Vector coding method, encoder using the same and decoder therefor

A method of encoding an input vector through the use of M codebooks 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) multiplying representative vectors, each selected from one of said M codebooks, by M weighting coefficient vectors of the same number of dimensions as those of said representative vectors of said M codebooks to obtain an M groups of weighted representative vectors, and determining M straight lines for approximating said M groups of weighted representative vectors, respectively, said M weighted coefficient vectors each having at least one maximum component in a different dimension;
• (b) projecting said input vector on said M straight lines on an M-dimensional coordinate system and pre-selecting pluralities of weighted representative vectors adjacent or close to said projections, respectively, to form M subgroups;
• (c) selecting weighted representative vectors from said M 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 calculate said distance;
• and
• (f) determining labels in said M codebooks which correspond to said weighted representative vectors of the combination that was minimum in said distance, and outputting said labels as encoded results of said input signal vector.

A method of encoding an input vector through the use of M codebooks 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) pregenerating M weighted codebooks each having weighted representative vectors obtained by multiplying representative vectors of said M codebooks by M predetermined weighting coefficient vectors of the same number of dimensions as those of said representative vectors, said M weighting coefficient vectors each having at least one maximum component in a different dimension and at least one of the components of each weighting coefficient vector being different from at least one of the other components of said weighting coefficient vector;
• (b) selecting one representative vectors one by one vector from each of said M weighted codebooks;
• (c) adding all of said M weighted representative codebooks to generate a combined representative vector;
• (d) calculating the distance between said input vector and said combined representative vector;
• (e) repeating steps (b), (c) and (d) to search for and determine said combined representative vector which has the minimum distance between it and said input vector;
• and
• (f) encoding and outputting labels attached to representative vectors of said codebooks which provided said combined representative vectors of said minimum distance.

A method of encoding an input vector through the use of M codebooks 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) multiplying representative vectors of said M codebooks by M weighting coefficient vectors of the same number of dimensions as those of said representative vectors to obtain an M groups of weighted representative vectors, said M weighting coefficient vectors each having at least one maximum component in a different dimension;
• (b) determining M straight lines for approximating said M groups of weighted representative vector, respectively;
• (c) projecting said input vector on said M straight lines on an M-dimensional coordinate system and pre-selecting pluralities of weighted representative vectors adjacent or close to said projections, respectively, to form M subgroups;
• (d) selecting weighted representative vectors from said M subgroups, respectively, and adding them to obtain a combined representative vector;
• (e) calculating the distance between said combined representative vector and said input vector;
• (f) repeating said steps (d) and (e) for each combination of weighted representative vectors of said M subgroups to calculate said distance;
• and
• (g) determining labels in said M codebooks which correspond to said weighted representative vectors of the combination which was minimum in said distance, and outputting said labels as encoded results of said input vector.

A method of encoding the vector of an input acoustic signal through the use of M excitation source codebooks each having a plurality of labeled excitation vectors, said M being an integer equal to or greater than 2, said method comprising the steps of:

• (a) calculating spectrum envelope parameters of said vector of said input acoustic signal, quantizing said spectrum envelope parameters and setting said quantized parameters as filter coefficients of a synthesis filter;
• (b) selecting M excitation vectors from said M excitation source codebooks so that distortion of an acoustic signal synthesized by said synthesis filter, from said input acoustic signal, is minimized;
• (c) selecting one gain vector from each of M gain codebooks each having a plurality of labeled M-dimensional gain vectors;
• (d) multiplying said gain vectors selected from said M gain codebooks by M predetermined M-dimensional weighting coefficient vectors, respectively, to generate M weighted gain vectors, said M weighting coefficient vectors each having at least one maximum component in a different dimension and at least one of the components of each weighting coefficient vector being different from at least one of the other components of said weighting coefficient vector;
• (e) adding all of said M weighted gain vectors to obtain a combined gain vector, and defining first to M-th components of said combined gain vector as first to M-th gains;
• (f) providing said first to M-th gains to said M determined excitation vectors, respectively;
• (g) adding said M gain-provided excitation vectors and exciting said synthesis filter by said added output to generate a synthesized acoustic signal;
• (h) calculating distortion of said synthesized acoustic signal from said input acoustic signal;
• (i) repeating steps (c) to (h) for every combination of respective gain vectors of said M gain codebooks to search for and determine M gain vectors that minimize said distortion, and obtaining M gain labels corresponding to said M gain vectors;
• and
• (j) outputting, as at least one part of encoded results of said input acoustic signal, said M gain labels obtained in said step (i) and the labels of said excitation codebooks obtained in said step (b).

A method of encoding the vector of an input acoustic signal through the use of M excitation source codebooks each having a plurality of labeled excitation vectors, said M being an integer equal to or greater than 2, said method comprising the steps of:

• (a) calculating spectrum envelope parameters of said vector of said input acoustic signal, quantizing said spectrum envelope parameters and setting said quantized parameters as filter coefficients of a synthesis filter;
• (b) selecting M excitation vectors from said M excitation source codebooks, respectively, so that distortion of synthesized speech by said synthesis filter, from said input acoustic signal, is minimized;
• (c) providing first to M-th gains to said M determined excitation vectors, respectively, adding them to generate an excitation signal vector for excitation of said synthesis filter to generate said synthesized acoustic signal and, for each combination of said excitation vectors, searching for optimum values of said first to M-th gains which minimize said distortion of said synthesized acoustic signal from said input acoustic signal;
• (d) multiplying respective gain vectors of M gain codebooks each having a plurality of labeled M-dimensional gain vectors by M predetermined M-dimensional weighting coefficient vectors for said M gain codebooks to obtain M groups of weighted gain vectors, and determining M straight lines for approximating said M groups of weighted gain vectors, said M weighting coefficient vectors each having at least one maximum component in a different dimension;
• (e) projecting a vector, composed of said optimum first to M-th gains, on said M straight lines of an M-dimensional coordinate system, and pre-selecting from said M groups pluralities of weighted gain vectors adjacent or close to said projections to form M subgroup;
• (f) selecting said one weighted representative vectors one by one vector from each of said M subgroups, adding them to obtain an M-dimensional combined gain vector, and defining first to M-th gain components of said combined gain vector as first to M-th gains;
• (g) multiplying said M excitation vectors, determined in step (b), by said first to M-th gain components of said combined gain vector, respectively, and adding them to generate an excitation signal vector;
• (h) applying said excitation signal vector, generated in said step (g), to said synthesis filter to synthesize an acoustic signal and calculating distortion of said acoustic signal from said input acoustic signal;
• (i) repeating said steps (f), (g) and (h) for every combination of weighted representative vectors of said M subgroups to calculate said distortion, and determining gain labels in said M codebooks which correspond to said combination of weighted gain vectors which was minimum in said distortion;
• and
• (j) outputting labels of said M excitation vectors and said gain labels as at least one part of encoded results of said vector of said input acoustic vector.

A method of encoding the vector of an input acoustic signal through the use of M excitation source codebooks each having a plurality of labeled excitation vectors, said M being an integer equal to or greater than 2, said method comprising the steps of:

• (a) calculating spectrum envelope parameters of said vector of said input acoustic signal, quantizing said spectrum envelope parameters and setting said quantized parameters as filter coefficients of a synthesis filter;
• (b) selecting said excitation vectors one by one from said M excitation source codebooks so that distortion of a synthesized acoustic signal by said synthesis filter, from said input acoustic signal, is minimized;
• (c) pregenerating M weighted gain codebooks each having M groups of labeled weighted gain vectors obtained by multiplying M-dimensional gain vectors of M gain codebooks by M predetermined M-dimensional weighting coefficient vectors, respectively, said M weighting coefficient vectors each having at least one maximum component in a different dimension and at least one of the components of each weighting coefficient vector being different from at least one of the other components of said weighting coefficient vector;
• (d) selecting one representative weighted gain vectors one by one vector from each of said M weighted codebooks;
• (e) adding all of said M weighted gain vectors to generate a combined gain vector, and defining first to M-th components of said combined gain vector as first to M-th gains;
• (f) providing said first to M-th gains to said M excitation vectors respectively;
• (g) adding said M gain-provided excitation vectors, and exciting said synthesis filter by said added output to generate a synthesized acoustic signal;
• (h) calculating distortion of said synthesized acoustic signal from said input acoustic signal;
• (i) repeating said steps (d) to (h) for every combination of said weighted gain vectors of said M weighted codebooks to search for and determine M weighted gain vectors which minimize said distortion, and obtaining M labels corresponding to said M weighted gain vectors;
• and
• (j) outputting, as at least one part of encoded results of said input acoustic signal, said M labels of said M weighted gain codebooks obtained in said step (i) and labels of said M excitation codebooks obtained in said step (b).

A method of encoding the vector of an input acoustic signal through the use of M excitation source codebooks each having a plurality of labeled excitation vectors, said M being an integer equal to or greater than 2, said method comprising the steps of:

• (a) calculating, spectrum envelope parameters of said vector of said input acoustic signal, quantizing said spectrum envelope parameters and setting said quantized parameters as filter coefficients of a synthesis filter;
• (b) selecting said excitation vectors one by one from said M excitation source codebooks so that distortion of a synthesized acoustic signal by said synthesis filter, from said input acoustic signal, is minimized;
• (c) providing first to M-th gains to said selected M excitation vectors, adding them to generate an excitation signal vector, and searching for and determining optimum values of said first to M-th gains such that distortion of a synthesized acoustic signal from said synthesis filter excited by said excitation signal vector, from said input acoustic signal, is minimized;
• (d) pregenerating M weighted codebooks each having M groups of labeled gain vectors obtained by multiplying M-dimensional gain vectors of M gain codebooks by M predetermined M-dimensional weighting coefficient vectors, respectively, said M weighting coefficient vectors each having at least one maximum component in a different dimension, and determining M straight lines for approximating said M groups of weighted gain vectors, respectively;
• (e) projecting a vector, composed of said optimum first to M-th gains, said M straight lines on an M-dimensional coordinate system, and pre-selecting from said M groups pluralities of weighted gain vectors adjacent or close to said projections to form M subgroups;
• (f) selecting said one weighted gain vectors one by one vector from each of said M subgroups, adding them to obtain an M-dimensional combined gain vector, and defining first to M-th gain components of said combined gain vector as first to M-th gains,
• (g) multiplying said M excitation vectors, determined in said step (b), by said first to M-th gain components of said combined gain vector, and adding them to generate an excitation signal vector;
• (h) applying said excitation signal vector to said synthesis filter to synthesize an acoustic signal, and calculating its distortion from said input acoustic signal;
• (i) repeating said steps (f), (g) and (h) for every combination of weighted representative vectors of said M subgroups to calculate said distortion, and searching for and determining gain labels in said M codebooks which correspond to weighted gain vectors of the combination which was minimum in said distortion;
• and
• (j) outputting labels of said M determined excitation vectors and said determined gain labels as at least one part of encoded results of said vector of said input acoustic signal vector.

An encoder for encoding the vector of an input signal through the use of a plurality of codebooks, comprising:

• M codebook each having a plurality of labeled representative vectors;
• multiplying means for multiplying representative vectors selected from said M codebooks by M predetermined but different weighting coefficient vectors to generate weighted representative vectors, and an M being an integer equal to or greater than 2 and said M weighting coefficient vectors each having at least one maximum component in a different dimension and at least one of the components of each weighting coefficient vector being different from at least one of the other components of said weighting coefficient vector;
• a vector combining part for adding said M weighted vectors to generate a combined representative vector;
• a distance calculating part for calculating the distance between said combined representative vector from said vector combining part and said input vector;
• and
• a control part for operating said vector combining part and said distance calculating part while changing the selection of weighted representative vectors from said M weighted codebooks, for determining a combination of weighted representative vectors of said M codebooks which minimizes said distance, and for outputting their corresponding labels as encoded results of the vector of said input signal.

An encoder for encoding the vector of an input signal through the use of a plurality of codebooks, comprising:

• M weighted representative codebooks each having M groups of weighted representative vectors generated by multiplying representative vectors of M groups by M different weighting coefficient vectors, respectively, said M being an integer equal to or greater than 2 and said weighting coefficient vectors each having at least one maximum component in a different dimension and at least one of the components of each weighting coefficient vector being different from at least one of the other components of said weighting coefficient vector;
• a vector combining part for adding M weighted vectors respectively selected from said M weighted codebooks to generate a combined representative vector;
• a distance calculating part for calculating the distance between said combined representative vector from said vector combining part and the vector of said input signal;
• and
• a control part for operating said vector combining part and said distance calculating part while changing the selection of weighted representative vectors from said M weighted codebooks, for determining a combination of weighted representative vectors of said M weighted codebooks which minimizes said distance, and for outputting their corresponding labels as encoded results of the vector of said input signal.

An encoder for encoding the vector of an input acoustic signal through the use of a plurality of codebooks, comprising:

• M excitation source codebooks 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 for multiplying said M excitation vectors from said M excitation source codebooks by first to M-th gains, respectively;
• an adding part for adding said M gain-provided excitation vectors from said first to M-th gain providing parts to generate an excitation signal vector;
• filter coefficient generating means which analyzes said input acoustic signal to obtain parameters representing its spectrum envelope and quantizes said parameters to generate filter coefficients;
• a synthesis filter which has said filter coefficients set therein and is excited by said excitation signal vector to synthesize an acoustic signal;
• distortion calculating means 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 from said input acoustic signal;
• M gain codebooks each having a plurality of labeled M-dimensional gain vectors;
• multiplying means for multiplying gain vectors respectively selected from said M gain codebooks by M predetermined M-dimensional weighting coefficient vectors to generate weighted gain vectors, said M weighting coefficient vectors each having at least one maximum component in a different dimension and at least one of the components of each weighting coefficient vector being different from at least one of the other components of said weighting coefficient vector;
• a vector combining part for adding said M weighted gain vectors to generate an M-dimensional combined gain vector and for setting 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, respectively;
• and
• control means for controlling the selection of said M excitation vectors from said M excitation source codebooks, for determining a combination of said M excitation vectors which minimizes said distortion of said synthesized acoustic signal from said input acoustic signal, for calculating said distortion by calculating means for every combination of gain vectors respectively selected from said M gain codebooks to determine a combination of M gain vectors which minimizes said distortion, and for outputting labels of said M gain codebooks corresponding to said M determined gain vectors and labels corresponding to said M determined excitation vectors as at least one part of encoded results of said input acoustic signal.

An encoder for encoding the vector of an input acoustic signal through the use of a plurality of codebooks, comprising:

• M excitation source codebooks 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 for multiplying said M excitation vectors from said M excitation source codebooks, respectively;
• an adding part for adding said M gain-provided excitation vectors from said first to M-th gain providing parts to generate an excitation signal vector;
• filter coefficient generating means which analyzes said input acoustic signal to obtain parameters representing its spectrum envelope and quantizes said parameters to generate filter coefficients;
• a synthesis filter which has said filter coefficients set therein and is excited by said excitation signal vector to synthesize an acoustic signal;
• distortion calculating means which calculates the difference between said input acoustic signal and said synthesized acoustic signal and calculates from said difference the distortion of said synthesized acoustic signal from said input acoustic signal;
• M weighted gain codebooks each having M groups of weighted gain vectors generated by multiplying M groups of M-dimensional gain vectors by M-dimensional weighting coefficient vectors, said M weighting coefficient vectors having at least one maximum component in a different dimension and at least one of the components of each weighting coefficient vector being different from at least one of the other components of said weighting coefficient vector;
• a vector combining part which adds M weighted vectors respectively selected from said M weighted codebooks to generate a combined representative vector and sets 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, respectively;
• and
• control means which controls the selection of said M excitation vectors from said M excitation source codebooks, determines a combination of said M excitation vectors which minimizes said distortion of said synthesized acoustic signal from said input acoustic signal, obtains by calculating means said distortion for each combination of weighted gain vectors selected from said M weighted gain codebooks, determines a combination of M weighted gain vectors which minimizes said distortion, and outputs labels of said M weighted gain codebooks corresponding to said M determined weighted gain vectors and labels corresponding to said M determined excitation vectors as at least one part of encoded results of said input acoustic signal.

A decoder for decoding an inputted code by referring to a plurality of codebooks, comprising:

• M codebook each having a plurality of labeled representative vectors, said M being an integer equal to or greater than 2;
• multiplying means which selects representative vectors corresponding to respective labels in said input code from the corresponding ones of said codebooks and multiplies said selected representative vectors by M weighting coefficient vectors predetermined for said M codebooks to generate M weighted representative vectors;
• and
• a vector combining part which combines said M weighted representative vectors into a reconstructed vector;
• wherein said M weighting coefficient vectors each has at least one maximum component in a different dimension and at least one of the components of each weighting coefficient vector is different from at least one of the other components of said weighting coefficient vector.

A decoder for decoding an inputted code by referring to a plurality of codebooks, comprising:

• M weighted codebooks each having a plurality of labeled weighted representative vectors, said M being an integer equal to or greater than 2;
• and
• a vector combining part which selects from said M weighted codebooks weighted representative vectors corresponding to M labels in said input code and combines them into a reconstructed vector;
• wherein said M weighting coefficient vectors each have at least one maximum component in a different dimension and at least one of the components of each weighting coefficient vector is different from at least one of the other components of said weighting coefficient vector.

A method of encoding an input vector through the use of M codebooks, each including a plurality of representative vectors, each representative vector having a corresponding label, said M being an integer equal to or greater than 2, and the method comprising the steps of:

• searching a combination of M representative vectors, each representative vector of the M representative vectors being selected from a different one of the M codebooks, for a combined minimum vector of the M selected representative vectors which provides a minimum distance between the input vector and itself;
• and
• outputting the corresponding labels for the M selected representative vectors of the combined minimum vector,
• wherein the representative vectors of a respective codebook of the M codebooks have a distribution which is concentrated close to a respective dimensional axis of the representative vectors in the respective codebook, said respective dimensional axis being different for each of the M codebooks.

A method of encoding an input vector through the use of M codebooks, each including a plurality of representative vectors, each representative vector having a corresponding label, said M being an integer equal to or greater than 2, and the method comprising the steps of:

• projecting the input vector onto M straight lines, each straight line approximating a respective distribution of the representative vectors in a respective codebook of the M codebooks;
• preselecting a predetermined number of representative vectors present around each projection of the input vector onto the M straight lines so as to form M sets of pre-selected representative vectors;
• searching a combination of the preselected representative vectors, each preselected vector being selected from a different one of the M sets or preselected representative vectors, for a combined minimum vector which provides a minimum distance between the input vector and itself;
• and
• outputting the corresponding labels for the preselected representative vectors of the combined minimum vector,
• wherein the representative vectors of a respective codebook of the M codebooks have a distribution which is concentrated close to a respective dimensional axis of the representative vectors in the respective codebook, said respective dimensional axis being different for each of the M codebooks.

An encoder for encoding an input vector, comprising:

• M codebooks, each including a plurality of representative vectors, each representative vector having a corresponding label, in which the representative vectors of a respective codebook of the M codebooks have a distribution which is concentrated close to a respective dimensional axis of the representative vectors in the respective codebook, said respective dimensional axis being different from each of the M codebooks, and said M being an integer equal to or greater than 2;
• a determining mechanism configured to search a combination of M representative vectors, each representative vector of the M representative vectors being selected from a different one of the M codebooks, for a combined minimum vector of the M selected representative vectors which provides a minimum distance between the input vector and itself;
• and
• a controlling mechanism configured to output the corresponding labels for the M selected representative vectors of the combined minimum vector.

An encoder for encoding an input vector through the use of M codebooks, each including a plurality of representative vectors, each representative vector having a corresponding label, said M being an integer equal to or greater than 2, the encoder comprising:

• a projecting mechanism configured to project the input vector onto M straight lines, each M straight line approximating a respective distribution of the representative vectors in a respective codebook of the M codebooks;
• a preselecting mechanism configured to preselect a predetermined number of representative vectors present around each projection of the input vector onto the M straight lines so as to form M sets of preselected representative vectors;
• a searching mechanism configured to search a combination of the preselected representative vectors, each preselected representative vector being selected from a different one of the M sets of preselected representative vectors, for a combined minimum vector which provides a minimum distance between the input vector and itself;
• and
• a controlling mechanism configured to output the corresponding labels for the preselected representative vectors of the combined minimum vector,
• wherein the representative vectors of a respective codebook of the M codebooks have a distribution which is concentrated close to a respective dimensional axis of the representative vectors in the respective codebook, said respective dimensional axis being different for each of the M codebooks.

A method of decoding an input code through the use of M codebooks, each including a plurality of representative vectors, each representative vector having a corresponding label, said M being an integer equal to or greater than 2, and the method comprising the steps of:

• selecting one representative vector from each of the M codebooks which correspond to labels in the input code;
• and
• obtaining a reconstructed vector by combining the representative vectors selected in the selecting step,
• wherein the representative vectors of a respective codebook of the M codebooks have a distribution which is concentrated close to a respective dimensional axis of the representative vectors in the respective codebook, said respective dimensional axis being different for each of the M codebooks.

A decoder for decoding an input code comprising:

• M codebooks each including a plurality of representative vectors, each representative vector having a corresponding label, in which the representative vectors of a respective codebook of the M codebooks have a distribution which is concentrated close to a respective dimensional axis of the representative vectors in the respective codebook, said respective dimensional axis being different for each of the M codebooks, and said M being an integer equal to or greater than 2;
• a selecting mechanism configured to select one representative vector from each of the M codebooks which correspond to labels in the input code;
• and
• a vector combining mechanism configured to reconstruct a vector by combining the representative vectors selected by the selecting mechanism.

A speech coding method, comprising the steps of:

• calculating spectrum envelope parameters of an inputted speech signal;
• quantizing the spectrum envelope parameters;
• setting the quantized spectrum envelope parameters as filter coefficients of a synthesis filter;
• selecting a pitch-excitation vector from a pitch-excitation source codebook which stores pitch-excitation vectors respectively containing different pitch-period components;
• providing the selected pitch-excitation vector with a first gain;
• selecting a random-excitation vector from a random-excitation source codebook;
• providing the random-excitation vector with a second gain;
• adding the pitch-excitation vector provided with the first gain and the random-excitation vector provided with the second gain;
• and
• reproducing a synthesized speech signal by driving said synthesis filter having the filter coefficients with the added signal,
• wherein a gain combined vector including M components is calculated by combining representative vectors each selected from a different codebook of the M codebooks, each codebook including a plurality of representative vectors, each representative vector having a corresponding label, in which the representative vectors of a respective codebook of the M codebooks have a distribution which is concentrated close to a respective dimensional axis of the representative vectors in the respective codebook, said respective dimensional axis being different for each of the M codebooks, and said M being an integer equal to or greater than 2,
• wherein the first and second gains are respectively provided by using each component of the gain combined vector, and
• wherein the gain combined vector is selected to minimize the distortion of the synthesized speech signal with respect to the inputted speech signal.

A speech encoder, comprising:

• a calculating mechanism configured to calculate spectrum envelope parameters of an inputted speech signal;
• a quantizing mechanism configured to quantize the spectrum envelope parameters;
• a synthesis filter having filter coefficients set as the quantized spectrum envelope parameters;
• a selecting mechanism configured to select a pitch-excitation vector from a pitch-excitation source codebook which stores pitch-excitation vectors containing different pitch-period components, the selected pitch-excitation vector being provided with a first gain;
• a selecting mechanism configured to select a random-excitation vector from a random-excitation source codebook, the random-excitation vector being provided with a second gain;
• and
• an adding mechanism configured to add the pitch-excitation vector provided with the first gain and the random-excitation vector provided with the second gain,
• wherein a synthesized speech signal is reproduced by driving said synthesis filter with the added signal,
• wherein a gain combined vector including M components is calculated by combining representative vectors each selected from a different codebook of M codebooks, each codebook including a plurality of representative vectors, each representative vector having a corresponding label, in which the representative vectors of a respective codebook of the M codebooks have a distribution which is concentrated close to a respective dimensional axis of the representative vectors in the respective codebook, said respective dimensional axis being different for each of the M codebooks, and said M being an integer equal to or greater than 2,
• wherein the first and second gains are respectively provided by using each component of the gain combined vector, and
• wherein the synthesized representative vector is selected to minimize the distortion of the synthesized speech signal with respect to the inputted speech signal.