G711編碼原理及程式碼
阿新 • • 發佈：2018-12-10
G711編碼的聲音清晰度好，語音自然度高，但壓縮效率低，資料量大常在32Kbps以上。常用於電話語音(推薦使用64Kbps)，sampling rate為8K，壓縮率為2，即把S16格式的資料壓縮為8bit，分為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，便於數字裝置進行快速運算。
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） 組合為seeewxyz，將seeewxyz逢偶數為取補數，編碼完畢
示例：
輸入pcm資料為3210，二進位制對應為（0000 1100 1000 1010）
二進位制變換下排列組合方式（0 0001 1001 0001010）
（1） 獲取符號位最高位為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） 組合為11001001，逢偶數為取反為10011100
編碼完畢。
u-law也叫g711u,使用在北美和日本，輸入的是14位，編碼演算法就是查表，沒啥複雜演算法，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）輸出為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; }