Low bit rate voice coding method and system

Claim 2

A process for low bit rate block encoding a sampled voice signal s(n) based on code-excited encoding providing at least one prestored table address k and associated gain coefficient G, for each block of samples to be encoded, said process including: preemphasizing said s(n) signal into an sp(n) signal and deriving partial autocorrelation coefficients ai therefrom; filtering the voice signal s(n) using a short-term predictive filter tuned by said ai coefficients and deriving therefrom a short-term residual r(n); processing said r(n) to derive a gain factor "b" and a long-term delay M; deriving a weighted and delayed synthesized short-term residual br'(n-M) from previously subtracting br'(n-M) from r(n) to derive a long-term residual signal e(n); splitting e(n) into consecuive blocks; and, code excited encoding each block of said long-term residual e(n) into at least one TABLE address reference k and one gain coefficient G.

Claim 3

A low bit rate block encoding system for encoding a sampled voice originated signal s(n) into a d(n) data flow including for each block of samples a prestored TABLE address k and a gain coefficient G, said system including: means for preemphasizing said s(n) signal; partial correlation means for deriving partial autocorrelation (PARCOR) coefficients k i from said preemphasized s(n), and deriving PARCOR related coefficients a i from said preemphasized s(n) signal; short-term predictive filter means tuned with said a i coefficients and fed with s(n) to derive a short term residuals signal r(n) therefrom; means for splitting r(n) into consecutive blocks of samples; computing means connected to receive r(n) and derive a pitch related long-term delay parameter M and a gain factor b; first adding means having a(+) input fed with said r(n) signal and a(-) input, and providing a long term residual signal e(n); means for splitting e(n) into blocks of predetermined length; code excited encoding means for converting each block of samples e(n) into a TABLE reference (k) and a gain coefficient (G), said reference representing the TABLE address of the codeword best matching the considered e(n) block multiplied by gain g; a second adding means having a first (+) input fed with said selected codeword multiplied by gain G, and a second (+) input, said second adding means providing a synthesized short term residual r'(n); delay means having an input fed with said synthesized residual r'(n) and for deriving a delayed synthesized residual r'(n-M); multiplying means for multiplying sand delayed synthesized residual r'(n-M) by said gain factor b and derive b·r'(n-M) therefrom; means for feeding said b·r'(n-M) into said first adder (-) input and said second adder (+) input; and, multiplexing means for multiplexing for each block of r(n) samples said gain coefficient G, table address K, gain factor b, long-term delay M and PARCOR coefficient a i .

Claim 4

A low bit rate block encoding system for encoding a sampled voice originated signal s(n) into a d(n) data flow including for each block of samples a prestored TABLE address k and a gain coefficient G, said system including: means for preemphasizing said s(n) signal; partial correlation means for deriving partial autocorrelation (PARCOR) coefficients k i from said preemphasized s(n), and deriving PARCOR related coefficients a i from said preemphasized s(n) signal; short-term predictive filter means tuned with said a i coefficients and fed with s(n) to derive a short term residuals signal r(n) therefrom; means for splitting r(n) into consecutive blocks of samples; computing means connected to receive r(n) and derive a pitch related long-term delay parameter M and a gain factor b; first adding means having a(+) input fed with said r(n) signal and a(-) input, and providing a long term residual signal e(n); means for splitting e(n) into blocks of predetermined length; code excited encoding means for converting each block of samples e(n) into a TABLE reference (k) and a gain coefficient (G), said reference representing the TABLE address of the codeword best matching the considered e(n) block multiplied by gain g; a second adding means having a first (+) input fed with said selected codeword multiplied by gain G, and a second (+) input, said second adding means providing a synthesized short term residual r'(n); delay means having an input fed with said synthesized residual r'(n) and for deriving a delayed synthesized residual r'(n-M); multiplying means for multiplying sand delayed synthesized residual r'(n-M) by said gain factor b and derive b·r'(n-M) therefrom; means for feeding said b·r'(n-M) into said first adder (-) input and said second adder (+) input; and, multiplexing means for multiplexing for each block of r(n) samples said gain coefficient G, table address K, gain factor b, long-term delay M and PARCOR coefficient a i .

Claim 5

A low bit rate encoding system according to claim 2 wherein said TABLE is made to store a normalized initial set of e(n) samples into a sequence (Y k ) with n-1,2, . . . , L wherein L is an integer number n defining a codeword length, and k=1,2, . . . , K, with K being the total number of codewords stored into said table.

Claim 6

A low bit rate encoding system according to claim 3 wherein said Code-Excited encoding means include: means for setting a predefined (1) samples long window; means for shifting said window over said TABLE, from a sample position to the next; autocorrelation means sensitive to said window shifting means for deriving Ek correlation terms: ##EQU17## wherein SIGMA symbolizes a summing operation; selecting means sensitive to said Ek terms for detecting the maximum Ek term, whereby said gain coefficient G is set equal to Ek and k is selected for being the table reference for the processed block of samples.

Claim 7

A low bit rate encoding system according to claim 2 or 3 or 4 wherein said a i computing means include: first computing means for computing ##EQU18## wherein j, is a predetermined integer value, e.g., : j'=160; second computing means for computing ##EQU19## means for converting said s(n) signal into sp(n)=S(N)-(R2/R1)·s(n-1), said sp(n) being then used to derive said ai coefficients set therefrom.

Claim 8

A low bit rate encoding system according to claim 2 or 3 or 4 wherein said system further includes: low pass filtering means connected to said short term predictive filter and providing a low frequency bandwidth signal r(n) to be Code-ExCited coded into (G, k)'s; and means for coding the energy of the removed high frequency bandwidth and for feeding said coded energy into said multiplexing means.