syntfilter.c

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

    iLBC Speech Coder ANSI-C Source Code

    syntFilter.c 

    Copyright (C) The Internet Society (2004). 
    All Rights Reserved.

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

#include "iLBC_define.h"
#include "syntFilter.h"

/*----------------------------------------------------------------*
 *  LP synthesis filter.
 *---------------------------------------------------------------*/
    
void syntFilter(
    float *Out,     /* (i/o) Signal to be filtered */
    float *a,       /* (i) LP parameters */
    int len,    /* (i) Length of signal */
    float *mem      /* (i/o) Filter state */
){
    int i, j;
    float *po, *pi, *pa, *pm;

    po=Out;

    /* Filter first part using memory from past */

    for (i=0; i<LPC_FILTERORDER; i++) {
        pi=&Out[i-1];
        pa=&a[1];
        pm=&mem[LPC_FILTERORDER-1];
        for (j=1; j<=i; j++) {
            *po-=(*pa++)*(*pi--);
        }
        for (j=i+1; j<LPC_FILTERORDER+1; j++) {
            *po-=(*pa++)*(*pm--);
        }
        po++;
    }

    /* Filter last part where the state is entierly in 
       the output vector */
    
    for (i=LPC_FILTERORDER; i<len; i++) {


        pi=&Out[i-1];
        pa=&a[1];
        for (j=1; j<LPC_FILTERORDER+1; j++) {
            *po-=(*pa++)*(*pi--);
        }
        po++;
    }

    /* Update state vector */

    memcpy(mem, &Out[len-LPC_FILTERORDER], 
        LPC_FILTERORDER*sizeof(float));
}