main/alaw.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/alaw.h"
  31
  32 #define AMI_MASK 0x55
  33
  34 static inline unsigned char linear2alaw (short int linear)
  35 {
  36     int mask;
  37     int seg;
  38     int pcm_val;
  39     static int seg_end[8] =
  40     {
  41          0xFF, 0x1FF, 0x3FF, 0x7FF, 0xFFF, 0x1FFF, 0x3FFF, 0x7FFF
  42     };
  43
  44     pcm_val = linear;
  45     if (pcm_val >= 0)
  46     {
  47         /* Sign (7th) bit = 1 */
  48         mask = AMI_MASK | 0x80;
  49     }
  50     else
  51     {
  52         /* Sign bit = 0 */
  53         mask = AMI_MASK;
  54         pcm_val = -pcm_val;
  55     }
  56
  57     /* Convert the scaled magnitude to segment number. */
  58     for (seg = 0;  seg < 8;  seg++)
  59     {
  60         if (pcm_val <= seg_end[seg])
  61             break;
  62     }
  63     /* Combine the sign, segment, and quantization bits. */
  64     return  ((seg << 4) | ((pcm_val >> ((seg)  ?  (seg + 3)  :  4)) & 0x0F)) ^ mask;
  65 }
  66 /*- End of function --------------------------------------------------------*/
  67
  68 static inline short int alaw2linear (unsigned char alaw)
  69 {
  70     int i;
  71     int seg;
  72
  73     alaw ^= AMI_MASK;
  74     i = ((alaw & 0x0F) << 4) + 8 /* rounding error */;
  75     seg = (((int) alaw & 0x70) >> 4);
  76     if (seg)
  77         i = (i + 0x100) << (seg - 1);
  78     return (short int) ((alaw & 0x80)  ?  i  :  -i);
  79 }
  80
  81 unsigned char __ast_lin2a[8192];
  82 short __ast_alaw[256];
  83
  84 void ast_alaw_init(void)
  85 {
  86         int i;
  87         /*
  88          *  Set up mu-law conversion table
  89          */
  90         for(i = 0;i < 256;i++)
  91            {
  92                 __ast_alaw[i] = alaw2linear(i);
  93            }
  94           /* set up the reverse (mu-law) conversion table */
  95         for(i = -32768; i < 32768; i++)
  96            {
  97                 __ast_lin2a[((unsigned short)i) >> 3] = linear2alaw(i);
  98            }
  99
 100 }
 101