/* * PCM - A-Law conversion * Copyright (c) 2000 by Abramo Bagnara * * Wrapper for linphone Codec class by Simon Morlat * * * This program is free software; you can redistribute it and/or modify * it under the terms of the GNU General Public License as published by * the Free Software Foundation; either version 2 of the License, or * (at your option) any later version. * * This program is distributed in the hope that it will be useful, * but WITHOUT ANY WARRANTY; without even the implied warranty of * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the * GNU General Public License for more details. * * You should have received a copy of the GNU General Public License * along with this program; if not, write to the Free Software * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA */ static inline int val_seg(int val) { int r = 0; val >>= 7; /*7 = 4 + 3*/ if (val & 0xf0) { val >>= 4; r += 4; } if (val & 0x0c) { val >>= 2; r += 2; } if (val & 0x02) r += 1; return r; } /* * s16_to_alaw() - Convert a 16-bit linear PCM value to 8-bit A-law * * s16_to_alaw() 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. * G711 is designed for 13 bits input signal, this function add extra shifting to take this into account. */ static inline unsigned char s16_to_alaw(int pcm_val) { int mask; int seg; unsigned char aval; if (pcm_val >= 0) { mask = 0xD5; } else { mask = 0x55; pcm_val = -pcm_val; if (pcm_val > 0x7fff) pcm_val = 0x7fff; } if (pcm_val < 256) /*256 = 32 << 3*/ aval = pcm_val >> 4; /*4 = 1 + 3*/ else { /* Convert the scaled magnitude to segment number. */ seg = val_seg(pcm_val); aval = (seg << 4) | ((pcm_val >> (seg + 3)) & 0x0f); } return aval ^ mask; } /* * alaw_to_s16() - Convert an A-law value to 16-bit linear PCM * */ static inline int alaw_to_s16(unsigned char a_val) { int t; int seg; a_val ^= 0x55; t = a_val & 0x7f; if (t < 16) t = (t << 4) + 8; else { seg = (t >> 4) & 0x07; t = ((t & 0x0f) << 4) + 0x108; t <<= seg -1; } return ((a_val & 0x80) ? t : -t); } /* * s16_to_ulaw() - 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. */ static inline unsigned char s16_to_ulaw(int pcm_val) /* 2's complement (16-bit range) */ { int mask; int seg; unsigned char uval; if (pcm_val < 0) { pcm_val = 0x84 - pcm_val; mask = 0x7f; } else { pcm_val += 0x84; mask = 0xff; } if (pcm_val > 0x7fff) pcm_val = 0x7fff; /* Convert the scaled magnitude to segment number. */ seg = val_seg(pcm_val); /* * Combine the sign, segment, quantization bits; * and complement the code word. */ uval = (seg << 4) | ((pcm_val >> (seg + 3)) & 0x0f); return uval ^ mask; } /* * ulaw_to_s16() - 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 inline int ulaw_to_s16(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 & 0x0f) << 3) + 0x84; t <<= (u_val & 0x70) >> 4; return ((u_val & 0x80) ? (0x84 - t) : (t - 0x84)); }