autohf.h

g729 coding ipaddressing

/**************************************************************************
*
* ROUTINE
*				autohf
*
* FUNCTION
*				LPC autocorrelation analysis with high frequency compensation
*
*
* SYNOPSIS
*				subroutine autohf(si, w, n, p, omega, a, rc)
*
*	formal 
*						data	I/O
*		name			type	type	function
*		-------------------------------------------------------------------
*		si(n)			float	i		signal input
*		w(n)			float	i		window (i.e., Hamming)
*		n				int 	i		length of input sequence
*		p				int 	i		order of LPC polynomial
*		omega			float	i		bandwidth expansion factor
*		a				float	o		LPC coefficients (1 to m+1)
*		rc				float	o		reflection coefficients (1 to m)
*										(voiced-> +rc1)
*
*	external 
*						data	I/O
*		name			type	type	function
*		-------------------------------------------------------------------
*		frame			int 	i		
*
***************************************************************************
*		
* DESCRIPTION
*
*		Subroutine to perform HF corrected autocorrelation LPC analysis.
*		First, autocorrelation coefficients are calculated and high
*		frequency corrected to partially compensate for the analog
*		antialiasing filter*.  (Traditionally, this technique has only been
*		applied to covariance analysis, but it applies to autocorrelation
*		analysis as well).	Next, the autocorrelation function is converted
*		to reflection coefficients by the Schur recursion (aka LeRoux &
*		Guegen).  Then, the reflection coefficients are converted to LPC
*		predictor coefficients.  Finally, the predictors are bandwidth
*		expanded by omega.
*
*       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
*
*		celp
*
* CALLS
*
*		actorc	bwexp	cor 	pctorc	rctopc
*
***************************************************************************
*		
* REFERENCES
*
*		*Atal & Schroeder, Predictive Coding of Speech Signals
*		 and Subjective Error Criteria, IEEE TASSP, June 1979.
*
**************************************************************************/

static void autohf(float si[], float w[], int n, int p, float omega, float a[], float rc[])
{
  int i, unstable;
  float c0, c[MAXNO], atemp[MAXNO+1], s[MAXLL];
  
  setr(MAXNO+1, 0.0, atemp);
  for (i = 0;  i < n ; i++)
	s[i] = si[i] * w[i];				/* apply window 				*/
  unstable = FALSE; 	
  cor(s, n, p, &c0, c); 				/* calculate autocorrelations	*/
  if (c0 < 0.0) 
	unstable = TRUE;
  atemp[0] = 1.0;                       /* convert autocorrelations to pc's  */
  durbin(c0, c, &atemp[1], p);

  bwexp(omega, atemp, a, p);            /* expand corrected pc's        */
  pctorc(a, rc, p);                     /* match rc's to expanded pc's  */
										/* and test for stability		*/
  for (i = 0; i < p ; i++)
  {
	if (fabs(rc[i]) > 1.0)
	  unstable = TRUE;
  }
  if (unstable)
  {
#ifdef CELPDIAG
    fprintf(stderr, "autohf:  unstable lpc analysis at frame %d\n",frame);
#endif
	for (i = 0; i < p; i++)
	{
	  a[i+1] = 0.0;
	  rc[i] = 0.0;
	}
  }
}