dec.c

基于TI xdais算法标准的音频ADPCM算法

#include <stdio.h>
#include "adpcm.h"
#include "evmdm642_echocfg.h"
#ifndef __STDC__
#define signed
#endif

extern far LOG_Obj trace;
extern far int stepsizeTable[];
extern far int indexTable[];

void decode(char *indata, short *outdata, int len, AdpcmState *state)
{
    signed char *inp;	/* Input buffer pointer */
    short *outp;		/* output buffer pointer */
    int sign;			/* Current adpcm sign bit */
    int delta;			/* Current adpcm output value */
    int step;			/* Stepsize */
    int valpred;		/* Predicted value */
    int vpdiff;			/* Current change to valpred */
    int index;			/* Current step change index */
    int inputbuffer;	/* place to keep next 4-bit value */
    int bufferstep;		/* toggle between inputbuffer/input */
    

    outp = outdata;
    inp = (signed char *)indata;

    valpred = state->valprev;
    index = state->index;
    step = stepsizeTable[index];

    bufferstep = 0;
    
    for ( ; len > 0 ; len-- ) 
    {
	
		/* Step 1 - get the delta value */
		if ( bufferstep ) {
		    delta = inputbuffer & 0xf;
		} else {
	    	inputbuffer = *inp++;
	    	delta = (inputbuffer >> 4) & 0xf;
		}
		
		

		/* Step 2 - Find new index value (for later) */
		index += indexTable[delta];
		if ( index < 0 ) index = 0;
		if ( index > 88 ) index = 88;

		/* Step 3 - Separate sign and magnitude */
		sign = delta & 8;
		delta = delta & 7;

	/* Step 4 - Compute difference and new predicted value */
	/*
	** Computes 'vpdiff = (delta+0.5)*step/4', but see comment
	** in adpcm_coder.
	*/
		vpdiff = step >> 3;
		if ( delta & 4 ) vpdiff += step;
		if ( delta & 2 ) vpdiff += step>>1;
		if ( delta & 1 ) vpdiff += step>>2;

		if ( sign )
	  	valpred -= vpdiff;
		else
	  	valpred += vpdiff;

		/* Step 5 - clamp output value */
		if ( valpred > 32767 )
	  		valpred = 32767;
		else if ( valpred < -32768 )
	  	valpred = -32768;

		/* Step 6 - Update step value */
		step = stepsizeTable[index];

		/* Step 7 - Output value */
			*outp = valpred;
			outp=outp+32;
  	
		    
    	bufferstep = !bufferstep;
    }

    state->valprev = valpred;
    state->index = index;
 
    //write to file
    //fwrite(outdata, 1, len, fpDecoder);
}