VECTOR ENCODING METHOD AND ENCODER/DECODER USING THE METHOD

Abstract: Representative vectors z 1i and z 2j are selected from codebooks CB1 and CB1, respetively, and multiplied by weighting coefficient vectors w 1 and w 2 of the same number of dimensions as those of the representative vectors by multipliers 21 and 22, whereby weighted representative vectors z 1i w 1 and z 2j w 2 are generated. These weighted representative vectors are vector combined by a combining part 3 into a combined vector y ij , and a combination of the representative vectors is seleced by a control part in such a manner as to minimize the distance between the combined vector y ij and an input vector X. The weighting coefficient vectors w 1 and w 2 each have a maximum component in a different dimension and are selected so that the sum of diagonal matrixes W 1 and W 2 using components of the weighting coefficient vectors as their diagonal elements becomes a constant multiple of the unit matrix.

Description: BACKGROUND OF THE INVENTION The present invention relates to a vector coding method that is used to encode speech, images and various other pieces of information and is particularly suited to encoding of information that is transmitted over an error-prone channel such as a mobile radio channel and encodes an input vector through the use of a plurality of codebooks each composed of plural representative vectors. The invention also pertains to a vector encoder using the above-mentioned vector coding method and a vector decoder for decoding codes encoded by the vector encoder. Methods that have been proposed to transmit vectors over channels prone to channel errors are to set representative vectors in anticipation of possible channel errors and to take into account the channel errors when labeling representative vectors. These methods are disclosed in Kumazawa, Kasahara and Namekawa, "A Communication of Vector Quantizers for Noisy Channels," Transactions of the Institute of Electronics, Information and Communication Engineers of Japan, Vol. J67-B, No. 1, pp. 1-8,1984, Zeger and Gersho, "Pseudo-Gray Coding," IEEE Trans. on Comm., Vol. 38, No. 12, pp. 2147-2158, 1990, and other literature. These methods hold all representative vectors directly in one codebook, and hence require large storage capacity for storing the codebook. As a method that does not need large storage capacity in transmitting vectors over channels prone to channel errors, there has been proposed to transmit vectors after quantizing them through the use of two structured codebooks. This is disclosed in Moriya, "Two-Channel Vector Quantizer Applied to Speech coding," Transactions of the Institute of Electronics, Information and Communication Engineers of Japan, IT87-106, pp. 25-30, 1987 and other literature. This method has two small-scale codebooks and uses two representative vectors in combination to reduce the storage capacity needed and transmits two labels indicative of the two representative vectors to lessen the influence of channel errors. This method will be described with reference to Figs. 1A and 1B. The representative vectors of the codebooks are generated beforehand by learning, for instance. In an encoder depicted in Fig. 1A, one representative vector z 1i is fetched from a codebook CB1 and one representative vector z2j from a codebook CB2, then they are added together in a vector combining part 3 to generate a vector sum y ij =z 1i +z 2j , and the distance, d(X,y ij ), between the combined representative vector y ij and an input vector X via an input terminal 4 is calculated, as distortion, in a distance calculating part 5. A control part 6 controls representative vector select switches 7 and 8 for the codebooks CB1 and CB2 and searches them for the representative vectors z 1i and z 2j that minimize the output d(X, y ij ) from the distance calculating part 5. The control part 6 provides, as an encoded output to an output terminal 9, labels i and j of the representative vectors z 1i and z 2j that provide minimum distance. In a decoder shown in Fig. 1B, the control part 12 controls representative vector select switches 13 and 14 in accordance with the labels i and j in the input code applied via an input terminal 11 and reads out representative vectors z 1i and z 2j from codebooks CB3 and CB4, respectively. The thus read-out representative vectors z 1i and z 2j are combined in a vector combining part 17 into a reconstructed vector y ij = z 1i + z 2j , which is provided to an output terminal 18. Incidentally, the codebooks CB3 and CB4 are identical with those CB1 and CB2, respectively. The method described above in respect of Figs. 1A and 1B saves storage capacity of the codebooks for storing the representative vectors and lessens the influence of channel errors by combining the vectors in the decoder through utilization of the two labels corresponding thereto. A problem of this method is that the amount of processing required is very large because it involves the calculation of the distance d(X, y ij ) for every combination of representative vectors of the two codebooks in search for the pair of representative vectors that minimizes the distance. TOMOYUKI OHYA et al. in "Revised TC-WVQ speech coder for mobile communication system", Proc. of the Int. Conf. on Spoken Language Processing (ICSLP), Kobe, November. 18-22, 1990, vol. 1, pp. 125-128, ACOUSTIC SOCIETY OF JAPAN, disclose a method according to the pre-characterizing portion of claim 1 and an encoder (decoder) according to the pre-characterizing portion of claim 14 (22). In this prior art, a subgroup of representative vectors is taken from each codebook. Each vector from a respective subgroup is multiplied by a respective sign vector whose components are either +1 or -1. The combined vector is then multiplied with a gain. More particularly, all odd components of the combined vector are multiplied with a first gain factor and all even components of that combined vector are multiplied with a second gain factor to obtain a weighted combined vector. With these methods, however, if an error arises in the labels during transmission over the channel, then distortion will occur in all elements of the received vector. According to the circumstances, the error will cause an abnormally large amount of distortion in the decoded output. SUMMARY OF THE INVENTION An object of the present invention is to provide a vector coding method that prevents an error in the input code to the decoder from causing serious distortion of its output. Another object of the present invention is to provide a vector coding method that prevents an error in the input code to the decoder from causing serious distortion of its output and permits reduction of the amount of processing required. Another object of the present invention is to provide a vector encoder that embodies the above-mentioned vector coding method. Still another object of the present invention is to provide a vector decoder that decodes a vector encoded by the vector coding method that serves the above-mentioned objects. These objects are achieved with a method as claimed in claims 1 to 4, an encoder as claimed in claims 16, 18, 20 and 22, and a decoder as claimed in claims 25 and 27, respectively. Preferred embodiments of the invention are subject-matter of the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1A is a block diagram showing the configuration of an encoder embodying a conventional vector coding method; Fig. 1B is a block diagram showing a conventional decoder for use with the encoder of Fig. 1A; Fig. 2A is a block diagram illustrating an example of the encoder embodying the present invention; Fig. 2B is a block diagram illustrating an embodiment of the decoder according to the present invention; Fig. 3A is a graph showing each representative vector z 1i of a codebook CB1; Fig. 3B is a graph showing a vector obtained by multiplying each representative vector z 1i by a weighting coefficient vector w 1 ; Fig. 3C is a graph showing each representative vector z 2j of a codebook CB2; Fig. 3D is a graph showing a vector obtained by multiplying each representative vector z 2j by a weighting coefficient vector w 2 ; Fig. 3E is a graph showing examples of a combined vector and an erroneous combined vector in the present invention; Fig. 3F is a graph showing examples of a combined vector and an erroneous combined vector in the prior art; Fig. 4 is a graph showing a set of weighted representative vectors and a straight line for approximation, for explaining the vector coding method of the present invention; Fig. 5 is a block diagram illustrating an example of the encoder of the present invention applied to the CELP scheme; Fig. 6A is a table showing a first codebook with weighted gain vectors stored therein; Fig. 6B is a table showing a second codebook with weighted gain vector stored therein; Fig. 7 is a graph showing weighted gain vectors of Figs. 6A and 6B on a coordinate system; Fig. 8 is a block diagram illustrating the encoder of the present invention applied to the quantization of random excitation vectors; Fig. 9 is a block diagram illustrating the encoder of the present invention applied to the quantization of pitch excitation vectors; Fig. 10 is a block diagram illustrating the encoder of the present invention applied to the VSELP scheme; Fig. 11 is a graph showing segmental SN ratio of reconstructed speech with respect to a channel error rate in the cases of using one gain codebook and two gain codebooks for vector encoding of gains g p and g c in Fig. 5; Fig. 12 is a graph showing, as an equivalent Q value, the MOS of the reconstructed speech with respect to the channel error rate in the two cases of Fig. 11; and Fig. 13 is a graph showing the segmental SN ratio of reconstructed speech with respect to the channel-error rate in the case of the present invention that uses two weighted gain codebooks for vector encoding of the gains g p and g c in Fig. 5. DESCRIPTION OF THE PREFERRED EMBODIMENTS In Figs. 2A and 2B there is illustrated in block form an embodiment of the present invention, in which the parts corresponding to those in Figs. 1A and 1B are identified by the same reference numerals. In the encoder of Fig. 2A, multipliers 21 and 22 are provided between the representative selection switches 7, 8 and the vector combining part 3, by which components of L-dimensional (where L is an integer equal to or greater than 2) representative vectors z 1i =(z 1i1 , z 1i2 , ..., z 1iL ) and z 2j =(z 2j1 , z 2j2 , ..., z 2jL ) selected from the codebooks CB1 and CB2 are multiplied by the corresponding components of L-dimensional weighting coefficient vectors w 1 =(w 11 , w 12 , ..., w 1L ) and w 2 =(w 21 , w 22 , ..., w 2L ), respectively. One of the L components forming each of the weighting coefficient vectors w 1 and w 2 is larger than the others and the positions of these maximum components in the respective weighting coefficient vectors w 1 and w 2 differ with the codebooks CB1 and CB2. According to the present invention, letting the weighting coefficient vectors w 1 and w 2 be represented by the following weighting coefficient matrixes W 1 and W 2 that have, as diagonal elements, the values w 11 , w 12 , ..., w 1L of the components by which the respective components of the representative vectors are multiplied: the weighting coefficient vectors w 1 and w 2 may preferably be selected so that the sum of the weighting coefficient matrixes W 1 and W 2 of the codebooks CB1 and CB2 becomes a constant multiple of the unit matrix as follows: where K is a predetermined constant. Vectors w 1 z 1i and w 2 z 2j , obtained by multiplying the representative vectors z 1i and z 2j by the weighting coefficient vectors w 1 and w 2 , respectively, are combined in the vector combining part 3, and the codebooks CB1 and CB2 are searched for representative vectors z 1i and z 2j that minimize the distance between the combined vector y ij and the input vector X. With such a configuration as described above, for example, when L=2, the representative vectors z 1i and z 2j are expressed by two-dimensional vectors z 1i =(z 1i1 ,z 1i2 ) and z 2j =(z 2j1 ,z 2j2 ), respectively. Suppose that k=2 and that the weighting coefficients which satisfy Eq. (3) are w 1 =(w 11 =1.8, w 12 =0.2) and w 2 =(w 21 =0.2, w 22 =1.8). Assuming that representative vectors z 11 , z 12 , ... of the codebook CB1 are distributed substantially uniformly all over a plane of a certain two-dimensional range defined by z 1i1 in the first-dimensional direction and z 1i2 in the second-dimensional direction as shown in Fig. 3A, weighted representative vectors z 11' and z 12' , ..., obtained by multiplying each representative vector z 1i =(z 1i1 , z 1i2 ) by the weighting coefficient vector w 1 =(1.8, 0.2), are concentrated close to the first-dimensional axis as shown in Fig. 3B. Similarly, assuming that representative vectors z 21 , z 22 , ... of the codebook CB2 are distributed substantially uniformly all over the plane of a certain two-dimensional range defined by two axes as depicted in Fig. 3C, weighted representative vectors, obtained by multiplying the representative vectors z 21 , z 22 , ... by the weighting coefficient vectors w 2 =(0.2, 1.8), are concentrated close to the second-dimensional axis as shown in Fig. 3D. Suppose, for example, that when it is judged at the transmitting side that the weighted combined vector y ij of the representative vectors z 2j and z 1i has the minimum distortion with respect to the input signal X, the label of the one weighted representative vector z 1i becomes z 1i ' because of a channel error as shown in Fig. 3E. In this instance, the combined vector y ij changes to y ij ' at the receiving side. There is a possibility that the weighted representative vector z 1i changes to any other weighted representative vectors z 1i ', but since the vector z 1i has a biased distribution, the value of the second-dimensional component of an error vector, Δ y =Δ y ij - y ij ', between the combined vectors y ij and y ij ' is relatively small, no matter how much the vector z 1i may change. In contrast to this, in the case where the combined vector is not multiplied by the weighting coefficient, if the one representative vector z 1i changes to a representative vector z 1i ', combined vectors of these vectors z 1i and z 1i' and the other representative vector z 2j become y ij and y ij' , respectively, as shown in Fig. 3F. Since there is a likelihood of the representative vector z 1i changing to any of the representative vectors of the codebook CB1 and since the representative vectors z 11 , z 12 , ... are distributed over a wide range, the error vector Δy between the combined vector y ij and the changed combined vector y ij ' is likely to have appreciably large first- and second-dimensional components. In other words, in the example of Fig. 3E, when the weighted representative vector w 1 z 1i =(w 11 z 1i1 , w 12 z 1i2 ) becomes w 1 z 1i '=(w 11 z 1i1' , w 12 z 1i1' ) because of a channel error, distortion is concentrated on the first-dimensional component w 11 z 1i1' to keep down distortion of the second-dimensional component w 12 z 1i2 ', by which distortion is reduced as a whole. Fig. 2B illustrates in block form an embodiment of the decoder of the present invention, which is supplied with the labels i and j and the weighting coefficient vectors w 1 and w 2 from the encoder of Fig. 2A and decodes the code y ij . The decoder has the same codebooks CB3 and CB4 as those CB1 and CB2 in Fig. 2A, reads out representative vectors of the labels i and j of the inputted code from the codebooks CB3 and CB4 and combines them as is the case with Fig. 113. In this embodiment, multipliers 24 and 25 are provided between the switches 13, 14 and the vector combining part 17, by which representative vectors z 1i and z 2j read out of the codebooks CB3 and CB4 are multiplied by the same weighting coefficient vectors w 1 and w 2 as those used by the corresponding multipliers 21 and 22 of the Fig. 2A encoder. The thus multiplied representative vectors w 1 z 1i and w 2 z 2j are combined in the vector combining part 17 into the reconstructed vector y ij . As will be evident from the above, it is also possible to omit the multipliers 21, 22, 24 and 25 by prestoring in the codebooks CB1, CB3 and CB2, CB4 in Figs. 2A and 2B weighted representative vectors obtained by multiplying the representative vectors z 1i and z 2j by the weighting coefficient vectors w 1 and w 2 , respectively. In the encoder of Fig. 2A, the combined vector y ij is determined for every combination of representative vectors z 1i and z 2j prestored in the codebooks CB1 and CB2, then the distortion of each combined vector is calculated with respect to the input signal vector X, and a decision is made of which pair of representative vectors z 1i and z 2j provides the minimum distortion. With this method, however, the number of calculations increases sharply as the codebooks CB1 and CB2 increase in size. Next, a description will be given of a scheme which pre-selects small numbers of representative vectors z 1i and z 2j and determines the pair of representative vectors of the minimum distortion among them, thereby reducing the computational complexity and hence shortening the operation time. Let it be assumed, for example, that the codebooks CB1 and CB2 in Fig. 2A have 8 and 16 representative vectors, respectively, and that the vectors z 1i and z 2j are all two-dimensional. In Fig. 4, eight weighted representative vectors, obtained by multiplying the eight representative vectors z 1i of the codebook CB1 by the weighting coefficient vector w 1 =(w 11 =1.8, w 12 =0.2), are indicated by crosses; 16 weighted representative vectors, similarly obtained by multiplying the 16 representative vectors z 2j of the codebook CB2 by the weighting coefficient vectors w 2 =(w 21 =0.2, w 22 =1.8), are indicated by white circles. The input signal vector is indicated by X, which is composed of a predetermined number of signal samples of each frame, two samples in this example. The two-dimensional weighting coefficient vectors w 1 and w 2 for the two-dimensional vectors z 1i and z 2j are determined such that they satisfy Eq. (3); w 11 +w 21 =w 12 +w 22 =2 in this example. As depicted in Fig. 4, the weighted representative vectors marked with the white circles and the weighted representative vectors marked with the crosses are distributed separately on the sides opposite with respect to a straight line of a 45°-gradient and passing through the origin (0, 0). The following description will be given on the assumption that there are stored such weighted representative vectors in the codebooks CB1 and CB2 in Fig. 2A and in the CB3 and CB4 in Fig. 2B, with the multipliers 21, 22, 24 and 25 left out. With this scheme, the set of weighted representative vectors (indicated by the crosses) of the codebook CB1 is approximated with a straight line 27. That is, the straight line 27 is determined so that the sum of distances, D 11 , D 12 , ..., D 18 , between it and the respective crosses (or distances in the second-dimensional axis direction) is minimum. Likewise, the set of weighted representative vectors (indicated by the white circles) of the codebook CB2 is approximated with a straight line 28. The straight line 28 is also determined so that the sum of distances, D 21 , D 22 , ..., D 216 , between it and the respective white circles (or distances in the first-dimensional axis direction) is minimum. The input vector X is projected on the approximating straight lines 27 and 28 and pluralities of weighted representative vectors present around the projections are selected. That is, a calculation is made of the value on the abscissa and hence a first-dimensional value p 1x at the intersection P 1 of a straight line 29 passing through the input signal vector X and parallel to the approximating straight line 28 and the approximating straight line 27, then the value P 1x and first-dimensional values (values of first components) of the cross-marked weighted representative vectors having a wide first-dimensional distribution are compared, and a predetermined number of, for example, three, weighted representative vectors are selected, as a subgroup H 1 , in increasing order of the difference between the value p 1x and the first-dimensional value of the respective weighted representative vector. In this way, the weighted representative vectors are pre-selected for the codebook CB1. Similarly, a calculation is made of a value on the ordinate and hence a second-dimensional value p 2y at the intersection P 2 of a straight line 31 passing through the input signal vector X and parallel to the approximating straight line 27 and the approximating straight line 28, then the second-dimensional value p 2y and second-dimensional values (values of second components) of the white-circled weighted representative vectors having a wide second-dimensional distribution, and a predetermined number of, for example, three, weighted representative vectors are selected, as a subgroup H 2 , in the order of increasing differences between the value p 2y and the second-dimensional values of the weighted representative vectors. This is the pre-selection of the weighted representative vectors for the codebook CB2. Only the weighted representative vectors thus pre-selected from the codebooks CB1 and CB2 are searched for a pair of weighted representative vectors that provides the minimum distance between their combined vector and the input signal vector. In this example, since three weighted representative vectors are pre-selected from each of the codebooks CB1 and CB2, the number of their combinations is nine, and hence the number of combined vectors is nine. When the pre-selection scheme is not adopted, the number of combinations of the weighted representative vectors (the number of combined representative vectors) is 8×16=128, and when the pre-selection is made, the number of calculations for the distance to the input signal vector X is reduced down to 9/128 of the number of calculations needed when no pre-selection takes place. In this pre-selection scheme, when M codebooks are used, the number of dimensions of representative vectors is also set to M. The M weighting coefficient vectors each have at least one maximum component at a different component position (that is, in a different dimension), and by multiplying the representative vector by the weighting coefficient vector, that dimension is emphasized more than the other dimensions. Fig. 5 illustrates an embodiment of the coding method of the present invention applied to speech coding of the CELP (Code-Excited Linear Prediction Coding) system. In the CELP system, as disclosed by M.R. Schroeder and B.S. Atal in "Code-Excited Linear Prediction (CELP): High-Quality Speech at Very Low Bit Rates", Proc. ICASSP'85, pp. 937-940, 1985, for instance, pitch excitation vectors read out of a pitch excitation source codebook and random excitation vectors read out of a random excitation source codebook are respectively provided with gains and are combined in pairs, then the combined vectors are each fed as an excitation signal to a synthesis filter to obtain synthesized speech, then two vectors and two gains therefor are determined which minimize distortion of the synthesized speech with respect to input speech, and labels of these vectors and labels of the gains are outputted as encoded results of the input speech, together with the filter coefficients of the synthesis filter. By applying the vector coding method of the present invention to the encoding of the gains for the two vectors in the CELP system, it is possible to prevent the occurrence of gross distortion in the decoded or reconstructed speech by a channel error in the codes representing the gains. The input speech signal X fed via an input terminal 34 is sampled with a fixed period and is expressed as a vector that is provided as a sequence of digital sample values for each frame period. The input signal vector X of every frame is subjected to, for example, an LPC analysis in a filter-coefficient determining part 35, from which linear predictive coefficients are provided. The linear predictive coefficients are used to calculate a spectrum envelope parameter, which is quantized in a filter coefficient quantizing part 36, and the quantized value is set as the filter coefficient of a synthesis filter 37. In a pitch excitation source codebook 39 there are stored sequences of sample values of waveforms respectively containing different pitch period components and labeled as pitch excitation vectors. In a random-excitation source codebook 43 there are stored sequences of sample values of various random waveforms respectively labeled as random-excitation vectors. The pitch-excitation vectors and the random-excitation vectors stored in the pitch-excitation source codebook 39 and the random-excitation source codebook 43 are each composed of components of the same number as that of samples of one frame. A selection switch 38 is controlled by the control part 6 to select one of the pitch-excitation vectors in the pitch-excitation source codebook 39, and the selected pitch-excitation vector is multiplied by a given gain in a gain providing part 41, thereafter being applied to the synthesis filter 37. The difference between synthesized speech Xp from the synthesis filter 37 and the input speech signal X is calculated by a subtractor 48, and in the distortion calculating part 5 the difference is used to calculate distortion D as D=∥X-Xp∥ 2 . Similarly, the other pitch-excitation vectors are sequentially taken out from the pitch-excitation source codebook 39 via the switch 38 under the control of the control part 6, then the above-mentioned distortion is calculated for each pitch-excitation vector, and the pitch-excitation vector of the minimum distortion is determined. Next, one of the random-excitation vectors stored in the random-excitation source codebook 43 is taken out through a switch 42 and amplified by a given gain in a gain providing part 46, thereafter being fed to an adder 47 wherein it is combined to the already determined pitch-excitation vector into an excitation signal vector E. The excitation signal vector E is provided to the synthesis filter 37 to generate synthesized speech and its distortion with respect to the input speech signal is similarly calculated. Likewise, such distortion is also calculated for each of the other remaining random-excitation vectors in the random-excitation source codebook 43 and the random excitation vector of the minimum distortion is determined. - After the selection of the pitch excitation vector and the random excitation vector as mentioned above, gains g p and g c of the gain providing parts 41 and 46 are so determined as to minimize the distortion as described hereafter. In gain coebooks CB1 and CB2 there are stored gain vectors z 1i (where i=1, ..., a) and z 2j (where j=1, ..., b), respectively. The gain vectors z 1i and z 2j are each composed of two components and expressed as z 1i =(z 1i1 , z 1i2 ) and z 2j =(z 2j1 , z 2j2 ), respectively. The gain vectors z 1i and z 2j taken out of the gain codebooks CB1 and CB2 are multiplied by weighting coefficient vectors w 1 =(w 11 , w 12 ) and w 2 =(w 21 , w 22 ) by multipliers 21 and 22, respectively, from which weighted gain vectors y i =(y i1 , y i2 ) and y j =(y ji , y j2 ) are provided. Here, y i1 =z 1i1 w 11 , y i2 =z 1i2 w 12 , y j1 =z 2j1 w 21 , y j2 =z 2j2 w 22 In a vector combining part 3 the weighted gain vectors y i and y j are combined into a combined gain vector G=(g p , g c ) which is composed of such components as follows: g p =y i1 +y j1 , g c =y i2 +y j2 The first and second components gp and g c of the combined gain vector G are provided as first and second gains to the gain providing parts 41 and 46, wherein they are used to multiply the pitch excitation vector C P and the random excitation vector C R from the pitch excitation source codebook 39 and the random excitation source codebook 46, respectively. The pitch excitation vector g p C P and the random excitation vector g c C R multiplied by the gains g p and C R in the gain providing parts 41 and 46, respectively, are added together by the adder 47 and the added output is fed as an excitation vector E=g p C P +g c C R to the synthesis filter 37 to synthesize speech X and. The difference between the synthesized speech X and and the input speech signal X is calculated by the subtractor 48 and the difference is provided to the distortion calculating part 5, wherein D=∥X-X and∥ 2 is calculated as the distortion of the synthesized speech. X and with respect to the input speech signal X. The control part 6 controls the selection switches 7 and 8 to control the selection of the gain vectors of the gain codebooks CB1 and CB2, and the selected gain vectors z 1i and z 2j are multiplied by the different weighting coefficient vectors w 1 and w 2 by the multipliers 21 and 22, respectively, thereafter being provided to the vector combining part 3. The weighting coefficient vectors w 1 and w 2 are two-dimensional vectors that satisfy Eq. (3), and the two elements of each vector differ from each other. The gain vectors of. the codebooks CB1 and CB2 are selected so that they minimize the distortion which is calculated in the distortion calculating part 5. Upon selection of the gain vectors that minimize the distortion, gain labels indicating the selected gain vectors of the gain codebooks CB1 and CB2, labels indicating the excitation vector and the random excitation vector of the pitch excitation source codebook 39 and the random excitation source, codebook 43 determined as described previously and labels indicating the filter coefficient set in the synthesis filter 37 are outputted, as encoded results of the input speech signal X, from a code outputting part 49. In the Fig. 5 embodiment, the distortion is calculated for every combination of gain vectors selected from the gain codebooks CB1 and CB2 so as to determine the pair of gain vectors that provides the minimum distortion. As referred to previously with respect to Fig. 4, however, the pair of gain vectors, which provides the minimum distortion, may also be determined by pre-selecting pluralities of candidates of gain vectors z 1i and z 2j and calculating the distortion for every combination of such pre-selected gain vectors in the case where they are combined with the pitch excitation vector C P and the random excitation vector C R . In this instance, as is the case with Fig. 4, weighted gain codebooks CB1' and CB2' are prepared, for example, as Tables I and II as shown in Figs. 6A and 6B, by precalculating weighted gain vectors y 1i =z 1i w 1 =(z 1i1 w 11 , z 1i2 w 12 )=(y 1i1 ,y 1i2 ) y 2j =z 2j w 2 =(z 2j1 w 21 , z 2j2 w 22 )=(y 2j1 , y 2j2 ) which are the products of the gain vectors z 1i and z 2j of the gain codebooks CB1 and CB2 and the weighting coefficient vectors w 1 =(w 11 , w 12 ) and w 2 =(w 21 ,w 22 ), and the codebooks CB1' and CB2' are used as substitutes for the codebooks CB1 and CB2 in Fig. 5 and the multipliers are left out. As in the case of Fig. 4, all two-dimensional weighted vectors y 1i of the weighted gain codebook CB1' are plotted as points on a two-dimensional coordinate system as indicated by black circles in Fig. 7 and a straight line L 1 closest to the group of such points is precalculated by the method of least squares. Similarly, all two-dimensional weighted vectors y 2j of the weighted gain codebook CB2' are plotted as points on the two-dimensional coordinate system as indicated by white circles in Fig. 7 and a straight line L2 closest to the group of such points is precalculated by the method of least squares. As is the case with the Fig. 5 example, the gains of the gain providing parts 41 and 46 are set arbitrarily and the pitch excitation vector C P that provides the minimum distortion is determined, which is followed by the determination of the random excitation vector C R that provides the minimum distortion. Next, the output X and P from the synthesis filter 37 is measured when only the pitch excitation vector C P is applied thereto as an excitation signal with the gains g p =1 and g c =0. Likewise, the output X and R from the synthesis filter 37 is measured when only the random excitation vector C R is applied thereto as an excitation signal with the gains g p =0 and g c =1. Since the synthesized speech output X and from the synthesis filter 37, which is provided when the vectors C P and C R selected from the codebooks 39 and 43 are multiplied by the gains g p and g c , respectively, is expressed as X and =g p X and p+g c X and R , the distortion D of the synthesized speech X and with respect to the input speech signal X is given by the following equation: D=∥X-g p X P -g c X R ∥ 2 =X t X+g p 2 X P t X P +g c 2 X R t X R -2g p X t X P -2g c X t X R -2g p g c X P t X R where t indicates a transposition. By partially differentiating the above equation by the gains g p and g c to obtain those which minimize the distortion D, the following equations are obtained: ∂D/∂g p =2g p X P t X P - 2 X t X P -2g c X P t X R ∂D/∂g c =2g c X R t X R -2 X t X R -2g p X P t X R Since the distortion D is a downward convex function with respect to each of the gains g p and g c as is evident from Eq. (6), the gains g p and g c that minimize the distortion D are values when ∂D/∂g p =0 and ∂D/∂g c =0. Hence, we have g p X P t X P -g c X P t X R = X t X P -g c X P t X R +g c X R t X R = X t X R from Eqs. (7) and (8), respectively. From the following equation, the gains g p and g c that satisfy Eqs. (9) and (10) simultaneously are expressed as follows: Expanding Eq(12), the gains g p and g c are given by the following equations, respectively: g p =k{ X R t X R · X t X P + X P t X R · X t X R } g c =k{ X P t X P · X t X R + X P t X R · X t X P } where k=1/(X and P t X and P ·X and R t X and R -2X and P t X and R ). The pair of gains {g p , g c } thus obtained is the combination of gains that minimizes the distortion D. This pair is plotted as the optimum gain vector as indicated by a point P 0 in Fig. 7 and straight lines parallel to the straight lines L 2 and L 1 are drawn from the point P 0 , determining their intersections P 1 and P 2 with the lines L 1 and L 2 . A plurality of weighted gain vectors y 1i =(y 1i1 , y 1i2 ) whose ordinate values are close to the point P 1 are selected from the codebook CB1' (Table I shown in Fig. 6A) to form a first subgroup. Similarly, a plurality of weighted gain vectors y 2j =(y 2j1 , y 2j2 ) whose abscissa values are close to the point P 2 are selected from the codebook CB2' (Table II shown in Fig. 6B) to form a second subgroup. In this case, predetermined numbers of weighted gain vectors (for example, four from Table I with respect to the point P 1 and eight from Table II with respect to the point P 2 ) are selected in increasing order of distance from the points P 1 and P 2 . Another method is to select weighted gain vectors that lie within predetermined distances d 1 and d 2 from the points P 1 and P 2 , respectively. Alternatively, since eight vectors y 1i are prestored in the codebook CB1', mean values of i=n-th and i=(n+4)th ones of the gain components y 1i2 are calculated for n=1, 2, 3, 4, respectively, and the thus obtained values are set as threshold values Th 1 , Th 2 , Th3 and Th 4 . If the ordinate value P1c at the intersection P 1 is P 1c ≤Th 1 , then (i=1 ..., 4)th weighted gain vectors are selected, and if Th n <p 1c ≤Th n+1 where n=1, 2, 3, 4, then (i=n+1, ..., n+4)th weighted gain vectors are selected. Similarly, since the number of vectors y 2j stored in the codebook CB2' is 16, mean values of (j=m)th and (j=m+8)th ones of gain components y 2ji are calculated for m=1, ..., 8, respectively, and the values thus obtained are set as threshold values Th 1 , ..., Th 8 . If the abscissa value p 2p at the intersection P 2 is p 2p <Th 1 , (j=1, ..., 8)th weighted gain vectors are selected, and if Th m <p 2p ≤Th m+1 where m=1, ..., 8, (j=m+1, ..., m+8)th vectors are selected. It is also possible to use various other selecting methods. A description will be given of still another method of pre-selecting candidates of vectors from the codebooks without using the afore-mentioned approximating straight lines L 1 and L 2 based on the method of least squares. At first, the synthesized speech signal X P is measured when only the pitch excitation vector C P from the pitch excitation source codebook 39 is provided as the excitation signal vector E to the synthesis filter 37, with the gain vectors set to g p =1 and g c =0. Similarly, the synthesized speech signal X C is measured when only the random excitation vector C R from the random excitation source codebook 43 is provided as the excitation signal vector E, with the gain vectors set to g p =0 and g c =1. For the pre-selection of the gain vectors stored in the gain codebook CB1, a value D 1 (i) is calculated for every i as follows: D 1 (i)=∥X-w 11 z 1i1 X P ∥ 2 Then, a predetermined number of, for example, three gain vectors are pre-selected from the gain codebook CB1 in increasing order of the value D 1 (i). For the pre-selection of the gain vectors stored in the gain codebook CB2, a value D 2 (j) is similarly calculated for every j as follows: D 2 (j)=∥X-w 11 z 2j2 X C ∥ 2 Then, a predetermined number of, for example, three gain vectors are pre-selected from the gain codebook CB2 in increasing order of the value D 2 (j). Only for the triplets of gain vectors z 1i and z 2j thus pre-selected from the codebooks CB1 and CB2, a value D(i, j) is calculated as follows: D(i, j)=∥X-(w 11 z 1i1 +w 12 z 1i2 ) X P -(w 12 z 2j1 +w 11 z 2j2 ) X C ∥ 2 Then, i and j that minimize the value are provided as encoded outputs. This method also permits reduction of the computational complexity involved. All pairs of thus pre-selected weighted gain vectors that are selected one by one from the first and second subgroups, respectively, are searched for a pair of weighted gain vectors that provide the minimum distortion from the input speech signal X, that is, an optimum pair of first and second gains g p and g c is thus determined and combined. Then, labels, which represent the pair of pitch excitation vector and random excitation vector determined previously and the combined gain vector (g p , g c ), that is, a label of the pitch excitation vector in the pitch excitation source codebook 39, label of the random excitation vector in the random excitation source codebook 43, labels of the gain vectors in the weighted gain codebooks CB1' and CB2' (or gain codebooks CB1 and CB2) and labels produced by quantizing the filter coefficients are outputted as encoded results of the input speech vector X. At any rate, the multiplication of the weighting coefficient vectors w 1 and w 2 by the multipliers 21 and 22 begins to bring about the effect of preventing a channel error from seriously distorting the decoded output when the ratio of corresponding components, for example, w 11 and w 21 of the weighting coefficient vectors exceeds a value 2:1. Even if the ratio is set to 10:1 or more, however, the distortion by the channel error cannot appreciably be improved or suppressed; on the contrary, the decoded output is rather distorted seriously when no channel error arises, that is, when the channel is normal. In the Fig. 5 embodiment, the weighting coefficient vectors, the weighted gain vectors and the combined gain vector have all been described to be two-dimensional vectors with a view to providing gains to the excitation vectors read out from both of the pitch excitation source codebook 39 and the random excitation source codebook 43. In the CELP system, however, there are cases where pluralities of pitch excitation source codebooks and random excitation source codebooks (which will hereinafter be referred to simply as excitation source codebooks) are provided and excitation vectors read out of the excitation codebooks are respectively multiplied by gains and combined into the excitation signal vector E. In general, according to the present invention, when M excitation gain codebooks are used, M gain codebooks (or weighted gain codebooks) are prepared to provide gains to M excitation vectors and the combined gain vector, the weighting coefficient vectors and the weighted gain vectors are all set to M-dimensional vectors accordingly. In the Fig. 5 embodiment, the random excitation source codebook 43 may be formed by a plurality of codebooks. For example, as shown in Fig. 8, the random excitation source codebook 43 is formed by two codebooks 43a and 43b; in this instance, one random excitation vector is selected from either of the codebooks 43a and 43b and the thus selected random excitation vectors are multiplied by weighting coefficient vectors w Ra and w Rb by weighting coefficient multipliers 51a and 51b, respectively. The weighting coefficient vectors w Ra and w Ra are selected such that they bear the same relationship as that between the weighting coefficient vectors w 1 and w 2 described previously with reference to Fig. 2A. The outputs from the multipliers 51a and 51b are combined in a random vector combining part 52 and the combined output is provided, as the random excitation vector selected from the random excitation source codebook 43 in Fig. 5, to the gain providing part 46. As described previously with respect to Fig. 5, random excitation vectors are selected from the random excitation source codebooks 43a and 43b under the control of the control part 6 in such a manner that the distortion of the synthesized speech signal X and from the input speech signal X becomes minimum. As is the case with the encoding of the random excitation vector, the present invention is also applicable to the encoding of the pitch excitation vector in the configuration of Fig. 5. That is to say, as shown in Fig. 9, the pitch excitation source codebook is formed by two codebooks 39a and 39b; one pitch excitation vector selected from either of the codebooks 39a and 39b, then they are multiplied by weighting coefficient vectors w Pa and w Pb by weighting coefficient multipliers 53a and 53b, respectively, then these multiplied outputs are combined in a pitch excitation vector combining part 54, and the combined output is provided, as the pitch excitation vector selected from the pitch excitation source vector codebook 43 in Fig. 5, to the multiplier 41. The weighting coefficient vectors wpa and w Pb that are set in the multipliers 53a and 53b are determined in the same fashion as the weighting coefficient vectors w 1 and w 2 in Fig. 2A. The present invention can be applied to the quantization in the filter coefficient quantizing part shown in Fig. 5 by configuring the filter coefficient quantizing part 36 in the same manner as depicted in Fig. 2A. That is, representative spectrum envelope vectors are prestored in the codebooks CB1 and CB2 in Fig. 2A, then one representative spectrum envelope vector selected from either of the codebooks CB1 and CB2 is multiplied by the corresponding one of the weighting coefficient vectors w 1 and w 2 , and the multiplied vectors are combined in the vector combining part 3. The representative spectrum envelope vectors that are selected from the codebooks CB1 and CB2 are searched for a combination of representative spectrum envelope vectors that provide the minimum distance between their combined vector and the input spectrum envelope vector from the filter coefficient determining part 35 (Fig. 5). The vector coding method of the present invention is applicable to the VSELP system as well. Fig. 10 illustrates the principal parts of its embodiment. In this instance, the random excitation source codebook 43 in Fig. 5 is formed by a number of basic vector codebooks 43 1 to 43 n in each of which there is stored one random excitation vector. The random excitation vectors read out of the basic vector codebooks 43 1 to 43 n are polarized positive or negative in polarity control parts 56 1 to 56 n and the polarity-controlled random excitation vectors are multiplied by weighting coefficient vectors w R1 to w Rn by weighting coefficient multipliers 57 1 to 57 n . The multiplied outputs are added together by an adder 58 and the added output is provided as the random excitation vector to the multiplier 46 in Fig. 5. The polarity control parts 56 1 to 56 n are individually controlled by the control part 6 in Fig. 5 so that the distortion of the synthesized speech signal from the input speech signal is minimized. In other words, each pair of basic vector codebook 43 i (where i=1, 2, ..., n) and polarity control part 56 i constitutes one random excitation source codebook and one of two positive and negative random excitation vectors is selected by the control part 6. The weighting coefficient vectors w R1 to w Rn of the weighting coefficient multipliers 57 1 to 57 n are set to bear the same relationship as that between the weighting coefficient vectors referred to previously with respect to Fig. 2A. As will be understood from the description given above with reference to Fig. 10, the random excitation source codebook 43 in Fig. 5 may also be substituted with the basic vector codebooks 43 1 to 43 n and the polarity control parts 56 1 to 56 n in Fig. 10. The same goes for the codebooks 43a and 43b in Fig. 8. The pitch excitation source codebook 39 in Fig. 5 may also be formed by what is called an adaptive codebook which adaptively generates the pitch excitation vector from the pitch period obtained by analyzing the input speech signal and the excitation signal vector E of the previous frame. This adaptive codebook can be used as the pitch excitation source codebook 39 when the configuration of Fig. 8 or 10 is employed as a substitute for the random excitation source codebook 43. Furthermore, the present invention is also applicable to an arbitrary combination of the vector coding of the power of a speech signal, the vector coding of a spectrum envelope parameter, the vector coding of a pitch excitation source codebook and the vector coding of a random excitation source codebook. As described previously, the multipliers 21 and 22 in Fig. 2A may be omitted by prestoring, as the representative vectors, weighted representative vectors w 1 z 1i and w 2 z 2j obtained by multiplying the representative vectors in the codebooks CB1 and CB2 by the weighting coefficient vectors w 1 and w 2 . Similarly, the multipliers 24 and 25 in Fig. 2B may be omitted by prestoring the weighted representative vectors w 1 z 1i and w 2 z 2j in the codebooks CB3 and CB4, respectively. Also in the Fig. 5 embodiment, the multipliers 21 and 22 may be omitted by prestoring weighted gain vectors in the gain codebooks CB1 and CB2. In Figs. 8 and 9, too, the multipliers 51a, 51b and 53a and 53b may be omitted by prestoring weighted vectors in the codebooks 43a, 43b and 39a, 39b. While in the above representative vectors read out of two codebooks are vector-combined, the present invention is also applicable to a system wherein representative vectors read out of three or more codebooks are vector-combined. Moreover, the Fig. 5 embodiment of the present invention has been described as being applied to the coding of the input speech signal, but it is needless to say that the invention is applicable to the encoding of ordinary acoustic signals as well as to the encoding of the speech signal. Next, a description will be given of characteristics that are obtained in the cases of applying conventional techniques and the present invention to the vector coding of the gains g p and g c in the CELP speech coding shown in Fig. 5. (A) A first conventional technique substitutes the two gain codebooks CB1 and CB2 in Fig. 5 with one gain codebook which specifies one two-dimensional gain vector by seven-bit label and has 2 7 =128 labels. The one component of the two-dimensional vector read out of the gain codebook is used as the gain gp for the pitch excitation vector and the other element as the gain gc for the random excitation vector. (B) A second conventional technique uses the two gain codebooks CB1 and CB2 shown in Fig. 5 but does not use the weighting coefficient vectors. The codebook CB1 prestores therein 2 3 two-dimensional vectors each of which is specified by a three-bit label, and the codebook CB2 prestores therein 2 4 two-dimensional vectors each of which is specified by a four-bit label. The vectors selected from the two codebooks, respectively, are combined into one two-dimensional combined vector; the one element of the combined vector is used as the gain g p and the other element as the gain g c . (C) In the example of the present invention applied to the encoding of gain vectors in Fig. 5, the gain vectors read out of the gain codebooks CB1 and CB2 in the above-mentioned case (B) are multiplied by the weighting coefficient vectors w 1 =(1.8, 0.2) and w 2 =(0.2, 1.8) and then added together into a combined vector. The gain codebooks CB1' and CB2', which store weighted gain vectors obtained by multiplying the gain vectors read out of the two codebooks CB1 and CB2 by the weighting coefficient vectors w 1 and w 2 , respectively, are the same as those in Figs. 6A and 6B. In Fig. 11 there are indicated, by the curves A11 and B11, measured results of the segmental SN ratio of reconstructed speech to the error rate when a channel error occurred in the gain labels in the encoded outputs of speech by the configurations of Cases (A) and (B) in Fig. 5. The segmental SN ratio is obtained by measuring the SN ratio of each frame for several minutes and averaging the measured values. The use of two codebooks (curve B11) attains a more excellent segmental SN ratio to the channel error than in the case of using one codebook (curve A11). In Fig. 12 there are shown, by curves A12 and B12, equivalent Q values converted from mean opinion scores of 24 ordinary people about the reconstructed speech with respect to the channel error rate of the gain labels in the case of Fig. 11. As is evident from Fig. 12, the use of two codebooks is preferable from the viewpoint of the channel error rate, and even if the two cases shown in Fig. 11 do not greatly differ in their SN ratio characteristic, they greatly differ psycho-acoustically. Fig. 13 shows, by the curve C13, measured values of the segmental SN ratio of reconstructed speech to the channel error rate in the Case (C) as in the case of Fig. 11, the curve B11 in the Case (B) being shown for comparison use. Apparently, the SN ratio of the reconstructed speech to the channel error rate in the case of the present invention is better than in the prior art. In view of the fact that the difference between the SN ratios shown in Figs. 11 and 12 exerts a great influence on the equivalent Q value, it is expected that the present invention, which uses two weighted codebooks, improves the equivalent Q value more than in the case of using two unweighted codebooks. As described above, according to the present invention, in the case of encoding vectors through the use of a plurality (M) of codebooks each having L-dimensional vectors, L-dimensional weighting coefficient vectors w 1 , ... , w M for the codebooks are selected so that the sum of weighting coefficient matrixes W 1 , ..., W M , each having components of the weighting coefficient vectors as diagonal elements, becomes a constant multiple of the unit matrix. As the result of this, the vector distribution of each codebook is deviated or biased by the L weighting coefficient vectors in such a manner that the individual vectors approach different coordinate axes of the L-dimensional coordinate system (that is, compress component values of other dimensions). In the case where a signal is encoded by a pair of such weighted vectors for each codebook in a manner to minimize the distortion and the labels of the M codebooks corresponding to the weighting coefficient vectors are transmitted, if an error occurs, for example, in one of the labels during transmission over the channel, there is a possibility that an error in the coordinate-axis direction of one dimension is large, but since errors in the coordinate-axis direction of all the other dimensions are compressed, the error of the absolute value of the combined vector does not become so large. Hence, the application of the present invention to the encoding of speech signals is effective in suppressing abnormalities that result from channel errors. Furthermore, according to the present invention, a plurality of weighted vectors of each codebook are pre-selected for input signal vectors and the distortion by encoding is calculated with respect to such pre-selected weighted vectors alone--this appreciably reduces the amount of calculations involved in the encoding and speeds up the encoding. The present invention is also applicable to what is called a CELP or VSELP speech coding scheme, in which case the invention can be applied to the vector coding of spectrum envelope parameters, the vector coding of power and the vector coding of each codebook individually or simultaneously. It will be apparent that many modifications and variations may be effected without departing from the scope of the novel concepts of the present invention.

Claims: A method of encoding an input vector through the use of M codebooks (CB1, CB2) each having a plurality of labeled representative vectors of the same number of dimensions, said M being an integer equal to or greater than 2, said method comprising the steps of: (a) selecting a set of M representative vectors, one from each codebook; (b) multiplying each representative vector in the set selected in step (a) with a respective one of M predetermined weighting coefficient vectors that have the same number of dimensions as said representative vectors, to generate M weighted representative vectors; (c) adding all of said M weighted representative vectors to generate a combined representative vector; (d) calculating the distance between said input vector and said combined representative vector; (e) repeating steps (a), (b), (c) and (d) to search for and determine the combined representative vector for which the distance calculated in step (d) assumes a minimum; and (f) outputting, as the encoded input vector, for each of the M weighted representative vectors that form the combined representative vector determined in step (e), the label of the corresponding representative vector in the respective one of said M codebooks (CB1, CB2); characterized in that each of said M weighting coefficient vectors has among its components one maximum component whose value is greater than that of all other components, the dimension of a respective weighting coefficient vector represented by its maximum component being different from the dimensions represented by the maximum components of the other M-1 weighting coefficient vectors, such that the weighted representative vectors derived from a respective codebook have a distribution which is concentrated much closer to one of the dimensional axes of these vectors than to any of the other dimensional axes of these vectors, said one dimensional axis being a different one for each of the M codebooks (CB1, CB2). A method of encoding an input vector through the use of M codebooks (CB1, CB2) each having a plurality of labeled representative vectors of the same number of dimensions, said M being an integer equal to or greater than 2, said method comprising the steps of: (a) obtaining from each of said M codebooks (CB1, CB2) a respective group of representative vectors and multiplying the representative vectors in each group with a respective one of M weighting coefficient vectors that have the same number of dimensions as said representative vectors, to obtain M groups of weighted representative vectors, wherein each of said M weighting coefficient vectors has among its components one maximum component whose value is greater than that of all other components, the dimension of a respective weighting coefficient vector represented by its maximum component being different from the dimensions represented by the maximum components of the other M-1 weighting coefficient vectors, such that the weighted representative vectors derived from a respective codebook have a distribution which is concentrated much closer to one of the dimensional axes of these vectors than to any of the other dimensional axes of these vectors, said one dimensional axis being a different one for each of said M groups; (b) determining M straight lines (27, 28) for approximating said M groups of weighted representative vectors, respectively; projecting said input vector onto said M straight lines in an M-dimensional coordinate system; and pre-selecting from each group of weighted representative vectors a respective subgroup of vectors adjacent or close to the point of projection of said input vector on the corresponding one of said M straight lines (27, 28); (c) selecting M weighted representative vectors, one from each of said subgroups, and adding them to obtain a combined representative vector; (d) calculating the distance between said combined representative vector and said input vector; (e) repeating said steps (c) and (d) for each combination of weighted representative vectors of said M subgroups to search and determine the combined representative vector for which the distance calculated in step (d) assumes a minimum; and (f) outputting, as the encoded input vector, for each of the M weighted representative vectors that form the combined representative vector determined in step (e), the label of the corresponding representative vector in the respective one of said M codebooks (CB1, CB2). A method of encoding an input vector through the use of M codebooks (CB1', CB2') each having a plurality of labeled weighted representative vectors of the same number of dimensions, said M being an integer equal to or greater than 2, said method comprising the steps of: (a) selecting a set of M weighted representative vectors, one from each codebook; (b) adding all of said M weighted representative vectors to generate a combined representative vector; (c) calculating the distance between said input vector and said combined representative vector; (d) repeating steps (a), (b) and (c) to search for and determine the combined representative vector for which the distance calculated in step (c) assumes a minimum; and (e) outputting, as the encoded input vector, for each of the M weighted representative vectors that form the combined representative vector determined in step (d), the label of that weighted representative vector in the respective one of said M codebooks (CB1', CB2'); wherein said weighted representative vectors in each of said M codebooks (CB1', CB2') are vectors that have a distribution which is concentrated much closer to one of the dimensional axes of these representative vectors than to any of the other dimensional axes of these vectors, said one dimensional axis being a different one for each of the M codebooks (CB1', CB2'). A method of encoding an input vector through the use of M codebooks (CB1', CB2') each having a plurality of labeled weighted representative vectors of the same number of dimensions, said M being an integer equal to or greater than 2, said method comprising the steps of: (a) projecting said input vector onto M straight lines (27, 28) in an M-dimensional coordinate system; and pre-selecting from each of said M codebooks a respective group of vectors adjacent or close to the point of projection of said input vector on a corresponding one of said M straight lines (27, 28), said M straight lines (27, 28) each approximating a distribution of weighted representative vectors in a respective one of said M codebooks; (b) selecting M weighted representative vectors, one from each of said groups, and adding them to obtain a combined representative vector; (c) calculating the distance between said combined representative vector and said input vector; (d) repeating said steps (b) and (c) for each combination of weighted representative vectors of said M groups to search for and determine the combined representative vector for which the distance calculated in step (c) assumes a minimum; and (e) outputting, as the encoded input vector, for each of the M weighted representative vectors that form the combined representative vector determined in step (d), the label of that weighted representative vector in the respective one of said M codebooks (CB1', CB2'); wherein said weighted representative vectors in each of said M codebooks (CB1', CB2') are vectors that have a distribution which is concentrated much closer to one of the dimensional axes of these vectors than to any of the other dimensional axes of these vectors, said one dimensional axis being a different one for each of the M codebooks (CB1', CB2'). The method of claim 1 or 2, wherein said M weighting coefficient vectors are selected so that the sum of M diagonal matrixes each having the components of a respective one of said weighting coefficient vectors as its diagonal elements, is a constant multiple of the unit matrix. The method of claim 2 wherein said pre-selecting in step (b) comprises selecting a predetermined number of said weighted representative vectors closest to the point of projection of said input vector on said corresponding one of said M straight lines (27, 28). The method of claim 4 wherein said pre-selecting in step (a) comprises selecting a predetermined number of said weighted representative vectors closest to the point of projection of said input vector on said corresponding one of said M straight lines (27, 28). The method of claim 2 wherein said pre-selecting in step (b) comprises selecting those weighted representative vectors that lie within the range of a predetermined distance from the point of projection of said input vector on said corresponding one of said M straight lines (27, 28). The method of claim 4 wherein said pre-selecting in step (a) comprises selecting those weighted representative vectors that lie within the range of a predetermined distance from the point of projection of said input vector on said corresponding one of said M straight lines (27, 28). The method of claim 2 wherein: when the direction of said one dimensional axis associated with a respective codebook (CB1, CB2; CB1', CB2') is denoted an emphasized direction and the component of a weighted representative vector that corresponds to the emphasized direction is denoted an emphasized component, said pre-selecting comprises for each of said codebooks (CB1, CB2; CB1', CB2'): determining to which of F+1 regions of values the coordinate Px, in said emphasized direction, of the point of projection of said input vector on the associated one of said M straight lines (27, 28) belongs, and selecting as said subgroup, weighted representative vectors whose emphasized component falls in the same region, wherein said F+ 1 regions are defined by F predetermined threshold values sub-dividing the entire range of values, arranged in increasing order, of the emphasized components of the weighted representative vectors derived from the respective codebook (CB1, CB2; CB1', CB2') into said F + 1 regions, F being an integer equal to or greater than 1. The method of claim 4 wherein: when the direction of said one dimensional axis associated with a respective codebook (CB1, CB2; CB1', CB2') is denoted an emphasized direction and the component of a weighted representative vector that corresponds to the emphasized direction is denoted an emphasized component, said pre-selecting comprises for each of said codebooks (CB1, CB2; CB1', CB2'): determining to which of F + 1 regions of values the coordinate Px, in said emphasized direction, of the point of projection of said input vector on the associated one of said M straight lines (27, 28) belongs, and selecting as said group, weighted representative vectors whose emphasized component falls in the same region, wherein said F + 1 regions are defined by F predetermined threshold values sub-dividing the entire range of values, arranged in increasing order, of the emphasized components of the weighted representative vectors derived from the respective codebook (CB1, CB2; CB1', CB2') into said F + 1 regions, F being an integer equal to or greater than 1. Use of the method of claim 1 or claim 5 as dependent on claim 1 for encoding an input acoustic vector representing an input acoustic signal, through the use of M excitation source codebooks (39, 43) each having a plurality of labeled excitation vectors, and M gain codebooks (CB1, CB2) each having a plurality of labeled M-dimensional gain vectors, said M being an integer equal to or greater than 2, comprising: (i) calculating spectrum envelope parameters of said input acoustic vector, quantizing said spectrum envelope parameters and setting said quantized parameters as filter coefficients in a synthesis filter (37); (ii) selecting M excitation vectors, one from each excitation source codebook (39, 43), so that the distortion of a synthesized acoustic signal from said synthesis filter (37) relative to said input acoustic signal is minimized; and (iii) performing steps (a) to (f) of claim 1 with said gain vectors as the representative vectors; wherein step (d) comprises (d1) multiplying said M excitation vectors selected in step (ii) with the first to M-th components of said combined representative vector as first to M -th gains, respectively; (d2) adding said M gain-multiplied excitation vectors to obtain a combined excitation vector, and exciting said synthesis filter (37) by this combined excitation vector to generate a synthesized acoustic signal; and (d3) calculating, as said distance, the distortion of said synthesized acoustic signal relative to said input acoustic signal; and wherein step (f) comprises outputting, as part of the encoded input acoustic vector, said labels as gain vector labels and the labels identifying the excitation vectors selected in said step (ii). Use of the method of claim 2 or any one of claims 5, 6, 8 and 10 as dependent on claim 2 for encoding an input acoustic vector representing an input acoustic signal, through the use of M excitation source codebooks (39, 43) each having a plurality of labeled excitation vectors, and M gain codebooks (CB1, CB2) each having a plurality of labeled M-dimensional gain vectors, said M being an integer equal to or greater than 2, comprising: (i) calculating spectrum envelope parameters of said input acoustic vector, quantizing said spectrum envelope parameters and setting said quantized parameters as filter coefficients in a synthesis filter (37); (ii) selecting M excitation vectors, one from each excitation source codebook (39, 43), so that the distortion of a synthesized acoustic signal from said synthesis filter (37) relative to said input acoustic signal is minimized; (iii) calculating M optimum gain factors for the M excitation vectors determined in step (ii), respectively, so as to minimize the distortion of said synthesized acoustic signal relative to the acoustic signal; and (iv) performing steps (a) to (f) of claim 2 with said gain vectors as the representative vectors and an input vector having said M gain factors calculated in step (iii) as its components, wherein step (d) comprises (d1) multiplying the M excitation vectors selected in step (ii) with the first to M-th components of said combined representative vector as first to M-th gains, respectively; (d2) adding said M gain-multiplied excitation vectors to generate a combined excitation vector, and exciting said synthesis filter (37) by this combined excitation vector to generate a synthesized acoustic signal; and (d3) calculating, as said distance, the distortion of said synthesized acoustic signal relative to said input acoustic signal; and wherein step (f) comprises outputting, as part of the encoded input acoustic vector, said labels as gain vector labels and the labels identifying the excitation vectors selected in said step (ii). Use of the method of claim 3 for encoding an input acoustic vector representing an input acoustic signal, through the use of M excitation source codebooks (39, 43) each having, a plurality of labeled excitation vectors, and M gain codebooks (CB1', CB2') each having a plurality of labeled M-dimensional weighted gain vectors, said M being an integer equal to or greater than 2, comprising the steps of: (i) calculating spectrum envelope parameters of said input acoustic vector, quantizing said spectrum envelope parameters and setting said quantized parameters as filter coefficients in a synthesis filter (37); (ii) selecting M excitation vectors, one from each excitation source codebook (39, 43), so that the distortion of a synthesized acoustic signal from said synthesis filter (37) relative to said input acoustic signal is minimized; and (iii) performing steps (a) to (e) of claim 3 with said weighted gain vectors as the weighted representative vectors; wherein step (c) comprises (c1) multiplying said M excitation vectors selected in step (ii) with the first to M-th components of said combined representative vector as first to M-th gains, respectively; (c2) adding said M gain-multiplied excitation vectors to obtain a combined excitation vector and exciting said synthesis filter (37) by this combined excitation vector to generate a synthesized acoustic signal; and (c3) calculating, as said distance, the distortion of said synthesized acoustic signal relative to said input acoustic signal; and wherein step (e) comprises outputting, as part of the encoded input acoustic vector, said labels as weighted gain vector labels and the labels identifying the excitation vectors selected in said step (ii). Use of the method of claim 4 or any one of claims 7, 9 and 11 as dependent on claim 4 for encoding an input acoustic vector representing an input acoustic signal, through the use of M excitation source codebooks (39, 43) each having a plurality of labeled excitation vectors, and M gain codebooks (CB1", CB2') each having a plurality of labeled M-dimensional weighted gain vectors, said M being an integer equal to or greater than 2, said method comprising the steps of: (i) calculating spectrum envelope parameters of said input acoustic vector, quantizing said spectrum envelope parameters and setting said quantized parameters as filter coefficients in a synthesis filter (37); (ii) selecting M excitation vectors, one from each excitation source codebook (39, 43), so that the distortion of a synthesized acoustic signal from said synthesis filter (37) relative to said input acoustic signal, is minimized; (iii) calculating M optimum gain factors for the M excitation vectors determined in step (ii), respectively, so as to minimize the distortion of said synthesized acoustic signal relative to the acoustic signal; and (iv) performing steps (a) to (e) of claim 4 with said weighted gain vectors as the weighted representative vectors and an input vector having said M gain factors calculated in step (iii) as its components, wherein step (c) comprises (c1) multiplying said M excitation vectors selected in step (ii) with the first to M-th components of said combined representative vector as first to M-th gains, respectively; (c2) adding said M gain-multiplied excitation vectors to obtain a combined excitation vector and exciting said synthesis filter (37) by this combined excitation vector to generate a synthesized acoustic signal; and (c3) calculating, as said distance, the distortion of said synthesized acoustic signal relative to said input acoustic signal; and wherein step (e) comprises outputting, as part of the encoded input acoustic vector, said labels as weighted gain vector labels and the labels identifying the excitation vectors selected in said step (ii). An encoder for encoding an input vector, comprising: M codebooks (CB1, CB2) each having a plurality of labeled representative vectors of the same number of dimensions; selection means (7, 8) for selecting a set of M representative vectors, one from each codebook; multiplying means (21, 22) for multiplying each representative vector in the set selected by said selection means (7, 8) with a respective one of M predetermined weighting coefficient vectors that have the same number of dimensions as said representative vectors, to generate M weighted representative vectors; a vector combining part (3) for adding said M weighted representative vectors to generate a combined representative vector; a distance calculating part (5) for calculating the distance between said combined representative vector and said input vector; and a control part (6) for operating said selection means (7, 8), said multiplying means (21, 22), said vector combining part (3) and said distance calculating part (5) to search for and determine a combined representative vector for which the distance calculated by said distance calculating part (5) assumes a minimum, and for outputting, as the encoded input vector, for each of the M weighted representative vectors that form the combined representative vector, for which said distance assumes a minimum, the label of the corresponding representative vector in the respective one of said M codebooks (CB1, CB2); characterized in that each of said M weighting coefficient vectors has among its components one maximum component whose value is greater than that of all other components, the dimension of a respective weighting coefficient vector represented by its maximum component being different from the dimensions represented by the maximum components of the other M-1 weighting coefficient vectors, such that the weighted representative vectors derived from a respective codebook have a distribution which is concentrated much closer to one of the dimensional axes of these vectors than to any of the other dimensional axes of these vectors, said one dimensional axis being a different one for each of the M codebooks (CB1, CB2). The encoder of claim 16, wherein said M weighting coefficient vectors are selected so that the sum of M diagonal matrixes each having the components of a respective one of said weighting coefficient vectors as its diagonal elements, is a constant multiple of the unit matrix. An encoder for encoding an input vector, comprising: M codebooks (CB1', CB2') each having a plurality of labeled representative vectors of the same number of dimensions; selection means (7, 8) for selecting a set of M representative vectors, one from each codebook; a vector combining part (3) for adding said M vectors in the set selected by said selection means to generate a combined representative vector; a distance calculating part (5) for calculating the distance between said combined representative vector and the input vector; and a control part (6) for operating said selection means, said vector combining part (3) and said distance calculating part (5) to search for and determine a combined representative vector for which the distance calculated by said distance calculating part (5) assumes a minimum, and for outputting, as the encoded input vector, for each of said M representative vectors that form the combined representative vector, for which said distance assumes a minimum, the label of that representative vector in the respective one of said M codebooks (CB1, CB2); characterized in that, said representative vectors in each of said M codebooks (CB1', CB2') are weighted representative vectors that have a distribution which is concentrated much closer to one of the dimensional axes of these vectors than to any of the other dimensional axes of these vectors, said one dimensional axis being a different one for each of the M codebooks (CB1', CB2'). The encoder of claim 16, 17 or 18 wherein said control part (6) includes: means for pre-selecting from each of said M codebooks , a group of weighted representative vectors adjacent or close to the point of projection of the input vector on a corresponding one of M straight lines (27, 281, said M straight lines (27, 28) being lines each closest to a respective distribution of weighted representative vectors in one of said M codebooks ; wherein said distance calculating part (5) is controlled to calculate said distance for every combination of M weighted representative vectors selected from said M groups so as to obtain the combination of weighted representative vectors which minimizes said distance. An encoder for encoding an input acoustic vector representing an input acoustic signal, comprising: M excitation source codebooks (39, 43) each having a plurality of excitation vectors, said M being an integer equal to or greater than 2; first to M-th gain providing parts (41, 46) for multiplying M excitation vectors, one from each excitation source codebook (39, 43), by first to M-th gains, respectively; an adding part (47) for adding said M gain-multiplied excitation vectors from said first to M-th gain providing parts (41, 46) to generate a combined excitation vector; a synthesis filter (37) adapted to be excited by said combined excitation vector to generate a synthesized acoustic signal; filter coefficient generating means (35, 36) for analyzing said input acoustic vector to obtain parameters representing its spectrum envelope, for quantizing said parameters and for setting said quantized parameters as filter coefficients in said synthesis filter (37); distortion calculating means (5) for calculating the difference between said input acoustic signal and said synthesized acoustic signal and for calculating from said difference the distortion of said synthesized acoustic signal relative to said input acoustic signal; M gain codebooks (CB1, CB2) each having a plurality of labeled M-dimensional gain vectors; multiplying means (21, 22) for multiplying gain vectors respectively selected from said M gain codebooks (CB1, CB2) by M predetermined M-dimensional weighting coefficient vectors to generate weighted gain vectors, wherein each of said M weighting coefficient vectors has among its components one maximum component whose value is greater than that of all other components, the dimension of a respective weighting coefficient vector represented by its maximum component being different from the dimensions represented by the maximum components of the other M-1 weighting coefficient vectors, such that the weighted gain vectors derived from a respective gain codebook have a distribution which is concentrated much closer to one of the dimensional axes of these vectors than to any of the other dimensional axes of these vectors, said one dimensional axis being a different one for each of the M gain codebooks (CB1, CB2); a vector combining part (3) for adding said M weighted gain vectors to generate an M-dimensional combined gain vector and for setting the first to M-th components of said combined gain vector as first to M-th gains in said first to M-th gain providing parts (41, 46), respectively; and control means (6) for controlling the selection of said M excitation vectors from said M excitation source codebooks (39, 43), for determining a combined excitation vector which minimizes said distortion of said synthesized acoustic signal relative to said input acoustic signal, for calculating said distortion by calculating means for each combination of gain vectors respectively selected from said M gain codebooks (CB1, CB2) to determine a combined gain vector which minimizes said distortion, and for outputting, as part of the encoded input acoustic vector, for each of the M weighted gain vectors that form the determined combined gain vector, the label of the corresponding gain vector in the respective one of said M gain codebooks (CB1, CB2), and for each of the M gain-multiplied excitation vectors that form the determined combined excitation vector, the label of the corresponding excitation vector in the respective one of said M excitation source codebooks (39, 43). The encoder of claim 20, wherein said M weighting coefficient vectors are selected so that the sum of M diagonal matrixes each having the components of a respective one of said weighting coefficient vectors as its diagonal elements, is a constant multiple of the unit matrix. An encoder for encoding an input acoustic vector representing an input acoustic signal, comprising: M excitation source codebooks (39, 43) each having a plurality of excitation vectors, said M being an integer equal to or greater than 2; first to M-th gain providing parts (41, 46) for multiplying M excitation vectors, one from each excitation source codebook (39, 43), by first to M-th gains, respectively; an adding part (47) for adding said M gain-multiplied excitation vectors from said first to M-th gain providing parts (41, 46) to generate a combined excitation vector; a synthesis filter (37) adapted to be excited by said combined excitation vector to generate a synthesized acoustic signal; filter coefficient generating means (35, 36) for analyzing said input acoustic vector to obtain parameters representing its spectrum envelope, for quantizing said parameters and for setting said quantized parameters as filter coefficients in said synthesis filter (37); distortion calculating means (5) for calculating the difference between said input acoustic signal and said synthesized acoustic signal and for calculating from said difference the distortion of said synthesized acoustic signal relative to said input acoustic signal; M weighted gain codebooks (CB1', CB2') each having a plurality of labeled M-dimensional weighted gain vectors that have a distribution which is concentrated much closer to one of the dimensional axes of these vectors than to any of the other dimensional axes of these vectors, said one dimensional axis being a different one for each of the M weighted gain codebooks (CB1', CB2'); a vector combing part (3) for adding M weighted gain vectors respectively selected from said M weighted gain codebooks (CB1', CB2') to generate a combined gain vector and for setting the first to M-th components of said combined gain vector as first to M-th gains in said first to M-th gain providing parts (41, 46), respectively; and control means (6) for controlling the selection of said M excitation vectors from said M excitation source codebooks (39, 43), for determining a combined excitation vector which minimizes said distortion of said synthesized acoustic signal relative to said input acoustic signal, for calculating said distortion by calculating means for each combination of weighted gain vectors respectively selected from said M weighted gain codebooks (CB1', CB2'), to determine a combined gain vector which minimizes said distortion, and for outputting, as part of the encoded input acoustic vector, for each of the M weighted gain vectors that form the determined combined gain vector, the label of that weighted gain vector in the respective one of said M weighted gain codebooks (CB1', CB2'), and for each of the M gain-multiplied excitation vectors that form the determined combined excitation vector, the label of the corresponding excitation vector in the respective one of said M excitation source codebooks (39, 43). The encoder of claim 20, 21 or 22 wherein said control means (6) includes: means for obtaining M groups of said weighted gain vectors, the vectors in each group derived from a respective one of the M codebooks (CB1, CB2; CB1', CB2'); means for controlling, when said M excitation vectors are being determined, said first to M-th gains to determine their optimum values which minimize said distortion; means for pre-selecting from each of said M codebooks, a group of weighted gain vectors adjacent or close to the points of projection of a vector, composed of said optimum first to M-th gains, onto M straight lines (27, 28), respectively, said M straight lines (27, 28) being lines each closest to a respective distribution of weighted representative vectors in one of said M codebooks ; and means for controlling said distortion calculating means (5) to calculate said distortion for each combination of M weighted gain vectors respectively selected from said M groups and for determining a combination of weighted gain vectors which minimizes said distortion. The encoder of claim 19 or 23 wherein: when the direction of said one dimensional axis associated with a respective codebook (CB1, CB2) is denoted an emphasized direction and the component of a weighted representative vector that corresponds to the emphasized direction is denoted an emphasized component, said means for pre-selecting comprises for each of said codebooks (CB1, CB2; CB1', CB2'): means for determining to which of F+1 regions of values the coordinate Px, in said emphasized direction, of the point of projection of said input vector on the associated one of said M straight lines (27, 28) belongs, and for selecting as said group, weighted representative vectors whose emphasized component falls in the same region, wherein said F+ 1 regions are defined by F predetermined threshold values sub-dividing the entire range of values, arranged in increasing order, of the emphasized components of the weighted representative vectors derived from the respective codebook (CB1, CB2; CB1', CB2'). A decoder for decoding an inputted code, comprising: M codebooks (CB3, CB4) each having a plurality of labeled representative vectors, said M being an integer equal to or greater than 2; multiplying means (24, 25) for selecting representative vectors corresponding to respective labels in said input code, from the corresponding ones of said codebooks (CB3, CB4) and for multiplying the selected representative vectors by M weighting coefficient vectors predetermined for said M codebooks (CB3, CB4) to generate M weighted representative vectors; and a vector combining part (17) for combining said M weighted representative vectors to a reconstructed vector; characterized in that each of said M weighting coefficient vectors has among its components one maximum component whose value is greater than that of all other components, the dimension of a respective weighting coefficient vector represented by its maximum component being different from the dimensions represented by the maximum components of the other M-1 weighting coefficient vectors, such that the weighted representative vectors derived from a respective codebook have a distribution which is concentrated much closer to one of the dimensional axes of these vectors than to any of the other dimensional axes of these vectors, said one dimensional axis being a different one for each of the M codebooks (CB1, CB2). The decoder of claim 25, wherein said M weighting coefficient vectors are selected so that the sum of M diagonal matrixes each having the components of a respective one of said weighting coefficient vectors as its diagonal elements, is a constant multiple of the unit matrix. A decoder for decoding an inputted code, comprising: M codebooks (CB3, CB4) each having a plurality of labeled representative vectors, said M being an integer equal to or greater than 2; and a vector combining part (17) for selecting from said M codebooks representative vectors corresponding to M labels in said input code and for combining the selected representative vectors to a reconstructed vector; characterized in that each of said M codebooks includes labeled weighted representative vectors that are vectors having a distribution which is concentrated much closer to one of the dimensional axes of these vectors than to any of the other dimensional axes of these vectors, said one dimensional axis being a different one for each of the M codebooks.

Inventors: IKEDO JOTARO, KATAOKA AKITOSHI, IKEDO, JOTARO, KATAOKA, AKITOSHI
Applicants: NIPPON TELEGRAPH & TELEPHONE, NIPPON TELEGRAPH AND TELEPHONE CORPORATION
Application year: 1995
Main classification: H03M7
IPC codes: G10L19, G06T9, H03M7, G10L15