本文转自【 http://blog.csdn.net/szfhy/article/details/52448906 】
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。
CONTENTS
a-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）
（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
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
实例代码：
#include <stdio.h>
#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. */
#define CLIP 8159
#define G711_A_LAW (0)
#define G711_MU_LAW (1)
#define DATA_LEN (16)
static short seg_aend[8] = {
0x1F, 0x3F, 0x7F, 0xFF,
0x1FF, 0x3FF, 0x7FF, 0xFFF
};
static short seg_uend[8] = {
0x3F, 0x7F, 0xFF, 0x1FF,
0x3FF, 0x7FF, 0xFFF, 0x1FFF
};
unsigned char _u2a[128] = {
/* u- to A-law conversions */
1,1,2,2,3,3,4,4,
5,5,6,6,7,7,8,8,
9,10,11,12,13,14,15,16,
17,18,19,20,21,22,23,24,
25,27,29,31,33,34,35,36,
37,38,39,40,41,42,43,44,
46,48,49,50,51,52,53,54,
55,56,57,58,59,60,61,62,
64,65,66,67,68,69,70,71,
72,73,74,75,76,77,78,79,
81,82,83,84,85,86,87,88,
89,90,91,92,93,94,95,96,
97,98,99,100,101,102,103,104,
105,106,107,108,109,110,111,112,
113,114,115,116,117,118,119,120,
121,122,123,124,125,126,127,128
};
unsigned char _a2u[128] = {
/* A- to u-law conversions */
1,3,5,7,9,11,13,15,
16,17,18,19,20,21,22,23,
24,25,26,27,28,29,30,31,
32,32,33,33,34,34,35,35,
36,37,38,39,40,41,42,43,
44,45,46,47,48,48,49,49,
50,51,52,53,54,55,56,57,
58,59,60,61,62,63,64,64,
65,66,67,68,69,70,71,72,
73,74,75,76,77,78,79,79,
80,81,82,83,84,85,86,87,
88,89,90,91,92,93,94,95,
96,97,98,99,100,101,102,103,
104,105,106,107,108,109,110,111,
112,113,114,115,116,117,118,119,
120,121,122,123,124,125,126,127
};
static short search(int val, short *table, int size)
{
int i;
for (i = 0; i < size; i++) {
if (val <= *table++)
return (i);
}
return (size);
}
/*
* linear2alaw() - Convert a 16-bit linear PCM value to 8-bit A-law
*
* linear2alaw() accepts an 16-bit integer and encodes it as A-law data.
*
*Linear Input CodeCompressed Code
*---------------------------------------
*0000000wxyza000wxyz
*0000001wxyza001wxyz
*000001wxyzab010wxyz
*00001wxyzabc011wxyz
*0001wxyzabcd100wxyz
*001wxyzabcde101wxyz
*01wxyzabcdef110wxyz
*1wxyzabcdefg111wxyz
*
* For further information see John C. Bellamy's Digital Telephony, 1982,
* John Wiley & Sons, pps 98-111 and 472-476.
*/
unsigned char linear2alaw(int pcm_val)/* 2's complement (16-bit range) */
{
int mask;
int seg;
unsigned char aval;
pcm_val = pcm_val >> 3;
if (pcm_val >= 0) {
mask = 0xD5;/* sign (7th) bit = 1 */
} else {
mask = 0x55;/* sign bit = 0 */
pcm_val = -pcm_val - 1;
}
/* Convert the scaled magnitude to segment number. */
seg = search(pcm_val, seg_aend, 8);
/* Combine the sign, segment, and quantization bits. */
if (seg >= 8)/* out of range, return maximum value. */
return (unsigned char) (0x7F ^ mask);
else {
aval = (unsigned char) seg << SEG_SHIFT;
if (seg < 2)
aval |= (pcm_val >> 1) & QUANT_MASK;
else
aval |= (pcm_val >> seg) & QUANT_MASK;
return (aval ^ mask);
}
}
/*
* alaw2linear() - Convert an A-law value to 16-bit linear PCM
*
*/
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);
}
/*
* linear2ulaw() - Convert a linear PCM value to u-law
*
* In order to simplify the encoding process, the original linear magnitude
* is biased by adding 33 which shifts the encoding range from (0 - 8158) to
* (33 - 8191). The result can be seen in the following encoding table:
*
*Biased Linear Input CodeCompressed Code
*---------------------------------------
*00000001wxyza000wxyz
*0000001wxyzab001wxyz
*000001wxyzabc010wxyz
*00001wxyzabcd011wxyz
*0001wxyzabcde100wxyz
*001wxyzabcdef101wxyz
*01wxyzabcdefg110wxyz
*1wxyzabcdefgh111wxyz
*
* Each biased linear code has a leading 1 which identifies the segment
* number. The value of the segment number is equal to 7 minus the number
* of leading 0's. The quantization interval is directly available as the
* four bits wxyz. * The trailing bits (a - h) are ignored.
*
* Ordinarily the complement of the resulting code word is used for
* transmission, and so the code word is complemented before it is returned.
*
* For further information see John C. Bellamy's Digital Telephony, 1982,
* John Wiley & Sons, pps 98-111 and 472-476.
*/
unsigned char linear2ulaw(short pcm_val)/* 2's complement (16-bit range) */
{
short mask;
short seg;
unsigned char uval;
/* Get the sign and the magnitude of the value. */
pcm_val = pcm_val >> 2;
if (pcm_val < 0) {
pcm_val = -pcm_val;
mask = 0x7F;
} else {
mask = 0xFF;
}
if (pcm_val > CLIP)
pcm_val = CLIP;/* clip the magnitude */
pcm_val += (BIAS >> 2);
/* Convert the scaled magnitude to segment number. */
seg = search(pcm_val, seg_uend, 8);
/*
* Combine the sign, segment, quantization bits;
* and complement the code word.
*/
if (seg >= 8)/* out of range, return maximum value. */
return (unsigned char) (0x7F ^ mask);
else {
uval = (unsigned char) (seg << 4) | ((pcm_val >> (seg + 1)) & 0xF);
return (uval ^ mask);
}
}
/*
* 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.
*/
short ulaw2linear(unsigned char u_val)
{
short 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));
}
/* A-law to u-law conversion */
unsigned char alaw2ulaw(unsigned char aval)
{
aval &= 0xff;
return (unsigned char) ((aval & 0x80) ? (0xFF ^ _a2u[aval ^ 0xD5]) :
(0x7F ^ _a2u[aval ^ 0x55]));
}
/* u-law to A-law conversion */
unsigned char ulaw2alaw(unsigned char uval)
{
uval &= 0xff;
return (unsigned char) ((uval & 0x80) ? (0xD5 ^ (_u2a[0xFF ^ uval] - 1)) :
(unsigned char) (0x55 ^ (_u2a[0x7F ^ uval] - 1)));
}
int encode(char *a_psrc, char *a_pdst, int in_data_len, unsigned char type)
{
int i;
short *psrc = (short *)a_psrc;
int out_data_len = in_data_len / sizeof(short);
if (a_psrc == NULL || a_pdst == NULL) {
return (-1);
}
if (in_data_len <= 0) {
return (-1);
}
if (type == G711_A_LAW) {
for (i = 0; i < out_data_len; i++) {
a_pdst[i] = (char)linear2alaw(psrc[i]);
}
} else {
for (i = 0; i < out_data_len; i++) {
a_pdst[i] = (char)linear2ulaw(psrc[i]);
}
}
return (i);
}
int decode(char *a_psrc, char *a_pdst, int in_data_len, unsigned char type)
{
int i;
short *pdst = (short *)a_pdst;
int out_data_len = in_data_len / sizeof(char);
if (a_psrc == NULL || a_pdst == NULL) {
return (-1);
}
if (type == G711_A_LAW) {
for (i = 0; i < out_data_len; i++) {
pdst[i] = (short)alaw2linear((unsigned char)a_psrc[i]);
}
} else {
for (i = 0; i < out_data_len; i++) {
pdst[i] = (short)ulaw2linear((unsigned char)a_psrc[i]);
}
}
return (i * sizeof(short));
}
int main(int argc, char **argv)
{
int i = 0;
int n = 0;
unsigned short pcm_buf[DATA_LEN] = {0}; /*store linear pcm data*/
unsigned short pcm_buf2[DATA_LEN] = {0}; /*store linear pcm data*/
unsigned char g711_buf[DATA_LEN] = {0};
FILE * fp_in = fopen("input.wav", "r");
FILE * fp_out = fopen("pcm.g711_alaw", "w");
FILE * fp_out_pcm = fopen("pcm2.wav", "w");
unsigned char header[128] = { 0 };
fread(header, 1, 0x2c, fp_in);
fwrite(header, 1, 0x2c, fp_out_pcm);
while (DATA_LEN * 2 == fread(pcm_buf, 1, DATA_LEN * 2, fp_in)) {
printf("encode %d was trans\n",
encode(pcm_buf, g711_buf, sizeof(pcm_buf), G711_A_LAW));
fwrite(g711_buf, 1, DATA_LEN, fp_out);
printf("decode %d was trans\n",
decode(g711_buf, pcm_buf2, sizeof(g711_buf), G711_A_LAW));
fwrite(pcm_buf2, 1, DATA_LEN*2, fp_out_pcm);
}
fclose(fp_in);
fclose(fp_out);
fclose(fp_out_pcm);
return 0;
}
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