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"
 
   /*----------------------------------------------------------------*
    *  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 entirely 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));
   }