void G721::adpcm_coder(short* indata,char* outdata,int len,adpcm_state* state)
{
short *inp; /* Input buffer pointer */
signed char *outp; /* output buffer pointer */
int val; /* Current input sample value */
int sign; /* Current adpcm sign bit */
int delta; /* Current adpcm output value */
int diff; /* Difference between val and valprev */
int step; /* Stepsize */
int valpred; /* Predicted output value */
int vpdiff; /* Current change to valpred */
int index; /* Current step change index */
int outputbuffer; /* place to keep previous 4-bit value */
int bufferstep; /* toggle between outputbuffer/output */
/* Step 1 - compute difference with previous value */
diff = val - valpred;
sign = (diff < 0) ? 8 : 0;
if(sign) diff = (-diff);
/* Step 2 - Divide and clamp
** This code *approximately* computes:
** delta = diff*4/step;
** vpdiff = (delta+0.5)*step/4;
** but in shift step bits are dropped. The net result of this is
** that even if you have fast mul/div hardware you cannot put it to
** good use since the fixup would be too expensive.
*/
delta = 0;
vpdiff = (step >> 3);
if(diff >= step)
{
delta = 4;
diff -= step;
vpdiff += step;
}
step >>= 1;
if(diff >= step)
{
delta |= 2;
diff -= step;
vpdiff += step;
}
step >>= 1;
if(diff >= step)
{
delta |= 1;
vpdiff += step;
}
/* Step 3 - Update previous value */
if( sign )
valpred -= vpdiff;
else
valpred += vpdiff;
/* Step 4 - Clamp previous value to 16 bits */
if( valpred > 32767 )
valpred = 32767;
else if(valpred < -32768)
valpred = -32768;
/* Step 5 - Assemble value, update index and step values */
delta |= sign;
index += indexTable[delta];
if(index < 0) index = 0;
if(index > 88) index = 88;
step = stepsizeTable[index];
/* Step 6 - Output value */
if(bufferstep)
{
outputbuffer = (delta << 4) & 0xf0;
}
else
{
*outp++ = (delta & 0x0f) | outputbuffer;
}
bufferstep = !bufferstep;
}
/* Output last step, if needed */
if(!bufferstep)
*outp++ = outputbuffer;
state->valprev = valpred;
state->index = index;
}
void G721::adpcm_decoder(char* indata,short* outdata,int len,adpcm_state* 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;
}
bufferstep = !bufferstep;
/* 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;
}
state->valprev = valpred;
state->index = index;
}
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