cbsearch.m

the implement of the encoder and the decoder of the CELP

function [synth_e0]=CBsearch(sube0)

% This search routine is based on algebraic ternary codebook search. No real codebook exists!
% Please refer to the following reference
% Y-H. Kao, Low Complexity CELP Speech Coding at 4.8 kbps, Master thesis 1992, University of Maryland

global h szsubf subvec nzero numsubv cindex cgain sub_cnt

% For the first sub-frame, which is empty, just assume a stoch code
if sub_cnt==1       
    cindex(sub_cnt)=512;
    cgain(sub_cnt)=-1;
else
    
    % Each subframe (60) is divided into 3 sub-vectors(20). Only 4 points in each vector are 1 or -1;
    offset=subvec/nzero;

    for i=1:szsubf/(subvec/nzero)                   
        wex(i)=sube0((i-1)*offset+1:i*offset)*h(1:offset)';           % Weight the error signal
    end

    % Compute cross-correlation of error signal and code on sub-vector level
    for i=1:numsubv
        sub(i,1)=wex((i-1)*nzero+1)+wex((i-1)*nzero+2)+wex((i-1)*nzero+3)+wex((i-1)*nzero+4);    
        sub(i,2)=wex((i-1)*nzero+1)+wex((i-1)*nzero+2)-wex((i-1)*nzero+3)-wex((i-1)*nzero+4);
        sub(i,3)=wex((i-1)*nzero+1)-wex((i-1)*nzero+2)+wex((i-1)*nzero+3)-wex((i-1)*nzero+4);
        sub(i,4)=wex((i-1)*nzero+1)-wex((i-1)*nzero+2)-wex((i-1)*nzero+3)+wex((i-1)*nzero+4);
        sub(i,5)=-wex((i-1)*nzero+1)+wex((i-1)*nzero+2)+wex((i-1)*nzero+3)-wex((i-1)*nzero+4);
        sub(i,6)=-wex((i-1)*nzero+1)+wex((i-1)*nzero+2)-wex((i-1)*nzero+3)+wex((i-1)*nzero+4);
        sub(i,7)=-wex((i-1)*nzero+1)-wex((i-1)*nzero+2)+wex((i-1)*nzero+3)+wex((i-1)*nzero+4);
        sub(i,8)=-wex((i-1)*nzero+1)-wex((i-1)*nzero+2)-wex((i-1)*nzero+3)-wex((i-1)*nzero+4);
    end
    
    % Compute cross-correlation of error signal and code on subframe level
    for i=1:8
        for j=1:8
            for k=1:8
                cor((i-1)*8*8+(j-1)*8+k)=sub(1,i)+sub(2,j)+sub(3,k);
            end
        end
    end

    % Find the best estimate
    eng=12*sum(h(1:5).^2);
    gain=cor/eng;
    [pk,cindex(sub_cnt)]=max(cor.*gain);

    cgain(sub_cnt)=gain(cindex(sub_cnt));
    
end

% Given the best estimate of stoch code and gain, synthesize the stochastic excitation
synth_e0=exc_syn(cindex(sub_cnt),cgain(sub_cnt));