原创 ADPCM压缩算法介绍

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。

ADPCM 压缩过程

　　首先我们认为声音信号都是从零开始的,那么需要初始化两个变量

　　　　int index=0,prev_sample=0;

　　下面的循环将依次处理声音数据流，注意其中的 getnextsample() 应该得到一个 16bit 的采样数据，而 outputdata() 可以将计算出来的数据保存起来，程序中用到的 step_table[],index_adjust[] 附在后面：

　　　　int index=0,prev_sample:=0;

　　　　while (还有数据要处理)
　　　　{
　　　　　　cur_sample=getnextsample();　　　　　　　　// 得到当前的采样数据
　　　　　　delta=cur_sample-prev_sample;　　　　　　　// 计算出和上一个的增量
　　　　　　if (delta<0) delta=-delta,sb=8;　　　　　　// 取绝对值
　　　　　　else sb = 0 ;　　　　　　　　　　　　　　　// sb 保存的是符号位
　　　　　　code = 4*delta / step_table[index];　　　　// 根据 steptable[]得到一个 0-7 的值
　　　　　　if (code>7) code=7;　　　　　　　　　　　　// 它描述了声音强度的变化量
　　　　　　index += index_adjust[code] ;　　　　　　　// 根据声音强度调整下次取steptable 的序号
　　　　　　if (index<0) index=0;　　　　　　　　　　　// 便于下次得到更精确的变化量的描述
　　　　　　else if (index>88) index=88;
　　　　　　prev_sample=cur_sample;
　　　　　　outputode(code|sb);　　　　　　　　　　　　// 加上符号位保存起来
　　　　}

　

ADPCM 解压缩过程

　　接压缩实际是压缩的一个逆过程，同样其中的 getnextcode() 应该得到一个编码，,而 outputsample() 可以将解码出来的声音信号保存起来。这段代码同样使用了同一个的 setp_table[] 和 index_adjust() 附在后面：

　　　　int index=0,cur_sample=0;

　　　　while (还有数据要处理)
　　　　{
　　　　　　　　code=getnextcode();　　　　　　　　　　　　　　　　　　　　　　　// 得到下一个数据
　　　　　　　　if ((code & 8) != 0) sb=1 else sb=0;
　　　　　　　　code&=7;　　　　　　　　　　　　　　　　　　　　　　　　　　　　// 将 code 分离为数据和符号
　　　　　　　　delta = (step_table[index]*code)/4+step_table[index]/8;　　　　　// 后面加的一项是为了减少误差
　　　　　　　　if (sb==1) delta=-delta;
　　　　　　　　cur_sample+=delta; 　　　　　　　　　　　　　　　　　　　　　　　// 计算出当前的波形数据
　　　　　　　　if (cur_sample>32767) output_sample(32767);
　　　　　　　　else if (cur_sample<-32768) output_sample(-32768);
　　　　　　　　else output_sample(cur_sample);
　　　　　　　　index+=index_adjust[code];
　　　　　　　　if (index<0) index=0;
　　　　　　　　if (index>88) index=88;
　　　　　}

附  ADPCM压缩算法代码实现

/***********************************************************
Copyright 1992 by Stichting Mathematisch Centrum, Amsterdam, The
Netherlands.

                        All Rights Reserved

Permission to use, copy, modify, and distribute this software and its
documentation for any purpose and without fee is hereby granted,
provided that the above copyright notice appear in all copies and that
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Centrum or CWI not be used in advertising or publicity pertaining to
distribution of the software without specific, written prior permission.

STICHTING MATHEMATISCH CENTRUM DISCLAIMS ALL WARRANTIES WITH REGARD TO
THIS SOFTWARE, INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND
FITNESS, IN NO EVENT SHALL STICHTING MATHEMATISCH CENTRUM BE LIABLE
FOR ANY SPECIAL, INDIRECT OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT
OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.

******************************************************************/

/*
** Intel/DVI ADPCM coder/decoder.
**
** The algorithm for this coder was taken from the IMA Compatability Project
** proceedings, Vol 2, Number 2; May 1992.
**
** Version 1.2, 18-Dec-92.
**
** Change log:
** - Fixed a stupid bug, where the delta was computed as
**   stepsize*code/4 in stead of stepsize*(code+0.5)/4.
** - There was an off-by-one error causing it to pick
**   an incorrect delta once in a blue moon.
** - The NODIVMUL define has been removed. Computations are now always done
**   using shifts, adds and subtracts. It turned out that, because the standard
**   is defined using shift/add/subtract, you needed bits of fixup code
**   (because the div/mul simulation using shift/add/sub made some rounding
**   errors that real div/mul don't make) and all together the resultant code
**   ran slower than just using the shifts all the time.
** - Changed some of the variable names to be more meaningful.
*/

#include "adpcm.h"
#include <stdio.h> /*DBG*/

#ifndef __STDC__
#define signed
#endif

/* 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(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;
}

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;
}