pctorc.h

g729 coding ipaddressing

/**************************************************************************
*
* NAME
*		pctorc 
*
* FUNCTION
*
*		Convert from lp-polynomial to reflection coefficients.
*
*		BEWARE: This code does not use memory efficiently.
*
* SYNOPSIS
*
*		subroutine pctorc(lpc, rc, n)
*
*	formal 
*						data	I/O
*		name			type	type	function
*		-------------------------------------------------------------------
*		lpc(n+1)		float	i		Array of n+1 coefficients
*										a(0)+a(1)z**(-1) + a(2)Z**(-2) +
*										.... + a(n)z**(-n)
*		rc(n)			float	i/o 	  reflection coefficients (voiced-> +rc1)
*		n				int 	i		Order of polynomial
*	  
***************************************************************************
*		
* DESCRIPTION
*
*		This routine uses the Levinson recursion to compute reflection
*		coefficients from the LPC coefficients.  The first LPC
*		coefficient is assumed to be 1, and although it is passed
*		to the routine, it is not used in the calculations.
*		Note:  the dimension of the internal array t limits the value
*		of the maximum order.
*
*       CELP's LPC predictor coefficient convention is:
*			   p+1		   -(i-1)
*		A(z) = SUM	 a	 z			where a  = +1.0
*			   i=1	  i 				   1
*
*		The sign convention used defines the first reflection coefficient
*		as the normalized first autocorrelation coefficient, which results
*		in positive values of rc(1) for voiced speech.
*
***************************************************************************
*
* CALLED BY
*
*		autohf	postfilter	specdist celp  intsynth
*
* CALLS
*
*
*
**************************************************************************/

static void pctorc(const float lpc[], float rc[], int n)
{
  float t[MAXNO+1], a[MAXNO+1];
  int i, j;
  
  for (i = 0; i <= n; i++)
	a[i] = lpc[i];
  for (i = n; i > 1; i--)
  {
	rc[i-1] = -a[i];
	for (j = 1; j < i; j++)
	  t[i-j] = (float) ((a[i-j] + rc[i-1] * a[j]) / (1.0 - rc[i-1] * rc[i-1]));
	for (j = 1; j < i; j++)
	  a[j] = t[j];
  }
  rc[0] = -a[1];
}