c_ivfilt.c

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#include "c_lpc10.h"

/**********************************************************/
/* 2nd order inverse filter, speech is decimated 4:1
Inputs:
.  lpbuf[] - Low pass filtered speech buffer (0:PWLEN-1 samples)
Output:
.  ivbuf[] - Inverse filtered speech buffer (0:PWLEN-1 samples,
.            last LFRAME written)
.  ivrc[2] - Inverse filter reflection coefficients (for voicing).
Constants:
.  LFRAME - speech frame length
.  PWLEN - pitch window length */

VOID ivfilt( FLOAT lpbuf[], FLOAT ivbuf[], FLOAT ivrc[2] )
{
    INDEX i, j, k;
    FLOAT r[3], pc1, pc2, tmp;

/* Calculate Autocorrelations */
    for (i = 0; i <= 2; i++) {
        k = i * 4;
        r[i] = (FLOAT)0.0;
        for (j = i * 4 + (PWLEN - LFRAME + 3); j < PWLEN; j += 2)
            r[i] += lpbuf[j] * lpbuf[j - k];
    }

/* Calculate predictor coefficients */
    pc1 = pc2 = ivrc[0] = ivrc[1] = (FLOAT)0.0;
    if (r[0] > (FLOAT)1e-6) {
        ivrc[0] = r[1] / r[0];
        tmp = ivrc[0] * r[1];
        pc2 = ivrc[1] = (r[2] - tmp) / (r[0] - tmp);
        pc1 = ivrc[0] - ivrc[0] * ivrc[1];
    }

/* Inverse filter {lpbuf} into {ivbuf} */
    for (i = PWLEN - LFRAME; i < PWLEN; i++)
        ivbuf[i] = lpbuf[i] - pc1 * lpbuf[i-4] - pc2 * lpbuf[i-8];
}

/**********************************************************/

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