syntfilter.c

Ilibc 语音编解码库算法。语音质量高。接口简单。

   
   /******************************************************************
   
       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 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));
   }