13 Nov 2012
This is a code to demonstrate CELP codecs of bitrate 9.6kbps and 16kbps.
function [ar,xi,kappa,ehat] = lpcana(x,M)
% lpcana --> Linear prediction analysis.
% [ar,xi,kappa,ehat] = lpcana(x,M)
% The function performs autocorrelation based LP analysis on the
% signal vector x using the Levinson-Durbin recursion. Thus, the
% function finds the coefficients, ar=[1 -a(1) ... -a(M)], of an
% M'th order forward linear predictor
% xhat(n) = a(1)*x(n-1) + a(2)*x(n-2) + ... + a(M)*x(n-M)
% such that the sum of the squares of the prediction errors
% ehat(n) = x(n) - xhat(n)
% is minimized. The reflection coefficients are returned in the
% vector kappa, and the prediction error energies for the 0'th to
% the M'th order solution are returned in the vector xi. Finally,
% the residual signal, ehat, is obtained by applying the inverse
% filter A(z) to the signal frame.
% Short-term autocorrelation.
[rx,eta] = xcorr(x,M,'biased');
% LP analysis based on Levinson-Durbin recursion.
[a,xi,kappa] = durbin(rx(M+1:2*M+1),M);
ar = [1; -a];
% Prediction error signal obtained by inverse filtering.
ehat = filter(ar,1,x);