G711編碼的聲音清晰度好,語音自然度高,但壓縮效率低,數據量大常在32Kbps以上。常用於電話語音(推薦使用64Kbps),sampling rate爲8K,壓縮率爲2,即把S16格式的數據壓縮爲8bit,分爲a-law和u-law。
a-law也叫g711a,輸入的是13位(其實是S16的高13位),使用在歐洲和其他地區,這種格式是經過特別設計的,便於數字設備進行快速運算。
運算過程如下:
(1) 取符號位並取反得到s,
(2) 獲取強度位eee,獲取方法如圖所示
(3) 獲取高位樣本位wxyz
(4) 組合爲seeewxyz,將seeewxyz逢偶數爲取補數,編碼完畢
示例:
輸入pcm數據爲3210,二進制對應爲(0000 1100 1000 1010)
二進制變換下排列組合方式(0 0001 1001 0001010)
(1) 獲取符號位最高位爲0,取反,s=1
(2) 獲取強度位0001,查表,編碼制應該是eee=100
(3) 獲取高位樣本wxyz=1001
(4) 組合爲11001001,逢偶數爲取反爲10011100
編碼完畢。
u-law也叫g711u,使用在北美和日本,輸入的是14位,編碼算法就是查表,沒啥複雜算法,就是基礎值+平均偏移值,具體示例如下:
pcm=2345
(1)取得範圍值
+4062 to +2015 in 16 intervals of 128 |
(2)得到基礎值0x90,
(3)間隔數128,
(4)區間基本值4062,
(5)當前值2345和區間基本值差異4062-2345=1717,
(6)偏移值=1717/間隔數=1717/128,取整得到13,
(7)輸出爲0x90+13=0x9D
Code如下
- #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;
- }