关于PCM音频和g711音频编码的转换。
PCM文件：模拟音频信号经模数转换（A/D变换）直接形成的二进制序列，该文件没有附加的文件头和文件结束标志。Windows的Convert工具能够把PCM音频格式的文件转换成Microsoft的WAV格式的文件。
将音频数字化。事实上就是将声音数字化。最常见的方式是透过脉冲编码调制PCM(Pulse Code Modulation) 。
运作原理例如以下:首先我们考虑声音经过麦克风，转换成一连串电压变化的信号。例如以下图所看到的。这张图的横座标为秒。纵座标为电压大小。要将这种信号转为 PCM 格式的方法，是使用三个參数来表示声音。它们是：声道数、採样位数和採样频率。
採样频率：即取样频率,指每秒钟取得声音样本的次数。採样频率越高,声音的质量也就越好,声音的还原也就越真实，但同一时候它占的资源比較多。因为人耳的分辨率非常有限,太高的频率并不能分辨出来。
在16位声卡中有22KHz、44KHz等几级,当中，22KHz相当于普通FM广播的音质，44KHz已相当于CD音质了，眼下的经常使用採样频率都不超过48KHz。
採样位数：即採样值或取样值（就是将採样样本幅度量化）。它是用来衡量声音波动变化的一个參数。也能够说是声卡的分辨率。
它的数值越大，分辨率也就越高。所发出声音的能力越强。
声道数：非常好理解，有单声道和立体声之分，单声道的声音仅仅能使用一个喇叭发声（有的也处理成两个喇叭输出同一个声道的声音）。立体声的PCM 能够使两个喇叭都发声（一般左右声道有分工） ，更能感受到空间效果。
以下再用图解来看看採样位数和採样频率的概念。让我们来看看这几幅图。图中的黑色曲线表示的是PCM 文件录制的自然界的声波，红色曲线表示的是PCM 文件输出的声波。横坐标便是採样频率；纵坐标便是採样位数。
这几幅图中的格子从左到右，逐渐加密，先是加大横坐标的密度，然后加大纵坐标的密度。显然，当横坐标的单位越小即两个採样时刻的间隔越小。则越有利于保持原始声音的真实情况，换句话说，採样的频率越大则音质越有保证；同理，当纵坐标的单位越小则越有利于音质的提高。即採样的位数越大越好。
在计算机中採样位数一般有8位和16位之分。但有一点请大家注意，8位不是说把纵坐标分成8份，而是分成2的8次方即256份； 同理16位是把纵坐标分成2的16次方65536份； 而採样频率一般有11025HZ（11KHz），22050HZ（22KHz）、44100Hz（44KHz）三种。
那么，如今我们就能够得到PCM文件所占容量的公式：存储量 = (採样频率*採样位数*声道)*时间/8(单位：字节数).
比如，数字激光唱盘(CD－DA。红皮书标准)的标准採样频率为44.lkHz。採样数位为16位，立体声(2声道)，能够差点儿无失真地播出频率高达22kHz的声音，这也是人类所能听到的最高频率声音。
激光唱盘一分钟音乐须要的存储量为：
(44.1*1000*l6*2)*60/8=10。584。000(字节)=10.584MBytes
这个数值就是PCM声音文件在硬盘中所占磁盘空间的存储量。
计算机音频文件的格式决定了其声音的品质，日常生活中电话、收音机等均为模拟音频信号。即不存在採样频率和採样位数的概念，我们能够这样比較一下：
- 44KHz，16BIT的声音称作：CD音质；
- 22KHz、16Bit的声音效果近似于立体声（FM Stereo）广播。称作：广播音质；
- 11kHz、8Bit的声音，称作：电话音质。
G711格式
G711编码的声音清晰度好，语音自然度高，但压缩效率低，数据量大常在32Kbps以上。常用于电话语音(推荐使用64Kbps)，sampling rate为8K，压缩率为2，即把S16格式的数据压缩为8bit，分为a-law和u-law。
a-law也叫g711a,输入的是13位（其实是S16的高13位），使用在欧洲和其他地区，这种格式是经过特别设计的，便于数字设备进行快速运算。
运算过程如下：
（1） 取符号位并取反得到s，
（2） 获取强度位eee，获取方法如图所示
（3） 获取高位样本位wxyz
（4） 组合为seeewxyz，将seeewxyz逢偶数为取补数，编码完毕
示例：
输入pcm数据为3210，二进制对应为（0000 1100 1000 1010）
二进制变换下排列组合方式（0 0001 1001 0001010）
（1） 获取符号位最高位为0，取反，s=1
（2） 获取强度位0001，查表，编码制应该是eee=100
（3） 获取高位样本wxyz=1001
（4） 组合为11001001，逢偶数为取反为10011100
编码完毕。
u-law也叫g711u,使用在北美和日本，输入的是14位，编码算法就是查表，没啥复杂算法，就是基础值+平均偏移值，具体示例如下：
pcm=2345
（1）取得范围值
| +4062 to +2015 in 16 intervals of 128 |
（2）得到基础值0x90，
（3）间隔数128，
（4）区间基本值4062，
（5）当前值2345和区间基本值差异4062-2345=1717，
（6）偏移值=1717/间隔数=1717/128，取整得到13，
（7）输出为0x90+13=0x9D
代码如下：
g711codec.h
/*
* G711 encode decode HEADER.
*/
#ifndef __G711CODEC_H__
#define __G711CODEC_H__
/*
* u-law, A-law and linear PCM conversions.
*/
#define SIGN_BIT (0x80) /* Sign bit for a A-law byte. */
#define QUANT_MASK (0xf) /* Quantization field mask. */
#define NSEGS (8) /* Number of A-law segments. */
#define SEG_SHIFT (4) /* Left shift for segment number. */
#define SEG_MASK (0x70) /* Segment field mask. */
#define BIAS (0x84) /* Bias for linear code. */
int PCM2G711a( char *InAudioData, char *OutAudioData, int DataLen, int reserve );
int PCM2G711u( char *InAudioData, char *OutAudioData, int DataLen, int reserve );
int G711a2PCM( char *InAudioData, char *OutAudioData, int DataLen, int reserve );
int G711u2PCM( char *InAudioData, char *OutAudioData, int DataLen, int reserve );
int g711a_decode(short amp[], const unsigned char g711a_data[], int g711a_bytes);
int g711u_decode(short amp[], const unsigned char g711u_data[], int g711u_bytes);
int g711a_encode(unsigned char g711_data[], const short amp[], int len);
int g711u_encode(unsigned char g711_data[], const short amp[], int len);
#endif /* g711codec.h */
g711codec.c
#include "g711codec.h"
static short seg_end[8] = {0xFF, 0x1FF, 0x3FF, 0x7FF,
0xFFF, 0x1FFF, 0x3FFF, 0x7FFF};
static int search(int val, short *table, int size)
{
int i;
for (i = 0; i < size; i++) {
if (val <= *table++)
return (i);
}
return (size);
}
/*
* alaw2linear() - Convert an A-law value to 16-bit linear PCM
*
*/
static int alaw2linear( unsigned char a_val )
{
int t;
int seg;
a_val ^= 0x55;
t = (a_val & QUANT_MASK) << 4;
seg = ( (unsigned)a_val & SEG_MASK ) >> SEG_SHIFT;
switch (seg)
{
case 0:
t += 8;
break;
case 1:
t += 0x108;
break;
default:
t += 0x108;
t <<= seg - 1;
}
return ((a_val & SIGN_BIT) ? t : -t);
}
/*
* ulaw2linear() - Convert a u-law value to 16-bit linear PCM
*
* First, a biased linear code is derived from the code word. An unbiased
* output can then be obtained by subtracting 33 from the biased code.
*
* Note that this function expects to be passed the complement of the
* original code word. This is in keeping with ISDN conventions.
*/
static int ulaw2linear(unsigned char u_val)
{
int t;
/* Complement to obtain normal u-law value. */
u_val = ~u_val;
/*
* Extract and bias the quantization bits. Then
* shift up by the segment number and subtract out the bias.
*/
t = ((u_val & QUANT_MASK) << 3) + BIAS;
t <<= ((unsigned)u_val & SEG_MASK) >> SEG_SHIFT;
return ((u_val & SIGN_BIT) ? (BIAS - t) : (t - BIAS));
}
/*
* linear2alaw() - Convert a 16-bit linear PCM value to 8-bit A-law
*
*/
unsigned char linear2alaw(int pcm_val) /* 2's complement (16-bit range) */
{
int mask;
int seg;
unsigned char aval;
if (pcm_val >= 0) {
mask = 0xD5; /* sign (7th) bit = 1 */
} else {
mask = 0x55; /* sign bit = 0 */
pcm_val = -pcm_val - 8;
}
/* Convert the scaled magnitude to segment number. */
seg = search(pcm_val, seg_end, 8);
/* Combine the sign, segment, and quantization bits. */
if (seg >= 8) /* out of range, return maximum value. */
return (0x7F ^ mask);
else {
aval = seg << SEG_SHIFT;
if (seg < 2)
aval |= (pcm_val >> 4) & QUANT_MASK;
else
aval |= (pcm_val >> (seg + 3)) & QUANT_MASK;
return (aval ^ mask);
}
}
/*
* linear2ulaw() - Convert a linear PCM value to u-law
*
*/
unsigned char linear2ulaw(int pcm_val) /* 2's complement (16-bit range) */
{
int mask;
int seg;
unsigned char uval;
/* Get the sign and the magnitude of the value. */
if (pcm_val < 0) {
pcm_val = BIAS - pcm_val;
mask = 0x7F;
} else {
pcm_val += BIAS;
mask = 0xFF;
}
/* Convert the scaled magnitude to segment number. */
seg = search(pcm_val, seg_end, 8);
/*
* Combine the sign, segment, quantization bits;
* and complement the code word.
*/
if (seg >= 8) /* out of range, return maximum value. */
return (0x7F ^ mask);
else {
uval = (seg << 4) | ((pcm_val >> (seg + 3)) & 0xF);
return (uval ^ mask);
}
}
int g711a_decode( short amp[], const unsigned char g711a_data[], int g711a_bytes )
{
int i;
int samples;
unsigned char code;
int sl;
for ( samples = i = 0; ; )
{
if (i >= g711a_bytes)
break;
code = g711a_data[i++];
sl = alaw2linear( code );
amp[samples++] = (short) sl;
}
return samples*2;
}
int g711u_decode(short amp[], const unsigned char g711u_data[], int g711u_bytes)
{
int i;
int samples;
unsigned char code;
int sl;
for (samples = i = 0;;)
{
if (i >= g711u_bytes)
break;
code = g711u_data[i++];
sl = ulaw2linear(code);
amp[samples++] = (short) sl;
}
return samples*2;
}
int g711a_encode(unsigned char g711_data[], const short amp[], int len)
{
int i;
for (i = 0; i < len; i++)
{
g711_data[i] = linear2alaw(amp[i]);
}
return len;
}
int g711u_encode(unsigned char g711_data[], const short amp[], int len)
{
int i;
for (i = 0; i < len; i++)
{
g711_data[i] = linear2ulaw(amp[i]);
}
return len;
}
decode.c
#include
#include
#include
#include
#include
#include "g711codec.h"
int main( int argc, char *argv[] )
{
if(argc < 3)
{
printf("==> Usage:\n\tdecode [src.g711a] [dest.pcm]\n");
//printf("==> Usage:\n\tdecode [src.g711u] [dest.pcm]\n");
return 0;
}
FILE *pInFile = fopen(argv[1], "rb");
FILE *pOutFile = fopen(argv[2], "wb");
if (NULL == pInFile || NULL == pOutFile)
{
printf("open file failed\n");
return 0;
}
struct stat s_buf;
int status = 0;
status = stat( argv[1], &s_buf );
printf("file_size = %d\n", s_buf.st_size);
int Ret = 0;
int Read = 0;
int DataLen = s_buf.st_size;
printf("datalen = %d, %s, %d\n", DataLen, __func__, __LINE__);
unsigned char ucInBuff[ DataLen + 1 ];
unsigned char ucOutBuff[ 2*DataLen + 1 ];
memset( ucInBuff, 0, sizeof(ucInBuff) );
memset( ucOutBuff, 0, sizeof(ucOutBuff) );
Read = fread( ucInBuff, 1, DataLen, pInFile );
printf("Read = %d, Ret = %d\n", Read, Ret);
if (Read)
{
Ret = G711a2PCM( (char *)ucInBuff, (char *)ucOutBuff, Read, 0 );
//Ret = G711u2PCM( (char *)ucInBuff, (char *)ucOutBuff, Read, 0 );
printf("Read = %d, Ret = %d, %s, %d\n", Read, Ret, __func__, __LINE__);
fwrite( ucOutBuff, 1, Ret, pOutFile );
memset( ucInBuff, 0, sizeof(ucInBuff) );
memset( ucOutBuff, 0, sizeof(ucOutBuff) );
}
else
{
printf("fread error !\n");
return -1;
}
fclose(pInFile);
fclose(pOutFile);
return 0;
}
encode.c
#include
#include
#include
#include
#include
#include "g711codec.h"
int main(int argc, char *argv[])
{
if(argc < 3)
{
printf("==> Usage:\n\tencode [src.pcm] [dest.g711a]\n");
//printf("==> Usage:\n\tencode [src.pcm] [dest.g711u]\n");
return 0;
}
FILE *pInFile = fopen(argv[1], "rb");
FILE *pOutFile = fopen(argv[2], "wb");
if (NULL == pInFile || NULL == pOutFile)
{
printf("open file failed\n");
return 0;
}
struct stat s_buf;
int status = 0;
status = stat( argv[1], &s_buf);
printf("file_size = %d\n", s_buf.st_size);
int Ret = 0;
int Read = 0;
int Len = s_buf.st_size;
printf("datalen = %d\n", s_buf.st_size);
unsigned char ucInBuff[ Len +1 ];
unsigned char ucOutBuff[ Len + 1 ];
memset(ucInBuff, 0, sizeof(ucInBuff));
memset(ucOutBuff, 0, sizeof(ucOutBuff));
Read = fread(ucInBuff, 1, Len, pInFile);
printf("Read = %d, Ret = %d\n", Read, Ret);
if (Read)
{
Ret = PCM2G711a( (char *)ucInBuff, (char *)ucOutBuff, Read, 0 );
//Ret = PCM2G711u( (char *)ucInBuff, (char *)ucOutBuff, Read, 0 );
printf("Read = %d, Ret = %d, %s, %d\n", Read, Ret, __func__, __LINE__);
fwrite(ucOutBuff, 1, Ret, pOutFile);
memset(ucInBuff, 0, sizeof(ucInBuff));
memset(ucOutBuff, 0, sizeof(ucOutBuff));
}
else
{
printf("fread error !\n");
return -1;
}
fclose(pInFile);
fclose(pOutFile);
return 0;
}
g711.c
#include
#include "g711codec.h"
/*
* function: convert PCM audio format to g711 alaw/ulaw.(zqj)
* InAudioData: PCM data prepared for encoding to g711 alaw/ulaw.
* OutAudioData: encoded g711 alaw/ulaw.
* DataLen: PCM data size.
* reserve: reserved param, no use.
*/
/*alaw*/
int PCM2G711a( char *InAudioData, char *OutAudioData, int DataLen, int reserve )
{
//check params.
if( (NULL == InAudioData) && (NULL == OutAudioData) && (0 == DataLen) )
{
printf("Error, empty data or transmit failed, exit !\n");
return -1;
}
printf("DataLen = %d, %s, %d\n", DataLen, __func__, __LINE__);
int Retaen = 0;
printf("G711a encode start......\n");
Retaen = g711a_encode( (unsigned char *)OutAudioData, (short*)InAudioData, DataLen/2 );
printf("Retaen = %d, %s, %d\n", Retaen, __func__, __LINE__);
return Retaen; //index successfully encoded data len.
}
/*ulaw*/
int PCM2G711u( char *InAudioData, char *OutAudioData, int DataLen, int reserve )
{
//check params.
if( (NULL == InAudioData) && (NULL == OutAudioData) && (0 == DataLen) )
{
printf("Error, empty data or transmit failed, exit !\n");
return -1;
}
printf("DataLen = %d, %s, %d\n", DataLen, __func__, __LINE__);
int Retuen = 0;
printf("G711u encode start......\n");
Retuen = g711u_encode( (unsigned char *)OutAudioData, (short*)InAudioData, DataLen/2 );
printf("Retuen = %d, %s, %d\n", Retuen, __func__, __LINE__);
return Retuen;
}
/*
* function: convert g711 alaw audio format to PCM.(zqj)
* InAudioData: g711 alaw data prepared for encoding to PCM.
* OutAudioData: encoded PCM audio data.
* DataLen: g711a data size.
* reserve: reserved param, no use.
*/
/*alaw*/
int G711a2PCM( char *InAudioData, char *OutAudioData, int DataLen, int reserve )
{
//check param.
if( (NULL == InAudioData) && (NULL == OutAudioData) && (0 == DataLen) )
{
printf("Error, empty data or transmit failed, exit !\n");
return -1;
}
printf("DataLen = %d, %s, %d\n", DataLen, __func__, __LINE__);
int Retade = 0;
printf("G711a decode start......\n");
Retade = g711a_decode( (short*)OutAudioData, (unsigned char *)InAudioData, DataLen );
printf("Retade = %d, %s, %d\n", Retade, __func__, __LINE__);
return Retade; //index successfully decoded data len.
}
/*ulaw*/
int G711u2PCM( char *InAudioData, char *OutAudioData, int DataLen, int reserve )
{
//check param.
if( (NULL == InAudioData) && (NULL == OutAudioData) && (0 == DataLen) )
{
printf("Error, empty data or transmit failed, exit !\n");
return -1;
}
printf("DataLen = %d, %s, %d\n", DataLen, __func__, __LINE__);
int Retude = 0;
printf("G711u decode start......\n");
Retude = g711u_decode( (short*)OutAudioData, (unsigned char *)InAudioData, DataLen );
printf("Retude = %d, %s, %d\n", Retude, __func__, __LINE__);
return Retude;
}