網上帖子一大推，都是照搬adpcm音頻算法代碼，實際使用時需要注意沒有相關細節，我在這裏補充一片防止大家走彎路
//ADPCM-DIV4 音頻幀標識頭
struct adpcm_state
{
//前一個採用點值：adpcm算法是根據前一個採樣點爲基準值，求後續壓縮值
short valprev;
char index;
char rev;
};
/* 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
};
/*
len: 表示採用點數。如16位音頻採用一個點是2個字節，但是長度是1
*/
int adpcm_coder(short *indata, signed 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 */
int outLen = 0;
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;
}*/
if ( bufferstep ) {
outputbuffer = delta & 0x0f;
} else {
*outp++ = ((delta << 4) & 0xf0) | outputbuffer;
outLen++;
}
bufferstep = !bufferstep;
}
/* Output last step, if needed */
if ( !bufferstep )
{
*outp++ = outputbuffer;
outLen++;
}
state->valprev = valpred;
state->index = index;
return outLen;
}
//16位 adpcm轉pcm
int adpcm_decode(char*inbuff,char*outbuff,int in_len)
{
struct adpcm_state adpcmState;
adpcmState.index =inbuff[2];
adpcmState.valprev =(short)inbuff[0]+((short)(inbuff[1]))*256; //每一個block裏面幀頭有一個未壓縮的數據 存儲時 先低後高
struct adpcm_state *state = &adpcmState;
inbuff += 4;
in_len -= 4;
int i=0,j=0;
char tmp_data;
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 */
int Samp;
unsigned char SampH,SampL;
unsigned char inCode;
/* Restore previous values of predicted sample and quantizer step
size index
*/
predsample =state->valprev;
index =state->index;
if(index <0)
index =0;
if(index >88)
index =88;
for(i=0;i<in_len*2;i++)
{
tmp_data=inbuff[i/2];
if(i%2)
inCode=(tmp_data&0xf0)>>4;
else
inCode=tmp_data &0x0f;
step =stepsizeTable[index];
/* Inverse quantize the ADPCM code into a predicted difference
using the quantizer step size
*/
diffq =step >>3;
if(inCode &4)
diffq +=step;
if(inCode &2)
diffq +=step >>1;
if(inCode &1)
diffq +=step >>2;
/* Fixed predictor computes new predicted sample by adding the
old predicted sample to predicted difference
*/
if(inCode &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[inCode];
/* Check for overflow of the new quantizer step size index
*/
if(index <0)
index =0;
if(index >88)
index =88;
/* Return the new ADPCM code */
Samp=predsample;
if(Samp>=0)
{
SampH=Samp/256;
SampL=Samp-256*SampH;
}
else
{
Samp=32768+Samp;
SampH=Samp/256;
SampL=Samp-256*SampH;
SampH+=0x80;
}
outbuff[j++]=SampL;
outbuff[j++]=SampH;
// hEncoder = NULL;
}
/* Save the predicted sample and quantizer step size index for
next iteration
*/
state->valprev =(short)predsample;
state->index =(char)index;
return j;
}
exmplor:
encode:
static struct adpcm_state state;
static char flag;
outBuf[outLen++] = inBuf[0];
outBuf[outLen++] = inBuf[1];
if(0 == flag)//編碼時第一幀index直接爲0
{
flag = 1;
outBuf[outLen++] = 0;
}
else //其他時候使用上一幀index
{
outBuf[outLen++] = state.index;
}
outBuf[outLen++] = 0;
outLen += adpcm_coder(inBuf, outbuf+4, len, &state);
//outLen 應該是164
decode：
outLen = adpcm_decode(inBuf, outBuf, inLen);
//inBuf必須是完整的adpcm數據幀164字節，inLen == 164
//outLen 應該是640