main.txt

这是一个在计算机上用C语言写的ADPCM编解码程序。

/* Table of index changes */
const int IndexTable[16] = {
0xff, 0xff, 0xff, 0xff, 2, 4, 6, 8,
0xff, 0xff, 0xff, 0xff, 2, 4, 6, 8
};
/* Quantizer step size lookup table */
const long 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
};
signed long diff; /* Difference between sample and predicted sample */
long step; /* Quantizer step size */
signed long predsample; /* Output of ADPCM predictor */
signed long diffq; /* Dequantized predicted difference */
int index; /* Index into step size table */
/*****************************************************************************
* ADPCMEncoder - ADPCM encoder routine *
******************************************************************************
* Input Variables: *
* signed long sample - 16-bit signed speech sample *
* Return Variable: *
* char - 8-bit number containing the 4-bit ADPCM code *
*****************************************************************************/
char ADPCMEncoder( signed long sample )
{
int code; /* ADPCM output value */
int tempstep; /* Temporary step size */
/* Restore previous values of predicted sample and quantizer step size index*/
predsample = state.prevsample;
index = state.previndex;
step = StepSizeTable[index];
/* Compute the difference between the actual sample (sample) and the the predicted sample (predsample)*/
diff = sample - predsample;
if(diff >= 0)
code = 0;
else
{
code = 8;
diff = -diff;
}
/* Quantize the difference into the 4-bit ADPCM code using the the quantizer step size*/
tempstep = step;
if( diff >= tempstep )
{
 code |= 4;
 diff -= tempstep;
}
tempstep >>= 1;
if( diff >= tempstep )
{
code |= 2;
diff -= tempstep;
}
tempstep >>= 1;
if( diff >= tempstep )
code |= 1;
/* Inverse quantize the ADPCM code into a predicted difference using the quantizer step size*/
diffq = step >> 3;
if( code & 4 )
diffq += step;
if( code & 2 )
diffq += step >> 1;
if( code & 1 )
diffq += step >> 2;
/* Fixed predictor computes new predicted sample by adding the old predicted sample to predicted difference*/
if( code & 8 )
predsample -= diffq;
else
predsample += diffq;
/* Check for overflow of the new predicted sample
*/
if( predsample > 32767 )
predsample = 32767;
else if( predsample < -32768 )
predsample = -32768;
/* Find new quantizer stepsize index by adding the old index
to a table lookup using the ADPCM code
*/
index += IndexTable[code];
/* Check for overflow of the new quantizer step size index
*/
if( index < 0 )
index = 0;
if( index > 88 )
index = 88;
/* Save the predicted sample and quantizer step size index for
next iteration
*/
state.prevsample = predsample;
state.previndex = index;
/* Return the new ADPCM code */
return ( code & 0x0f );
}