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));
   }