Efficient calculation of autocorrelation coefficients for CELP vocoder adaptive codebook

A method for CELP coding speech employing autocorrelation coefficients of vectors of an adaptive codebook identified by an index k, wherein analysis by synthesis initially utilizes M<L codebook values where L is the speech analysis frame length and m is an index running from 0 to M-1 describing the autocorrelation lag, comprising:

• calculating m=0 to m-1 autocorrelation coefficients of a first codebook vector k having n=1 to M values therein where n is an index of the code vector values;
• placing the m=0 to M-1 calculated autocorrelation coefficients in a temporary store;
• scaling the coefficients in the temporary store by a multiplying factor L/M and transferring the result to an output;
• multiplying codebook values for n=M+j where j=1 by codebook values for n=M+j down to n=1 and adding the products to the m=0 to M-1 autocorrelation coefficients, respectively, from the temporary store to produce a result;
• replacing the autocorrelation coefficients in the temporary store by the result;
• scaling the coefficients in the temporary store by a multiplying factor L/(M+j) and transferring the result to the output;
• repeating the multiplying, replacing, scaling and transferring steps for j=2 to j=k-1 and k=(L+1-M);
• and
• using the autocorrelation coefficients transferred to the output to determining which of the codebook vectors provides better CELP coding of speech.

An apparatus for CELP coding of speech employing autocorrelation coefficients of vectors of an adaptive codebook wherein analysis initially utilizes a subset of samples M in connection with a speech analysis frame of length L>M, comprising:

• means for determining autocorrelation coefficients U.sub.k' (m) of a first vector C.sub.k (n) of length M, where k=1 and m is an autocorrelation lag index and n is an index of successive samples in the codebook vector, according to, ##EQU20## for m=0 to T<M;
• means for determining autocorrelation coefficients U.sub.k (m) of remaining codebook vectors incrementally where k.gtoreq.2 according to,
• U'.sub.k (m)=[U'.sub.k-1 (m)+C.sub.k (M+k-1)C.sub.k (M+k-1+m)](2)
• for m=0 to T<M until (M+k-1)=L;
• means for scaling the result of Eq. (1) according to ##EQU21## and scaling the result for Eq. (2) according to ##EQU22## for m=0 to T<M to produce a result for each m and each k;
• and means for using the result to evaluate which codebook vector provides a least error compared to input speech.