Sat Aug 6 00:39:33 2011

Asterisk developer's documentation


alaw.c File Reference

u-Law to Signed linear conversion More...

#include "asterisk.h"
#include "asterisk/alaw.h"

Go to the source code of this file.

Defines

#define AMI_MASK   0x55

Functions

static short int alaw2linear (unsigned char alaw)
void ast_alaw_init (void)
static unsigned char linear2alaw (short int linear)

Variables

short __ast_alaw [256]
unsigned char __ast_lin2a [8192]


Detailed Description

u-Law to Signed linear conversion

Author:
Mark Spencer <markster@digium.com>

Definition in file alaw.c.


Define Documentation

#define AMI_MASK   0x55

Definition at line 32 of file alaw.c.

Referenced by alaw2linear(), and linear2alaw().


Function Documentation

static short int alaw2linear ( unsigned char  alaw  )  [inline, static]

Definition at line 68 of file alaw.c.

References AMI_MASK.

Referenced by ast_alaw_init().

00069 {
00070     int i;
00071     int seg;
00072 
00073     alaw ^= AMI_MASK;
00074     i = ((alaw & 0x0F) << 4) + 8 /* rounding error */;
00075     seg = (((int) alaw & 0x70) >> 4);
00076     if (seg)
00077         i = (i + 0x100) << (seg - 1);
00078     return (short int) ((alaw & 0x80)  ?  i  :  -i);
00079 }

void ast_alaw_init ( void   ) 

To init the ulaw to slinear conversion stuff, this needs to be run.

Definition at line 84 of file alaw.c.

References alaw2linear(), and linear2alaw().

Referenced by main().

00085 {
00086    int i;
00087    /* 
00088     *  Set up mu-law conversion table
00089     */
00090    for(i = 0;i < 256;i++)
00091       {
00092            __ast_alaw[i] = alaw2linear(i);
00093       }
00094      /* set up the reverse (mu-law) conversion table */
00095    for(i = -32768; i < 32768; i++)
00096       {
00097       __ast_lin2a[((unsigned short)i) >> 3] = linear2alaw(i);
00098       }
00099 
00100 }

static unsigned char linear2alaw ( short int  linear  )  [inline, static]

Definition at line 34 of file alaw.c.

References AMI_MASK.

Referenced by ast_alaw_init().

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     {
00047         /* Sign (7th) bit = 1 */
00048         mask = AMI_MASK | 0x80;
00049     }
00050     else
00051     {
00052         /* Sign bit = 0 */
00053         mask = AMI_MASK;
00054         pcm_val = -pcm_val;
00055     }
00056 
00057     /* Convert the scaled magnitude to segment number. */
00058     for (seg = 0;  seg < 8;  seg++)
00059     {
00060         if (pcm_val <= seg_end[seg])
00061        break;
00062     }
00063     /* Combine the sign, segment, and quantization bits. */
00064     return  ((seg << 4) | ((pcm_val >> ((seg)  ?  (seg + 3)  :  4)) & 0x0F)) ^ mask;
00065 }


Variable Documentation

short __ast_alaw[256]

help

Definition at line 82 of file alaw.c.

unsigned char __ast_lin2a[8192]

Definition at line 81 of file alaw.c.


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