Abstract:
Disclosed is an audio encoding device which removes unnecessary inter-channel parameters from the subject to be encoded, improving the encoding efficiency thereby. In this audio encoding device, a principal component analysis unit ( 301 ) converts an inputted left signal {L sb (f)} and an inputted right signal {R sb (f)} into a principal component signal {PC sb (f)} and an ambient signal {A sb (f)} and calculates for each sub-band, a rotation angle which indicates the degree of conversion; a monophonic encoding unit ( 303 ) encodes the principal component signal {Pc sb (f)); a rotation angle encoding unit ( 302 ) encodes the angle of rotation {θ b }; a local monophonic decoding unit ( 603 ) creates a decoded principal component signal; and a redundant parameter elimination unit ( 604 ) identifies the redundant parameters by analyzing the encoding quality of the decoded principal component signal and eliminates the redundant parameters from the signal to be encoded.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a speech coding apparatus and a speech coding method and more particularly relates to a speech coding apparatus and a speech coding method capable of deleting redundant inter-channel parameters. 
       BACKGROUND ART 
       [0002]    Generally, a stereo speech coding method or a multi-channel speech coding method include two methods. 
         [0003]    One is the method to individually encode different channel signals, and this method can be easily applied to stereo speech signals or multi-channel speech signals. However, since this method does not delete inter-channel redundancy, the entire coding bit rate becomes proportional to the number of channels, and hence results in a higher bit rate. 
         [0004]    The other is the method to parametrically encode a stereo speech signal or a multi-channel speech signal. The basic principle of this method is as follows. That is, at first, a coding side down-mixes or transforms an input signal into a signal of fewer channels than (or the same number as) those of the input signal. Next, the coding side encodes the down-mixed or transformed signal using the conventional speech coding method. In parallel with this, the coding side calculates inter-channel parameters representing inter-channel relationship from an original signal, encodes and then transmits the inter-channel parameters to a decoding side such that the decoding side can generate a stereo image or a multi-channel image. This method can encode inter-channel parameters with a smaller amount of coding than the amount of coding to encode a speech signal itself, thus making it possible to realize a lower bit rate. 
         [0005]    A parametric stereo coding system or a multi-channel coding system widely use a principal component analysis (PCA) (Non-Patent Literature 1), a binaural cue coding method (BCC) (Non-Patent Literature 2), an inter-channel prediction (ICP) (Non-Patent Literature 3), and intensity stereo (IS) (Non-Patent Literature 4). The above methods generate and then transmit certain inter-channel parameters to a decoding side. For example, a binaural cue coding method (BCC) generates inter-channel level difference (ICLD), inter-channel time difference (ICTD), and inter-channel coherence (ICC) as the inter-channel parameters. Also, as inter-channel parameters, an inter-channel prediction (ICP), intensity stereo (IS), and a principal component analysis (PCA) generate an inter-channel prediction coefficient, an energy scale coefficient, and a rotation angle, respectively. 
         [0006]    Since BCC, ICP, IS, and PCA require to obtain highly precise inter-channel parameters, it is general to calculate and encode the inter-channel parameters on a subband basis. 
         [0007]      FIG. 1  and  FIG. 2  simply illustrate configurations of parametric multi-channel codecs, and the meanings of signs in  FIG. 1  and  FIG. 2  are as follows. 
         [0008]    {X i     —     sb }: a series of multi-channel signals divided into a plurality of subbands (which represents signals in a frequency domain, a time domain, or a hybrid domain where the frequency domain and the time domain are combined) 
         [0009]    {Y i     —     sb }: a series of down-mixed or transformed signals calculated every subband (which are the signals in the same domain as {x i     —     sb }) 
         [0010]    {P i     —     sb }: a series of inter-channel parameters calculated every subband 
         [0011]    The following will be explained assuming that down-mixing is performed. 
         [0012]    At the coding side illustrated in  FIG. 1 , inter-channel parameter generating section  101  down-mixes input signals {x i     —     sb } by BCC, PCA or the like, and generates down-mixed signals {y i     —     sb } and inter-channel parameters {P i     —     sb }. 
         [0013]    Coding section  102  encodes down-mixed signal {y i     —     sb }, and coding section  103  (inter-channel parameter coding section), which is separately provided, encodes the inter-channel parameters {P i     —     sb }. 
         [0014]    Multiplexing section  104  multiplexes coding parameters of down-mixed signals {y i     —     sb } and coding parameters of inter-channel parameters {P i     —     sb }, which generates a bit stream. This bit stream is transmitted to a decoding side. 
         [0015]    At the decoding side illustrated in  FIG. 2 , demultiplexing section  201  demultiplexes the bit stream to obtain coding parameters of the down-mixed signals and the inter-channel parameters. 
         [0016]    Decoding section  202  performs decoding processing using the coding parameters of the down-mixed signals, and generates decoded down-mixed signals {y{tilde over ( )} i     —     sb }. 
         [0017]    Decoding section  203  (inter-channel parameter decoding section) performs decoding processing using the coding parameters of the inter-channel parameters, and generates decoded inter-channel parameters {P{tilde over ( )} i     —     sb }. 
         [0018]    Inter-channel parameter applying section  204  up-mixes decoded down-mixed signals {y{tilde over ( )} i     —     sb } using spatial information represented by the decoded inter-channel parameters {P{tilde over ( )} i     —     sb }, and generates decoded signals {x{tilde over ( )} i     —     sb }. 
         [0019]    Non-Patent Literature 1 describes a codec based on a principal component analysis (PCA) in the frequency domain.  FIG. 3  and  FIG. 4  illustrate configurations of a coding apparatus and a decoding apparatus based on PCA in Non-Patent Literature 1. The meanings of signs are as follows. 
         [0020]    {L sb (f)}: left signals divided into a plurality of subbands 
         [0021]    {R sb (f)}: right signals divided into a plurality of subbands 
         [0022]    {Pc sb (f)}: principal-component signals calculated every subband by a principal component analysis 
         [0023]    {A sb (f)}: ambient signals calculated every subband by a principal component analysis 
         [0024]    {θ sb }: rotation angles calculated every subband by a principal component analysis 
         [0025]    {PcAR sb }: energy ratios of principal component signals to ambient signals, the ratios calculated every subband 
         [0026]    At a coding side illustrated in  FIG. 3 , principal component analyzing section  301  transforms input left signals {L sb (f)} and input right signals {R sb (f)} into principal-component signals {Pc sb (f)} and ambient signals {A sb (f)}. In this transforming processing, the rotation angles each representing a transform degree are calculated every subband as the following. 
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         [0027]    The transform of a principal component analysis is performed as the following equation. 
         [0000]      (Equation 2) 
         [0000]        Pc   sb ( f )= L   sb ( f )*cos θ sb   +R   sb ( f )*sin θ sb  
 
         [0000]        A   sb ( f )= R   sb ( f )*cos θ sb   −L   sb ( f )*sin θ sb    [2]
 
         [0028]    Monaural coding section  303  encodes principal-component signals {Pc sb (f)}. 
         [0029]    Coding section  302  (rotation angle coding section) encodes rotation angles {θ sb }. 
         [0030]    Ambient signals {A sb (f)} are not regarded as important and thereby are not directly encoded. Energy parameter extracting section  304  calculates energy ratios {PcAR sb } of principal-component signals to ambient signals, and coding section  305  (energy ratio coding section) encodes the energy ratios {PcAR sb } and generates energy ratio coding parameters. The energy ratios {PcAR sb } are calculated as the following equation. 
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         [0031]    Multiplexing section  306  multiplexes coding parameters of principal-component signals {Pc sb (f)}, rotation angles {θ sb }, and energy ratios {PcAR sb }, and transmits a bit stream to a decoding side. 
         [0032]    At the decoding side illustrated in  FIG. 4 , demultiplexing section  401  demultiplexes the bit stream, and obtains coding parameters of the principal-component signals, coding parameters of the rotation angles, and coding parameters of the energy ratios. 
         [0033]    Decoding section  402  (rotation angle decoding section) decodes the coding parameters of the rotation angles and outputs the decoded rotation angles {θ{tilde over ( )} i     —     sb } to principal component combining section  406 . 
         [0034]    Monaural decoding section  403  decodes the coding parameters of the principal-component signals, generates and then outputs decoded principal-component signals {P{tilde over ( )}c sb (f)} to principal component combining section  406  and ambient signal combining section  405 . 
         [0035]    Decoding section  404  (energy ratio decoding section) decodes the coding parameters of the energy ratios and generates decoded energy ratios {P{tilde over ( )}cAR sb } of the principal-component signals to the ambient signals. 
         [0036]    By scaling the decoded principal-component signals {P{tilde over ( )}c sb (f)} by the decoded energy ratios, ambient signal combining section  405  generates decoded ambient signals {A{tilde over ( )} sb (f)}. 
         [0037]    Principal component combining section  406  inversely transforms decoded principal-component signals {P{tilde over ( )}c sb (f)} and decoded ambient signals {A{tilde over ( )} sb (f)} by decoded rotation angles {θ{tilde over ( )} i     —     sb }, and generates decoded left signals {L{tilde over ( )} sb (f)} and decoded right signals {R{tilde over ( )} sb (f)}. This inverse transformation is performed as the following equation. 
         [0000]      (Equation 4) 
         [0000]        {tilde over (L)}   sb ( f )= {tilde over (P)}c   sb ( f )*cos {tilde over (θ)} sb   −Ã   sb ( f )*sin {tilde over (θ)} sb  
 
         [0000]        {tilde over (R)}   sb ( f )= {tilde over (P)}c   sb ( f )*sin {tilde over (θ)} sb   +Ã   sb ( f )*cos {tilde over (θ)} sb    [4]
 
         [0038]    In the case that the ambient signals are not encoded, the inverse transformation is performed as the following equation. 
         [0000]      (Equation 5) 
         [0000]        {tilde over (L)}   sb ( f )= {tilde over (P)}c   sb ( f )*cos {tilde over (θ)} sb  
 
         [0000]        {tilde over (R)}   sb ( f )= {tilde over (P)}c   sb ( f )*sin {tilde over (θ)} sb    [5]
 
       CITATION LIST 
     Non-Patent Literature 
       [0000]    
       
         NPL 1 
         Manuel Briand, David Virette and Nadine Martin “Parametric coding of stereo audio based on principal component analysis”, Proc of the 9 th  International Conference on Digital Audio Effects, Montreal, Canada, Sep. 18-20, 2006. 
         NPL 2 
         Christof Faller and Frank Baumgarte “Binaural Cue Coding—Part II:Schemes and Applications”, IEEE Transactions on Speech and Audio Processing, Vol. 11, No 6, November 2003 
         NPL 3 
         Hendrik Fuchs “Improving Joint Stereo Audio Coding by Adaptive Inter-channel Prediction”, Proc of IEEE ASSP Workshop on Applications of Signal Processing to Audio and Acoustics, New Paltz, N.Y., USA, Oct. 17-20, 1993 
         NPL 4 
         Jurgen Herre, “From Joint Stereo to Spatial Audio Coding—Recent Progress and Standardization”, Proc of the 7th International Conference on Digital Audio Effects, Naples, Italy, Oct. 5-8, 2004. 
       
     
       SUMMARY OF INVENTION 
     Technical Problem 
       [0047]    Irrespective of coding quality or signal-level sizes of down-mixed signals {y i     —     sb }, the above conventional art encodes inter-channel parameters at a predetermined bit rate. Even when the down-mixed signals are not encoded at all in one or a plurality of subbands, the inter-channel parameter coding is performed irrespective of this situation. 
         [0048]    Here, let us consider, as an example, a case where down-mixed signals of one or a plurality of subbands are not encoded, in the case of an extremely low bit rate. In these subbands where down-mixed signals are not encoded, the inter-channel parameters are unnecessary in generating multi-channel speech signals, and coding of these unnecessary parameters results in wasting bits used in the coding processing. 
         [0049]    Hereinafter, a case will be described exemplifying the above codec based on a principal component analysis in the frequency domain. 
         [0050]    It is assumed that when input signals are represented as L(n) and R(n), these signals can be represented as L(n)=S(n)+C(n) and R(n)=S(n)+B(n) (S(n) means the main source signal, and C(n) and B(n) means certain ambient noise). 
         [0051]    In the case of the frequency domain, L(f)=S(f)+C(f) and R(f)=S(f)+B(f) hold true. In the subband where S(f) is not so strong, the ambient noise is dominant; that is, C(f) is dominant in L(f) and B(f) is dominant in R(f). In this case, these types of subbands are not so important in the whole spectrum that signals in these subbands are not encoded in the case of a low bit rate. Therefore, coding of rotation angles in these subbands is essentially not necessary. For this reason, the conventional art which always encodes the rotation angles of all subbands wastes the bits allocated to the coding of the rotation angles in these subbands. 
         [0052]    Referring to  FIG. 5  illustrating the above problematic case, under the condition of a low bit rate, the coding side does not encode principal-component signal Pc 2 (f) of the second subband of which energy of the principal-component signal is smaller than the energy of other subbands. Therefore, in the decoding side, the decoded principal-component signal of the second subband is 0. Since ambient signals are generated by scaling the principal-component signals, the ambient signal of the second subband also is 0. In this case, even if the rotation angle has any value, decoded left signal L{tilde over ( )} 2 (f) and decoded right signal R{tilde over ( )} 2 (f) of the second subband become 0. That is, the decoded left signal and the decoded right signal of the second subband are the same regardless of whether or not the rotation angle is transmitted. 
         [0053]    It is therefore an object of the present invention to provide a speech coding apparatus and a speech coding method capable of deleting the redundant inter-channel parameters. 
       Solution to Problem 
       [0054]    In the first aspect of the present invention, before encoding and transmitting inter-channel parameters, a coding apparatus analyzes signal characteristics of each subband signal and checks whether or not it is necessary to transmit inter-channel parameters. Then, the coding apparatus selects inter-channel parameters not necessary to be transmitted and deletes the parameters from coding targets. 
         [0055]    By this means, it is possible to delete the unnecessary inter-channel parameters from the coding targets and to prevent encoding the unnecessary parameters, which makes it possible to improve a coding efficiency without wasting bits. 
         [0056]    In the second aspect of the present invention, redundant parameters are selected by a closed loop method. Introduction of a local decoding section at the coding side and analysis of signal coding quality selects the redundant parameters. By analyzing the energy or amplitude of decoded down-mix signals generated via the local decoding section, the subband with small energy or amplitude is regarded as a subband having a redundant inter-channel parameter. Deletion of the inter-channel parameter of this subband from the coding targets prevents a possibility of decreasing sound quality. 
         [0057]    By this means, the local decoding section can select the subband having the redundant parameter (unimportant inter-channel parameter). 
         [0058]    In the third aspect of the present invention, the redundant parameters are selected by an open loop method. An analysis of the characteristics of transformed or down-mixed original signals selects the redundant parameters. 
         [0059]    Therefore, the present embodiment does not require a local decoding section and is useful in the condition incapable of using the local decoding section. Also, absence of the local decoding section can reduce the amount of calculations. 
         [0060]    In the fourth aspect of the present invention, after decoding, the decoding side analyzes the transformed or down-mixed signals and selects the subband without an inter-channel parameter. Therefore, flag signals are not required, the signals reporting to the decoding section that a specific subband does not include the inter-channel parameter. 
         [0061]    By this means, unnecessity of additional information representing the flag signals can improve the coding efficiency. 
         [0062]    The fifth aspect of the present invention uses the bits saved by applying the present invention in order to encode certain more important signals (for example, the coding parameters of the principal-component signals, and the coding parameters of the transformed or down-mixed signals). 
         [0063]    Thus, realization of more precise bit allocation can improve the coding efficiency. 
         [0064]    In the sixth aspect of the present invention, the decoding side predicts non-existent inter-channel parameters from parameters of adjacent subbands, parameters of a former frame, or both of them. The predicted value is used on inverse transformation or up-mixing. 
         [0065]    By this means, it is possible to predict non-existent inter-channel parameters and to maintain spatial images. 
         [0066]    The seventh aspect of the present invention applies the present invention for scalable coding. In each layer, before encoding and transmitting inter-channel parameters, the coding apparatus analyzes the characteristics of the transformed or down-mixed signals every subband signal, and checks whether or not it is necessary to transmit inter-channel parameters. Then, the coding apparatus selects the inter-channel parameter not necessary to be transmitted and deletes the parameter from the coding targets. In the case of a layer where inter-channel parameters are necessary to generate input signals, the coding apparatus transmits the inter-channel parameters. 
         [0067]    By this means, since the coding apparatus transmits the inter-channel parameters only in the case of the layer requiring the inter-channel parameters, it is possible to realize precise bit allocation. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0068]      FIG. 1  illustrates a coding side configuration in parametric multi-channel speech coding; 
           [0069]      FIG. 2  illustrates a decoding side configuration in parametric multi-channel speech coding; 
           [0070]      FIG. 3  illustrates a coding side configuration in stereo codec based on PCA; 
           [0071]      FIG. 4  illustrates a decoding side configuration in stereo codec based on PCA; 
           [0072]      FIG. 5  illustrates a problem in stereo codec based on PCA; 
           [0073]      FIG. 6  illustrates a configuration of a speech coding apparatus according to embodiment 1 of the present invention in stereo codec based on PCA; 
           [0074]      FIG. 7  illustrates a coding processing according to embodiment 1 of the present invention in stereo codec based on PCA; 
           [0075]      FIG. 8  illustrates a configuration of a speech decoding apparatus according to embodiment 1 of the present invention in stereo codec based on PCA; 
           [0076]      FIG. 9  illustrates decoding processing according to embodiment 1 of the present invention in stereo codec based on PCA; 
           [0077]      FIG. 10  illustrates a configuration of a speech coding apparatus according to embodiment 2 of the present invention in multi-channel speech coding; 
           [0078]      FIG. 11  illustrates coding processing according to embodiment 2 of the present invention in multi-channel speech coding; 
           [0079]      FIG. 12  illustrates a configuration of a speech decoding apparatus according to embodiment 2 of the present invention in multi-channel speech coding; 
           [0080]      FIG. 13  illustrates decoding processing according to embodiment 2 of the present invention in multi-channel speech coding; 
           [0081]      FIG. 14  illustrates a configuration of a speech decoding apparatus according to embodiment 3 of the present invention in multi-channel speech coding; 
           [0082]      FIG. 15  illustrates decoding processing according to embodiment 3 of the present invention in multi-channel speech coding; 
           [0083]      FIG. 16  illustrates a configuration of a speech coding apparatus according to embodiment 4 of the present invention in multi-channel speech coding; 
           [0084]      FIG. 17  illustrates coding processing according to embodiment 4 of the present invention in multi-channel speech coding; 
           [0085]      FIG. 18  illustrates a configuration of a speech decoding apparatus according to embodiment 4 of the present invention in multi-channel speech coding; 
           [0086]      FIG. 19  illustrates decoding processing according to embodiment 4 of the present invention in multi-channel speech coding; 
           [0087]      FIG. 20  illustrates a configuration of a speech coding apparatus according to embodiment 5 of the present invention in multi-channel speech coding; 
           [0088]      FIG. 21  illustrates coding processing according to embodiment 5 of the present invention in multi-channel speech coding; 
           [0089]      FIG. 22  illustrates a configuration of a speech decoding apparatus according to embodiment 5 of the present invention in multi-channel speech coding; and 
           [0090]      FIG. 23  illustrates decoding processing according to embodiment 5 of the present invention in multi-channel speech coding. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0091]    Embodiments of the present invention will now be described with reference to the accompanying drawings. 
       Embodiment 1 
       [0092]    The present embodiment will be described referring to  FIG. 6  to  FIG. 9 . 
         [0093]      FIG. 6  illustrates a configuration of speech coding apparatus  600  according to the present embodiment.  FIG. 6  additionally includes local monaural decoding section  603  and redundant parameter deleting section  604 , in comparison with  FIG. 3 . In  FIG. 6 , descriptions on the components as the same as those in  FIG. 3  will be omitted. 
         [0094]    Local monaural decoding section  603  generates decoded principal-component signals such that a coding side can confirm the coding quality of the principal-component signals. 
         [0095]    Through analysis of the coding quality of the decoded principal-component signals, redundant parameter deleting section  604  selects redundant parameters and deletes these parameters from coding targets. 
         [0096]    The coding processing according to the present embodiment will be described referring to  FIG. 7 . 
         [0097]    As illustrated in  FIG. 7 , spectra of the principal-component signals are encoded and decoded. Analyzing the decoded spectra after generating the decoded spectrum, shows that the principal component of the second subband is not encoded at all, and therefore the decoded spectrum of the second subband is 0. Thus, there is no need to encode the rotation angle of the second subband. For this reason, the rotation angle of the second subband is regarded as a redundant parameter, and this parameter is deleted from the coding targets before encoding. 
         [0098]      FIG. 8  illustrates a configuration of speech decoding apparatus  800  according to the present embodiment.  FIG. 8  additionally includes zero-value inserting section  804 , in comparison with  FIG. 4 . In  FIG. 8 , descriptions on the components as the same as those in  FIG. 4  will be omitted. 
         [0099]    Zero-value inserting section  804  analyzes the decoded principal-component signals, selects the subband without a rotation angle, and inserts a zero value to the subband, so that inverse transformation can be performed smoothly. 
         [0100]    The decoding processing according to the present embodiment will be described referring to  FIG. 9 . 
         [0101]    As illustrated in  FIG. 9 , analyzing the decoded principal-component signals after generating the decoded principal-component, shows that the decoded principal-component signal of the second subband is 0 and that the rotation angle in the second subband is not encoded. Therefore, the decoding side decodes only rotation angles of other subbands. Also, in order to perform decoding processing smoothly, the decoding side inserts a zero value as the decoded rotation angle of the second subband. 
         [0102]    The present invention can be applied to encoding of the energy ratios of principal-component signals to ambient signals. 
       Embodiment 2 
       [0103]    The present embodiment will be described referring to  FIG. 10  to  FIG. 13 . The meanings of signs in  FIG. 10  to  FIG. 13  are as follows. 
         [0104]    {X i     —     sb }: multi-channel signals divided into a plurality of subbands (which represents signals in a frequency domain, a time domain, or a hybrid domain where the frequency domain and the time domain are combined) 
         [0105]    {Y i     —     sb }: down-mixed or transformed signals divided into a plurality of subbands (which are the signals in the same domains as {x i     —     sb }) 
         [0106]    {P i     —     sb }: inter-channel parameters calculated every subband 
         [0107]    {X{tilde over ( )} i     —     sb }: decoded signals of {x i     —     sb } 
         [0108]    {Y{tilde over ( )} i     —     sb }: decoded signals of {y i     —     sb } 
         [0109]    {P{tilde over ( )} i     —     sb }: decoded inter-channel parameters 
         [0110]    The present embodiment deletes redundant parameters in multi-channel speech coding. 
         [0111]      FIG. 10  illustrates a configuration of speech coding apparatus  1000  according to the present embodiment. 
         [0112]    In speech coding apparatus  1000 , inter-channel parameter generating section  1001  transforms or down-mixes input signals {x i     —     sb } into {y i     —     sb } by BCC, PCA or the like. During transforming and down-mixing processing, inter-channel parameter generating section  1001  also generates inter-channel parameters {P i     —     sb }. 
         [0113]    Coding section  1002  encodes the transformed or down-mixed signals {y i     —     sb }. 
         [0114]    Local decoding section  1003  generates signals transformed or down-mixed after decoding, such that the coding side can identify coding quality of the transformed or down-mixed signals. 
         [0115]    By analyzing the coding quality of the transformed or down-mixed signals, deleting section  1004  selects redundant parameters and deletes these parameters from coding targets. 
         [0116]    Coding section  1005  (inter-channel parameter coding section) encodes the remaining inter-channel parameters {P′ i     —     sb } after the deletion of the redundant parameters. 
         [0117]    Multiplexing section  1006  multiplexes coding parameters of {y i     —     sb } and coding parameters of  {P′   i     —     sb }, generates and then transmits a bit stream to the decoding side. 
         [0118]    The coding processing according to the present embodiment will be described referring to  FIG. 11 . 
         [0119]    As illustrated in  FIG. 11 , spectra of the transformed or down-mixed signals are encoded and decoded. Analyzing the decoded spectra after generating the decoded spectra, shows that, since the transformed or down-mixed signal, for example in the second subband, is critically weak (in an extreme case, the second subband is not encoded at all), the decoded signal is 0. In this case, there is no need to encode the inter-channel parameter of the second subband. Therefore, the inter-channel parameter of the second subband is regarded as the redundant parameter, and deletes this parameter from the coding targets before encoding. 
         [0120]    There are many methods, such as the following two, to determine whether or not the decoded subband signals are sufficiently weak. However, the present invention is not limited to the following methods. 
         [0121]    &lt;Method 1&gt; Case Where Signal Energy of Subband is Extremely Lower than Adjacent Subbands 
         [0122]    Every subband, this method calculates energy {E sb } and energy ratios of the subband to the adjacent subbands, and then compares the energy ratios with a predetermined value E th  (E th &lt;1). When both energy ratios are smaller than E th , the subband signal is regarded as weak. For example, two energy ratios E 2 /E 1  and E 2 /E 3  are calculated in the second subband. If E 2 /E 1 &lt;E th  and E 2 /E 3 &lt;E th  hold true, the signal of the second subband is regarded as weak in this case. In this case, the inter-channel parameter of the second subband is regarded as the redundant parameter. 
         [0123]    &lt;Method 2&gt; Case Where Subband Signal is Close to or Lower than Masking Curve 
         [0124]    Every subband, this method calculates energy {E sb } and masking curve level {M sb }, and then compares the masking curve level with the subband energy. In this case, it is possible to define another threshold M th  (M th &gt;0). When the subband energy is smaller than or close to a masking curve, that is, E sb &lt;M sb +M th  holds true, the subband signal is regarded as weak. For example, subband energy E 2  is compared with masking curve level M 2 . If E 2 &lt;M 2 +M th  holds true, the signal of the second subband is regarded as weak. Therefore, the inter-channel parameter in the second subband is regarded as the redundant parameter. 
         [0125]      FIG. 12  illustrates a configuration of speech decoding apparatus  1200  according to the present embodiment. 
         [0126]    In speech decoding apparatus  1200 , demultiplexing section  1201  demultiplexes the bit stream. 
         [0127]    Decoding section  1202  decodes coding parameters of {y i     —     sb }, and generates transformed or down-mixed signals {y{tilde over ( )} i     —     sb }. 
         [0128]    Decoding section  1203  (inter-channel parameter decoding section) decodes coding parameters of {P′ i     —     sb }, and generates decoded inter-channel parameters {P{tilde over ( )}′ i     —     sb }. 
         [0129]    Zero-value inserting section  1204  analyzes the decoded spectra of the transformed or down-mixed signals, selects the subband without an inter-channel parameter, and inserts a zero value in the subband so that inverse transformation or up-mixing can be performed smoothly. 
         [0130]    By using spatial information represented by the decoded inter-channel parameters {P{tilde over ( )} i     —     sb }, inter-channel parameter applying section  1205  inversely transforms or up-mixes decoded signals {y{tilde over ( )} i     —     sb } to generate {x{tilde over ( )} i     —     sb }. 
         [0131]    The decoding processing according to the present embodiment will be described referring to  FIG. 13 . 
         [0132]    As illustrated in  FIG. 13 , analyzing the decoded spectra after generating the decoded spectra, shows that the decoded signal of the second subband is critically weak (in an extreme case, the decoded signal is 0). That is, the inter-channel parameter of the second subband is not encoded. Thus, only inter-channel parameters of other subbands are decoded. In order to perform the decoding processing smoothly, a zero value is inserted to the decoded inter-channel parameter of the second subband. The method of the decoding side to determine whether or not the inter-channel parameters are encoded is the same as the method of the coding side for the purpose of maintaining consistency with the coding side. 
         [0133]    As described above, before encoding and transmitting inter-channel parameters, the present embodiment analyzes the signal characteristics per signal transformed in each subband, and checks whether or not it is necessary to transmit the inter-channel parameters. Then, the inter-channel parameter not necessary to be transmitted is selected and deleted from the coding targets. 
         [0134]    Therefore, according to the present embodiment, by deleting unnecessary inter-channel parameters from the coding targets, it is possible to prevent encoding the unnecessary parameters and hence to improve a coding efficiency. 
         [0135]    Also, according to the present invention, the redundant parameters are selected by a closed loop method. That is, by analyzing the coding quality of signals, the local decoding section in the coding side selects redundant parameters. 
         [0136]    Thus, according to the present embodiment, the local decoding section can specify the subband including the redundant parameter (unimportant inter-channel parameter). Thus, the possibility of decreasing sound quality is avoided. 
         [0137]    Also, according to the present invention, the decoding side selects a subband in which no inter-channel parameter exists, by decoding and analyzing the transformed or down-mixed signals. Therefore, a flag signal reporting to the decoding section that no inter-channel parameter exists in a specific subband is not required. 
         [0138]    As mentioned above, according to the present embodiment, unnecessity of additional information to represent the flag signals can improve the coding efficiency. 
       Embodiment 3 
       [0139]    The present embodiment will be described referring to  FIG. 14  and  FIG. 15 . The meanings of signs in  FIG. 14  and  FIG. 15  are the same as those of embodiment 2. 
         [0140]    In the present embodiment, the decoding side predicts the non-existent inter-channel parameter, from parameters of adjacent subbands, parameters of the former frame, or both of them. The predicted value is used in performing inverse transformation or up-mixing. 
         [0141]      FIG. 14  illustrates a configuration of speech decoding apparatus  1400  according to the present embodiment. In  FIG. 14 , zero-value inserting section  1204  illustrated in  FIG. 12  is replaced with missing parameter predicting section  1404 . In  FIG. 14 , descriptions on the components as the same as those in  FIG. 12  will be omitted. 
         [0142]    In speech decoding apparatus  1400 , missing parameter predicting section  1404  predicts the non-existent inter-channel parameter by using the parameters of the adjacent subbands or the parameters of the former frame without insertion of a zero value into the non-existent inter-channel parameter. 
         [0143]    The decoding processing according to the present embodiment will be described referring to  FIG. 15 . 
         [0144]      FIG. 15  illustrates an example of a case where, because of the absence of the inter-channel parameter in the second subband in the decoding side, the decoding side predicts this inter-channel parameter from the parameters of the adjacent subbands or the parameters of the former frame. 
         [0145]    There are many other methods to predict non-existent inter-channel parameters. 
         [0146]    For example, as the following equation, there is a method to interpolate the non-existent inter-channel parameter using the parameters of the adjacent subbands. 
         [0000]    
       
         
           
             
               
                 
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         [0147]    Also, as the following equation, there is a method to predict a non-existent inter-channel parameter using the parameters of the former frame. This method is effective when the spatial image is stable in a time domain. 
         [0000]      (Equation 7) 
         [0000]      {tilde over (P)} i     —     2 ={tilde over (P)} i     —     2     —     old    [7]
 
         [0148]    As described above, according to the present embodiment, the decoding side predicts the non-existent inter-channel parameter from the parameters of the adjacent subbands, the parameters of the former frame, or both of them. The predicted value is used on performing inverse transformation or up-mixing. 
         [0149]    By this means, it is possible to predict the non-existent inter-channel parameters to maintain spatial images. 
       Embodiment 4 
       [0150]    The present embodiment will be described referring to  FIG. 16  to  FIG. 19 . The meanings of signs in  FIG. 16  to  FIG. 19  are as follows. 
         [0151]    {X i     —     sb }: multi-channel signals divided into a plurality of subbands (which represents signals in a frequency domain, a time domain, or a hybrid domain where the frequency domain and the time domain are combined) 
         [0152]    {Y i     —     sb }: down-mixed or transformed signals divided into a plurality of subbands (which are the signals in the same domain as {x i     —     sb }) 
         [0153]    {P i     —     sb }: inter-channel parameters calculated every subband 
         [0154]    {X{tilde over ( )} i     —     sb }: decoded signals of {x i     —     sb } 
         [0155]    {Y{tilde over ( )} i     —     sb }: decoded signals of {y i     —     sb } 
         [0156]    {P{tilde over ( )} i     —     sb }: decoded inter-channel parameters 
         [0157]    In the present invention, an open loop method selects redundant parameters. By analyzing the characteristics of the transformed or down-mixed original signal, the present embodiment selects the redundant inter-channel parameters and deletes the parameters from the coding targets. 
         [0158]      FIG. 16  illustrates a configuration of speech coding apparatus  1600  according to the present embodiment. 
         [0159]    In speech coding apparatus  1600 , inter-channel parameter generating section  1601  transforms or down-mixes input signal {x i     —     sb } into {y i     —     sb } by BCC, PCA or the like. During the transforming and down-mixing processing, inter-channel parameter generating section  1601  also generates inter-channel parameter {P i     —     sb }. 
         [0160]    Coding section  1602  encodes the transformed or down-mixed signal {y i     —     sb }. 
         [0161]    Signal analyzing section  1603  selects the redundant parameters by analyzing the signal characteristics of the transformed or down-mixed signal {y i     —     sb }. 
         [0162]    Redundant parameter deleting section  1604  selects the redundant parameters and deletes the parameters from the coding targets. 
         [0163]    Coding section  1605  (inter-channel parameter coding section) encodes remaining inter-channel parameters {P′ i     —     sb } after deleting the redundant parameters. 
         [0164]    Multiplexing section  1606  multiplexes coding parameters of {y i     —     sb } and coding parameters of {P′ i     —     sb }, generates and then transmits a bit stream to the decoding side. 
         [0165]    The coding processing according to the present embodiment will be described referring to  FIG. 17 . 
         [0166]    As illustrated in  FIG. 17 , the characteristics of the transformed or down-mixed signals are analyzed by an energy analysis, a psychoacoustic analysis, a bit allocating analysis, or the like. The analysis shows that the transformed or down-mixed signal is critically weak, for example, in the second subband. In this case, there is no need to encode the inter-channel parameters of the second subband. Therefore, the inter-channel parameters of the second subband is regarded as the redundant parameters, and deleted from the coding targets before encoding. 
         [0167]    There are many methods, such as the following two, to determine whether or not the subband signals are sufficiently weak. However, the present invention is not limited to the followings. 
         [0168]    &lt;Method 1&gt; Case Where Signal Energy of Subband is Extremely Lower than Adjacent Subbands 
         [0169]    Every subband, this method calculates energy {E sb } and energy ratios of the subband to the adjacent subbands, and then compares the energy ratios with a certain predetermined value E th  (E th &lt;1). When both energy ratios are smaller than E th , the subband signal is regarded as weak. For example, two energy ratios E 2 /E 1  and E 2 /E 3  are calculated in the second subband. If E 2 /E 1 &lt;E th  and E 2 /E 3 &lt;E th  hold true, the signal of the second subband is regarded as weak in this case. In this case, the inter-channel parameter of the second subband is regarded as the redundant parameter. 
         [0170]    &lt;Method 2&gt; Case Where Subband Signal is Close to or Lower than Masking Curve 
         [0171]    Every subband, this method calculates energy {E sb } and masking curve level {M sb }, and then compares the masking curve level with the subband energy. In this case, it is possible to define another threshold M th  (M th &gt;0). When the subband energy is smaller than or close to a masking curve, that is, E sb &lt;M sb +M th  holds true, the subband energy is regarded as weak. For example, when subband energy E 2  is compared with masking curve level M 2  and thereby E 2 &lt;M 2 +M th  holds true, the signal of the second subband is regarded as weak. The inter-channel parameter in the second subband is regarded as the redundant parameter. 
         [0172]      FIG. 18  illustrates a configuration of speech decoding apparatus  1800  according to the present embodiment. 
         [0173]    In speech decoding apparatus  1800 , demultiplexing section  1801  demultiplexes the bit stream. 
         [0174]    Decoding section  1802  decodes coding parameters of {y i     —     sb }, and generates the transformed or down-mixed signals {y{tilde over ( )} i     —     sb }. 
         [0175]    Decoding section  1803  (inter-channel parameter decoding section) decodes coding parameters of {P′ i     —     sb }, and generates decoded inter-channel parameters {P{tilde over ( )}′ i     —     sb }. 
         [0176]    Zero-value inserting section  1804  analyzes the decoded spectrum of the transformed or down-mixed signal, selects the subband without an inter-channel parameter, and inserts a zero value in the subband so that inverse transformation or up-mixing can be performed smoothly. 
         [0177]    By using spatial information represented by decoded inter-channel parameters {P{tilde over ( )} i     —     sb }, inter-channel parameter applying section  1805  inversely transforms or up-mixes the decoded signals {y{tilde over ( )} i     —     sb } to generate {x{tilde over ( )} i     —     sb }. 
         [0178]    The decoding processing according to the present embodiment will be described referring to  FIG. 19 . 
         [0179]    As illustrated in  FIG. 19 , analyzing the decoded spectra after generating the decoded spectra, shows that the decoded signal of the second subband is critically weak (in an extreme case, the decoded signal is 0). That is, the inter-channel parameter of the second subband is not encoded. Thus, only inter-channel parameters of other subbands are decoded. In order to perform the decoding processing smoothly, a zero value is inserted to the decoded inter-channel parameter of the second subband. The method of the decoding side to determine whether or not the inter-channel parameters are encoded is the same as the method of the coding side for the purpose of maintaining consistency with the coding side. 
         [0180]    According to the present invention, the redundant parameters are selected by an open loop method. That is, an analysis of the characteristics of transformed or down-mixed original signals selects the redundant parameters. 
         [0181]    Therefore, the present embodiment does not require a local decoding section. Thus, the present embodiment is useful in the condition incapable of using the local decoding section. Also, absence of the local decoding section can reduce the amount of calculations. 
       Embodiment 5 
       [0182]    The present embodiment will be described referring to  FIG. 20  to  FIG. 23 . The meanings of signs in  FIG. 20  to  FIG. 23  are as follows. 
         [0183]    {X i     —     sb }: multi-channel signals divided into a plurality of subbands (which represents signals in a frequency domain, a time domain, or a hybrid domain where the frequency domain and the time domain are combined) 
         [0184]    {Y i     —     sb }: down-mixed or transformed signals divided into a plurality of subbands (which are the signals in the same domain as {x i     —     sb }) 
         [0185]    {P i     —     sb }: inter-channel parameters calculated every subband 
         [0186]    {X{tilde over ( )} i     —     sb }: decoded signals of {x i     —     sb } 
         [0187]    {Y{tilde over ( )} i     —     sb }: decoded signals of {y i     —     sb } 
         [0188]    {P{tilde over ( )} i     —     sb }: decoded inter-channel parameters 
         [0189]    The present embodiment deletes redundant parameters in scalable codec. 
         [0190]      FIG. 20  illustrates a configuration of speech coding apparatus  2000  according to the present embodiment. 
         [0191]    In speech coding apparatus  2000 , inter-channel parameter generating section  2001  transforms or down-mixes input signals {x i     —     sb } into {y i     —     sb } by BCC, PCA or the like. During transforming and down-mixing processing, inter-channel parameter generating section  2001  also generates inter-channel parameters {P i     —     sb }. 
         [0192]    Scalable coding section  2002  encodes the transformed or down-mixed signals {y i     —     sb }. 
         [0193]    Scalable local decoding section  2003  generates decoded signals of layers, such that the coding side can identify coding quality of the transformed or down-mixed signals. 
         [0194]    By analyzing the coding quality of the transformed or down-mixed signal, scalable redundant parameter deleting section  2004  selects redundant parameters and deletes these parameters from coding targets. 
         [0195]    Coding section  2005  (inter-channel parameter coding section) encodes the remaining inter-channel parameters {P′ i     —     sb } after deleting the redundant parameters. 
         [0196]    Multiplexing section  2006  multiplexes the coding parameters of {y i     —     sb } and coding parameters of {P′ i     —     sb }, generates and then transmits a bit stream to the decoding side. 
         [0197]    The coding processing according to the present embodiment will be described referring to  FIG. 21 . 
         [0198]    As illustrated in  FIG. 21 , spectra of the transformed or down-mixed signals are encoded and decoded. Analyzing the decoded spectra after generating the decoded spectra, shows that since the transformed or down-mixed signals, for example, in the second subband in layer  1  of  FIG. 21 , are critically weak (in an extreme case, the second subband is not encoded at all), the decoded signal is 0. In this case, in layer  1 , there is no need to encode the inter-channel parameter of the second subband. Therefore, in layer  1 , the inter-channel parameter of the second subband is regarded as the redundant parameter, and deletes this parameter from the coding targets before encoding. 
         [0199]    On the other hand, in layer  2 , the decoded signal of the second subband is not weak, and hence it is necessary to encode the inter-channel parameter in order to prevent possible deterioration of sound quality. Therefore, it is layer  2  that firstly encodes the inter-channel parameter of the second subband. 
         [0200]    There are many methods, such as the following two, to determine whether or not the subband signal is extremely weak. However, the present invention is not limited to the followings. 
         [0201]    &lt;Method 1&gt; Case Where Signal Energy of Subband is Extremely Lower than Adjacent Subbands 
         [0202]    Every subband, this method calculates energy {E sb } and energy ratios of the subband to the adjacent subbands, and then compares the energy ratios with a certain predetermined value E th  (E th &lt;1). When both energy ratios are smaller than E th , the subband signal is regarded as weak. For example, two energy ratios E 2 /E 1  and E 2 /E 3  are calculated in the second subband. If E 2 /E 1 &lt;E th  and E 2 /E 3 &lt;E th  hold true, the signal of the second subband is regarded as weak. The inter-channel parameter of the second subband is regarded as the redundant parameter. 
         [0203]    &lt;Method 2&gt; Case Where Subband Signal is Close to or Lower than Masking Curve 
         [0204]    Every subband, this method calculates energy {E sb } and masking curve level {M sb }, and then compares the masking curve level with the subband energy. In this case, it is possible to define another threshold M th  (M th &gt;0) When the subband energy is smaller than or close to a masking curve, that is, when E sb &lt;M sb +M th  holds true, the subband energy is regarded as weak. For example, when subband energy E 2  is compared with masking curve level M 2  and thereby E 2 &lt;M 2 +M th  holds true, the signal of the second subband is regarded as weak. The inter-channel parameter in this second subband is regarded as the redundant parameter. 
         [0205]      FIG. 22  illustrates a configuration of speech decoding apparatus  2200  according to the present embodiment. 
         [0206]    In speech decoding apparatus  2200 , demultiplexing section  2201  demultiplexes the bit stream in each layer. 
         [0207]    Scalable decoding section  2202  decodes coding parameters of {y i     —     sb }, and generates transformed or down-mixed signals {y{tilde over ( )} i     —     sb }. 
         [0208]    Decoding section  2203  (inter-channel parameter decoding section) decodes coding parameters of {P′ i     —     sb }, and generates decoded inter-channel parameters {P{tilde over ( )}′ i     —     sb }. 
         [0209]    In each layer, zero-value inserting section  2204  analyzes the decoded spectrum of the transformed or down-mixed signal, selects the subband without an inter-channel parameter, and inserts a zero value in the subband so that inverse transformation or up-mixing can be performed smoothly. 
         [0210]    By using spatial information represented by inter-channel parameters {P{tilde over ( )} i     —     sb }, inter-channel parameter applying section  2205  inversely transforms or up-mixes decoded signals {y{tilde over ( )} i     —     sb } to generate {x{tilde over ( )} i     —     sb }. 
         [0211]    The decoding processing according to the present embodiment will be described referring to  FIG. 23 . 
         [0212]    As illustrated in  FIG. 23 , analyzing the decoded spectra after generating the decoded spectra, shows that, in layer  1 , the decoded signal of the second subband is critically weak (in an extreme case, the decoded signal is 0). That is, the inter-channel parameter of the second subband is not encoded. Thus, only inter-channel parameters of other subbands are decoded. In order to perform the decoding processing smoothly, a zero value is inserted to the decoded inter-channel parameter of the second subband. 
         [0213]    On the other hand, since the decoded signal of the second subband is not weak in layer  2 , it is necessary to encode the inter-channel parameter of the second subband. 
         [0214]    The method of the decoding side to determine whether or not the inter-channel parameters are encoded is the same as the method of the coding side for the purpose of maintaining consistency with the coding side. 
         [0215]    As described above, before encoding inter-channel parameters and transmitting the result, in each layer of scalable coding, the present embodiment analyzes the characteristics of transformed or down-mixed signals every subband and checks whether or not it is necessary to transmit the inter-channel parameters. Then, the inter-channel parameter not necessary to be transmitted is selected and deleted from the coding targets. Meanwhile, in the case of the layer requiring the inter-channel parameter so as to generate input signals, the inter-channel parameter is transmitted. 
         [0216]    Therefore, the present invention can realize precise bit allocation so as to transmit the inter-channel parameter only for the layer requiring the inter-channel parameter. 
         [0217]    The disclosure of Japanese Patent Application No. 2009-298321, filed on Dec. 28, 2009, including the specification, drawings and abstract, is incorporated herein by reference in its entirety. 
       INDUSTRIAL APPLICABILITY 
       [0218]    The present invention is suitable for a communication apparatus performing speech coding, a communication apparatus performing speech decoding, and particularly a wireless communication apparatus. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           600  Speech coding apparatus 
           603  Local monaural decoding section 
           604  Redundant parameter deleting section 
           800  Speech decoding apparatus 
           804  Zero-value inserting section