Abstract:
To provide an acoustic signal processing apparatus which can reduce the amount of calculation in matrix arithmetic. An acoustic signal processing apparatus converts down-mixed acoustic signals of NI channels to acoustic signals of NO channels, where NO&gt;NI. The acoustic signal processing apparatus includes: a first matrix arithmetic unit for performing arithmetic on a matrix with K rows and NI columns, where NO&gt;K≧NI, for the down-mixed acoustic signals of the NI channels, and outputting K signals obtained after the matrix arithmetic; K decorrelation units for generating signals incoherent, in terms of time characteristics, with the signals obtained after the matrix arithmetic, while maintaining frequency characteristics of the signals obtained after the matrix arithmetic; and a second matrix arithmetic unit for performing arithmetic on a matrix with NO rows and (NI+K) columns for the down-mixed acoustic signals of the NI channels and for the K incoherent signals, and outputting the acoustic signals of the NO channels.

Description:
TECHNICAL FIELD 
     The present invention relates to an acoustic signal processing apparatus, an acoustic signal processing method, and particularly to a technology for converting down-mixed acoustic signals of NI channels to acoustic signals of NO (NO&gt;NI) channels. 
     BACKGROUND ART 
     In recent years, a technology called Spatial Codec has been developed. This technology is designed to compress and encode multichannel realism on the basis of an extremely small amount of information. For example, the AAC method, which is a multichannel codec already widely used as an audio method for digital television, requires a bit rate such as 512 kbps or 384 kbps for 5.1 channels. On the other hand, the Spatial Codec aims to compress and encode multichannel signals at an extremely low bit rate such as 128 kbps, 64 kbps, or even 48 kbps. International standardization activities to achieve this aim are ongoing by the MPEG audio standardization conference, and so-called Reference Model Zero (also referred to as “RM0” hereafter) which is a basic processing method for the spatial audio codec is disclosed (see Non-patent document 1). 
     Here, an explanation is given as to a basic principle of the Spatial Codec. 
       FIG. 1  is a diagram for explaining the basic principle of the Spatial Codec in the case of two channels of L and R as an example. 
     In an encoding process, a spatial audio encoder obtains a down-mixed signal S (S=(L+R)/2), a level difference c, and a phase difference θ through complex calculations based on acoustic signals from the two channels of L and R, as shown in  FIG. 1(   a ). The down-mixed signal S is further encoded, together with the level difference c and the phase difference θ, by an encoding apparatus manufactured under the standard such as the MPEG AAC standard. 
     In a decoding process, a decorrelated signal D, which is orthogonal to the down-mixed signal S and carries reverberations, is generated as shown in  FIG. 1(   b ). 
     Then, as shown in  FIG. 1(   c ), the down-mixed signal S and the decorrelated signal D are mixed so that acoustic signals of the two channels of L and R that satisfy the relationship of a parallelogram shown in  FIG. 1(   a ) are generated on the basis of the decoded level difference c and the decoded phase difference θ. 
     The explanation has been given here for the case where two channels are down mixed to one channel and one channel is multiplied to two channels. By repeating this principle a plural number of times, 5.1 channels can be down mixed to two channels, and the two channels can be multiplied to the 5.1 channels, for example. 
     Next, an explanation is given as to a signal flow in the case of RM0. 
       FIG. 2  is a block diagram showing a functional structure of an acoustic signal processing apparatus  900  which converts two-channel signals to five-channel signals, the conversion being an example of a basic signal flow in the case of RM0. 
     Here, note that inputs of the two channels are down-mixed from original five-channel signals and that outputs of the five channels are restored to the original five-channel signals. Also note that the two-channel signals refer to signals usually outputted respectively from front left and right speakers and that the five-channel signals refer to signals usually outputted respectively from front left and right speakers, rear left and right speakers, and a front center speaker. 
     As shown in  FIG. 2 , the acoustic signal processing apparatus  900  includes a pre-mixing matrix M 1  ( 901 ), decorrelators (also described as “De correlators” or “Decorrelators”)  902  and  903 , and a post-mixing matrix M 2  ( 904 ). 
     The pre-mixing matrix M 1  ( 901 ) converts the inputs of an input  1  and an input  2  to five-channel signals through a process whereby matrix arithmetic related to gain control is performed on the inputs. Out of the five-channel signals, signals of two channels are respectively converted to incoherent signals through processes performed by the decorrelators  902  and  903 . The post-mixing matrix M 2  ( 904 ) generates the outputs of the five-channel signals through a process whereby matrix arithmetic related to phase control is performed on signals of five channels in total, including the signals of the two channels converted by the decorrelators  902  and  903  and the unconverted signals of the remaining three channels. 
       FIG. 3  is a block diagram showing a more detailed functional structure of the acoustic signal processing apparatus  900 . It should be noted here that although  FIG. 2  shows the signals flow from left to right,  FIG. 3  shows the signals flow from right to left. Since the insides of the pre-mixing matrix M 1  ( 901 ) and the post-mixing matrix M 2  ( 904 ) are defined by the matrix arithmetic, the diagram of  FIG. 3  is illustrated to show that the signals flow from right to left only in order for mathematical expressions of matrix arithmetic expressions to agree with the flow of the signals. Thus, the diagram is essentially the same as that of  FIG. 2 . 
     In addition to the pre-mixing matrix M 1  ( 901 ), the decorrelators  902  and  903 , and the post-mixing matrix M 2  ( 904 ) described above, the acoustic signal processing apparatus  900  further includes two determinant generation units  905  and  907 , and two interpolation units  906  and  908 . 
     As shown in  FIG. 3 , the signal processing for the pre-mixing matrix M 1  ( 901 ) is realized by a determinant of a five-row*two-column matrix. In general, a determinant shown below as Equation (1) is defined as an example of the pre-mixing matrix M 1  ( 901 ). 
     
       
         
           
             
               
                 
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     In Equation (1), α and β are values obtained from acoustic spatial coefficients called CPC (Channel Prediction Coefficients), and γ is a value obtained from an acoustic spatial coefficient called an ICC (Inter Channel Correlation). 
     Additionally, a superscript I indicates that the data comes from an I th  parameter set (an aggregate of compressed and encoded parameters). Also, a superscript m indicates that the data comes from an m th  frequency band. Details of their respective meanings are omitted here since they are not related to the scope of the present invention. 
     Equation (1) is a determinant of a five-row*three-column matrix, in which the third column has a meaning only when so-called Residual Coding described in Non-patent document 1 is performed. In most cases, Residual Coding is not performed usually in view of restriction on the bit rate and reduction in the decoding arithmetic load. In such a case, Equation (1) can be considered as Equation (2) below. 
     
       
         
           
             
               
                 
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     To be more specific, Equation (2) corresponds to the determinant shown on the right-hand part of  FIG. 3 . It is obvious that, when Residual Coding is performed, the determinant shown on the right-hand part of  FIG. 3  is to be a determinant of a five-row*three-column matrix according to Equation (1) and a Residual Signal is added as an input signal so that there would be three channels. 
     Out of the five-channel signals generated as described so far, signals of two channels are respectively converted to incoherent signals through processes performed by the decorrelators  902  and  903 . The signals of the five channels in total, including the signals of the two channels converted in this way and the unconverted signals of the remaining three channels, are converted through the process of the post-mixing matrix M 2  ( 904 ), so that the five-channel signals are generated as outputs. This signal processing is realized by a five-row*five-column matrix arithmetic expression. 
     For the sake of simplification, a five-row*five-column matrix arithmetic expression is given as one example here. Note that this is intended for the case of five channels including front two channels, rear two channels, and a center channel. Thus, when an LFE channel is added, the matrix of this determinant would have six rows and five columns. Moreover, when a decorrelator is used for a so-called Ttt Element described in Non-patent document 1, the matrix of this determinant would have six rows and six columns since one channel is added to the input side of the present matrix arithmetic. 
     Here, elements (coefficients) of each determinant in the matrix arithmetic are generated on the basis of parameters encoded from the channel level differences, the inter-channel correlations (phase differences), and the channel prediction coefficients among the original five-channel signals. 
     First, information of the encoded channel level differences, inter-channel correlations (phase differences), and channel prediction coefficients is decoded, so as to obtain the channel level differences, the inter-channel phase differences, and the prediction coefficients which are required when the determinant generation units  905  and  907  divide the two-channel signals into the five-channel signals. 
     These encoded signals are updated for each frame, which is a predetermined time interval. For this reason, the interpolation units  906  and  908  perform smoothing on the values of the level difference and the phase difference in order to smooth out variations between a current frame and a preceding frame. In this way, each element of the matrix arithmetic expressions of the pre-mixing matrix M 1  ( 901 ) and the post-mixing matrix M 2  ( 904 ) is determined. The process of determining each element of the matrix arithmetic expressions is not particularly related to the scope of the present invention and, therefore, the detailed explanation is omitted here. 
     Moreover, Non-patent document 1 describes that the processing performed by the decorrelators  902  and  903  is to generate a signal incoherent with the input signal in terms of temporal characteristics while maintaining frequency characteristics of the input signal, and also describes that lattice all-pass filters are used as a method.
     Non-patent document 1: J. Herre, et al, “The Reference Model Architecture for MPEG Spatial Audio Coding”, 118th AES Convention, Barcelona, May 28-31, 2005, Audio Engineering Society Convention Paper 6447.   

     SUMMARY OF THE INVENTION 
     Problems that Invention is to Solve 
     The above-described acoustic signal processing apparatus  900 , however, has the following problem. 
     To be more specific, since both the pre-mixing matrix M 1  ( 901 ) and the post-mixing matrix M 2  ( 904 ) are realized by the matrix arithmetic using the large-size determinants, a first problem is that an enormous amount of product-sum calculation is required. 
     Moreover, since the interpolation units  906  and  908  perform the smoothing for each frame with respect to the preceding frame, a second problem is that an enormous amount of calculation is required. 
     Furthermore, since the lattice all-pass filter used in the processing performed by the decorrelators  902  and  903  includes a multi-tap IIR filter, a third problem is that an enormous amount of calculation is required. 
     The present invention is conceived in view of the stated conventional problems, and a first object is to provide an acoustic signal processing apparatus and an acoustic signal processing method which can reduce the amount of calculation required for the matrix arithmetic. 
     Moreover, a second object is to provide an acoustic signal processing apparatus and an acoustic signal processing method which can reduce the amount of calculation required for the interpolation processing. 
     Furthermore, a third object is to provide an acoustic signal processing apparatus and an acoustic signal processing method which can reduce the amount of calculation required for the decorrelation processing. 
     Means to Solve the Problems 
     In order to solve the above-mentioned first problem, an acoustic signal processing apparatus of the present invention includes: a first matrix arithmetic unit which performs arithmetic on a matrix with K rows and NI columns, where NO&gt;K≧NI, for the down-mixed acoustic signals of the NI channels, and outputs K signals obtained after the matrix arithmetic; K decorrelation units which generate signals incoherent, in terms of time characteristics, with the signals obtained after the matrix arithmetic, while maintaining frequency characteristics of the signals obtained after the matrix arithmetic; and a second matrix arithmetic unit which performs arithmetic on a matrix with NO rows and (NI+K) columns for the down-mixed acoustic signals of the NI channels and for the K incoherent signals, and outputs the acoustic signals of the NO channels. 
     The number of rows of a determinant of the pre-mixing matrix M 1  in the conventional case of RM0 is NO which is always larger than K that is the number of decorrelators. However, according to the present invention, the number of rows of a determinant of the first matrix arithmetic unit is reduced to the same number as K which is the number of the decorrelators, thereby significantly reducing the amount of calculation. 
     Also, the acoustic signal processing apparatus according to the present invention can be characterized by that K is equal to NI. 
     Suppose that, in the case of RM0, the pre-mixing matrix M 1  calculates a determinant with a five-row*two-column size, for example, and that the post-mixing matrix M 2  calculates a determinant with a five-row*five-column size, for example. When applying this to the present invention, the first matrix arithmetic unit is to calculate a small-size determinant of a two-row*two-column matrix and the second matrix arithmetic unit is to calculate a small-size determinant of a five-row*four-column matrix. Thus, the amount of calculation can be further reduced. 
     Moreover, in order to solve the above-mentioned second problem, the acoustic signal processing apparatus of the present invention can be characterized by including a first determinant generation unit which generates each coefficient of a first determinant of the first matrix arithmetic unit from a parameter updated for each of frames separated by a predetermined time interval; a second determinant generation unit which generates each coefficient of a second determinant of the second matrix arithmetic unit from the parameter; and an interpolation unit which calculates each coefficient of the second determinant of the second matrix arithmetic unit by sequentially performing interpolation using a parameter of an immediately preceding frame or each coefficient of a second determinant of the immediately preceding frame. 
     With this, the interpolation processing for each element of a determinant is performed only on the second determinant of the second matrix arithmetic unit. To be more specific, the interpolation processing for each element of the first determinant of the first matrix arithmetic unit, which is unnecessary in terms of the hearing sense, is skipped. Therefore, the amount of calculation can be further reduced. 
     Furthermore, in order to solve the above-mentioned third problem, the acoustic signal processing apparatus of the present invention can be characterized by that the K decorrelation units perform a process to rotate a phase of an input signal by 90 degrees. 
     With this, K number of decorrelation units can be structured in an extremely simple manner. Thus, the amount of calculation can be further reduced. 
     Also, the acoustic signal processing apparatus according to the present invention can be characterized by that: the first determinant with K rows and NI columns used in the matrix arithmetic of the first matrix arithmetic unit is formed only by minimum-unit coefficients that are related to gain control and are necessary to the decorrelation units, the coefficients being obtained by separating coefficients that are related to the gain control and are unnecessary to the decorrelation units from coefficients related to the gain control; and the second determinant of NO rows and (NI+K) columns used in the matrix arithmetic of the second matrix arithmetic unit is formed by coefficients which are obtained by combining: the coefficients that are related to the gain control and are unnecessary to the decorrelation units; and coefficients related to phase control. 
     With this, while the amount of calculation is reduced, high-quality acoustic signals of NO channels can be outputted without crosstalk into other channels. 
     It should be noted here that the present invention can be realized not only as such an acoustic signal processing apparatus, but also as: an acoustic signal processing method which has the characteristic units of the acoustic signal processing apparatus as its steps; and a program which causes a computer to execute these steps. It should be obvious that such a program can be distributed via a recording medium such as a CD-ROM or via a transmission medium such as the Internet. 
     Effects of the Invention 
     As apparent from the above explanation, the acoustic signal processing apparatus and the acoustic signal processing method according to the present invention have the effect of reducing the amount of calculation and thus allowing even a processor with low arithmetic performance to reproduce high-quality surround sound. 
     Thus, according to the present invention, places for watching and listening are not limited to fixed locations, and can be mobile units such as an automobile. On the account of this, the practical value of the present invention is extremely high in these days where distribution of contents, such as music, has become widespread. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram for explaining about the basic principle of Spatial Codec in the case of two channels of L and R as an example. 
         FIG. 2  is a block diagram showing a functional structure of the conventional acoustic signal processing apparatus  900  in the case of RM0. 
         FIG. 3  is a block diagram showing a more detailed functional structure of the acoustic signal processing apparatus  900 . 
         FIG. 4  is a diagram showing an overall structure of an audio content distribution system  1  which uses an acoustic signal processing apparatus of a first embodiment according to the present invention. 
         FIG. 5  is a block diagram showing detailed structures of an audio encoder  10  and an audio decoder  20  shown in  FIG. 4 . 
         FIG. 6  is a block diagram showing a functional structure of an acoustic signal processing apparatus  24  shown in  FIG. 5 . 
         FIG. 7  is a diagram showing a main flow of the signal processing according to the conventional technology. 
         FIG. 8  is a diagram showing that a matrix arithmetic expression of a pre-mixing matrix M 1  shown in  FIG. 7  is expanded by the insertion of “0”. 
         FIG. 9  is a diagram showing that the expanded determinant shown in  FIG. 8  is divided into two determinants by the insertion of “1”. 
         FIG. 10  is a diagram showing that a sequence of the signal processing is changed with respect to the sequence shown in  FIG. 9 . 
         FIG. 11  is a diagram showing that what is shown in  FIG. 10  is rationalized. 
         FIG. 12  is a flowchart showing an operation of processing performed by units of the acoustic signal processing apparatus  24 . 
         FIG. 13  is a diagram showing an idea of applying the technology of the present invention, for the case where a one-channel signal is converted to five-channel signals by an acoustic signal processing apparatus of a second embodiment according to the present invention. 
     
    
    
     NUMERICAL REFERENCES 
     
         
         
           
               24  acoustic signal processing apparatus 
               241  first matrix arithmetic unit 
               242 ,  243  decorrelators 
               244  second matrix arithmetic unit 
               245  first determinant generation unit 
               246  second determinant generation unit 
               247  interpolation unit 
           
         
       
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following is a description of embodiments of the present invention, with reference to the drawings. 
     First Embodiment 
       FIG. 4  is a diagram showing an overall structure of an audio content distribution system  1  which uses an acoustic signal processing apparatus of the first embodiment according to the present invention. 
     As shown in  FIG. 4 , the audio content distribution system  1  includes: an audio encoder  10 ; an audio decoder  20 ; and a communication path  40  which connects the audio encoder  10  and the audio decoder  20  for mutual communications. The audio encoder  10  sends audio content via one segment of the communication path  40 . While receiving the audio content, the audio decoder  20  performs streaming reproduction at a predetermined bit rate. It should be noted here that an explanation is given in the first embodiment on the assumption that the audio encoder  10  is placed in a broadcast station or the like and the audio decoder  20  is placed in an automobile. 
     The communication path  40  includes: an Internet  42  as a center; an Internet Service Provider (also referred to as the “ISP” hereafter)  43  which is connected to the Internet  42 ; a gateway  45  and a base station  44  which build a cellular phone network; and a plurality of access points  46   a  to  46   n  which build a wireless LAN. These access points  46   a  to  46   n  are successively placed along a road so that the communication is available even while the automobile is moving. 
     The audio encoder  10  is connected to the Internet  42  via the ISP  43 . The audio decoder  20  is connected to the Internet  42  via the cellular phone network and the wireless LAN. 
       FIG. 5  is a block diagram showing detailed structures of the audio encoder  10  and the audio decoder  20  shown in  FIG. 4 . Note that the communication path  40  is not shown in  FIG. 5 . 
     The audio encoder  10  processes audio signals of a plurality of channels (audio signals of five channels, for example) for each frame representing 1024 samples or 2048 samples, for instance. The audio encoder  10  includes a down-mixing unit  11 , a binaural cue detection unit  12 , an encoder  13 , a multiplexing unit  14 , and a communication unit  15  for connecting to the communication path  40 . 
     The down-mixing unit  11  generates down-mixed signals Ms down mixed to two channels, by calculating an average of audio signals of five channels that are expressed spectrally. 
     The binaural cue detection unit  12  generates BC information (a binaural cue) to convert the down-mixed signals Ms back to the five-channel audio signals, by comparing the five-channel audio signals and the down-mixed signals Ms for each spectral band. 
     The BC information includes: a CPC which is a value obtained from an acoustic spatial coefficient; correlation information ICC which shows inter-channel coherence/correlation; and a channel level intensity difference CLD which is a value obtained from an acoustic spatial coefficient. 
     Here, the correlation information ICC shows a similarity among the five audio signals whereas the channel level intensity difference CLD shows a relative intensity among the five-channel audio signals. In general, the channel level intensity difference CLD is information used for controlling balance and localization of sounds, and the correlation information ICC is used for controlling width and diffusion of a sound image. Both of these pieces of information are spatial parameters to help listeners create auditory scenes in their minds. 
     The audio signals of the five channels expressed spectrally and the down-mixed signals Ms are usually divided into a plurality of groups including “parameter bands”. Thus, the BC information is calculated for each parameter band. It should be noted here that the “BC information” and the “spatial parameters” are often used synonymously with each other. 
     The encoder  13  compresses and encodes the down-mixed signals Ms according to MP3 (MPEG Audio Layer-3), AAC (Advanced Audio Coding), or the like. 
     The multiplexing unit  14  generates a bitstream by multiplexing the down-mixed signals Ms and quantized BC information, and then outputs the bitstream as the encoded signals described above. 
     The audio decoder  20  includes: a communication unit  21  for connecting to a communication path  21 ; an inverse-multiplexing unit  22 ; a decoder  23 ; and an acoustic signal processing apparatus  24 . 
     The inverse-multiplexing unit  22  acquires the above bitstream, divides the bitstream into the quantized BC information and the encoded down-mixed signals Ms, and then outputs the resulting BC information and the down-mixed signals Ms. Note that the inverse-multiplexing unit  22  performs inverse quantization on the quantized BC information, and then outputs the resulting BC information. 
     The decoder  23  decodes the encoded down-mixed signals Ms and outputs the decoded down-mixed signals Ms to the acoustic signal processing apparatus  24 . 
     The acoustic signal processing apparatus  24  acquires the down-mixed signals Ms outputted from the decoder  23  and the BC information outputted from the inverse-multiplexing unit  22 . Then, the acoustic signal processing apparatus  24  reconstructs the five audio signals from the down-mixed signals Ms, using the BC information. 
     It should be noted here that although the audio content distribution system has been explained with an example where the audio signals of five channels are encoded and then decoded, the audio content distribution system can also encode and decode audio signals of more than two channels (for example, audio signals of six channels making up a 5.1-channel sound source). 
     Note that, in order to show how to improve the technology disclosed by RM0, the first embodiment is contrasted with the RM0 technology whereby the two-channel input signals are converted into the five-channel output signals as explained in the above Background Art. Although the present embodiment is described for the case where inputs are two channels and outputs are five channels, this is just one example. Thus, it is obvious that the outputs may be 5.1 channels or the like. 
       FIG. 6  is a block diagram showing a functional structure of the acoustic signal processing apparatus  24  shown in  FIG. 5 . 
     As shown in  FIG. 6 , the acoustic signal processing apparatus  24  includes: a first matrix arithmetic unit  241  for performing arithmetic on a two-row*two-column matrix; two decorrelators  242  and  243 ; a second matrix arithmetic unit  244  for performing arithmetic on a five-row*four-column matrix; a first determinant generation unit  245  for calculating each element of a first determinant of the first matrix arithmetic unit  241 , on the basis of the BC information transmitted for each of frames separated by a predetermined time interval; a second determinant generation unit  246  for calculating each element of a second determinant of the second matrix arithmetic unit  244 , on the basis of the BC information transmitted for each of the frames separated by the predetermined time interval; and an interpolation unit  247  for smoothing out the values generated by the second determinant generation unit  246  by performing interpolation between the frames. 
     The first matrix arithmetic unit  241 , the first and second decorrelators  242  and  243 , the second matrix arithmetic unit  244 , the first determinant generation unit  245 , the second determinant generation unit  246 , and the interpolation unit  247  as described above are realized by a program previously stored in a ROM, a digital signal processor (DSP) executing the program, a memory providing a work area for execution of the program, and so forth. 
     The following is an explanation of an operation performed by the acoustic signal processing apparatus  24  structured as described above. Before the explanation, a reason is given as to why the determinant shown in  FIG. 3  according to the conventional technology can be changed to the determinant shown in the structure of  FIG. 6 , with reference to  FIGS. 7 to 11 . 
       FIG. 7  is a diagram of part showing a main signal flow that is extracted from  FIG. 3 . Thus, the signal flow is the same as explained in the above Background Art, that is, the two-channel signals are inputted from the right-hand side and then the five-channel signals are outputted eventually. 
       FIG. 8  is a diagram showing that the matrix arithmetic expression of the pre-mixing matrix M 1  shown in  FIG. 7  is expanded by the insertion of “0”. 
     With this expansion of the determinant, the input signals of original two channels are respectively copied so as to be expanded to four signals. However, as apparent from the determinant shown on the right-hand side, the significance of the signal processing is mathematically exactly the same as shown in  FIG. 7 . 
       FIG. 9  is a diagram showing that the expanded determinant shown in  FIG. 8  is divided into two determinants by the insertion of “1”. 
     Here, the determinant is simply divided into two. Accordingly, as apparent from the determinants shown on the right-hand side, it is mathematically exactly the same as shown in  FIG. 7 . 
       FIG. 10  is a diagram showing that a sequence of the signal processing is changed with respect to the sequence shown in  FIG. 9 . 
     To be more specific, the process for the left-side determinant out of the divided determinants and the process by the decorrelators in  FIG. 9  are interchanged. 
       FIG. 11  is a diagram showing that what is shown in  FIG. 10  is rationalized. 
     To be more specific, the diagram shows that: the two determinants shown on the left-hand side in  FIG. 10  are combined into one by previously performing matrix arithmetic on the determinants; and the size of the matrix shown on the right-hand side in  FIG. 10  is reduced by deleting the elements whose coefficients are “1” from the determinant. For example, an element w 0  in the first row and the first column of the left-side determinant of  FIG. 11  can be calculated as follows, according to the usual manner of matrix arithmetic:
 
 w 0= c 0 *a 0 +d 0 *a 1 +e 0 *a 2 +f 0*0+ g 0*0
 
     The other elements are calculated in the same way according to the usual manner of matrix arithmetic. 
     In this way, as shown in  FIGS. 7 to 11 , the flow of the signal processing in the case of RM0 can be changed to the flow of the signal processing of the present invention shown in  FIG. 6 , by dividing the determinant, interchanging the sequence of the processes, and combining the determinants. 
     Accordingly, while the amount of calculation is reduced, the acoustic signals of NO channels with a high sound quality can be outputted without signal crosstalk into the other channels. 
     Next, the following is an explanation as to an operation performed by the units of the acoustic signal processing apparatus  24  structured as shown in  FIG. 6 . 
     When converting the down-mixed signals of two channels into the signals of five channels, the DSP first executes preprocessing (S 11 ). 
     This preprocessing includes making a decision so that the first determinant of the first matrix arithmetic unit  241  is formed only by minimum-unit coefficients that are related to gain control and are necessary to the first and second decorrelators  242  and  243 , these coefficients being obtained by separating coefficients that are related to the gain control and are unnecessary to the first and second decorrelators  242  and  243 , from the coefficients related to the gain control. Also, the preprocessing includes making a decision so that the second determinant of the second matrix arithmetic unit  244  is formed by coefficients which are obtained by combining: the coefficients that are related to the gain control and are unnecessary to the first and second decorrelators  242  and  243 ; and coefficients related to phase control. Moreover, the preprocessing includes making a decision to simplify the processing performed by the first and second decorrelators  242  and  243  (a 90-degree phase rotation, for example). Furthermore, the preprocessing includes making a decision to skip the interpolation processing for the coefficients generated by the first determinant generation unit  245 . 
     After the preprocessing is finished, the DSP repeatedly executes the processing for each frame (S 12  to S 19 ). 
     In this processing performed for each frame, the DSP first causes the first determinant generation unit  245  to calculate each element of the first determinant of the first matrix arithmetic unit  241  from the inter-channel coherence information, the channel level difference, and the channel prediction coefficient transmitted for each of the frames separated by the predetermined time interval (S 13 ). 
     To be more specific, the elements a 3 , b 3 , a 4 , and b 4  of the determinant of the first matrix arithmetic unit  241  are calculated. Here, the values of a 3 , b 3 , a 4 , and b 4  have the same significance as the values of a 3 , b 3 , a 4 , and b 4  of  FIG. 3 . For this reason, the calculation method can be the same as the method defined by RM0. More specifically, using characters employed by RM0, the determinant shown on the right-hand side of  FIG. 6  is expressed as the following Equation (3) which is a determinant of a two-row*two-column matrix. 
     
       
         
           
             
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     3 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                       
                   
                   ⁢ 
                   
                     
                       R 
                       1 
                       
                         l 
                         , 
                         m 
                       
                     
                     = 
                     
                       
                         γ 
                         
                           l 
                           , 
                           m 
                         
                       
                       ⁢ 
                       
                         
                           1 
                           3 
                         
                         ⁡ 
                         
                           [ 
                           
                             
                               
                                 
                                   
                                     α 
                                     
                                       l 
                                       , 
                                       m 
                                     
                                   
                                   + 
                                   2 
                                 
                               
                               
                                 
                                   
                                     β 
                                     
                                       l 
                                       , 
                                       m 
                                     
                                   
                                   - 
                                   1 
                                 
                               
                             
                             
                               
                                 
                                   
                                     α 
                                     
                                       l 
                                       , 
                                       m 
                                     
                                   
                                   - 
                                   1 
                                 
                               
                               
                                 
                                   
                                     β 
                                     
                                       l 
                                       , 
                                       m 
                                     
                                   
                                   + 
                                   2 
                                 
                               
                             
                           
                           ] 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     It should be obvious that Equation (3) is an example where so-called Residual Coding is not performed. When Residual Coding is performed, the determinant would be the following Equation (4) which is a determinant with a two-row*three-column matrix. 
     
       
         
           
             
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     4 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                       
                   
                   ⁢ 
                   
                     
                       R 
                       1 
                       
                         l 
                         , 
                         m 
                       
                     
                     = 
                     
                       
                         γ 
                         
                           l 
                           , 
                           m 
                         
                       
                       ⁢ 
                       
                         
                           1 
                           3 
                         
                         ⁡ 
                         
                           [ 
                           
                             
                               
                                 
                                   
                                     α 
                                     
                                       l 
                                       , 
                                       m 
                                     
                                   
                                   + 
                                   2 
                                 
                               
                               
                                 
                                   
                                     β 
                                     
                                       l 
                                       , 
                                       m 
                                     
                                   
                                   - 
                                   1 
                                 
                               
                               
                                 1 
                               
                             
                             
                               
                                 
                                   
                                     α 
                                     
                                       l 
                                       , 
                                       m 
                                     
                                   
                                   - 
                                   1 
                                 
                               
                               
                                 
                                   
                                     β 
                                     
                                       l 
                                       , 
                                       m 
                                     
                                   
                                   + 
                                   2 
                                 
                               
                               
                                 1 
                               
                             
                           
                           ] 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     Note that, however, the values of a 3 , b 3 , a 4 , and b 4  in  FIG. 3  are obtained after the processing of the interpolation unit  247  and are thus different from the values of the elements a 3 , b 3 , a 4 , and b 4  of the determinant of the first matrix arithmetic unit  241  in  FIG. 6  that are obtained before the processing of the interpolation unit  247 . In either case, the calculation method can be the same as the method defined by RM0. 
     Next, an explanation is given as to a main signal flow with reference to  FIG. 6 . 
     For an input  1  and an input  2 , the first matrix arithmetic unit  241  performs matrix arithmetic for each element. More specifically, the DSP executes the arithmetic processing for the first determinant of the first matrix arithmetic unit  241  (S 14 ). The signals generated in this way are processed by the first and second decorrelators  242  and  243 . To be more specific, the DSP executes the decorrelation processing in the first and second decorrelators  242  and  243  (S 15 ). 
     These first and second decorrelators  242  and  243  perform processing to generate signals which are incoherent with the input signals in terms of temporal characteristics while maintaining frequency characteristics of the input signals. Although a lattice all-pass filter is used as a method in the case of RM0, a simplified method whereby the phase of the input signal is rotated 90 degrees can be employed. This is because, when the phase of the input signal is rotated 90 degrees, the frequency characteristics of the signal are completely maintained and a signal which is completely mathematically-incoherent can be generated. In addition, when there are a plurality of input signals, the processing can be realized by exchanging a real number term and an imaginary number term and then inverting one of the codes. On account of this, the structures of the first and second decorrelators  242  and  243  can be simplified and the amount of calculation can be thus extremely small. 
     After the completion of the decorrelation processing, the DSP causes the second determinant generation unit  246  to calculate values as the basis of the elements in the determinant of the second matrix arithmetic unit  244 , from the inter-channel coherence information and the channel level difference transmitted for each of the frames separated by the predetermined time interval (S 16 ). 
     To be more specific, the second determinant generation unit  246  acquires two determinants shown on the left-hand side in  FIG. 10  and additionally executes a process to combine these two determinants. Here, the values of a 0 , b 0 , a 1 , b 1 , a 2 , and b 2  shown in  FIG. 10  have the same significance as the values of a 0 , b 0 , a 1 , b 1 , a 2 , and b 2  shown in  FIG. 3 . On account of this, the calculation method can be the same as the method defined by RM0. 
     More specifically, when using characters employed by RM0, the right-hand determinant out of the two determinants shown on the left-hand side in  FIG. 10  is expressed as the following Equation (5) which is a determinant of a five-row*four-column matrix. 
     
       
         
           
             
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     5 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                       
                   
                   ⁢ 
                   
                     
                       R 
                       1 
                       
                         l 
                         , 
                         m 
                       
                     
                     = 
                     
                       
                         γ 
                         
                           l 
                           , 
                           m 
                         
                       
                       ⁢ 
                       
                         
                           1 
                           3 
                         
                         ⁡ 
                         
                           [ 
                           
                             
                               
                                 
                                   
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                                   + 
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                                   - 
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                                     ( 
                                     
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                                           l 
                                           , 
                                           m 
                                         
                                       
                                     
                                     ) 
                                   
                                   ⁢ 
                                   
                                     2 
                                   
                                 
                               
                               
                                 
                                   
                                     ( 
                                     
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                                   ⁢ 
                                   
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                                 0 
                               
                               
                                 0 
                               
                               
                                 0 
                               
                               
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                           ] 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     It is obvious that Equation (5) is an example where: so-called Residual Coding is not performed; so-called Ttt Decorrelator processing is not performed; and an LFE channel is omitted. When these are all performed, the determinant would be the following Equation (6). 
     
       
         
           
             
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     6 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     R 
                     1 
                     
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                       , 
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                   = 
                   
                     
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                         1 
                         3 
                       
                       ⁡ 
                       
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                               0 
                             
                             
                               0 
                             
                             
                               0 
                             
                             
                               
                                   
                               
                             
                             
                               1 
                             
                             
                               0 
                             
                             
                               0 
                             
                           
                           
                             
                               0 
                             
                             
                               0 
                             
                             
                               0 
                             
                             
                               
                                   
                               
                             
                             
                               0 
                             
                             
                               1 
                             
                             
                               0 
                             
                           
                           
                             
                               0 
                             
                             
                               0 
                             
                             
                               0 
                             
                             
                               - 
                             
                             
                               0 
                             
                             
                               0 
                             
                             
                               1 
                             
                           
                         
                         ] 
                       
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     Note that, however, although the values of a 0 , b 0 , a 1 , b 1 , a 2 , and b 2  in  FIG. 3  are obtained after the processing of the interpolation unit  247 , the values of a 0 , b 0 , a 1 , b 1 , a 2 , and b 2  used here are obtained before the processing of the interpolation unit  247 . 
     Moreover, the values of c 0  to c 4 , d 0  to d 4 , e 0  to e 4 , f 0  to f 4 , and g 0  to g 4  shown in  FIG. 10  have the same significance as the values of c 0  to c 4 , d 0  to d 4 , e 0  to e 4 , f 0  to f 4 , and g 0  to g 4  shown in  FIG. 3 . On account of this, the calculation method can be the same as the method defined by RM0. Note that, however, although the values of c 0  to c 4 , d 0  to d 4 , e 0  to e 4 , f 0  to f 4 , and g 0  to g 4  in  FIG. 3  are obtained after the processing of the interpolation unit  247 , the values of c 0  to c 4 , d 0  to d 4 , e 0  to e 4 , f 0  to f 4 , and g 0  to g 4  used here are obtained before the processing of the interpolation unit  247 . According to the usual manner of matrix arithmetic, the values of a 0 , b 0 , a 1 , b 1 , a 2 , b 2 , and c 0  to c 4 , d 0  to d 4 , e 0  to e 4 , f 0  to f 4 , and g 0  to g 4  calculated in this way are combined into one determinant where the values are shown as w 0  to w 4 , x 0  to x 4 , y 0  to y 4 , and z 0  to z 4  in  FIG. 11 . 
     Next, the DSP smoothes out the values of w 0  to w 4 , x 0  to x 4 , y 0  to y 4 , and z 0  to z 4  in order to prevent the elements of the determinant from abruptly changing between the frames. For doing so, the DSP has the interpolation unit  247  interpolate between the above-mentioned w 0  to w 4 , x 0  to x 4 , y 0  to y 4 , and z 0  to z 4  generated by the second determinant generation unit  246  and these values generated in the immediately preceding processed frame (S 17 ). The values obtained according to this manner are shown as w 0 ^ to w 4 ^, x 0 ^ to x 4 ^, y 0 ^ to y 4 ^, and z 0 ^ to z 4 ^ in the second matrix arithmetic  244  of  FIG. 6   
     Here, a symbol “^” is assigned to each element to indicate that the current value is obtained after the interpolation processing. The way how the signal processing is altered was shown earlier with reference to  FIGS. 7 to 11 , and “^” is not assigned to the final elements of the left-hand determinant in  FIG. 11  because the drawing only aims to mathematically show how the signal processing is altered. On the other hand, the elements of the left-hand determinant in  FIG. 6  are obtained after the interpolation processing and, for this reason, the symbol “^” is assigned to make a clear distinction. 
     It should be noted that the interpolation unit  247  may be removed for the purpose of reducing the amount of calculation. Moreover, although the coefficients of the determinant generated by the first determinant generation unit  245  are not processed by the interpolation unit  247  in  FIG. 6 , these coefficients may be smoothed out in the interpolation processing. 
     However, in view of influence on the sound quality, the coefficients of the determinant generated by the first matrix arithmetic  245  do not have to be smoothed out as shown in  FIG. 6  since there is little influence on the sound quality. 
     The reason is explained. The outputs of the first matrix arithmetic unit  241  are all inputted to the immediately succeeding first and second decorrelators  242  and  243 . The first and second decorrelators  242  and  243  perform the processing whereby reverberation components are given to the sound according to RM0. Thus, even when the determinant abruptly changes because the smoothing is not performed, the effect by the first and second decorrelators  242  and  243  to blur the sound can weaken a sense of discontinuity at changing points of the determinant. 
     In this way, the signals of four channels in total including the two-channel signals converted by the first and second decorrelators  242  and  243  and the signals of the input  1  and the input  2  are processed by the second matrix arithmetic  244 , so that the five-channel signals are generated as the outputs. To be more specific, the DSP executes the arithmetic processing using the second determinant of the second matrix arithmetic unit  244  (S 18 ). Here, take notice that each element of the determinant of the second matrix arithmetic unit  244  is sequentially interpolated. 
     For example, in the case where one frame time has a time length lasting for 32 units of time, the elements of the determinant of the first matrix arithmetic  241  respectively maintain the same values during the 32 units of time whereas the elements of the determinant of the second matrix arithmetic  244  are sequentially changed for each unit of time. For example, take the value of w 0  of the first row and the first column in the determinant of the second matrix arithmetic  244 . When the value of w 0  in the current frame generated by the second determinant generation unit  246  is w 0 ( t ) and the value of w 0  in the preceding frame generated by the second determinant generation unit  246  is w 0 ( t −1), the interpolation unit  247  interpolates between w 0 ( t −1) and w 0 ( t ) for each unit of time so that the value smoothly shifts from w 0 ( t −1) to w 0 ( t ). 
     As described so far, the first embodiment includes: the first matrix arithmetic  241  for performing matrix arithmetic on N rows; an NI number of the first and second decorrelators  242  and  243 ; and the second matrix arithmetic  244  for performing matrix arithmetic on NO rows. Thus, the amount of calculation can be reduced by having: NI-channel signals as the inputs of the first matrix arithmetic unit  241 ; the output signals of the first matrix arithmetic unit  241  as the inputs of the first and second decorrelators  242  and  243 ; and the input signals of the first matrix arithmetic unit  241  and the output signals of the first and second decorrelators  242  and  243  as the inputs of the second matrix arithmetic unit  244 . 
     Suppose a case of RM0 where the pre-mixing matrix M 1  performs matrix arithmetic on a five-row*two-column matrix and the post-mixing matrix M 2  performs matrix arithmetic on a five-row*five-column matrix, for example. When applying the technology of the present invention to this case, the first matrix arithmetic is to be performed on a two-row*two-column matrix and the second matrix arithmetic is to be performed on a five-row*four-column matrix. In this way, the amount of calculation can be reduced. 
     Moreover, the present embodiment includes the determinant generation unit  245  for generating each coefficient of the determinants of the first matrix arithmetic unit  241  and the second matrix arithmetic unit  244  on the basis of the parameters updated for each of the frames separated by the predetermined time interval. The coefficients of the determinant of the first matrix arithmetic  241  are constant in each frame whereas the coefficients of the determinant of the second matrix arithmetic  244  are calculated by sequentially performing interpolation using the parameters of the immediately preceding frame or the coefficients of the determinant of the immediately preceding frame. Thus, the interpolation processing for each element of the determinant can be performed only for the second matrix arithmetic expression and, as a result, the amount of calculation can be reduced. 
     Also, the first and second decorrelators  242  and  243  may rotate the phases of the input signals by 90 degrees as their processing to perform. Then, the structures of the first and second decorrelators  242  and  243  can be remarkably simplified. 
     In the first embodiment, the process to calculate the coefficients of the second determinant (S 16 ) and the process to execute the interpolation processing for the coefficients of the second determinant (S 17 ) are performed after the decorrelation processing. However, these processes may be executed between Step S 13  and Step S 14 . This can separate the process for calculating the coefficients and the main process for converting the signals to the five-channel acoustic signals. 
     Moreover, the first embodiment describes the processing flow in the case of generating the multichannel outputs corresponding to the two-channel inputs. However, the present invention can be applied to the case of generating multichannel outputs corresponding to a one-channel input. 
     Second Embodiment 
     For example, an explanation is given as to a case where the number of output channels is five corresponding to an input of one channel, with reference to  FIG. 13 . 
     The purpose of the present invention is to make the amount of calculation required for the first matrix arithmetic unit  241  smaller than the amount of calculation required for the pre-mixing matrix M 1  disclosed in RM0, by equalizing the number of rows in the determinant of the first matrix arithmetic unit  241  with the number of decorrelators. 
     The top drawing of  FIG. 13 , which is illustrated as  FIG. 13(   a ), shows a signal flow of generating the multichannel outputs corresponding to the one-channel input in the case of RM0. In the second and third drawings from the top, which are illustrated as  FIG. 13(   b ) and  FIG. 13(   c ), what is shown in  FIG. 13(   a ) is mathematically expanded and divided. The concepts were described above with reference to  FIGS. 8 and 9 . 
     In the fourth drawing from the top, which is illustrated as  FIG. 13(   d ), the processes performed by the decorrelators and the process for matrix arithmetic are interchanged. The concept was described above with reference to  FIG. 10 . 
     In the bottom drawing, which is illustrated as  FIG. 13(   e ), the amount of calculation is reduced in comparison with the fourth drawing from the top, by combining the left-hand two determinants in advance and by minimizing (optimizing) the right-hand determinant. 
     As a result, the determinant of the first matrix arithmetic unit  241  becomes a determinant of a four-row*one-column matrix, and the number of rows is equal to the number of decorrelators. Accordingly, the amount of calculation can be reduced. 
     Moreover, the outputs of the first matrix arithmetic unit  241  are all inputted to the decorrelators, which add the reverberation components. On this account, the abrupt variations in the elements of the determinant of the first matrix arithmetic unit  241  between the frames are never a problem acoustically. In addition, there is an advantage that the smoothing processing by the interpolation unit is not necessary to the elements of the first determinant. 
     In the present example, the number of channels as outputs is five. However, it should be obvious that the number of channels may be six in consideration of an LFE channel. In this case, the number of rows in the left-hand determinant is six. 
     INDUSTRIAL APPLICABILITY 
     The acoustic signal processing apparatus according to the present invention can perform the processing of decoding the down-mixed signals back to the original multichannel signals with the small amount of calculation. On account of this, the present invention can be applied to low bit-rate music broadcast service and low bit-rate music distribution service, and to receiving apparatuses for receiving such service, for example.