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alaw.c

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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  */
00024 
00025 #include "asterisk.h"
00026 
00027 ASTERISK_FILE_VERSION(__FILE__, "$Revision: 7221 $")
00028 
00029 #include "asterisk/alaw.h"
00030 
00031 #define AMI_MASK 0x55
00032 
00033 static inline unsigned char linear2alaw (short int linear)
00034 {
00035     int mask;
00036     int seg;
00037     int pcm_val;
00038     static int seg_end[8] =
00039     {
00040          0xFF, 0x1FF, 0x3FF, 0x7FF, 0xFFF, 0x1FFF, 0x3FFF, 0x7FFF
00041     };
00042     
00043     pcm_val = linear;
00044     if (pcm_val >= 0)
00045     {
00046         /* Sign (7th) bit = 1 */
00047         mask = AMI_MASK | 0x80;
00048     }
00049     else
00050     {
00051         /* Sign bit = 0 */
00052         mask = AMI_MASK;
00053         pcm_val = -pcm_val;
00054     }
00055 
00056     /* Convert the scaled magnitude to segment number. */
00057     for (seg = 0;  seg < 8;  seg++)
00058     {
00059         if (pcm_val <= seg_end[seg])
00060        break;
00061     }
00062     /* Combine the sign, segment, and quantization bits. */
00063     return  ((seg << 4) | ((pcm_val >> ((seg)  ?  (seg + 3)  :  4)) & 0x0F)) ^ mask;
00064 }
00065 /*- End of function --------------------------------------------------------*/
00066 
00067 static inline short int alaw2linear (unsigned char alaw)
00068 {
00069     int i;
00070     int seg;
00071 
00072     alaw ^= AMI_MASK;
00073     i = ((alaw & 0x0F) << 4);
00074     seg = (((int) alaw & 0x70) >> 4);
00075     if (seg)
00076         i = (i + 0x100) << (seg - 1);
00077     return (short int) ((alaw & 0x80)  ?  i  :  -i);
00078 }
00079 
00080 unsigned char __ast_lin2a[8192];
00081 short __ast_alaw[256];
00082 
00083 void ast_alaw_init(void)
00084 {
00085    int i;
00086    /* 
00087     *  Set up mu-law conversion table
00088     */
00089    for(i = 0;i < 256;i++)
00090       {
00091            __ast_alaw[i] = alaw2linear(i);
00092       }
00093      /* set up the reverse (mu-law) conversion table */
00094    for(i = -32768; i < 32768; i++)
00095       {
00096       __ast_lin2a[((unsigned short)i) >> 3] = linear2alaw(i);
00097       }
00098 
00099 }
00100 

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