alaw.c File Reference

a-Law to Signed linear conversion More...

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

Include dependency graph for alaw.c:

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 38 of file alaw.c.

Referenced by alaw2linear(), and linear2alaw().


Function Documentation

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

Definition at line 114 of file alaw.c.

References AMI_MASK.

Referenced by ast_alaw_init().

00115 {
00116    int i;
00117    int seg;
00118 
00119    alaw ^= AMI_MASK;
00120    i = ((alaw & 0x0F) << 4) + 8 /* rounding error */;
00121    seg = (((int) alaw & 0x70) >> 4);
00122    if (seg) {
00123       i = (i + 0x100) << (seg - 1);
00124    }
00125    return (short int) ((alaw & 0x80) ? i : -i);
00126 }

void ast_alaw_init ( void   ) 

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

Definition at line 154 of file alaw.c.

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

Referenced by main().

00155 {
00156    int i;
00157    /*
00158     *  Set up mu-law conversion table
00159     */
00160 #ifndef G711_NEW_ALGORITHM
00161    for (i = 0; i < 256; i++) {
00162       __ast_alaw[i] = alaw2linear(i);
00163    }
00164    /* set up the reverse (mu-law) conversion table */
00165    for (i = -32768; i < 32768; i++) {
00166       __ast_lin2a[((unsigned short)i) >> 3] = linear2alaw(i);
00167    }
00168 #else
00169    for (i = 0; i < 256; i++) {
00170       __ast_alaw[i] = alaw2linear(i);
00171    }
00172    /* set up the reverse (a-law) conversion table */
00173    for (i = 0; i <= 32768; i += AST_ALAW_STEP) {
00174       AST_LIN2A_LOOKUP(i) = linear2alaw(i, 0 /* half-cooked */);
00175    }
00176 #endif
00177 
00178 #ifdef TEST_CODING_TABLES
00179    for (i = -32768; i < 32768; ++i) {
00180 #ifndef G711_NEW_ALGORITHM
00181       unsigned char e1 = linear2alaw(i);
00182 #else
00183       unsigned char e1 = linear2alaw(i, 1);
00184 #endif
00185       short d1 = alaw2linear(e1);
00186       unsigned char e2 = AST_LIN2A(i);
00187       short d2 = alaw2linear(e2);
00188       short d3 = AST_ALAW(e1);
00189 
00190       if (e1 != e2 || d1 != d3 || d2 != d3) {
00191          ast_log(LOG_WARNING, "a-Law coding tables test failed on %d: e1=%u, e2=%u, d1=%d, d2=%d\n",
00192                i, (unsigned)e1, (unsigned)e2, (int)d1, (int)d2);
00193       }
00194    }
00195    ast_log(LOG_NOTICE, "a-Law coding tables test complete.\n");
00196 #endif /* TEST_CODING_TABLES */
00197 
00198 #ifdef TEST_TANDEM_TRANSCODING
00199    /* tandem transcoding test */
00200    for (i = -32768; i < 32768; ++i) {
00201       unsigned char e1 = AST_LIN2A(i);
00202       short d1 = AST_ALAW(e1);
00203       unsigned char e2 = AST_LIN2A(d1);
00204       short d2 = AST_ALAW(e2);
00205       unsigned char e3 = AST_LIN2A(d2);
00206       short d3 = AST_ALAW(e3);
00207 
00208       if (e1 != e2 || e2 != e3 || d1 != d2 || d2 != d3) {
00209          ast_log(LOG_WARNING, "a-Law tandem transcoding test failed on %d: e1=%u, e2=%u, d1=%d, d2=%d, d3=%d\n",
00210                i, (unsigned)e1, (unsigned)e2, (int)d1, (int)d2, (int)d3);
00211       }
00212    }
00213    ast_log(LOG_NOTICE, "a-Law tandem transcoding test complete.\n");
00214 #endif /* TEST_TANDEM_TRANSCODING */
00215 
00216 }

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

Definition at line 40 of file alaw.c.

References AMI_MASK.

Referenced by ast_alaw_init().

00041 {
00042    int mask;
00043    int seg;
00044    int pcm_val;
00045    static int seg_end[8] =
00046       {
00047          0xFF, 0x1FF, 0x3FF, 0x7FF, 0xFFF, 0x1FFF, 0x3FFF, 0x7FFF
00048       };
00049 
00050    pcm_val = linear;
00051    if (pcm_val >= 0) {
00052       /* Sign (7th) bit = 1 */
00053       mask = AMI_MASK | 0x80;
00054    } else {
00055       /* Sign bit = 0 */
00056       mask = AMI_MASK;
00057       pcm_val = -pcm_val;
00058    }
00059 
00060    /* Convert the scaled magnitude to segment number. */
00061    for (seg = 0; seg < 8; seg++) {
00062       if (pcm_val <= seg_end[seg]) {
00063          break;
00064       }
00065    }
00066    /* Combine the sign, segment, and quantization bits. */
00067    return ((seg << 4) | ((pcm_val >> ((seg) ? (seg + 3) : 4)) & 0x0F)) ^ mask;
00068 }


Variable Documentation

short __ast_alaw[256]

help

Definition at line 152 of file alaw.c.

unsigned char __ast_lin2a[8192]

converts signed linear to alaw

Definition at line 148 of file alaw.c.


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