adpcm音频压缩算法源代码

#include <stdio.h>

#ifndef __STDC__
#define signed
#endif

struct adpcm_state
{
short valprev; /* Previous output value */
char index; /* Index into stepsize table */
};

/* Intel ADPCM step variation table */
static int indexTable[16] = {
-1, -1, -1, -1, 2, 4, 6, 8,
-1, -1, -1, -1, 2, 4, 6, 8,
};

static int stepsizeTable[89] = {
7, 8, 9, 10, 11, 12, 13, 14, 16, 17,
19, 21, 23, 25, 28, 31, 34, 37, 41, 45,
50, 55, 60, 66, 73, 80, 88, 97, 107, 118,
130, 143, 157, 173, 190, 209, 230, 253, 279, 307,
337, 371, 408, 449, 494, 544, 598, 658, 724, 796,
876, 963, 1060, 1166, 1282, 1411, 1552, 1707, 1878, 2066,
2272, 2499, 2749, 3024, 3327, 3660, 4026, 4428, 4871, 5358,
5894, 6484, 7132, 7845, 8630, 9493, 10442, 11487, 12635, 13899,
15289, 16818, 18500, 20350, 22385, 24623, 27086, 29794, 32767
};

/*-------------adpcm_ecoder---------------------------------------------*/
void
adpcm_coder(indata, outdata, len, state)
short indata[];
char outdata[];
int len;
struct 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 */

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

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

bufferstep = 1;

for ( ; len > 0 ; len-- ) {
val = *inp++;

/* Step 1 - compute difference with previous value */
diff = val - valpred;
sign = (diff < 0) ? 8 : 0;
if ( sign ) diff = (-diff);

/* Step 2 - Divide and clamp */
/* Note:
** 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;
}

/*-------------adpcm_decoder---------------------------------------------*/
void
adpcm_decoder(indata, outdata, len, state)
char indata[];
short outdata[];
int len;
struct 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 */