adpcm音频压缩算法源代码

admin

#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 */
    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;
}