Thu Jul 9 13:40:43 2009

Asterisk developer's documentation


alaw.c File Reference

a-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)
 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 33 of file alaw.c.

Referenced by alaw2linear(), and linear2alaw().


Function Documentation

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

Definition at line 109 of file alaw.c.

References AMI_MASK.

Referenced by ast_alaw_init().

00110 {
00111    int i;
00112    int seg;
00113 
00114    alaw ^= AMI_MASK;
00115    i = ((alaw & 0x0F) << 4) + 8 /* rounding error */;
00116    seg = (((int) alaw & 0x70) >> 4);
00117    if (seg) {
00118       i = (i + 0x100) << (seg - 1);
00119    }
00120    return (short int) ((alaw & 0x80) ? i : -i);
00121 }

void ast_alaw_init ( void   ) 

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

Definition at line 149 of file alaw.c.

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

Referenced by main().

00150 {
00151    int i;
00152    /* 
00153     *  Set up mu-law conversion table
00154     */ 
00155 #ifndef G711_NEW_ALGORITHM
00156    for (i = 0; i < 256; i++) {
00157       __ast_alaw[i] = alaw2linear(i);
00158    }
00159    /* set up the reverse (mu-law) conversion table */
00160    for (i = -32768; i < 32768; i++) {
00161       __ast_lin2a[((unsigned short)i) >> 3] = linear2alaw(i);
00162    }
00163 #else
00164    for (i = 0; i < 256; i++) {
00165       __ast_alaw[i] = alaw2linear(i);
00166    }
00167    /* set up the reverse (a-law) conversion table */
00168    for (i = 0; i <= 32768; i += AST_ALAW_STEP) {
00169       AST_LIN2A_LOOKUP(i) = linear2alaw(i, 0 /* half-cooked */);
00170    }
00171 #endif
00172    
00173 #ifdef TEST_CODING_TABLES
00174    for (i = -32768; i < 32768; ++i) {
00175 #ifndef G711_NEW_ALGORITHM
00176       unsigned char e1 = linear2alaw(i);
00177 #else
00178       unsigned char e1 = linear2alaw(i, 1);
00179 #endif
00180       short d1 = alaw2linear(e1);
00181       unsigned char e2 = AST_LIN2A(i);
00182       short d2 = alaw2linear(e2);
00183       short d3 = AST_ALAW(e1);
00184       
00185       if (e1 != e2 || d1 != d3 || d2 != d3) {
00186          ast_log(LOG_WARNING, "a-Law coding tables test failed on %d: e1=%u, e2=%u, d1=%d, d2=%d\n",
00187                i, (unsigned)e1, (unsigned)e2, (int)d1, (int)d2);
00188       }
00189    }
00190    ast_log(LOG_NOTICE, "a-Law coding tables test complete.\n");
00191 #endif /* TEST_CODING_TABLES */
00192    
00193 #ifdef TEST_TANDEM_TRANSCODING
00194    /* tandem transcoding test */
00195    for (i = -32768; i < 32768; ++i) {
00196       unsigned char e1 = AST_LIN2A(i);
00197       short d1 = AST_ALAW(e1);
00198       unsigned char e2 = AST_LIN2A(d1);
00199       short d2 = AST_ALAW(e2);
00200       unsigned char e3 = AST_LIN2A(d2);
00201       short d3 = AST_ALAW(e3);
00202       
00203       if (e1 != e2 || e2 != e3 || d1 != d2 || d2 != d3) {
00204          ast_log(LOG_WARNING, "a-Law tandem transcoding test failed on %d: e1=%u, e2=%u, d1=%d, d2=%d, d3=%d\n",
00205                i, (unsigned)e1, (unsigned)e2, (int)d1, (int)d2, (int)d3);
00206       }
00207    }
00208    ast_log(LOG_NOTICE, "a-Law tandem transcoding test complete.\n");
00209 #endif /* TEST_TANDEM_TRANSCODING */
00210    
00211 }

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

Definition at line 35 of file alaw.c.

References AMI_MASK.

Referenced by ast_alaw_init().

00036 {
00037    int mask;
00038    int seg;
00039    int pcm_val;
00040    static int seg_end[8] =
00041       {
00042          0xFF, 0x1FF, 0x3FF, 0x7FF, 0xFFF, 0x1FFF, 0x3FFF, 0x7FFF
00043       };
00044 
00045    pcm_val = linear;
00046    if (pcm_val >= 0) {
00047       /* Sign (7th) bit = 1 */
00048       mask = AMI_MASK | 0x80;
00049    } else {
00050       /* Sign bit = 0 */
00051       mask = AMI_MASK;
00052       pcm_val = -pcm_val;
00053    }
00054 
00055    /* Convert the scaled magnitude to segment number. */
00056    for (seg = 0; seg < 8; seg++) {
00057       if (pcm_val <= seg_end[seg]) {
00058          break;
00059       }
00060    }
00061    /* Combine the sign, segment, and quantization bits. */
00062    return ((seg << 4) | ((pcm_val >> ((seg) ? (seg + 3) : 4)) & 0x0F)) ^ mask;
00063 }


Variable Documentation

short __ast_alaw[256]

help

Definition at line 147 of file alaw.c.

unsigned char __ast_lin2a[8192]

converts signed linear to alaw

Definition at line 143 of file alaw.c.


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