1.**adpcm介绍 **
ADPCM (ADPCM Adaptive Differential Pulse Code Modulation)，是一种针对16bit (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。而压缩/解压缩算法非常的简单，所以是一种低空间消耗，高质量声音获得的好途径。
2.**adpcm源码 **
adpcm.h
/*
** adpcm.h - include file for adpcm coder.
**
** Version 1.0, 7-Jul-92.
*/
#ifndef __IMA_ADPCM_H
#define __IMA_ADPCM_H
typedef struct adpcm_state
{
short valprev; /* Previous output value */
char index; /* Index into stepsize table */
}adpcm_state_t;
/*
*
*/
void adpcm_coder(short indata[], char outdata[], unsigned short len, adpcm_state_t *state);
void adpcm_decoder(char indata[], short outdata[], unsigned short len, adpcm_state_t *state);
#endif
adpcm.m
/***********************************************************
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
both that copyright notice and this permission notice appear in
supporting documentation, and that the names of Stichting Mathematisch
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"
extern struct adpcm_state AdpcmState;
/* Intel ADPCM step variation table for 4bit*/
static const signed short indexTable[16] = {
-1, -1, -1, -1, 2, 4, 6, 8,
-1, -1, -1, -1, 2, 4, 6, 8,
};
static const signed short 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
};
/***********************************************************************************************************************
* Function Name:
* Description :
* Arguments :
***********************************************************************************************************************/
void adpcm_coder(short indata[], char outdata[], unsigned short len, adpcm_state_t *state)
{
short *inp; /* Input buffer pointer */
char *outp; /* output buffer pointer */
short val; /* Current input sample value */
short sign; /* Current adpcm sign bit */
short delta; /* Current adpcm output value */
short diff; /* Difference between val and valprev */
short step; /* Stepsize */
short valpred; /* Predicted output value */
short vpdiff; /* Current change to valpred */
short index; /* Current step change index */
short outputbuffer = 0; /* place to keep previous 4-bit value */
short bufferstep; /* toggle between outputbuffer/output */
outp = 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 )
{
bufferstep=0;
outputbuffer = delta & 0x0f;
}
else
{
bufferstep=1;
*outp++ = (char)((delta<<4 & 0xf0) | outputbuffer);
}
}
/* Output last step, if needed */
if ( !bufferstep )
{*outp++ = (char)outputbuffer;}
state->valprev = valpred;
state->index = (char)index;
}
/***********************************************************************************************************************
* Function Name:
* Description :
* Arguments :
***********************************************************************************************************************/
void adpcm_decoder(char indata[], short outdata[], unsigned short len, adpcm_state_t *state)
{
char *inp; /* Input buffer pointer */
short *outp; /* output buffer pointer */
short sign; /* Current adpcm sign bit */
short delta; /* Current adpcm output value */
short step; /* Stepsize */
short valpred; /* Predicted value */
short vpdiff; /* Current change to valpred */
short index; /* Current step change index */
short inputbuffer = 0; /* place to keep next 4-bit value */
short bufferstep; /* toggle between inputbuffer/input */
outp = outdata;
inp = 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 */
//œ»»°µÕ4Œª OK
if ( bufferstep )
{
delta = (inputbuffer >> 4) & 0x0f;
}
else
{
inputbuffer = *inp++;
delta = inputbuffer & 0x0f;
}
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 = (char)index;
}
3.**使用 **
typedef struct adpcm_state
{
short valprev; /* Previous output value */
char index; /* Index into stepsize table */
}adpcm_state_t;
adpcm_decoder(encoded_data, (short *)decoded_data, sizeof(encoded_data), &adpcm_state);
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