/*
Copyright 1992, 1993, 1994 by Jutta Degener and Carsten Bormann,
Technische Universitaet Berlin
Any use of this software is permitted provided that this notice is not
removed and that neither the authors nor the Technische Universitaet Berlin
are deemed to have made any representations as to the suitability of this
software for any purpose nor are held responsible for any defects of
this software. THERE IS ABSOLUTELY NO WARRANTY FOR THIS SOFTWARE.
As a matter of courtesy, the authors request to be informed about uses
this software has found, about bugs in this software, and about any
improvements that may be of general interest.
Berlin, 28.11.1994
Jutta Degener
Carsten Bormann
Code slightly modified by Jean-Marc Valin
Speex License:
This library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
This library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with this library; if not, write to the Free Software
Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
*/
/* LPC- and Reflection Coefficients
*
* The next two functions calculate linear prediction coefficients
* and/or the related reflection coefficients from the first P_MAX+1
* values of the autocorrelation function.
*/
/* Invented by N. Levinson in 1947, modified by J. Durbin in 1959.
*/
#include "lpc.h"
float /* returns minimum mean square error */
wld(
float * lpc, /* [0...p-1] LPC coefficients */
const float * ac, /* in: [0...p] autocorrelation values */
float * ref, /* out: [0...p-1] reflection coef's */
int p
)
{
int i, j; float r, error = ac[0];
if (ac[0] == 0) {
for (i = 0; i < p; i++) ref[i] = 0; return 0; }
for (i = 0; i < p; i++) {
/* Sum up this iteration's reflection coefficient.
*/
r = -ac[i + 1];
for (j = 0; j < i; j++) r -= lpc[j] * ac[i - j];
ref[i] = r /= error;
/* Update LPC coefficients and total error.
*/
lpc[i] = r;
for (j = 0; j < i/2; j++) {
float tmp = lpc[j];
lpc[j] += r * lpc[i-1-j];
lpc[i-1-j] += r * tmp;
}
if (i % 2) lpc[j] += lpc[j] * r;
error *= 1.0 - r * r;
}
return error;
}
/* Compute the autocorrelation
* ,--,
* ac(i) = > x(n) * x(n-i) for all n
* `--'
* for lags between 0 and lag-1, and x == 0 outside 0...n-1
*/
void autocorr(
const float * x, /* in: [0...n-1] samples x */
float *ac, /* out: [0...lag-1] ac values */
int lag, int n)
{
float d; int i;
while (lag--) {
for (i = lag, d = 0; i < n; i++) d += x[i] * x[i-lag];
ac[lag] = d;
}
}