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| package org.xiph.speex;
/**
* 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.
*/
public class Lpc
{
/**
* Returns minimum mean square error
* @param lpc - float[0...p-1] LPC coefficients
* @param ac - in: float[0...p] autocorrelation values
* @param ref - out: float[0...p-1] reflection coef's
* @param p
*/
public static float wld(float[] lpc, float[] ac, float[] ref, 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) != 0)
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
* @param x - in: float[0...n-1] samples x
* @param ac - out: float[0...lag-1] ac values
* @param lag
* @param n
*/
public static void autocorr(float[] x, float[] ac, int lag, int n)
{
float d;
int i;
while (lag-- > 0)
{
for (i=lag, d=0; i<n; i++)
d += x[i] * x[i-lag];
ac[lag] = d;
}
}
} |
