wcelp.asm

文件为美军4800bps语音压缩编码标准在TI的DSP芯片TMS320C5X上的实时实现源代码。

/**************************************************************************
*
* ROUTINE   bwexp
*
* FUNCTION
*		Bandwidth expansion of LPC predictor coefficients
*
* SYNOPSIS
*		bwexp(float alpha, float *pc, float *pcexp, int n)
*
*   formal
*                   data    I/O
*       name        type    type    function
*  -------------------------------------------------------------------
*	     alpha       float    i      Bandwidth expansion factor
*	     pc          float    i      predictor coefficients
*	     pcexp       float    o      expanded predictor coefficients
*	     n           int      i      predictor order
*
***************************************************************************
*
* DESCRIPTION
*
*  	Subroutine to perform bandwidth modification by moving the poles
*	(or zeros) radially in the z plane.  If the bandwidth expansion
*	factor (alpha) is less than unity, the bandwidths are expanded by
*	shifting the poles (or zeros) toward the origin of the z plane.
*	The predictor coefficients are scaled directly according to:
*
*		              i-1
*		a' = a  alpha		where i = 1, . . . , order+1
*		 i    i
*
*	Resulting in a bandwidth expansion of:
*
*		-(fs/pi)ln(alpha) Hz
*
*	(e.g., fs = 8 kHz, alpha = 0.994127 -> 15 Hz bandwidth expansion)
*
*	CELP's LPC predictor coefficient convention is:
*              p+1         -(i-1)
*       A(z) = SUM   a   z          where a  = +1.0
*              i=1    i                    1
*
***************************************************************************
*
* CALLED BY
* CALLS        none
*
**************************************************************************/
void bwexp(float alpha, float *pc, float *pcexp, int n)
{
  int i;
  int j,ax0,ax1;
  long acc,p32;
  float tpc[10];
   for (i = 0; i <= n; i++)
	 tpc[i] = pc[i]*pow(alpha,(double)(i));
   ax1=alpha*0x8000u;	//alpha:1.15
   for (i = 0; i <= n; i++)
     {	//pcexp[i] = pc[i]*pow(alpha,(double)(i)); pc: 3.13
	if(pc[i]>=4.0) {printf("pc>4.0 at bwexp.c .\n"); exit(0); }
	if(i==0) { pcexp[i]=pc[i]; continue; }
	ax0=pc[i]*0x2000u;	//pc:3.13;
	p32=ax0;
	for(j=0;j<i; j++)
	  { p32=p32*ax1; //3.13*1.15->3.13
	    p32>>=15;
	  }
	pcexp[i]=(float)p32/0x2000u;
     }
   i=i;
}