main/ulaw.c

   1 /*
   2  * Asterisk -- An open source telephony toolkit.
   3  *
   4  * Copyright (C) 1999 - 2005, Digium, Inc.
   5  *
   6  * Mark Spencer <markster@digium.com>
   7  *
   8  * See http://www.asterisk.org for more information about
   9  * the Asterisk project. Please do not directly contact
  10  * any of the maintainers of this project for assistance;
  11  * the project provides a web site, mailing lists and IRC
  12  * channels for your use.
  13  *
  14  * This program is free software, distributed under the terms of
  15  * the GNU General Public License Version 2. See the LICENSE file
  16  * at the top of the source tree.
  17  */
  18
  19 /*! \file
  20  *
  21  * \brief u-Law to Signed linear conversion
  22  *
  23  * \author Mark Spencer <markster@digium.com>
  24  */
  25
  26 #include "asterisk.h"
  27
  28 ASTERISK_FILE_VERSION(__FILE__, "$Revision$")
  29
  30 #include "asterisk/ulaw.h"
  31
  32 #define ZEROTRAP    /*!< turn on the trap as per the MIL-STD */
  33 #define BIAS 0x84   /*!< define the add-in bias for 16 bit samples */
  34 #define CLIP 32635
  35
  36 unsigned char __ast_lin2mu[16384];
  37 short __ast_mulaw[256];
  38
  39
  40 static unsigned char linear2ulaw(short sample)
  41 {
  42         static int exp_lut[256] = {
  43                 0,0,1,1,2,2,2,2,3,3,3,3,3,3,3,3,
  44                 4,4,4,4,4,4,4,4,4,4,4,4,4,4,4,4,
  45                 5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,
  46                 5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,
  47                 6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,
  48                 6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,
  49                 6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,
  50                 6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,
  51                 7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,
  52                 7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,
  53                 7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,
  54                 7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,
  55                 7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,
  56                 7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,
  57                 7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,
  58                 7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7 };
  59         int sign, exponent, mantissa;
  60         unsigned char ulawbyte;
  61
  62         /* Get the sample into sign-magnitude. */
  63         sign = (sample >> 8) & 0x80;          /* set aside the sign */
  64         if (sign != 0)
  65                 sample = -sample;              /* get magnitude */
  66         if (sample > CLIP)
  67                 sample = CLIP;             /* clip the magnitude */
  68
  69         /* Convert from 16 bit linear to ulaw. */
  70         sample = sample + BIAS;
  71         exponent = exp_lut[(sample >> 7) & 0xFF];
  72         mantissa = (sample >> (exponent + 3)) & 0x0F;
  73         ulawbyte = ~(sign | (exponent << 4) | mantissa);
  74 #ifdef ZEROTRAP
  75         if (ulawbyte == 0)
  76                 ulawbyte = 0x02;   /* optional CCITT trap */
  77 #endif
  78
  79         return ulawbyte;
  80 }
  81
  82 /*!
  83  * \brief  Set up mu-law conversion table
  84  */
  85 void ast_ulaw_init(void)
  86 {
  87         int i;
  88         for (i = 0; i < 256; i++) {
  89                 short mu, e, f, y;
  90                 static short etab[] = {0,132,396,924,1980,4092,8316,16764};
  91
  92                 mu = 255 - i;
  93                 e = (mu & 0x70) / 16;
  94                 f = mu & 0x0f;
  95                 y = f * (1 << (e + 3));
  96                 y += etab[e];
  97                 if (mu & 0x80)
  98                         y = -y;
  99                 __ast_mulaw[i] = y;
 100         }
 101         /* set up the reverse (mu-law) conversion table */
 102         for (i = -32768; i < 32768; i++) {
 103                 __ast_lin2mu[((unsigned short)i) >> 2] = linear2ulaw(i);
 104         }
 105 }
 106