G.711 u律；A律压缩算法

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G.711 u律；A律压缩算法

/********************************************************************** * g711.c * 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. */ static short seg_end[8] = {0xFF, 0x1FF, 0x3FF, 0x7FF, 0xFFF, 0x1FFF, 0x3FFF, 0x7FFF}; /* copy from CCITT G.711 specifications */ 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 int 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 Code Compressed Code * ----------------- ------------------ * 0000000wxyza 000wxyz * 0000001wxyza 001wxyz * 000001wxyzab 010wxyz * 00001wxyzabc 011wxyz * 0001wxyzabcd 100wxyz * 001wxyzabcde 101wxyz * 01wxyzabcdef 110wxyz * 1wxyzabcdefg 111wxyz * * 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; 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); } } /********************************************************************* * 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); } #define BIAS (0x84) /* Bias for linear code. */ /********************************************************************* * 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 Code Compressed Code * ------------------------ --------------- * 00000001wxyza 000wxyz * 0000001wxyzab 001wxyz * 000001wxyzabc 010wxyz * 00001wxyzabcd 011wxyz * 0001wxyzabcde 100wxyz * 001wxyzabcdef 101wxyz * 01wxyzabcdefg 110wxyz * 1wxyzabcdefgh 111wxyz * * 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(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); } } /********************************************************************* * 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. *********************************************************************/ 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)); } /* A-law to u-law conversion */ unsigned char alaw2ulaw(unsigned char aval) { aval &= 0xff; return ((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 ((uval & 0x80) ? (0xD5 ^ (_u2a[0xFF ^ uval] - 1)): (0x55 ^ (_u2a[0x7F ^ uval] - 1))); }