00001 /* 00002 * SpanDSP - a series of DSP components for telephony 00003 * 00004 * g711.h - In line A-law and u-law conversion routines 00005 * 00006 * Written by Steve Underwood <steveu@coppice.org> 00007 * 00008 * Copyright (C) 2001 Steve Underwood 00009 * 00010 * All rights reserved. 00011 * 00012 * This program is free software; you can redistribute it and/or modify 00013 * it under the terms of the GNU Lesser General Public License version 2.1, 00014 * as published by the Free Software Foundation. 00015 * 00016 * This program is distributed in the hope that it will be useful, 00017 * but WITHOUT ANY WARRANTY; without even the implied warranty of 00018 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 00019 * GNU Lesser General Public License for more details. 00020 * 00021 * You should have received a copy of the GNU Lesser General Public 00022 * License along with this program; if not, write to the Free Software 00023 * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA. 00024 * 00025 * $Id: g711.h,v 1.15 2008/11/30 10:17:31 steveu Exp $ 00026 */ 00027 00028 /*! \file */ 00029 00030 /*! \page g711_page A-law and mu-law handling 00031 Lookup tables for A-law and u-law look attractive, until you consider the impact 00032 on the CPU cache. If it causes a substantial area of your processor cache to get 00033 hit too often, cache sloshing will severely slow things down. The main reason 00034 these routines are slow in C, is the lack of direct access to the CPU's "find 00035 the first 1" instruction. A little in-line assembler fixes that, and the 00036 conversion routines can be faster than lookup tables, in most real world usage. 00037 A "find the first 1" instruction is available on most modern CPUs, and is a 00038 much underused feature. 00039 00040 If an assembly language method of bit searching is not available, these routines 00041 revert to a method that can be a little slow, so the cache thrashing might not 00042 seem so bad :( 00043 00044 Feel free to submit patches to add fast "find the first 1" support for your own 00045 favourite processor. 00046 00047 Look up tables are used for transcoding between A-law and u-law, since it is 00048 difficult to achieve the precise transcoding procedure laid down in the G.711 00049 specification by other means. 00050 */ 00051 00052 #if !defined(_SPANDSP_G711_H_) 00053 #define _SPANDSP_G711_H_ 00054 00055 /* The usual values to use on idle channels, to emulate silence */ 00056 #define G711_ALAW_IDLE_OCTET 0x5D 00057 #define G711_ULAW_IDLE_OCTET 0xFF 00058 00059 enum 00060 { 00061 G711_ALAW = 0, 00062 G711_ULAW 00063 }; 00064 00065 typedef struct g711_state_s g711_state_t; 00066 00067 #if defined(__cplusplus) 00068 extern "C" 00069 { 00070 #endif 00071 00072 /* N.B. It is tempting to use look-up tables for A-law and u-law conversion. 00073 * However, you should consider the cache footprint. 00074 * 00075 * A 64K byte table for linear to x-law and a 512 byte table for x-law to 00076 * linear sound like peanuts these days, and shouldn't an array lookup be 00077 * real fast? No! When the cache sloshes as badly as this one will, a tight 00078 * calculation may be better. The messiest part is normally finding the 00079 * segment, but a little inline assembly can fix that on an i386, x86_64 and 00080 * many other modern processors. 00081 */ 00082 00083 /* 00084 * Mu-law is basically as follows: 00085 * 00086 * Biased Linear Input Code Compressed Code 00087 * ------------------------ --------------- 00088 * 00000001wxyza 000wxyz 00089 * 0000001wxyzab 001wxyz 00090 * 000001wxyzabc 010wxyz 00091 * 00001wxyzabcd 011wxyz 00092 * 0001wxyzabcde 100wxyz 00093 * 001wxyzabcdef 101wxyz 00094 * 01wxyzabcdefg 110wxyz 00095 * 1wxyzabcdefgh 111wxyz 00096 * 00097 * Each biased linear code has a leading 1 which identifies the segment 00098 * number. The value of the segment number is equal to 7 minus the number 00099 * of leading 0's. The quantization interval is directly available as the 00100 * four bits wxyz. * The trailing bits (a - h) are ignored. 00101 * 00102 * Ordinarily the complement of the resulting code word is used for 00103 * transmission, and so the code word is complemented before it is returned. 00104 * 00105 * For further information see John C. Bellamy's Digital Telephony, 1982, 00106 * John Wiley & Sons, pps 98-111 and 472-476. 00107 */ 00108 00109 //#define ULAW_ZEROTRAP /* turn on the trap as per the MIL-STD */ 00110 #define ULAW_BIAS 0x84 /* Bias for linear code. */ 00111 00112 /*! \brief Encode a linear sample to u-law 00113 \param linear The sample to encode. 00114 \return The u-law value. 00115 */ 00116 static __inline__ uint8_t linear_to_ulaw(int linear) 00117 { 00118 uint8_t u_val; 00119 int mask; 00120 int seg; 00121 00122 /* Get the sign and the magnitude of the value. */ 00123 if (linear >= 0) 00124 { 00125 linear = ULAW_BIAS + linear; 00126 mask = 0xFF; 00127 } 00128 else 00129 { 00130 linear = ULAW_BIAS - linear; 00131 mask = 0x7F; 00132 } 00133 00134 seg = top_bit(linear | 0xFF) - 7; 00135 00136 /* 00137 * Combine the sign, segment, quantization bits, 00138 * and complement the code word. 00139 */ 00140 if (seg >= 8) 00141 u_val = (uint8_t) (0x7F ^ mask); 00142 else 00143 u_val = (uint8_t) (((seg << 4) | ((linear >> (seg + 3)) & 0xF)) ^ mask); 00144 #ifdef ULAW_ZEROTRAP 00145 /* Optional ITU trap */ 00146 if (u_val == 0) 00147 u_val = 0x02; 00148 #endif 00149 return u_val; 00150 } 00151 /*- End of function --------------------------------------------------------*/ 00152 00153 /*! \brief Decode an u-law sample to a linear value. 00154 \param ulaw The u-law sample to decode. 00155 \return The linear value. 00156 */ 00157 static __inline__ int16_t ulaw_to_linear(uint8_t ulaw) 00158 { 00159 int t; 00160 00161 /* Complement to obtain normal u-law value. */ 00162 ulaw = ~ulaw; 00163 /* 00164 * Extract and bias the quantization bits. Then 00165 * shift up by the segment number and subtract out the bias. 00166 */ 00167 t = (((ulaw & 0x0F) << 3) + ULAW_BIAS) << (((int) ulaw & 0x70) >> 4); 00168 return (int16_t) ((ulaw & 0x80) ? (ULAW_BIAS - t) : (t - ULAW_BIAS)); 00169 } 00170 /*- End of function --------------------------------------------------------*/ 00171 00172 /* 00173 * A-law is basically as follows: 00174 * 00175 * Linear Input Code Compressed Code 00176 * ----------------- --------------- 00177 * 0000000wxyza 000wxyz 00178 * 0000001wxyza 001wxyz 00179 * 000001wxyzab 010wxyz 00180 * 00001wxyzabc 011wxyz 00181 * 0001wxyzabcd 100wxyz 00182 * 001wxyzabcde 101wxyz 00183 * 01wxyzabcdef 110wxyz 00184 * 1wxyzabcdefg 111wxyz 00185 * 00186 * For further information see John C. Bellamy's Digital Telephony, 1982, 00187 * John Wiley & Sons, pps 98-111 and 472-476. 00188 */ 00189 00190 #define ALAW_AMI_MASK 0x55 00191 00192 /*! \brief Encode a linear sample to A-law 00193 \param linear The sample to encode. 00194 \return The A-law value. 00195 */ 00196 static __inline__ uint8_t linear_to_alaw(int linear) 00197 { 00198 int mask; 00199 int seg; 00200 00201 if (linear >= 0) 00202 { 00203 /* Sign (bit 7) bit = 1 */ 00204 mask = ALAW_AMI_MASK | 0x80; 00205 } 00206 else 00207 { 00208 /* Sign (bit 7) bit = 0 */ 00209 mask = ALAW_AMI_MASK; 00210 linear = -linear - 1; 00211 } 00212 00213 /* Convert the scaled magnitude to segment number. */ 00214 seg = top_bit(linear | 0xFF) - 7; 00215 if (seg >= 8) 00216 { 00217 if (linear >= 0) 00218 { 00219 /* Out of range. Return maximum value. */ 00220 return (uint8_t) (0x7F ^ mask); 00221 } 00222 /* We must be just a tiny step below zero */ 00223 return (uint8_t) (0x00 ^ mask); 00224 } 00225 /* Combine the sign, segment, and quantization bits. */ 00226 return (uint8_t) (((seg << 4) | ((linear >> ((seg) ? (seg + 3) : 4)) & 0x0F)) ^ mask); 00227 } 00228 /*- End of function --------------------------------------------------------*/ 00229 00230 /*! \brief Decode an A-law sample to a linear value. 00231 \param alaw The A-law sample to decode. 00232 \return The linear value. 00233 */ 00234 static __inline__ int16_t alaw_to_linear(uint8_t alaw) 00235 { 00236 int i; 00237 int seg; 00238 00239 alaw ^= ALAW_AMI_MASK; 00240 i = ((alaw & 0x0F) << 4); 00241 seg = (((int) alaw & 0x70) >> 4); 00242 if (seg) 00243 i = (i + 0x108) << (seg - 1); 00244 else 00245 i += 8; 00246 return (int16_t) ((alaw & 0x80) ? i : -i); 00247 } 00248 /*- End of function --------------------------------------------------------*/ 00249 00250 /*! \brief Transcode from A-law to u-law, using the procedure defined in G.711. 00251 \param alaw The A-law sample to transcode. 00252 \return The best matching u-law value. 00253 */ 00254 uint8_t alaw_to_ulaw(uint8_t alaw); 00255 00256 /*! \brief Transcode from u-law to A-law, using the procedure defined in G.711. 00257 \param ulaw The u-law sample to transcode. 00258 \return The best matching A-law value. 00259 */ 00260 uint8_t ulaw_to_alaw(uint8_t ulaw); 00261 00262 int g711_decode(g711_state_t *s, 00263 int16_t amp[], 00264 const uint8_t g711_data[], 00265 int g711_bytes); 00266 00267 int g711_encode(g711_state_t *s, 00268 uint8_t g711_data[], 00269 const int16_t amp[], 00270 int len); 00271 00272 int g711_transcode(g711_state_t *s, 00273 uint8_t g711_out[], 00274 const uint8_t g711_in[], 00275 int g711_bytes); 00276 00277 /*! Initialise a G.711 encode or decode context. 00278 \param s The G.711 context. 00279 \param mode The G.711 mode. 00280 \return A pointer to the G.711 context, or NULL for error. */ 00281 g711_state_t *g711_init(g711_state_t *s, int mode); 00282 00283 /*! Free a G.711 encode or decode context. 00284 \param s The G.711 context. 00285 \return 0 for OK. */ 00286 int g711_release(g711_state_t *s); 00287 00288 #if defined(__cplusplus) 00289 } 00290 #endif 00291 00292 #endif 00293 /*- End of file ------------------------------------------------------------*/