postfilt.c

语音CELP压缩解压源代码（C语音)

/**************************************************************************
*
* ROUTINE
*               postfilt & postfilt2
*
* FUNCTION
*                
*               reduce coder noise (sample wise AGC version)
*
* SYNOPSIS
*
*               subroutine postfilt(s, l, alpha, beta, powerin
*				powerout, dp1, dp2, dp3)
*
*   formal 
*                       data    I/O
*       name            type    type    function
*       -------------------------------------------------------------------
*       s		float	i/o	speech input/postfiltered output
*	l		int	i	subframe size
*	alpha		float	i	filter parameter
*	beta		float	i	filter parameter
*	powerin		float	i/o	input power estimate
*	powerout	float	i/o	output power estimate
*	dp1		float	i/o	filter memory
*	dp2		float	i/o	filter memory
*	dp3		float	i/o	filter memory
*
*   external 
*                       data    I/O
*       name            type    type    function
*       -------------------------------------------------------------------
*	no		int	i
*	fci[]		float	i
*
***************************************************************************
*
* DESCRIPTION
*
*	Adaptive postfilter routine to reduce perceptual coder noise.
*	The postfilter emphasis the spectral regions predicted by the
*	short-term LPC analysis.  This tends to mask coder noise by
*	concentrating it under the formant peaks.  Unfortunately, acoustic
*	background noise may also be enhanced because LPC analysis often
*	models acoustic noise instead of speech formants.  In addtion,
*	postfiltering can be detrimental to tandem coding if not taken
*	into consideration.  (To overcome these problems, we hope to
*	eventually incorporate the postfilter's enhancement properties
*	into the analysis process.)
*
*	Adaptive spectral tilt compensation is applied to flatten the 
*	overall tilt of the postfilter.  [[Slight high frequency boost is 
*	applied for output shaping.  A pitch postfilter is used to reduce 
*	pitch buzz.]]  Finally, AGC compensates for the filter gains using 
*	a time constant set by parameter tc that should be dependent on
*	frame length.
*
*
***************************************************************************
*
* CALLED BY
*
*	celp
*
* CALLS
*
*	bwexp   zerofilt   polefilt   pctorc
*
***************************************************************************
*
* REFERENCES
*
*	Chen & Gersho, Real-Time Vector APC Speech Coding at 4800 bps
*	with Adaptive Postfiltering, ICASSP '87
*
*	Juin-Hwey (Raymond) Chen, "Low-Bit-Rate Predictive Coding of
*	Speech Waveforms Based on Vector Quantization," PhD Dissertation,
*	UCSB ECE Dept., March 1987.
*
*	Ramamoorthy, Jayant, Cox, & Sondhi, "Enhancement of ADPCM Speech
*	Coding with Backward-Adaptive Algorithms for Postfiltering and
*	Noise Feedback," IEEE JOSAIC, Feb. 1988, pp. 364-382.
*
**************************************************************************/
#define TC  0.01
#include <math.h>
#include "ccsub.h"
extern int no;
extern float fci[MAXNO+1];

postfilt(s, l, alpha, beta, powerin, powerout, dp1, dp2, dp3)
int l;
float s[], alpha, beta, *powerin, *powerout;
float dp1[], dp2[], dp3[];

{
  int n;
  float ast[2];
  float pcexp1[MAXNO + 1], pcexp2[MAXNO + 1], rcexp2[MAXNO];

#ifdef POSTFIL2
  float scale;
#else
  float newpowerin[MAXL + 1], newpowerout[MAXL + 1];
#endif

  /*			*estimate input power				*/

#ifdef POSTFIL2
  for (n = 0; n < l; n++)
    *powerin = *powerin * (1.0 - TC) + TC * s[n] * s[n];
#else
  newpowerin[0] = *powerin;
  for (n = 0; n < l; n++)
    newpowerin[n + 1] = (1.0 - TC) * newpowerin[n] + TC * s[n] * s[n];
  *powerin = newpowerin[l];
#endif

  /* *BW expansion							*/
  bwexp(beta, fci, pcexp1, no);
  bwexp(alpha, fci, pcexp2, no);

  /* *pole-zero postfilter						*/
  zerofilt(pcexp1, no, dp1, s, l);
  polefilt(pcexp2, no, dp2, s, l);

  /* *find spectral tilt (1st order fit) of postfilter
     *(denominator dominates the tilt)					*/
  pctorc(pcexp2, rcexp2, no);

  /* *tilt compensation by a scaled zero
     *(don't allow hF roll-off)						*/
  ast[0] = 1.0;
  ast[1] = (rcexp2[0] > 0.) ? -0.5 * rcexp2[0] : 0;
  zerofilt(ast, 1, dp3, s, l);

  /* *estimate output power						*/

#ifdef POSTFIL2
  for (n = 0; n < l; n++)
    *powerout = *powerout * (1.0 - TC) + TC * s[n] * s[n];

  /* *block wise automatic gain control					*/

  if (*powerout > 0.0)
    for (scale = sqrt(*powerin / *powerout), n = 0; n < l; n++)
      s[n] *= scale;
#else
  newpowerout[0] = *powerout;
  for (n = 0; n < l; n++)
    newpowerout[n + 1] = (1.0 - TC) * newpowerout[n] + TC * s[n] * s[n];
  *powerout = newpowerout[l];

  /* *sample wise automatic gain control				*/

  for (n = 0; n < l; n++)
  {
    if (newpowerout[n + 1] > 0.0)
      s[n] *= sqrt(newpowerin[n + 1] / newpowerout[n + 1]);
  }
#endif
}