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]; （取余运算） // 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;
}
附表
int index_adjust[8] = {-1,-1,-1,-1,2,4,6,8};
int step_table[] = { 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,
3327,3660,4026,4428,4871,5358,54,84,7132,7845,8630,9493,10442,11487,12635,139,
152,16818,18500,20350,22385,24623,27086,29794,32767 }
TCPMP原代码赏析
/*****************************************************************************
*
* This program is free software ; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
*
* $Id: adpcm.h 271 2005-08-09 08:31:35Z picard $
*
* The Core Pocket Media Player
* Copyright (c) 2004-2005 Gabor Kovacs
*
****************************************************************************/
#ifndef __ADPCM_H
#define __ADPCM_H
#define ADPCM_CLASS FOURCC('A','D','P','C')
#define ADPCM_MS_ID FOURCC('A','D','M','S')
#define ADPCM_IMA_ID FOURCC('A','D','I','M')
#define ADPCM_IMA_QT_ID FOURCC('A','D','I','Q')
#define ADPCM_G726_ID FOURCC('G','7','2','6')
extern void ADPCM_Init();
extern void ADPCM_Done();
#endif
/*****************************************************************************
*
* This program is free software ; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
*
* $Id: adpcm.c 565 2006-01-12 14:11:44Z picard $
*
* The Core Pocket Media Player
* Copyright (c) 2004-2005 Gabor Kovacs
*
****************************************************************************/
#include "../common/common.h"
#include "adpcm.h"
#include "g726/g72x.h"
typedef struct state
{
int Predictor;
int StepIndex;
int Step;
int Sample1;
int Sample2;
int CoEff1;
int CoEff2;
int IDelta;
} state;
typedef struct adpcm
{
codec Codec;
buffer Data;
int Channel; //IMA_QT
int16_t* Buffer;
state State[2];
g726_state G726[2];
} adpcm;
static const int IndexTable[16] =
{
-1, -1, -1, -1, 2, 4, 6, 8,
-1, -1, -1, -1, 2, 4, 6, 8,
};
static const int StepTable[] =
{
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,
54, 84, 7132, 7845, 8630, 9493, 10442, 11487, 12635, 139,
152, 16818, 18500, 20350, 22385, 24623, 27086, 29794, 32767
};
// AdaptationTable[], AdaptCoeff1[], and AdaptCoeff2[] are from libsndfile
static const int AdaptationTable[] =
{
230, 230, 230, 230, 307, 409, 512, 614,
768, 614, 512, 409, 307, 230, 230, 230
};
static const int AdaptCoeff1[] =
{
256, 512, 0, 192, 240, 460, 392
};
static const int AdaptCoeff2[] =
{
0, -256, 0, , 0, -208, -232
};
static _INLINE int IMA_Calc(state* s, int v)
{
int StepIndex;
int Predictor;
int Diff,Step;
Step = StepTable[s->StepIndex];
StepIndex = s->StepIndex + IndexTable[v];
if (StepIndex < 0)
StepIndex = 0;
else if (StepIndex > 88)
StepIndex = 88;
Diff = ((2 * (v & 7) + 1) * Step) >> 3;
Predictor = s->Predictor;
if (v & 8)
Predictor -= Diff;
else
Predictor += Diff;
if (Predictor > 32767)
Predictor = 32767;
else if (Predictor < -32768)
Predictor = -32768;
s->Predictor = Predictor;
s->StepIndex = StepIndex;
return Predictor;
}
static _INLINE int MS_Calc(state* s, int v)
{
int Predictor;
Predictor = ((s->Sample1 * s->CoEff1) + (s->Sample2 * s->CoEff2)) >> 8;
Predictor += ((v & 0x08) ? v-0x10:v) * s->IDelta;
if (Predictor > 32767)
Predictor = 32767;
else if (Predictor < -32768)
Predictor = -32768;
s->Sample2 = s->Sample1;
s->Sample1 = Predictor;
s->IDelta = (AdaptationTable[v] * s->IDelta) >> 8;
if (s->IDelta < 16)
s->IDelta = 16;
return Predictor;
}
static int Process(adpcm* p, const packet* Packet, const flowstate* State)
{
int i;
int Predictor;
const uint8_t* In;
const uint8_t* InEnd;
int16_t* Out = p->Buffer;
if (Packet)
{
if (Packet->RefTime >= 0)
p->Codec.Packet.RefTime = Packet->RefTime;
BufferPack(&p->Data,0);
BufferWrite(&p->Data,Packet->Data[0],Packet->Length,1024);
}
else
p->Codec.Packet.RefTime = TIME_UNKNOWN;
if (!BufferRead(&p->Data,&In,p->Codec.In.Format.Format.Audio.BlockAlign))
return ERR_NEED_MORE_DATA;
InEnd = In + p->Codec.In.Format.Format.Audio.BlockAlign;
switch (p->Codec.Node.Class)
{
case ADPCM_G726_ID:
{
g726_state *g1,*g2;
g1 = g2 = &p->G726[0];
if (p->Codec.In.Format.Format.Audio.Channels==2)
++g2;
switch (p->Codec.In.Format.Format.Audio.Bits)
{
case 2:
for (;In Out[0] = (int16_t)g726_16_decoder(In[0] >> 6,g1); Out[1] = (int16_t)g726_16_decoder(In[0] >> 4,g2); Out[2] = (int16_t)g726_16_decoder(In[0] >> 2,g1); Out[3] = (int16_t)g726_16_decoder(In[0],g2); } break; case 3: InEnd -= 2; for (;In Out[0] = (int16_t)g726_24_decoder(In[0] >> 5,g1); Out[1] = (int16_t)g726_24_decoder(In[0] >> 2,g2); Out[2] = (int16_t)g726_24_decoder((In[0] << 1) | (In[1] >> 7),g1); Out[3] = (int16_t)g726_24_decoder(In[1] >> 4,g2); Out[4] = (int16_t)g726_24_decoder(In[1] >> 1,g1); Out[5] = (int16_t)g726_24_decoder((In[1] << 2) | (In[2] >> 6),g2); Out[6] = (int16_t)g726_24_decoder(In[2] >> 3,g1); Out[7] = (int16_t)g726_24_decoder(In[2] >> 0,g2); } break; case 4: for (;In Out[0] = (int16_t)g726_32_decoder(In[0] >> 4,g1); Out[1] = (int16_t)g726_32_decoder(In[0],g2); } break; case 5: InEnd -= 4; for (;In Out[0] = (int16_t)g726_40_decoder(In[0] >> 3,g1); Out[1] = (int16_t)g726_40_decoder((In[0] << 2) | (In[1] >> 6),g2); Out[2] = (int16_t)g726_40_decoder(In[1] >> 1,g1); Out[3] = (int16_t)g726_40_decoder((In[1] << 4) | (In[2] >> 4),g2); Out[4] = (int16_t)g726_40_decoder((In[2] << 1) | (In[3] >> 7),g1); Out[5] = (int16_t)g726_40_decoder(In[3] >> 2,g2); Out[6] = (int16_t)g726_40_decoder((In[3] << 3) | (In[4] >> 5),g1); Out[7] = (int16_t)g726_40_decoder(In[4] >> 0,g2); } break; } break; } case ADPCM_IMA_QT_ID: { int No,Ch; Ch = p->Codec.In.Format.Format.Audio.Channels; for (No=0;No state *s; s = &p->State[0]; s->Predictor = (int16_t)((In[1] & 0x80) | (In[0] << 8)); s->StepIndex = In[1] & 0x7F; if (s->StepIndex > 88) s->StepIndex = 88; In+=2; InEnd=In+32; Out = p->Buffer+No; for (;In *Out = (int16_t)IMA_Calc(s, In[0] & 0x0F); Out+=Ch; *Out = (int16_t)IMA_Calc(s, In[0] >> 4); Out+=Ch; } } Out = p->Buffer+Ch*; break; } case ADPCM_IMA_ID: { state *s1,*s2; s1 = &p->State[0]; s1->Predictor = (int16_t)(In[0] | (In[1] << 8)); In+=2; s1->StepIndex = *In++; if (s1->StepIndex > 88) s1->StepIndex = 88; ++In; if (p->Codec.In.Format.Format.Audio.Channels == 2) { s2 = &p->State[1]; s2->Predictor = (int16_t)(In[0] | (In[1] << 8)); In+=2; s2->StepIndex = *In++; if (s2->StepIndex > 88) s2->StepIndex = 88; ++In; for (i=4;In Out[0] = (int16_t)IMA_Calc(s1, In[0] & 0x0F); Out[1] = (int16_t)IMA_Calc(s2, In[4] & 0x0F); Out[2] = (int16_t)IMA_Calc(s1, In[0] >> 4); Out[3] = (int16_t)IMA_Calc(s2, In[4] >> 4); if (--i==0) { i=4; In+=4; } } } else { for (;In Out[0] = (int16_t)IMA_Calc(s1, In[0] & 0x0F); Out[1] = (int16_t)IMA_Calc(s1, In[0] >> 4); } } break; } case ADPCM_MS_ID: { state *s1,*s2; s1 = &p->State[0]; s2 = p->Codec.In.Format.Format.Audio.Channels==2 ? &p->State[1] : s1; Predictor = *In++; if (Predictor > 7) Predictor = 7; s1->CoEff1 = AdaptCoeff1[Predictor]; s1->CoEff2 = AdaptCoeff2[Predictor]; if (s2 != s1) { Predictor = *In++; if (Predictor > 7) Predictor = 7; s2->CoEff1 = AdaptCoeff1[Predictor]; s2->CoEff2 = AdaptCoeff2[Predictor]; } s1->IDelta = (int16_t)(In[0] | (In[1] << 8)); In+=2; if (s2 != s1) { s2->IDelta = (int16_t)(In[0] | (In[1] << 8)); In+=2; } s1->Sample1 = (int16_t)(In[0] | (In[1] << 8)); In+=2; if (s2 != s1) { s2->Sample1 = (int16_t)(In[0] | (In[1] << 8)); In+=2; } s1->Sample2 = (int16_t)(In[0] | (In[1] << 8)); In+=2; if (s2 != s1) { s2->Sample2 = (int16_t)(In[0] | (In[1] << 8)); In+=2; } *Out++ = (int16_t)s1->Sample1; if (s2 != s1) *Out++ = (int16_t)s2->Sample1; *Out++ = (int16_t)s1->Sample2; if (s2 != s1) *Out++ = (int16_t)s2->Sample2; for (;In Out[0] = (int16_t)MS_Calc(s1, In[0] >> 4); Out[1] = (int16_t)MS_Calc(s2, In[0] & 0x0F); } break; } } p->Codec.Packet.Length = (uint8_t*)Out - (uint8_t*)p->Buffer; return ERR_NONE; } static int UpdateInput(adpcm* p) { BufferClear(&p->Data); free(p->Buffer); p->Buffer = NULL; if (p->Codec.In.Format.Type == PACKET_AUDIO) { PacketFormatPCM(&p->Codec.Out.Format,&p->Codec.In.Format,16); if (!p->Codec.In.Format.Format.Audio.BlockAlign) p->Codec.In.Format.Format.Audio.BlockAlign = 1024; if (p->Codec.Node.Class == ADPCM_IMA_QT_ID) p->Codec.In.Format.Format.Audio.BlockAlign = (32+2)*p->Codec.In.Format.Format.Audio.Channels; if (p->Codec.Node.Class == ADPCM_G726_ID) { p->Codec.In.Format.Format.Audio.BlockAlign = 120; g726_init_state(&p->G726[0]); g726_init_state(&p->G726[1]); } p->Buffer = (int16_t*) malloc(sizeof(int16_t)*4*p->Codec.In.Format.Format.Audio.BlockAlign); if (!p->Buffer) return ERR_OUT_OF_MEMORY; p->Codec.Packet.Data[0] = p->Buffer; } return ERR_NONE; } static int Flush(adpcm* p) { if (p->Codec.Node.Class == ADPCM_G726_ID) { g726_init_state(&p->G726[0]); g726_init_state(&p->G726[1]); } BufferDrop(&p->Data); p->Channel = 0; return ERR_NONE; } static int Create(adpcm* p) { p->Codec.Process = (packetprocess)Process; p->Codec.UpdateInput = (nodefunc)UpdateInput; p->Codec.Flush = (nodefunc)Flush; return ERR_NONE; } static const nodedef ADPCM = { sizeof(adpcm)|CF_ABSTRACT, ADPCM_CLASS, CODEC_CLASS, PRI_DEFAULT, (nodecreate)Create, NULL, }; static const nodedef ADPCM_MS = { 0, //parent size ADPCM_MS_ID, ADPCM_CLASS, PRI_DEFAULT, NULL, NULL, }; static const nodedef ADPCM_IMA = { 0, //parent size ADPCM_IMA_ID, ADPCM_CLASS, PRI_DEFAULT, NULL, NULL, }; static const nodedef ADPCM_IMA_QT = { 0, //parent size ADPCM_IMA_QT_ID, ADPCM_CLASS, PRI_DEFAULT, NULL, NULL, }; static const nodedef ADPCM_G726 = { 0, //parent size ADPCM_G726_ID, ADPCM_CLASS, PRI_DEFAULT, NULL, NULL, }; void ADPCM_Init() { NodeRegisterClass(&ADPCM); NodeRegisterClass(&ADPCM_MS); NodeRegisterClass(&ADPCM_IMA); NodeRegisterClass(&ADPCM_IMA_QT); NodeRegisterClass(&ADPCM_G726); } void ADPCM_Done() { NodeUnRegisterClass(ADPCM_MS_ID); NodeUnRegisterClass(ADPCM_IMA_ID); NodeUnRegisterClass(ADPCM_IMA_QT_ID); NodeUnRegisterClass(ADPCM_G726_ID); NodeUnRegisterClass(ADPCM_CLASS); }
