mexcite.c

语音CELP压缩解压源代码（C语音)

#define mmax(A,B)      ((A)>(B)?(A):(B))
#define mmin(A,B)       ((A)<(B)?(A):(B))
/**************************************************************************
*
* NAME
*		mexcite
*
* FUNCTION
*		Modify the stochastic code book excitation gain
*
* SYNOPSIS
*		subroutine mexcite1(l)
*		subroutine mexcite2(l)
*		subroutine mexcite3(cgain)
*
*   formal 
*                       data    I/O
*       name            type    type    function
*       ------------------------------------------------------------------
*       l		int	i	length of error signal
*	cgain		float	i/o	stochastic code book gain
*
*   global 
*                       data    I/O
*       name            type    type    function
*       ------------------------------------------------------------------
* 	/ccsub/		see description include file
*       e0		float	i	error signal array
*	
***************************************************************************
*
* DESCRIPTION
*
*	Depending on the current system state, the stochastic code book
*	excitation is reduced to a level that is low enough to produce
*	positive perceptual effects, yet is high enough so as not to upset
*	the dynamics of the system.  The main effect of the method is that
*	during sustained voiced sounds, the excitation level is attenuated
*	and in unvoiced and transition regions the level is amplified to a
*	level slightly more than that of standard CELP.
*
*	The relative adaptive code book excitation component is
*	increased in voiced regions by decreasing the stochastic code book
*	excitation component.  The amount of decrease in the stochastic
*	component depends on the efficiency of the adaptive component.
*	More reconstruction burden is placed on the adaptive component as
*	its efficiency increases.  The efficiency is measured by the
*	closeness (in the crosscorrelation sense) of the residual signals
*       before and after pitch prediction.  When the efficiency is high
*       (e.g., > 0.9), the stochastic component is amplified slightly
*       (e.g., one quantizer level). 
*
*	The procedure for modifying the stochastic gain outside the
*	search loop is:
*	1)  Measure the efficiency of the adaptive component (ccor)
*	2)  Search the stochastic code book for the optimum codeword
*	3)  Modify the stochastic code book gain
*
*	This method is compatible with Federal Standard 1016.
*
*
*
***************************************************************************
*
* CALLED BY
*
*	csub csearch
*
* CALLS
*
***************************************************************************
* REFERENCES
*
*	Shoham, Yair, "Constrained-Stochastic Excitation Coding of Speech
*	at 4.8 kbps," in Advances in Speech Coding, ed. B. Atal, V.
*	Cuperman, and A. Gersho, submitted to Kluwer Academic Publishers.
*
*	Shoham, Yair, "Constrained-Stochastic Excitation Coding of Speech," 
*	Abstracts of the IEEE Workshop on Speech Coding for
*	Telecommunications, 1989, p. 65.
*
***************************************************************************/
#include <math.h>
#include "ccsub.h"
#ifdef SUNGRAPH
extern int gain_vid, ccor_vid;
#endif

extern float e0[MAXLP];

static float ccor;
static float e1, e0save[60];

mexcite1(l)
int l;
{
  int i;

  /* *e1 = Euclidean norm of the first error signal		*/
  /* (note: the error signal array e0 is reused)		*/

  e1 = 1e-6;
  for (i = 0; i < l; i++)
  {
    e0save[i] = e0[i];
    e1 += e0[i] * e0[i];
  }
}

mexcite2(l)
int l;
{
  int i;

  /* *ccor = crosscorrelation of the residual signals before 	*/
  /* *and after pitch prediction				*/
  /* *(note: the error signal array e0 is reused		*/

  ccor = 1e-6;
  for (i = 0; i < l; i++)
    ccor += e0[i] * e0save[i];

  /* *normalize the crosscorrelation				*/

  ccor = ccor / e1;
}

mexcite3(cgain)
float *cgain;
{
  float scale;

  /* *square root crosscorrelation scaling			*/

  scale = sqrt(fabs(ccor));

  /* *modify scale						*/

  if (scale < 0.2)
    scale = 0.2;
  else if (scale > 0.9) 
    scale = scale * 1.4;

  /* *modify the stochastic component				*/

  *cgain = *cgain * scale;

#ifdef SUNGRAPH
  save_sg(gain_vid, &scale, 1, "save gain_vid");
  save_sg(ccor_vid, &ccor, 1, "save ccor_vid");
#endif

}