Patent Publication Number: US-7904292-B2

Title: Scalable encoding device, scalable decoding device, and method thereof

Description:
TECHNICAL FIELD 
     The present invention relates to a scalable encoding apparatus that performs scalable encoding on a stereo speech signal using a CELP method (hereinafter referred to simply as CELP encoding), a scalable decoding apparatus, and a method used by the scalable encoding apparatus and scalable decoding apparatus. 
     BACKGROUND ART 
     In speech communication of a mobile communication system, communication using a monaural scheme (monaural communication) is a mainstream, such as communication using mobile telephones. However, if a transmission rate increases further as in the fourth-generation mobile communication system, it is possible to maintain an adequate bandwidth for transmitting a plurality of channels. It is therefore expected that communication using a stereo system (stereo communication) will be widely used in speech communication as well. 
     For example, considering the increasing number of users who enjoy stereo music by storing music in portable audio players that are equipped with a HDD (hard disk) and attaching stereo earphones, headphones, or the like to the player, it is anticipated that mobile telephones will be combined with music players in the future, and that a lifestyle of using stereo earphones, headphones, or other equipments and performing speech communication using a stereo system will become prevalent. In order to realize realistic conversation in the environment such as in currently popularized TV conference, it is anticipated that stereo communication is used. 
     Even when stereo communication becomes common, it is assumed that monaural communication will also be used. This is because monaural communication has a low bit rate, and a lower cost of communication can therefore be expected. Further, a mobile telephone which supports only monaural communication has a smaller circuit scale and is therefore inexpensive. Users who do not need high-quality speech communication will purchase mobile telephones which support only monaural communication. Accordingly, in a single communication system, mobile telephones which support stereo communication and mobile telephones which support monaural communication will coexist. Therefore, the communication system will have to support both stereo communication and monaural communication. 
     In the mobile communication system, communication data is exchanged using radio signals, a part of the communication data is sometimes lost according to the propagation path environment. Therefore, if the mobile telephone has a function of restoring the original communication data from the residual received data even in this case, it is extremely useful. 
     There is scalable encoding composed of a stereo signal and a monaural signal. This type of encoding can support both stereo communication and monaural communication and is capable of restoring the original communication data from residual received data even when a part of the communication data is lost. An example of a scalable encoding apparatus that has this function is disclosed in Non-patent Document 1, for example.
     Non-patent Document 1: ISO/IEC 14496-3:1999 (B.14 Scalable AAC with core coder)   

     DISCLOSURE OF INVENTION 
     Problems to Be Solved by the Invention 
     However, the scalable encoding apparatus disclosed in Non-patent Document 1 is designed for an audio signal and does not assume a speech signal, and therefore there is a problem of decreasing encoding efficiency when the scalable encoding is applied to a speech signal as is. Specifically, for a speech signal, it is required to apply CELP encoding which is capable of efficient encoding, but Non-patent Document 1 does not disclose the specific configuration for the case where a CELP method is applied, particularly where CELP encoding is applied in an extension layer. Even when CELP encoding optimized for the speech signal which is not assumed to that apparatus is applied as is, the desired encoding efficiency is difficult to obtain. 
     It is therefore an object of the present invention to provide a scalable encoding apparatus capable of realizing scalable encoding of a stereo speech signal using a CELP method and improving encoding efficiency, a scalable decoding apparatus, and a method used by the scalable encoding apparatus and scalable decoding apparatus. 
     Means for Solving the Problem 
     The scalable encoding apparatus of the present invention has: a generating section that generates a monaural speech signal from a stereo speech signal that includes a first channel signal and a second channel signal; a monaural encoding section that encodes the monaural speech signal using a CELP method; a calculating section that calculates encoding distortion of the second channel signal that occurs by the CELP encoding; and a first encoding section that encodes the first channel signal using the CELP method and obtains an encoded parameter of the first channel signal so as to minimize the sum of the encoding distortion of the first channel signal that occurs in the encoding, and the encoding distortion of the second channel signal calculated by the calculating section. 
     ADVANTAGES EFFECT OF THE INVENTION 
     According to the present invention, it is possible to perform scalable encoding of a stereo speech signal using CELP encoding and improve encoding efficiency. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram showing the main configuration of the scalable encoding apparatus according to embodiment 1; 
         FIG. 2  shows the relationship of the monaural signal, the first channel signal and the second channel signal; 
         FIG. 3  is a block diagram showing the main internal configuration of the monaural signal CELP encoder according to embodiment 1; 
         FIG. 4  is a block diagram showing the main internal configuration of the first channel signal encoder according to embodiment 1; 
         FIG. 5  is a block diagram showing the main configuration of the scalable decoding apparatus according to embodiment 1; 
         FIG. 6  is a block diagram showing the main configuration of the scalable encoding apparatus according to embodiment 2; 
         FIG. 7  is a block diagram showing the main internal configuration of the first channel signal encoder according to embodiment 2; and 
         FIG. 8  is a block diagram showing the main configuration of the scalable decoding apparatus according to embodiment 2. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Embodiments of the present invention will be described in detail hereinafter with reference to the accompanying drawings. The case will be described as an example where the stereo speech signal formed with two channels is encoded, wherein the first channel and the second channel described hereinafter are an L channel and an R channel, respectively, or an R channel and an L channel, respectively. 
     Embodiment 1 
       FIG. 1  is a block diagram showing the main configuration of scalable encoding apparatus  100  according to embodiment 1 of the present invention. Scalable encoding apparatus  100  is provided with adder  101 , multiplier  102 , monaural signal CELP encoder  103  and first channel signal encoder  104 . 
     Each section of scalable encoding apparatus  100  performs the operation described below. 
     Adder  101  adds first channel signal CH 1  and second channel signal CH 2  which are inputted to scalable encoding apparatus  100  to generate a sum signal. Multiplier  102  multiplies the sum signal by ½ to divide the scale in half and generates monaural signal M. Specifically, adder  101  and multiplier  102  calculate the average signal of first channel signal CH 1  and second channel signal CH 2  and set the average signal as monaural signal M. 
     Monaural signal CELP encoder  103  performs CELP encoding on monaural signal M and outputs a CELP encoded parameter obtained for each sub-frame to outside of scalable encoding apparatus  100 . Monaural signal CELP encoder  103  outputs synthesized monaural signal M′, which is synthesized (for each sub-frame) using the CELP encoded parameter for each sub-frame, to first channel signal encoder  104 . The term “CELP encoded parameter” used herein is an LPC (LSP) parameter, an adaptive excitation codebook index, an adaptive excitation gain, a fixed excitation codebook index and a fixed excitation gain. 
     First channel signal encoder  104  performs encoding described later on first channel signal CH 1  inputted to scalable encoding apparatus  100  using second channel signal CH 2  inputted to scalable encoding apparatus  100  in the same way and synthesized monaural signal M′ outputted from monaural signal CELP encoder  103 , and outputs the CELP encoded parameter of the obtained first channel signal to outside of scalable encoding apparatus  100 . 
     One of the characteristics of scalable encoding apparatus  100  is that adder  101 , multiplier  102 , and monaural signal CELP encoder  103  form a first layer, and first channel signal encoder  104  forms a second layer, wherein the encoded parameter of the monaural signal is outputted from the first layer, and the encoded parameter with which a stereo signal can be obtained by decoding together with a decoded signal of the first layer (monaural signal) at the decoding side is outputted from the second layer. Specifically, the scalable encoding apparatus according to this embodiment performs scalable encoding that is composed of a monaural signal and a stereo signal. 
     According to this configuration, the decoding apparatus which acquires the encoded parameters composed of the above mentioned first layer and second layer can decode a monaural signal although at a low quality, even if the decoding apparatus cannot acquire the encoded parameter of the second layer and can only acquire the encoded parameter of the first layer due to deterioration of the transmission path environment. When the decoding apparatus can acquire the encoded parameters of the first layer and second layer, it is possible to decode a stereo signal at a high quality using these parameters. 
     The principle by which the decoding apparatus can decode a stereo signal using the encoded parameters of the first layer and second layer outputted from scalable encoding apparatus  100  will be described hereinafter.  FIG. 2  shows the relationship of the monaural signal, the first channel signal and the second channel signal. 
     As shown in  FIG. 2A , monaural signal M prior to encoding can be calculated by multiplying the sum of first channel signal CH 1  and second channel signal CH 2  by ½, that is, by the following Equation (1).
 
 M =(CH1+CH2)/2  (Equation 1)
 
Therefore, second channel signal CH 2  can be calculated when monaural signal M and first channel signal CH 1  are known.
 
     However, in reality, when monaural signal M and first channel signal CH 1  are encoded, encoding distortion occurs as a result of encoding, and therefore, Equation (1) no longer holds. More specifically, when the difference between first channel signal CH 1  and monaural signal M is referred to as first channel signal difference ΔCH 1 , and the difference between second channel signal CH 2  and monaural signal M is referred to as second channel signal difference ΔCH 2 , a difference occurs between ΔCH 1  and ΔCH 2  as shown in  FIG. 2B  as a result of encoding, and the relationship of Equation (1) is no longer satisfied. Therefore, even when monaural signal M and first channel signal CH 1  can be obtained by decoding, it is subsequently no longer possible to correctly calculate second channel signal CH 2 . In order to prevent the degradation of the speech quality of the decoded signal, it is necessary to consider an encoding method taking into consideration the difference between the two encoding distortions. 
     In order to further improve the decoding accuracy of CH 1  and CH 2 , scalable encoding apparatus  100  according to this embodiment minimizes the encoding distortion of CH 1  upon encoding of CH 1  so that the encoding distortion of CH 2  is minimized, and determines the encoded parameter of CH 1 . By this means, it is possible to prevent the degradation of the speech quality of the decoded signal. 
     On the other hand, the decoded CH 2  is generated in the decoding apparatus from the decoded signal of CH 1  and the decoded signal of the monaural signal. Equation (2) below is obtained from the above Equation (1), and CH 2  can therefore be generated according to Equation (2).
 
CH2=2× M− CH1  (Equation 2)
 
       FIG. 3  is a block diagram showing the main internal configuration of monaural signal CELP encoder  103 . 
     Monaural signal CELP encoder  103  is provided with LPC analyzing section  111 , LPC quantizing section  112 , LPC synthesis filter  113 , adder  114 , perceptual weighting section  115 , distortion minimizing section  116 , adaptive excitation codebook  117 , multiplier  118 , fixed excitation codebook  119 , multiplier  120 , gain codebook  121  and adder  122 . 
     LPC analyzing section  111  performs linear prediction analysis on monaural signal M outputted from multiplier  102 , and outputs the LPC parameter which is the analysis result to LPC quantizing section  112  and perceptual weighting section  115 . 
     LPC quantizing section  112  quantizes the LSP parameter after converting the LPC parameter outputted from LPC analyzing section  111  to an LSP parameter which is suitable for quantization, and outputs the obtained quantized LSP parameter (CL) to outside of monaural signal CELP encoder  103 . The quantized LSP parameter is one of the CELP encoded parameters obtained by monaural signal CELP encoder  103 . LPC quantizing section  112  reconverts the quantized LSP parameter to a quantized LPC parameter, and outputs the quantized LPC parameter to LPC synthesis filter  113 . 
     LPC synthesis filter  113  uses the quantized LPC parameter outputted from LPC quantizing section  112  to perform synthesis by LPC synthesis filter using an excitation vector generated by adaptive excitation codebook  117  and fixed excitation codebook  119  (described hereinafter) as excitation. The obtained synthesized signal M′ is outputted to adder  114  and first channel signal encoder  104 . 
     Adder  114  inverts the polarity of the synthesized signal outputted from LPC synthesis filter  113 , calculates an error signal by adding to monaural signal M, and outputs the error signal to perceptual weighting section  115 . This error signal corresponds to the encoding distortion. 
     Perceptual weighting section  115  uses a perceptual weighting filter configured based on the LPC parameter outputted from LPC analyzing section  111  to perform perceptual weighting for the encoding distortion outputted from adder  114 , and the signal is outputted to distortion minimizing section  116 . 
     Distortion minimizing section  116  indicates various types of parameters to adaptive excitation codebook  117 , fixed excitation codebook  119  and gain codebook  121  so as to minimize the encoding distortion that is outputted from perceptual weighting section  115 . Specifically, distortion minimizing section  116  indicates indices (C A , C D , C G ) to adaptive excitation codebook  117 , fixed excitation codebook  119  and gain codebook  121 . 
     Adaptive excitation codebook  117  stores the previously generated excitation vector for LPC synthesis filter  113  in an internal buffer, generates a single sub-frame portion from the stored excitation vector based on an adaptive excitation lag that corresponds to the index indicated from distortion minimizing section  116 , and outputs the single sub-frame portion to multiplier  118  as an adaptive excitation vector. 
     Fixed excitation codebook  119  outputs the excitation vector, which corresponds to the index indicated from distortion minimizing section  116 , to multiplier  120  as a fixed excitation vector. 
     Gain codebook  121  generates a gain that corresponds to the index indicated from distortion minimizing section  116 , that is, a gain for the adaptive excitation vector from adaptive excitation codebook  117 , and a gain for the fixed excitation vector from fixed excitation codebook  119 , and outputs the gains to multipliers  118  and  120 . 
     Multiplier  118  multiplies the adaptive excitation gain outputted from gain codebook  121  by the adaptive excitation vector outputted from adaptive excitation codebook  117 , and outputs the result to adder  122 . 
     Multiplier  120  multiplies the fixed excitation gain outputted from gain codebook  121  by the fixed excitation vector outputted from fixed excitation codebook  119 , and outputs the result to adder  122 . 
     Adder  122  adds the adaptive excitation vector outputted from multiplier  118  and the fixed excitation vector outputted from multiplier  120 , and outputs the added excitation vector as excitation to LPC synthesis filter  113 . Adder  122  also feeds back the obtained excitation vector of the excitation to adaptive excitation codebook  117 . 
     As previously described, the excitation vector outputted from adder  122 , that is, the excitation vector generated by adaptive excitation codebook  117  and fixed excitation codebook  119 , is synthesized as excitation by LPC synthesis filter  113 . 
     In this way, a series of processing of obtaining the encoding distortion using the excitation vectors generated by adaptive excitation codebook  117  and fixed excitation codebook  119  is a closed loop (feedback loop). Distortion minimizing section  116  indicates adaptive excitation codebook  117 , fixed excitation codebook  119 , and gain codebook  121  so as to minimize the encoding distortion. Distortion minimizing section  116  outputs various types of CELP encoded parameters (C A , C D , C G ) that minimize the encoding distortion to outside of scalable encoding apparatus  100 . 
       FIG. 4  is a block diagram showing the main internal configuration of first channel signal encoder  104 . 
     In first channel signal encoder  104 , the configurations of LPC analyzing section  131 , LPC quantizing section  132 , LPC synthesis filter  133 , adder  134 , distortion minimizing section  136 , adaptive excitation codebook  137 , multiplier  138 , fixed excitation codebook  139 , adder  140 , gain codebook  141  and adder  142  are the same as those of LPC analyzing section  111 , LPC quantizing section  112 , LPC synthesis filter  113 , adder  114 , distortion minimizing section  116 , adaptive excitation codebook  117 , multiplier  118 , fixed excitation codebook  119 , multiplier  120 , gain codebook  121  and adder  122  in monaural signal CELP encoder  103 . These components are therefore not described. 
     Second channel signal error component calculating section  143  is an entirely new component. The basic operations of perceptual weighting section  135  and distortion minimizing section  136  are the same as those of perceptual weighting section  115  and distortion minimizing section  116  in monaural signal CELP encoder  103 . However, perceptual weighting section  135  and distortion minimizing section  136  receive the output of second channel signal error component calculating section  143  and perform operations that differ from those of monaural signal CELP encoder  103  as described below. 
     When CH 1  is encoded in a second layer, that is, in first channel signal encoder  104 , scalable encoding apparatus  100  according to this embodiment decides an encoded parameter of CH 1  so as to minimize the sum of the encoding distortion of CH 1  and the encoding distortion of CH 2 . A high-quality speech can thereby be achieved by simultaneously optimizing the encoding distortions of CH 1  and CH 2 . 
     Second channel signal error component calculating section  143  calculates an error component for a case where CELP encoding is temporarily performed on the second channel signal, that is, calculates the above-described encoding distortion of CH 2 . Specifically, second channel synthesis signal generating section  144  in second channel signal error component calculating section  143  calculates a synthesized second channel signal CH 2 ′ by doubling synthesized monaural signal M′ and subtracting synthesized first channel signal CH 1 ′ from the calculated value. Second channel synthesis signal generating section  144  does not perform CELP encoding of the second channel signal. Adder  145  then calculates the difference between second channel signal CH 2  and synthesized second channel signal CH 2 ′. 
     Perceptual weighting section  135  performs perceptual weighting on the difference between first channel signal CH 1  and synthesized first channel signal CH 1 ′, that is, the encoding distortion of the first channel, in the same way as perceptual weighting section  115  in monaural signal CELP encoder  103 . Perceptual weighting section  135  also performs perceptual weighting of the difference between second channel signal CH 2  and synthesized second channel signal CH 2 ′, that is, the encoding distortion of the second channel. 
     Distortion minimizing section  136  decides the optimal adaptive excitation vector, the fixed excitation vector and the gain of the vectors using the algorithm described below so as to minimize the perceptual-weighted encoding distortion, that is, the sum of the encoding distortion for the first channel signal and the encoding distortion for the second channel signal. 
     Hereinafter, the algorithm used in distortion minimizing section  136  which minimizes encoding distortion will be described. CH 1  and CH 2  are input signals, CH 1 ′ is the synthesized signal of CH 1 , CH 2 ′ is the synthesized signal of CH 2 , and M′ is the synthesized monaural signal. 
     Sum d of the encoding distortions of the first channel signal and the second channel signal can be expressed by Equation (3) below.
 
 d =∥CH1−CH1′∥ 2 +∥CH2−CH2′∥ 2   (Equation 3)
 
     From the relationship of the monaural signal, the first channel signal and the second channel signal, CH 2 ′ can be expressed by already-encoded monaural synthesized signal M′ and first channel synthesized signal CH 1 ′ as shown in Equation (4) below.
 
CH2′=2× M ′−CH1′  (Equation 4)
 
Equation (3) can thus be rewritten as Equation (5) below.
 
 d =∥CH1−CH1′∥ 2 +∥CH2−(2× M′ −CH1′)∥ 2   (Equation 5)
 
Specifically, the scalable encoding apparatus according to this embodiment obtains through search the CELP encoded parameter of the first channel signal for obtaining CH 1 ′ that minimizes encoding distortion d expressed by Equation (5).
 
     Specifically, the LPC parameter for the first channel is first analyzed/quantized. The adaptive excitation codebook, the fixed excitation codebook and the excitation gain are then searched so as to minimize the encoding distortion expressed by Equation (5) above, and an adaptive excitation codebook index, a fixed excitation codebook index and an excitation gain index are determined. 
     Specifically, although the sum of the encoding distortion of CH 1  and the encoding distortion of CH 2  is minimized, it is only necessary to consider the encoding distortion of CH 1  in the process of encoding. The encoding distortion for CH 2  is thereby simultaneously considered. 
     By optimizing the encoding (adaptive excitation codebook index and fixed excitation codebook index) of the first channel parameter, it is possible to perform encoding so as to minimize the encoding distortion not only for the first channel signal, but also for the second channel signal. 
     Another variation of the algorithm used in distortion minimizing section  136  that minimizes the encoding distortion will next be described. A case will be described where the encoding distortion of the first channel signal and the encoding distortion of the second channel signal are weighted in accordance with the degree of accuracy when it is desired that the encoding distortion of the first channel signal and the encoding distortion of the second channel signal are perceptual-weighted at perceptual weighting section  135 , and either of the channel signals is encoded at high accuracy. Herein, α and β are weighting coefficients with respect to the encoding distortion of perceptual-weighted CH 1  and CH 2 , respectively. 
     Sum d′ of the encoding distortions for the first channel signal and the second channel signal is expressed by Equation (6) below.
 
 d ′=α×∥CH1−CH1′∥ 2 +β×∥CH2−CH2′∥ 2   (Equation 6)
 
     From the relationship of the monaural signal, the first channel signal and the second channel signal, CH 2 ′ can be expressed by already-encoded monaural synthesized signal M′ and first channel synthesized signal CH 1 ′ as shown in Equation (7) below.
 
CH2′=2× M ′−CH1′  (Equation 7)
 
Equation (6) thus becomes Equation (8) below.
 
 d ′=α×∥CH1−CH1′∥ 2 +β×∥CH2−(2× M ′−CH1′)∥ 2   (Equation 8)
 
The scalable encoding apparatus according to this embodiment obtains through search the first channel CELP encoded parameter so as to obtain CH 1 ′ that minimizes encoding distortion d′ expressed by Equation (8).
 
     Specifically, the LPC parameter for the first channel is first analyzed/quantized. The adaptive excitation codebook, the fixed excitation codebook and the excitation gain are then searched so as to minimize the encoding distortion expressed by Equation (8) above, and an adaptive excitation codebook index, a fixed excitation codebook index and a excitation gain index are determined. 
     Specifically, although the sum of the encoding distortion of CH 1  and the encoding distortion of CH 2  is minimized, it is only necessary to consider the encoding distortion of CH 1  in the process of encoding. The encoding distortion for CH 2  is thereby simultaneously considered. 
     Simultaneous consideration herein does not necessarily mean that the encoding distortions are considered in equal ratios. For example, when the first channel signal and the second channel signal are completely independent signals (for example, a speech signal and a separate music signal, the speech by person A and the speech by person B, or another case), and higher accuracy encoding of the first channel signal is desired, by setting weighting coefficient α for the distortion signal of the first channel signal so as to be larger than β, it is possible to make the distortion of the first channel signal smaller than the second channel signal. 
     In this way, by optimizing the encoding (adaptive excitation codebook index and fixed excitation codebook index) of the first channel parameter, it is possible to perform encoding so as to minimize the encoding distortion not only for the first channel signal, but also for the second channel signal. 
     The values of α and β may be determined by preparing the values in advance in a table according to a type of the input signal (such as a speech signal and a music signal), or the values may be determined by calculating an energy ratio of signals in a fixed interval (such as frame and sub-frame). 
       FIG. 5  is a block diagram showing the main configuration of scalable decoding apparatus  150  that decodes the encoded parameter generated by scalable encoding apparatus  100 , that is, corresponds to scalable encoding apparatus  100 . 
     Monaural signal CELP decoder  151  synthesizes monaural signal M′ from the CELP encoded parameter of the monaural signal. First channel signal decoder  152  synthesizes the first channel signal CH 1 ′ from the CELP encoded parameter of the first channel signal. 
     Second channel signal decoder  153  calculates second channel signal CH 2 ′ according to Equation (9) below from monaural signal M′ and first channel signal CH 1 ′.
 
CH2′=2 ×M ′−CH1′  (Equation 9)
 
     According to this embodiment, when CH 1  is encoded, the encoded parameter of CH 1  is determined so as to minimize the sum of the encoding distortion of CH 1  and the encoding distortion of CH 2 , so that it is possible to improve the decoding accuracy of CH 1  and CH 2  and prevent the degradation of the speech quality of the decoded signal. 
     In this embodiment, the encoded parameter of CH 1  is determined so as to minimize the sum of the encoding distortion of CH 1  and the encoding distortion of CH 2 , but the encoded parameter of CH 1  may also be determined so as to minimize both the encoding distortion of CH 1  and the encoding distortion of CH 2 . 
     Embodiment 2 
       FIG. 6  is a block diagram showing the main configuration of scalable encoding apparatus  200  according to embodiment 2 of the present invention. Scalable encoding apparatus  200  has the same basic configuration as scalable encoding apparatus  100  of embodiment 1. Components that are the same will be assigned the same reference numerals without further explanations. 
     In this embodiment, when CH 1  is encoded in the second layer, a difference parameter of CH 1  relative to the monaural signal is encoded. More specifically, first channel signal encoder  104   a  performs encoding in accordance with CELP encoding, that is, encoding using linear prediction analysis and adaptive excitation codebook search, on the first channel signal CH 1  inputted to scalable encoding apparatus  200 , and obtains a difference parameter between an encoded parameter obtained in the process and a CELP encoded parameter of the monaural signal outputted from monaural signal CELP encoder  103 . When this encoding is also referred to simply as CELP encoding, the above-described processing corresponds to obtaining a difference in the level (stage) of the CELP encoded parameter for monaural signal M and first channel signal CH 1 . First channel signal encoder  104   a  encodes the above-described difference parameter. By this means, the difference parameter is quantized, so that it is possible to perform more efficient encoding. 
     In the same way as in embodiment 1, monaural signal CELP encoder  103  performs CELP encoding on the monaural signal generated from the first channel signal and the second channel signal, and extracts and outputs a CELP encoded parameter of the monaural signal. The CELP encoded parameter of the monaural signal is also inputted to first channel signal encoder  104   a . Monaural signal CELP encoder  103  also outputs synthesized monaural signal M′ to first channel signal encoder  104   a.    
     The input of first channel signal encoder  104   a  is first channel signal CH 1 , second channel signal CH 2 , synthesized monaural signal M′, and the CELP encoded parameter of the monaural signal. First channel signal encoder  104   a  encodes the difference between the first channel signal and the monaural signal and outputs the CELP encoded parameter of the first channel signal. The monaural signal herein is already CELP-encoded, and the encoded parameter is extracted. Therefore, the CELP encoded parameter of the first channel signal is the difference parameter with respect to the CELP encoded parameter of the monaural signal. 
       FIG. 7  is a block diagram showing the main internal configuration of first channel signal encoder  104   a.    
     LPC quantizing section  132  calculates the difference LPC parameter between an LPC parameter of first channel signal CH 1  obtained by LPC analyzing section  131  and an LPC parameter of the monaural signal already calculated by monaural signal CELP encoder  103 , and quantizes the difference to obtain the final LPC parameter of the first channel. 
     The excitation is searched as follows. Adaptive excitation codebook  137   a  indicates the adaptive codebook lag of first channel CH 1  as the adaptive codebook lag of the monaural signal and a difference lag parameter with respect to the adaptive codebook lag of the monaural signal. Fixed excitation codebook  139   a  uses the fixed excitation codebook index for monaural signal M which is used in fixed excitation codebook  119  of monaural signal CELP encoder  103 A, as the fixed excitation codebook index of CH 1 . Specifically, fixed excitation codebook  139   a  uses the same index as that obtained in encoding of the monaural signal as the fixed excitation vector. 
     The excitation gain is expressed by the product of the adaptive excitation gain obtained by encoding monaural signal M, and a gain multiplier multiplied by the adaptive excitation gain; or the product of the fixed excitation gain obtained by encoding monaural signal M, and again multiplier (which is the same as that multiplied by the adaptive excitation gain) to be multiplied by the fixed excitation gain. This gain multiplier is encoded. 
       FIG. 8  is a block diagram showing the main configuration of scalable decoding apparatus  250  that corresponds to scalable encoding apparatus  200  described above. 
     First channel signal decoder  152   a  synthesizes first channel signal CH 1 ′ from both the CELP encoded parameter of the monaural signal and the CELP encoded parameter of the first channel signal. 
     In this way, according to this embodiment, when CH 1  is encoded in the second layer, the difference parameter relative to the monaural signal is encoded, so that it is possible to perform more efficient encoding. 
     Embodiments 1 and 2 according to the present invention were described above. 
     The scalable encoding apparatus and scalable decoding apparatus according to the present invention are not limited to the embodiments described above, and may include various types of modifications. 
     The scalable encoding apparatus and scalable decoding apparatus according to the present invention can also be provided in a communication terminal apparatus and a base station apparatus in a mobile communication system. By this means, it is possible to provide a communication terminal apparatus and a base station apparatus that have the same operational advantages as those described above. 
     In the embodiments described above, monaural signal M was the average signal of CH 1  and CH 2 , but this is by no means limiting. 
     The adaptive excitation codebook is also sometimes referred to as an adaptive codebook. The fixed excitation codebook is also sometimes referred to as a fixed codebook, a noise codebook, a stochastic codebook or a random codebook. 
     The case has been described as an example where the present invention is implemented with hardware, the present invention can be implemented with software. 
     Furthermore, each function block used to explain the above-described embodiments is typically implemented as an LSI constituted by an integrated circuit. These may be individual chips or may partially or totally contained on a single chip. 
     Here, each function block is described as an LSI, but this may also be referred to as “IC”, “system LSI”, “super LSI”, “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 a programmable FPGA (Field Programmable Gate Array) or a reconfigurable processor in which connections and settings of circuit cells within 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 development of semiconductor technology or a derivative other technology, it is naturally also possible to carry out function block integration using this technology. Application in biotechnology is also possible. 
     The present application is based on Japanese Patent Application No. 2004-288327, filed on Sep. 30, 2004, the entire content of which is expressly incorporated by reference herein. 
     INDUSTRIAL APPLICABILITY 
     The scalable encoding apparatus, scalable encoding apparatus, and method according to the present invention can be applied to a communication terminal apparatus, a base station apparatus, or other apparatus that perform scalable encoding on a stereo signal using CELP encoding in a mobile communication system.