alaw.c

00001 /*
00002  * Asterisk -- An open source telephony toolkit.
00003  *
00004  * Copyright (C) 1999 - 2005, Digium, Inc.
00005  *
00006  * Mark Spencer <markster@digium.com>
00007  *
00008  * See http://www.asterisk.org for more information about
00009  * the Asterisk project. Please do not directly contact
00010  * any of the maintainers of this project for assistance;
00011  * the project provides a web site, mailing lists and IRC
00012  * channels for your use.
00013  *
00014  * This program is free software, distributed under the terms of
00015  * the GNU General Public License Version 2. See the LICENSE file
00016  * at the top of the source tree.
00017  */
00018 
00019 /*! \file
00020  *
00021  * \brief u-Law to Signed linear conversion
00022  *
00023  * \author Mark Spencer <markster@digium.com> 
00024  */
00025 
00026 #include "asterisk.h"
00027 
00028 ASTERISK_FILE_VERSION(__FILE__, "$Revision: 51499 $")
00029 
00030 #include "asterisk/alaw.h"
00031 
00032 #define AMI_MASK 0x55
00033 
00034 static inline unsigned char linear2alaw (short int linear)
00035 {
00036     int mask;
00037     int seg;
00038     int pcm_val;
00039     static int seg_end[8] =
00040     {
00041          0xFF, 0x1FF, 0x3FF, 0x7FF, 0xFFF, 0x1FFF, 0x3FFF, 0x7FFF
00042     };
00043     
00044     pcm_val = linear;
00045     if (pcm_val >= 0) {
00046        /* Sign (7th) bit = 1 */
00047        mask = AMI_MASK | 0x80;
00048     } else {
00049        /* Sign bit = 0 */
00050        mask = AMI_MASK;
00051        pcm_val = -pcm_val;
00052     }
00053 
00054     /* Convert the scaled magnitude to segment number. */
00055     for (seg = 0;  seg < 8;  seg++) {
00056        if (pcm_val <= seg_end[seg])
00057           break;
00058     }
00059     /* Combine the sign, segment, and quantization bits. */
00060     return  ((seg << 4) | ((pcm_val >> ((seg)  ?  (seg + 3)  :  4)) & 0x0F)) ^ mask;
00061 }
00062 /*- End of function --------------------------------------------------------*/
00063 
00064 static inline short int alaw2linear (unsigned char alaw)
00065 {
00066     int i;
00067     int seg;
00068 
00069     alaw ^= AMI_MASK;
00070     i = ((alaw & 0x0F) << 4);
00071     seg = (((int) alaw & 0x70) >> 4);
00072     if (seg)
00073        i = (i + 0x100) << (seg - 1);
00074     return (short int) ((alaw & 0x80)  ?  i  :  -i);
00075 }
00076 
00077 unsigned char __ast_lin2a[8192];
00078 short __ast_alaw[256];
00079 
00080 void ast_alaw_init(void)
00081 {
00082    int i;
00083    /* 
00084     *  Set up mu-law conversion table
00085     */
00086    for (i = 0; i < 256; i++) {
00087            __ast_alaw[i] = alaw2linear(i);
00088    }
00089    /* set up the reverse (mu-law) conversion table */
00090    for (i = -32768; i < 32768; i++) {
00091       __ast_lin2a[((unsigned short)i) >> 3] = linear2alaw(i);
00092    }
00093 
00094 }
00095

Asterisk developer's documentation :: Codename Pineapple

alaw.c