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5
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1 /*
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2 * SpanDSP - a series of DSP components for telephony
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3 *
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4 * g711.h - In line A-law and u-law conversion routines
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5 *
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6 * Written by Steve Underwood <steveu@coppice.org>
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7 *
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8 * Copyright (C) 2001 Steve Underwood
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9 *
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10 * All rights reserved.
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11 *
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12 * This program is free software; you can redistribute it and/or modify
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13 * it under the terms of the GNU General Public License version 2, as
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14 * published by the Free Software Foundation.
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15 *
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16 * This program is distributed in the hope that it will be useful,
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17 * but WITHOUT ANY WARRANTY; without even the implied warranty of
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18 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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19 * GNU General Public License for more details.
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20 *
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21 * You should have received a copy of the GNU General Public License
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22 * along with this program; if not, write to the Free Software
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23 * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
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24 *
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25 * $Id: g711.h,v 1.3 2006/10/24 13:45:28 steveu Exp $
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26 */
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27
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28 /*! \file */
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29
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30 /*! \page g711_page A-law and mu-law handling
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31 Lookup tables for A-law and u-law look attractive, until you consider the impact
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32 on the CPU cache. If it causes a substantial area of your processor cache to get
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33 hit too often, cache sloshing will severely slow things down. The main reason
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34 these routines are slow in C, is the lack of direct access to the CPU's "find
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35 the first 1" instruction. A little in-line assembler fixes that, and the
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36 conversion routines can be faster than lookup tables, in most real world usage.
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37 A "find the first 1" instruction is available on most modern CPUs, and is a
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38 much underused feature.
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39
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40 If an assembly language method of bit searching is not available, these routines
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41 revert to a method that can be a little slow, so the cache thrashing might not
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42 seem so bad :(
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43
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44 Feel free to submit patches to add fast "find the first 1" support for your own
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45 favourite processor.
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46
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47 Look up tables are used for transcoding between A-law and u-law, since it is
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48 difficult to achieve the precise transcoding procedure laid down in the G.711
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49 specification by other means.
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50 */
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51
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52 #if !defined(_G711_H_)
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53 #define _G711_H_
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54
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55 #ifdef __cplusplus
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56 extern "C" {
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57 #endif
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58
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59 /* N.B. It is tempting to use look-up tables for A-law and u-law conversion.
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60 * However, you should consider the cache footprint.
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61 *
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62 * A 64K byte table for linear to x-law and a 512 byte table for x-law to
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63 * linear sound like peanuts these days, and shouldn't an array lookup be
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64 * real fast? No! When the cache sloshes as badly as this one will, a tight
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65 * calculation may be better. The messiest part is normally finding the
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66 * segment, but a little inline assembly can fix that on an i386, x86_64 and
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67 * many other modern processors.
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68 */
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69
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70 /*
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71 * Mu-law is basically as follows:
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72 *
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73 * Biased Linear Input Code Compressed Code
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74 * ------------------------ ---------------
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75 * 00000001wxyza 000wxyz
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76 * 0000001wxyzab 001wxyz
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77 * 000001wxyzabc 010wxyz
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78 * 00001wxyzabcd 011wxyz
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79 * 0001wxyzabcde 100wxyz
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80 * 001wxyzabcdef 101wxyz
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81 * 01wxyzabcdefg 110wxyz
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82 * 1wxyzabcdefgh 111wxyz
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83 *
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84 * Each biased linear code has a leading 1 which identifies the segment
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85 * number. The value of the segment number is equal to 7 minus the number
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86 * of leading 0's. The quantization interval is directly available as the
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87 * four bits wxyz. * The trailing bits (a - h) are ignored.
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88 *
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89 * Ordinarily the complement of the resulting code word is used for
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90 * transmission, and so the code word is complemented before it is returned.
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91 *
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92 * For further information see John C. Bellamy's Digital Telephony, 1982,
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93 * John Wiley & Sons, pps 98-111 and 472-476.
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94 */
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95
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96 //#define ULAW_ZEROTRAP /* turn on the trap as per the MIL-STD */
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97 #define ULAW_BIAS 0x84 /* Bias for linear code. */
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98
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99 /*! \brief Encode a linear sample to u-law
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100 \param linear The sample to encode.
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101 \return The u-law value.
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102 */
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103 static __inline__ uint8_t linear_to_ulaw(int linear)
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104 {
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105 uint8_t u_val;
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106 int mask;
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107 int seg;
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108
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109 /* Get the sign and the magnitude of the value. */
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110 if (linear < 0)
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111 {
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112 linear = ULAW_BIAS - linear;
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113 mask = 0x7F;
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114 }
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115 else
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116 {
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117 linear = ULAW_BIAS + linear;
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118 mask = 0xFF;
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119 }
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120
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121 seg = top_bit(linear | 0xFF) - 7;
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122
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123 /*
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124 * Combine the sign, segment, quantization bits,
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125 * and complement the code word.
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126 */
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127 if (seg >= 8)
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128 u_val = (uint8_t) (0x7F ^ mask);
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129 else
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130 u_val = (uint8_t) (((seg << 4) | ((linear >> (seg + 3)) & 0xF)) ^ mask);
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131 #ifdef ULAW_ZEROTRAP
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132 /* Optional ITU trap */
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133 if (u_val == 0)
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134 u_val = 0x02;
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135 #endif
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136 return u_val;
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137 }
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138 /*- End of function --------------------------------------------------------*/
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139
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140 /*! \brief Decode an u-law sample to a linear value.
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141 \param ulaw The u-law sample to decode.
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142 \return The linear value.
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143 */
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144 static __inline__ int16_t ulaw_to_linear(uint8_t ulaw)
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145 {
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146 int t;
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147
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148 /* Complement to obtain normal u-law value. */
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149 ulaw = ~ulaw;
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150 /*
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151 * Extract and bias the quantization bits. Then
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152 * shift up by the segment number and subtract out the bias.
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153 */
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154 t = (((ulaw & 0x0F) << 3) + ULAW_BIAS) << (((int) ulaw & 0x70) >> 4);
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155 return (int16_t) ((ulaw & 0x80) ? (ULAW_BIAS - t) : (t - ULAW_BIAS));
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156 }
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157 /*- End of function --------------------------------------------------------*/
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158
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159 /*
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160 * A-law is basically as follows:
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161 *
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162 * Linear Input Code Compressed Code
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163 * ----------------- ---------------
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164 * 0000000wxyza 000wxyz
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165 * 0000001wxyza 001wxyz
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166 * 000001wxyzab 010wxyz
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167 * 00001wxyzabc 011wxyz
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168 * 0001wxyzabcd 100wxyz
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169 * 001wxyzabcde 101wxyz
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170 * 01wxyzabcdef 110wxyz
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171 * 1wxyzabcdefg 111wxyz
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172 *
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173 * For further information see John C. Bellamy's Digital Telephony, 1982,
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174 * John Wiley & Sons, pps 98-111 and 472-476.
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175 */
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176
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177 #define ALAW_AMI_MASK 0x55
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178
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179 /*! \brief Encode a linear sample to A-law
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180 \param linear The sample to encode.
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181 \return The A-law value.
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182 */
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183 static __inline__ uint8_t linear_to_alaw(int linear)
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184 {
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185 int mask;
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186 int seg;
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187
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188 if (linear >= 0)
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189 {
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190 /* Sign (bit 7) bit = 1 */
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191 mask = ALAW_AMI_MASK | 0x80;
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192 }
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193 else
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194 {
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195 /* Sign (bit 7) bit = 0 */
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196 mask = ALAW_AMI_MASK;
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197 linear = -linear - 8;
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198 }
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199
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200 /* Convert the scaled magnitude to segment number. */
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201 seg = top_bit(linear | 0xFF) - 7;
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202 if (seg >= 8)
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203 {
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204 if (linear >= 0)
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205 {
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206 /* Out of range. Return maximum value. */
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207 return (uint8_t) (0x7F ^ mask);
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208 }
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209 /* We must be just a tiny step below zero */
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210 return (uint8_t) (0x00 ^ mask);
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211 }
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212 /* Combine the sign, segment, and quantization bits. */
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213 return (uint8_t) (((seg << 4) | ((linear >> ((seg) ? (seg + 3) : 4)) & 0x0F)) ^ mask);
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214 }
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215 /*- End of function --------------------------------------------------------*/
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216
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217 /*! \brief Decode an A-law sample to a linear value.
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218 \param alaw The A-law sample to decode.
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219 \return The linear value.
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220 */
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221 static __inline__ int16_t alaw_to_linear(uint8_t alaw)
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222 {
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223 int i;
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224 int seg;
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225
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226 alaw ^= ALAW_AMI_MASK;
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227 i = ((alaw & 0x0F) << 4);
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228 seg = (((int) alaw & 0x70) >> 4);
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229 if (seg)
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230 i = (i + 0x108) << (seg - 1);
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231 else
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232 i += 8;
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233 return (int16_t) ((alaw & 0x80) ? i : -i);
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234 }
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235 /*- End of function --------------------------------------------------------*/
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236
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237 /*! \brief Transcode from A-law to u-law, using the procedure defined in G.711.
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238 \param alaw The A-law sample to transcode.
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239 \return The best matching u-law value.
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240 */
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241 uint8_t alaw_to_ulaw(uint8_t alaw);
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242
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243 /*! \brief Transcode from u-law to A-law, using the procedure defined in G.711.
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244 \param alaw The u-law sample to transcode.
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245 \return The best matching A-law value.
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246 */
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247 uint8_t ulaw_to_alaw(uint8_t ulaw);
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248
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249 #ifdef __cplusplus
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250 }
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251 #endif
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252
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253 #endif
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254 /*- End of file ------------------------------------------------------------*/
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