bwexp.h

g729 coding ipaddressing

/**************************************************************************
*
* ROUTINE
*				bwexp
*
* FUNCTION
*				Bandwidth expansion of LPC predictor coefficients
*
* SYNOPSIS
*				subroutine bwexp(alpha, pc, pcexp, 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
*
*		autohf	confg	impls	postfilter
*
* CALLS
*
*		
*
**************************************************************************/

static void bwexp(float alpha, float pc[], float pcexp[], int n)
{
  int i;
  for (i = 0; i <= n; i++)
	pcexp[i] = (float) (pc[i]*pow(alpha,(double)(i)));
}