Sat Mar 10 01:54:29 2012

Asterisk developer's documentation


alaw.c File Reference

a-Law to Signed linear conversion More...

#include "asterisk.h"
#include "asterisk/alaw.h"
#include "asterisk/logger.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)
 To init the alaw to slinear conversion stuff, this needs to be run.
static unsigned char linear2alaw (short int linear)

Variables

short __ast_alaw [256]
unsigned char __ast_lin2a [8192]
 converts signed linear to alaw


Detailed Description

a-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 34 of file alaw.c.

Referenced by alaw2linear(), and linear2alaw().


Function Documentation

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

Definition at line 110 of file alaw.c.

References AMI_MASK.

Referenced by ast_alaw_init().

00111 {
00112    int i;
00113    int seg;
00114 
00115    alaw ^= AMI_MASK;
00116    i = ((alaw & 0x0F) << 4) + 8 /* rounding error */;
00117    seg = (((int) alaw & 0x70) >> 4);
00118    if (seg) {
00119       i = (i + 0x100) << (seg - 1);
00120    }
00121    return (short int) ((alaw & 0x80) ? i : -i);
00122 }

void ast_alaw_init ( void   ) 

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

Definition at line 150 of file alaw.c.

References alaw2linear(), AST_ALAW, AST_ALAW_STEP, AST_LIN2A, ast_log(), linear2alaw(), LOG_NOTICE, and LOG_WARNING.

Referenced by main().

00151 {
00152    int i;
00153    /*
00154     *  Set up mu-law conversion table
00155     */
00156 #ifndef G711_NEW_ALGORITHM
00157    for (i = 0; i < 256; i++) {
00158       __ast_alaw[i] = alaw2linear(i);
00159    }
00160    /* set up the reverse (mu-law) conversion table */
00161    for (i = -32768; i < 32768; i++) {
00162       __ast_lin2a[((unsigned short)i) >> 3] = linear2alaw(i);
00163    }
00164 #else
00165    for (i = 0; i < 256; i++) {
00166       __ast_alaw[i] = alaw2linear(i);
00167    }
00168    /* set up the reverse (a-law) conversion table */
00169    for (i = 0; i <= 32768; i += AST_ALAW_STEP) {
00170       AST_LIN2A_LOOKUP(i) = linear2alaw(i, 0 /* half-cooked */);
00171    }
00172 #endif
00173 
00174 #ifdef TEST_CODING_TABLES
00175    for (i = -32768; i < 32768; ++i) {
00176 #ifndef G711_NEW_ALGORITHM
00177       unsigned char e1 = linear2alaw(i);
00178 #else
00179       unsigned char e1 = linear2alaw(i, 1);
00180 #endif
00181       short d1 = alaw2linear(e1);
00182       unsigned char e2 = AST_LIN2A(i);
00183       short d2 = alaw2linear(e2);
00184       short d3 = AST_ALAW(e1);
00185 
00186       if (e1 != e2 || d1 != d3 || d2 != d3) {
00187          ast_log(LOG_WARNING, "a-Law coding tables test failed on %d: e1=%u, e2=%u, d1=%d, d2=%d\n",
00188                i, (unsigned)e1, (unsigned)e2, (int)d1, (int)d2);
00189       }
00190    }
00191    ast_log(LOG_NOTICE, "a-Law coding tables test complete.\n");
00192 #endif /* TEST_CODING_TABLES */
00193 
00194 #ifdef TEST_TANDEM_TRANSCODING
00195    /* tandem transcoding test */
00196    for (i = -32768; i < 32768; ++i) {
00197       unsigned char e1 = AST_LIN2A(i);
00198       short d1 = AST_ALAW(e1);
00199       unsigned char e2 = AST_LIN2A(d1);
00200       short d2 = AST_ALAW(e2);
00201       unsigned char e3 = AST_LIN2A(d2);
00202       short d3 = AST_ALAW(e3);
00203 
00204       if (e1 != e2 || e2 != e3 || d1 != d2 || d2 != d3) {
00205          ast_log(LOG_WARNING, "a-Law tandem transcoding test failed on %d: e1=%u, e2=%u, d1=%d, d2=%d, d3=%d\n",
00206                i, (unsigned)e1, (unsigned)e2, (int)d1, (int)d2, (int)d3);
00207       }
00208    }
00209    ast_log(LOG_NOTICE, "a-Law tandem transcoding test complete.\n");
00210 #endif /* TEST_TANDEM_TRANSCODING */
00211 
00212 }

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

Definition at line 36 of file alaw.c.

References AMI_MASK.

Referenced by ast_alaw_init().

00037 {
00038    int mask;
00039    int seg;
00040    int pcm_val;
00041    static int seg_end[8] =
00042       {
00043          0xFF, 0x1FF, 0x3FF, 0x7FF, 0xFFF, 0x1FFF, 0x3FFF, 0x7FFF
00044       };
00045 
00046    pcm_val = linear;
00047    if (pcm_val >= 0) {
00048       /* Sign (7th) bit = 1 */
00049       mask = AMI_MASK | 0x80;
00050    } else {
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       if (pcm_val <= seg_end[seg]) {
00059          break;
00060       }
00061    }
00062    /* Combine the sign, segment, and quantization bits. */
00063    return ((seg << 4) | ((pcm_val >> ((seg) ? (seg + 3) : 4)) & 0x0F)) ^ mask;
00064 }


Variable Documentation

short __ast_alaw[256]

help

Definition at line 148 of file alaw.c.

unsigned char __ast_lin2a[8192]

converts signed linear to alaw

Definition at line 144 of file alaw.c.


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