anafilter.c

来自「视频监控vc源代码.对于做视频系统的朋友们很有帮助」· C语言 代码 · 共 72 行

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

    iLBC Speech Coder ANSI-C Source Code

    anaFilter.c 

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

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

#include <string.h>
#include "anaFilter.h"
#include "iLBC_define.h"

/*----------------------------------------------------------------*
 *  LP analysis filter.


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

    po = Out;

    /* Filter first part using memory from past */
    
    for (i=0; i<LPC_FILTERORDER; i++) {
        pi = &In[i];
        pm = &mem[LPC_FILTERORDER-1];
        pa = a;
        *po=0.0;
        for (j=0; 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 input vector */

    for (i=LPC_FILTERORDER; i<len; i++) {
        pi = &In[i];
        pa = a;
        *po=0.0;
        for (j=0; j<LPC_FILTERORDER+1; j++) {
            *po+=(*pa++)*(*pi--);
        }
        po++;
    }

    /* Update state vector */

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