ADPCM编解码简介

     ADPCM(Adaptive Differential Pulse Code Modulation)，是一种针对 16bits( 或8bits或者更高) 声音波形数据的一种有损压缩算法,它将声音流中每次采样的 16bit 数据以 4bit 存储,所以压缩比 1:4. 而且压缩/解压缩算法非常简单,所以是一种低空间消耗，高质量高效率声音获得的好途径。保存声音的数据文件后缀名为 .AUD 的大多用ADPCM压缩。
     ADPCM主要是针对连续的波形数据的，保存的是波形的变化情况，以达到描述整个波形的目的，由于它的编码和解码的过程却很简洁，列在后面，相信大家能够看懂。
　　8bits采样的声音人耳是可以勉强接受的，而 16bit 采样的声音可以算是高音质了。ADPCM算法却可以将每次采样得到的 16bit 数据压缩到 4bit 。需要注意的是，如果要压缩/解压缩得是立体声信号，采样时，声音信号是放在一起的，需要将两个声道分别处理。

     原理性的东西就不介绍过多了，网上有些相关的c代码，但整理的有点乱，下面给出我使用过的ADPCM压缩代码，只针对单声道的pcm数据。

 

1、adpcm.h

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

typedef struct adpcm_state adpcm_state_t;

void adpcm_coder(short [], char [], int, struct adpcm_state *);
void adpcm_decoder(char [], short [], int, struct adpcm_state *);

2、adpcm.c

 

#include "adpcm.h"
#include <stdio.h>

/* 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
};

void adpcm_coder(short indata[], char outdata[], int len, adpcm_state_t *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;
    len /= 2;

    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;
}

void adpcm_decoder(char indata[], short outdata[], int len, adpcm_state_t *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;
    len *= 2;            /* !!!! verify important TODO (FIX ME) JGF*/

    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;
}