a monaural signal generating section that generates a representative value in a form of a monaural signal, the representative value being acquired using a first channel signal and second channel signal of a stereo speech signal formed with two channel signals; a synthesis ratio adjusting section that adjusts a first channel synthesis ratio and a second channel synthesis ratio; an adaptive synthesis section that generates a first channel synthesis signal using the first channel synthesis ratio adjusted in the synthesis ratio adjusting section, the first channel signal and the second channel signal, and generates a second channel synthesis signal using the second channel synthesis ratio adjusted in the synthesis ratio adjusting section, the first channel signal and the second channel signal; and an inter-channel prediction section that performs an inter-channel prediction for a first channel using the monaural signal and the first channel synthesis signal, and further performs an inter-channel prediction for a second channel using the monaural signal and the second channel synthesis signal, and in which the synthesis ratio adjusting section adjusts the first channel synthesis ratio based on a correlation between the monaural signal and the first channel synthesis signal, and further adjusts the second channel synthesis ratio based on a correlation between the monaural signal and the second channel synthesis signal. 
     The stereo speech coding method of the present invention includes: a step of generating a representative value in a form of a monaural signal, the representative value being acquired using a first channel signal and second channel signal of a stereo speech signal formed with two channel signals; an synthesis ratio adjusting step of adjusting a first channel synthesis ratio and a second channel synthesis ratio; a step of generating a first channel synthesis signal using the first channel synthesis ratio adjusted in the synthesis ratio adjusting step, the first channel signal and the second channel signal, and generating a second channel synthesis signal using the second channel synthesis ratio adjusted in the synthesis ratio adjusting step, the first channel signal and the second channel signal; and a step of performing an inter-channel prediction for the first channel using the monaural signal and the first channel synthesis signal, and further performing an inter-channel prediction for the second channel using the monaural signal and the second channel synthesis signal, and in which the synthesis ratio adjusting step comprises adjusting the first channel synthesis ratio based on a correlation between the monaural signal and the first channel synthesis signal, and further adjusting the second channel synthesis ratio based on a correlation between the monaural signal and the second channel synthesis signal. 
     ADVANTAGEOUS EFFECT OF INVENTION 
     According to the present invention, it is possible to improve the ICP performance for speech signals having low inter-channel correlation in stereo speech coding while suppressing the bit rate. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram showing the main components of a stereo speech coding apparatus according to an embodiment of the present invention; 
         FIG. 2  is a flowchart showing the steps of adjusting a synthesis ratio in a stereo speech coding apparatus according to an embodiment of the present invention; 
         FIG. 3  is a block diagram showing the main components of a stereo speech decoding apparatus according to an embodiment of the present invention; 
         FIG. 4  is a block diagram showing a variation example of the main components of a stereo speech coding apparatus according to an embodiment of the present invention; 
         FIG. 5  is block diagram showing a variation example of the main components of a stereo speech coding apparatus according to an embodiment of the present invention; and 
         FIG. 6  is a block diagram showing a variation example the main components of a stereo speech decoding apparatus according to an embodiment of the present invention. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     An embodiment of the present invention will be explained below in detail with reference to the accompanying drawings. 
       FIG. 1  is a block diagram showing the main components of stereo speech coding apparatus  100  according to an embodiment of the present invention. An example case will be explained below where a stereo signal is comprised of two channels of the left channel and right channel. Here, the notation of “left channel,” “right channel,” “L” and “R” are used for ease of explanation and do not necessarily limit the positional conditions of right and left. 
     In  FIG. 1 , stereo speech coding apparatus  100  is provided with monaural signal generating section  101 , LPC (Linear Prediction Coefficients) analysis section  102 , adaptive synthesis section  103 , LPC analysis section  104 , synthesis ratio adjusting section  105 , ICP analysis section  106 , ICP coefficient quantizing section  107 , LPC coefficient quantizing section  108 , monaural signal encoding section  109 , correlation calculating section  110  and multiplexing section  111 . 
     Monaural signal generating section  101  generates monaural signal M from a stereo speech signal received as input in stereo speech coding apparatus  100 , that is, from the left channel signal L and right channel signal R, and outputs the monaural signal M to LPC analysis section  102  and monaural signal encoding section  109 . As an example in the present embodiment, the monaural signal M is generated by calculating the average value of the left channel signal L and right channel signal R according to following equation 1. 
         M= ( L+R )/2  (Equation 1) 
     LPC analysis section  102  performs an LPC analysis using the monaural signal M received as input from monaural signal generating section  101 , determines the linear prediction residual signal M e  with respect to the monaural signal M using the linear prediction coefficients acquired by analysis, and outputs the linear prediction residual signal M e  to synthesis ratio adjusting section  105  and ICP analysis section  106 . 
     Using the left channel synthesis ratio α that is adaptively adjusted in synthesis ratio adjusting section  105 , adaptive synthesis section  103  applies the left channel signal L and right channel signal R received as input in stereo speech coding apparatus  100 , to following equation 2, and generates the left channel synthesis signal L 2 ″. Further, adaptive synthesis section  103  adjusts the energy of the resulting left channel synthesis signal L 2 ″, according to following equation 3, and outputs the left channel synthesis signal L 2  with adjusted energy, to LPC analysis section  104 . 
     
       
         
           
             
               
                 
                   
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     As shown in equation 2, the left channel synthesis ratio α represents the ratio between the left channel signal L and right channel signal R included in the left channel synthesis signal L 2 . In equation 3, “frame size” represents the number of samples in one frame. In the energy adjustment represented by equation 3, the energy of the left channel synthesis signal L 2  is equal to the energy of the left channel signal L. 
     Similarly, using the right channel synthesis ratio β that is adaptively adjusted in synthesis ratio adjusting section  105 , adaptive synthesis section  103  applies the left channel signal L and right channel signal R received as input in stereo speech coding apparatus  100 , to following equation 4, and generates the right channel synthesis signal R 2 ″. Further, adaptive synthesis section  103  adjusts the energy of the resulting right channel synthesis signal R 2 ″, according to following equation 5, and outputs the right channel synthesis signal R 2  with adjusted energy, to LPC analysis section  104 . 
     
       
         
           
             
               
                 
                   
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     LPC analysis section  104  performs an LPC analysis of the left channel synthesis signal L 2  received as input from adaptive synthesis section  103 , and outputs the resulting linear prediction coefficients for the left channel, LPC L , to LPC coefficient quantizing section  108 . Similarly, LPC analysis section  104  performs an LPC analysis of the right channel synthesis signal R 2  received as input from adaptive synthesis section  103 , and outputs the resulting linear prediction coefficients for the right channel, LPC R , to LPC coefficient quantizing section  108 . Further, using the resulting linear prediction coefficients for the left channel, LPC L , LPC analysis section  104  determines and outputs the linear prediction residual signal L 2e  with respect to the left channel synthesis signal L 2 , to synthesis ratio adjusting section  105  and ICP analysis section  106 . Similarly, using the resulting linear prediction coefficients for the right channel, LPC R , LPC analysis section  104  determines and outputs the linear prediction residual signal R 2e  with respect to the right channel synthesis signal R 2 , to synthesis ratio adjusting section  105  and ICP analysis section  106 . 
     First, synthesis ratio adjusting section  105  initializes the left channel synthesis ratio α to “1.0.” Next, if the correlation value per frame, Corr L  (L 2e , M e ), between the linear prediction residual signal L 2e  received as input from LPC analysis section  104  and the linear prediction residual signal M e  received as input from LPC analysis section  102 , is lower than a predetermined threshold, synthesis ratio adjusting section  105  reduces and outputs the left channel synthesis ratio α to adaptive synthesis section  103 . Similarly, first, synthesis ratio adjusting section  105  initializes the right channel synthesis ratio β to “1.0.” Next, if the correlation value per frame, Corr R  (R 2e , M e ), between the linear prediction residual signal R 2e  received as input from LPC analysis section  104  and the linear prediction residual signal M e  received as input from LPC analysis section  102 , is lower than a predetermined threshold, synthesis ratio adjusting section  105  reduces and outputs the right channel synthesis ratio β to adaptive synthesis section  103 . Thus, synthesis ratio adjusting section  105  performs loop processing for adjusting the synthesis ratios α and β together with adaptive synthesis section  103  and LPC analysis section  104 , until the correlation values Corr L  (L 2e , M e ) and Corr R  (R 2e , M e ) are both equal to or higher than a predetermined threshold. Synthesis ratio adjusting section  105  calculates the correlation values Corr L  (L 2e , M e ) and Corr R  (R 2e , M e ) according to following equations 6 and 7, respectively. 
     
       
         
           
             
               
                 
                   
                     
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     ICP analysis section  106  calculates the left channel ICP coefficient h L , using the linear prediction residual signal L 2e  received as input from LPC analysis section  104  and the linear prediction residual signal M e  received as input from LPC analysis section  102 , and outputs the left channel ICP coefficient h L , to ICP coefficient quantizing section  107 . This left channel ICP coefficient h L , is the FIR coefficient of the N-th order for predicting the linear prediction residual signal L 2e  from the linear prediction residual signal M e , and, when the prediction signal with respect to the linear prediction residual signal L 2e  is L̂ 2e , the prediction signal is represented by following equation 8. 
     
       
         
           
             
               
                 
                   
                     
                       
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     In equation 8, “n” represents the sample numbers of the linear prediction residual signals M e  and L 2e , and “N” represents the order of FIR filter coefficients. Here, the FIR filter coefficient h L (i) is determined based on the least mean squared error. To be more specific, h L (i) is a value that minimizes the mean squared error ε represented by following equation 9, and that therefore satisfies following equation 10. By calculating equation 10, h L , represented by equation 11 is acquired. 
     
       
         
           
             
               
                 
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     Further, using the linear prediction residual signal R 2e  received as input from LPC analysis section  104  and linear prediction residual signal M e  received as input from LPC analysis section  102 , ICP analysis section  106  determines the right channel ICP coefficient h R  in the same way as the method of determining the left channel ICP coefficient h L , and outputs the right channel ICP coefficient h R  to ICP coefficient quantizing section  107 . 
     ICP coefficient quantizing section  107  quantizes the left channel ICP coefficient h L , and right channel ICP coefficient h R  received as input from ICP analysis section  106 , and outputs the ICP coefficient coded parameter for the left channel and the ICP coefficient coded parameter for the right channel to multiplexing section  111 . 
     LPC coefficient quantizing section  108  quantizes the linear prediction coefficients for the left channel, LPC L , and the linear prediction coefficients for the right channel, LPC R  received as input from LPC analysis section  104 , and outputs the LPC coded parameter for the left channel and the LPC coded parameter for the right channel to multiplexing section  111 . 
     Monaural signal encoding section  109  encodes the monaural signal M received as input from monaural signal generating section  101  by an arbitrary coding scheme, and outputs the resulting monaural signal coded parameter to multiplexing section  111 . 
     Correlation value calculating section  110  calculates the correlation value per frame, Corr (L, R), between the left channel signal L and right channel signal R received as input in stereo speech coding apparatus  100 , according to following equation 12, and outputs the results to multiplexing section  111 . 
     
       
         
           
             
               
                 
                   
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     Multiplexing section  111  multiplexes the ICP coefficient coded parameter for the left channel and ICP coefficient coded parameter for the right channel received as input from ICP coefficient quantizing section  107 , the LPC coded parameter for the left channel and LPC coded parameter for the right channel received as input from LPC coefficient quantizing section  108 , the monaural signal coded parameter received as input from monaural signal encoding section  109  and the correlation value Corr (L, R) received as input from correlation value calculating section  110 , and outputs the resulting bit stream to stereo speech decoding apparatus  200 , which will be described later. 
       FIG. 2  is a flowchart showing the steps of adjusting the synthesis ratio α and β in stereo speech coding apparatus  100 . Here, although the steps of adjusting the left channel synthesis ratio α will be explained with this figure as an example, the steps of adjusting the right channel synthesis ratio β are basically the same as the steps in this figure, where α, L 2 ″, L 2e  and h L  are replaced with β, R 2 ″, R 2e  and h R , respectively. 
     In step (hereinafter abbreviated to “ST”)  1010 , synthesis ratio adjusting section  105  initializes the synthesis ratio α to “1.0.” 
     Next, in ST  1020 , adaptive synthesis section  103  generates the synthesis signal L 2 ″ according to equation 2. 
     Next, in ST  1030 , adaptive synthesis section  103  performs an energy adjustment of the synthesis signal L 2 ″ according to equation 3 and acquires the synthesis signal L 2 . 
     Next, in ST  1040 , LPC analysis section  104  performs an LPC analysis of the synthesis signal L 2  and generates the linear prediction residual signal L 2e . 
     Next, in ST  1050 , synthesis ratio adjusting section  105  calculates the correlation value Corr L  (L 2e , M e ) between the linear prediction residual signal L 2e  received as input from LPC analysis section  104  and the linear prediction residual signal M e  received as input from LPC analysis section  102 . 
     Next, in ST  1060 , synthesis ratio adjusting section  105  decides whether or not the correlation value Corr L  (L 2e , M e ) is lower than a predetermined threshold. 
     In ST  1060 , if the correlation value Corr L , (L 2e , M e ) is decided to be lower than the predetermined threshold (“YES” in ST  1060 ), in ST  1070 , synthesis ratio adjusting section  105  adjusts the synthesis ratio α to α=α−0.1. 
     Next, in ST  1080 , synthesis ratio adjusting section  105  decides whether or not the synthesis ratio α is higher than “0.5.” 
     In ST  1080 , if the synthesis ratio α is decided to be higher than “0.5” (“YES” in ST  1080 ), the processing step proceeds to ST  1020 . 
     By the decision processing in this step, the synthesis ratio α is limited to the range of 0.5≦α≦1.0. Here, when the value of the synthesis ratio α is equal to “1.0,” the synthesis signal L 2  and monaural signal M are the most different from each other, and therefore the ICP prediction performance degrades most significantly. By contrast, when the value of the synthesis ratio α is closer to “0.5,” the synthesis signal L 2  and monaural signal M become closer to each other, so that the ICP prediction performance is improved. Here, it is needless to say that the value to compare with synthesis ratios is not limited to “0.5” in the above and can be set to an appropriate value adequately. 
     On the other hand, if the correlation value Corr L  (L 2e , M e ) is decided to be equal to or higher than a threshold in ST  1060  (“NO” in ST  1060 ) or if the synthesis ratio α is decided to be equal to or lower than “0.5” in ST  1080  (“NO” in ST  1080 ), ICP analysis section  106  calculates the ICP coefficient h L , using the linear prediction residual signal L 2e  received as input from LPC analysis section  104  and the linear prediction residual signal M e  received as input from LPC analysis section  102 . 
       FIG. 3  is a block diagram showing the main components of stereo speech decoding apparatus  200  according to the present embodiment. 
     In  FIG. 3 , stereo speech decoding apparatus  200  is provided with demultiplexing section  201 , monaural signal decoding section  202 , LPC analysis section  203 , ICP coefficient decoding section  204 , ICP synthesis section  205 , LPC coefficient decoding section  206 , LPC synthesis section  207  and stereo signal reconstructing section  208 . 
     Demultiplexing section  201  demultiplexes the bit stream transmitted from stereo speech coding apparatus  100  into the monaural signal coded parameter, the ICP coefficient coded parameter for the left channel, the ICP coefficient coded parameter for the right channel, the LPC coded parameter for the left channel, the LPC coded parameter for the right channel and the correlation value Corr (L, R). Further, demultiplexing section  201  outputs the monaural signal coded parameter to monaural signal decoding section  202 , the ICP coefficient coded parameter for the left channel and the ICP coefficient coded parameter for the right channel to ICP coefficient decoding section  204 , the LPC coded parameter for the left channel and the LPC coded parameter for the right channel to LPC coefficient decoding section  206 , and the correlation value Corr (L, R) to stereo signal reconstructing section  208 . 
     Monaural signal decoding section  202  performs decoding by a scheme supporting the coding scheme on the coding side, using the monaural signal coded parameter received as input from demultiplexing section  201 , outputs the acquired decoded monaural signal M′ to LPC analysis section  203 , and, if necessary, outputs it to the outside of stereo speech decoding apparatus  200 . 
     LPC analysis section  203  performs an LPC analysis using the decoded monaural signal M′ received as input from monaural signal decoding section  202 , determines the decoded linear prediction residual signal M e ′ with respect to the decoded monaural signal M′ using the linear prediction coefficients acquired by the analysis, and outputs the decoded linear prediction residual signal M e ′ to ICP synthesis section  205 . 
     ICP coefficient decoding section  204  decodes the ICP coefficient coded parameter for the left channel and the ICP coefficient coded parameter for the right channel received as input from demultiplexing section  201 , and outputs the resulting decoded ICP coefficients h L ′ and h R ′ to ICP synthesis section  205 . 
     ICP synthesis section  205  performs an ICP synthesis using the decoded linear prediction residual signal M e ′ received as input from LPC analysis section  203  and the decoded ICP coefficient h L ′ received as input from ICP coefficient decoding section  204 , and outputs the resulting linear prediction residual signal L 2e ′ to LPC synthesis section  207 . Similarly, ICP synthesis section  205  performs an ICP synthesis using the decoded linear prediction residual signal M e ′ received as input from LPC analysis section  203  and the decoded ICP coefficient h R ′ received as input from ICP coefficient decoding section  204 , and outputs the resulting linear prediction residual signal R 2e ′ to LPC synthesis section  207 . 
     LPC coefficient decoding section  206  decodes the LPC coded parameter for the left channel and the LPC coded parameter for the right channel received as input from demultiplexing section  201 , and outputs the resulting decoded linear prediction coefficients LPC L ′ and LPC R ′ to LPC synthesis section  207 . 
     LPC synthesis section  207  performs an LPC synthesis using the linear prediction residual signal L 2e ′ received as input from ICP synthesis section  205  and the decoded linear prediction coefficient LPC L ′ received as input from LPC coefficient decoding section  206 , and outputs the resulting decoded synthesis signal L 2 ′ to stereo signal reconstructing section  208 . Further, LPC synthesis section  207  performs an LPC synthesis using the linear prediction residual signal R 2 ′ received as input from ICP synthesis section  205  and the decoded linear prediction coefficient LPC R ′ received as input from LPC coefficient decoding section  206 , and outputs the resulting decoded synthesis signal R 2 ′ to stereo signal reconstructing section  208 . 
     Stereo signal reconstructing section  208  reconstructs the decoded left channel signal L′ and decoded right channel signal R′ forming a stereo signal, using the decoded synthesis signals L 2 ′ and R 2 ′ received as input from LPC synthesis section  207  and the correlation value Corr (L, R) received as input from demultiplexing section  201 , and outputs the decoded left channel signal L′ and decoded right channel signal R′ to the outside of stereo speech decoding apparatus  200 . 
     Processing of reconstructing stereo signals in stereo signal reconstructing section  208  will be explained below in detail. 
     The correlation value Corr (L 2 ′, R 2 ′) between the decoded synthesis signal L 2 ′ and decode synthesis signal R 2 ′ received as input in stereo signal reconstructing section  208 , is generally higher than the correlation value Corr (L, R) received as input from demultiplexing section  201 . 
     Here, when the correlation between the right and left channels of a stereo signal is higher, the stereo sound image of the stereo signal becomes narrower. Therefore, stereo signal reconstructing section  208  further adds perceptually orthogonal reverberation components to the decoded synthesis signal L 2 ′ and decoded synthesis signal R 2 ′, using the correlation value Corr (L, R) received as input from demultiplexing section  201 , and outputs the results in the form of a stereo signal. Here, the reverberation components are the components for spatial enhancement of a stereo signal, and can be calculated by allpass filters or allpass filter lattices. For example, stereo signal reconstructing section  208  reconstructs the left channel signal L′ and right channel signal R′ according to following equations 13 and 14. 
         L′=c·L   2 ′+√{square root over (1 −c   2 )}· AP   1 ( L   2 ′)  (Equation 13) 
         R′=c·R   2 ′+√{square root over (1 −c   2 )}· AP   2 ( R   2 ′)  (Equation 14) 
     In equations 13 and 14, AP 1 (L 2 ′) and AP 2 (R 2 ′) represent the transfer functions of two different allpass filters, and “c” represents the value shown in following equation 15. Here, to improve a stereo sound image, it may be preferable to divide the left and right channel signals of a stereo signal into a plurality of frequency bands and apply respective allpass filters to the frequency bands. 
     
       
         
           
             
               
                 
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     Thus, according to the present embodiment, the stereo speech coding apparatus generates a synthesis signal of a left channel signal and right channel signal and performs an ICP using the monaural signal and synthesis signal such that the correlation value between a monaural signal and the synthesis signal is equal to or higher than a predetermined threshold, so that, without increasing the ICP order, it is possible to suppress the bit rate, improve ICP performance with respect to a stereo signal having lower inter-channel correlation, and, consequently, improve the sound quality of the decoded speech signal. 
     Here, although an example case has been described above with the present embodiment where “0.1” is used in the step of adjusting the synthesis ratio α, the present invention is not limited to this, and it is equally possible to use an arbitrary value in the step of adjusting the synthesis ratio α such as a smaller value, “0.05.” 
     Also, to avoid sound instability in very dynamic speech signals, it is possible to set the adjustment range of the synthesis ratio α of the current frame to α prev     —     frame −ρ≦α≦α prev     —     frame +ρ, based on the synthesis ratio α prev     —     frame  used in ICP of the previous frame. Here, ρ is a real number. 
     Also, although a case has been described above with the present embodiment where monaural signal encoding section  109  performs coding by an arbitrary coding scheme, when monaural signal encoding section  109  is an encoder adopting a CELP (Code Excited Linear Prediction) scheme or an arbitrary encoder which provides processing of generating a linear prediction residual signal (i.e. excitation signal), stereo speech coding apparatus  100  needs not to have LPC analysis section  102 . 
     Also, although an example case has been described above with the present embodiment where synthesis ratio adjusting section  105  adjusts the synthesis ratio α based on the correlation value between the linear prediction residual signal L 2e  and the linear prediction residual signal M e , the present invention is not limited to this, and, as in stereo speech coding apparatus  300  shown in  FIG. 4 , synthesis ratio adjusting section  105   a  may adjust the synthesis ratio α based on the correlation value between the synthesis signal L 2  and the monaural signal M. The same applies to the synthesis ratio β. 
     Also, an example case has been described above with the present embodiment where stereo speech coding apparatus  100  further performs an LPC analysis before performing coding by an ICP scheme, the stereo speech coding apparatus according to the present invention is not limited to this, and may employ a configuration not performing an LPC analysis as in stereo speech coding apparatus  400  shown in  FIG. 5 , thereby simplifying coding processing and reducing the amount of calculations. In this case, the configuration of stereo speech decoding apparatus  500  is as shown in  FIG. 6 . 
     Also, an example case has been described above with the present embodiment where a stereo signal is comprised of two channel signals of the left channel signal L as the first channel signal and the right channel signal R as the second channel signal, the present invention is not limited to this, and “L” and “R” may be reversed, or a stereo signal may be comprised of three or more signals. In this case, the average value of three or more channel signals is generates in a form of the monaural signal M, and the synthesis signal L 2  is generated using three or more channel signals. Here, although M represents an average value with the present embodiment, the present invention is not limited to this, and M may be a representative value that can be adequately calculated using L and R. 
     Also, although the stereo speech decoding apparatus of the present embodiment has been described to perform processing using a bit stream transmitted from the stereo speech coding apparatus according to the present embodiment, the present invention is not limited to this, and, if the bit stream includes necessary parameters and data, the processing is possible even when the bit stream is not transmitted from the stereo speech coding apparatus according to the present embodiment. 
     The stereo speech coding apparatus and stereo speech decoding apparatus according to the present invention can be mounted on a communication terminal apparatus in a mobile communication system, so that it is possible to provide a communication terminal apparatus having the same operational effect as described above. Also, the stereo speech coding apparatus and stereo speech coding method according to the present embodiment are available in a communication system of a wired system. 
     Also, although an example case has been described above with this description where the preset invention is applied to monaural-to-stereo scalable coding, it is equally possible to employ a configuration where the present invention is applied to coding/decoding per band upon performing band split coding of stereo signals. 
     Although a case has been described above with the above embodiments as an example where the present invention is implemented with hardware, the present invention can be implemented with software. For example, by describing the stereo speech coding method according to the present invention in a programming language, storing this program in a memory and making the information processing section execute this program, it is possible to implement the same function as the stereo speech coding apparatus according to the present invention. 
     Furthermore, each function block employed in the description of each of the aforementioned embodiments may typically be implemented as an LSI constituted by an integrated circuit. These may be individual chips or partially or totally contained on a single chip. 
     “LSI” is adopted here but this may also be referred to as “IC,” “system LSI,” “super LSI,” or “ultra LSI” depending on differing extents of integration. 
     Further, the method of circuit integration is not limited to LSI&#39;s, and implementation using dedicated circuitry or general purpose processors is also possible. After LSI manufacture, utilization of an FPGA (Field Programmable Gate Array) or a reconfigurable processor where connections and settings of circuit cells in an LSI can be reconfigured is also possible. 
     Further, if integrated circuit technology comes out to replace LSI&#39;s as a result of the advancement of semiconductor technology or a derivative other technology, it is naturally also possible to carry out function block integration using this technology. Application of biotechnology is also possible. 
     The disclosure of Japanese Patent Application No. 2007-111864, filed on Apr. 20, 2007, including the specification, drawings and abstract, is incorporated herein by reference in its entirety. 
     INDUSTRIAL APPLICABILITY 
     The stereo speech coding apparatus and stereo speech coding method according to the present embodiment are applicable to, for example, a communication terminal apparatus in a mobile communication system.