ulaw.c File Reference

u-Law to Signed linear conversion More...

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

Include dependency graph for ulaw.c:

Go to the source code of this file.

Macros
#define	BIAS   0x84
#define	CLIP   32635

Functions
void	ast_ulaw_init (void)
	Set up mu-law conversion table.
static unsigned char	linear2ulaw (short sample)

Variables
unsigned char	__ast_lin2mu [16384]
	converts signed linear to mulaw
short	__ast_mulaw [256]

Detailed Description

u-Law to Signed linear conversion

Author

Mark Spencer marks.nosp@m.ter@.nosp@m.digiu.nosp@m.m.co.nosp@m.m

Definition in file ulaw.c.

Macro Definition Documentation

BIAS

#define BIAS   0x84

define the add-in bias for 16 bit samples

Definition at line 44 of file ulaw.c.

CLIP

#define CLIP   32635

Definition at line 45 of file ulaw.c.

Function Documentation

ast_ulaw_init()

void ast_ulaw_init

(

void

)

Set up mu-law conversion table.

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

Definition at line 173 of file ulaw.c.

{
    int i;
 
    /*
     *  Set up mu-law conversion table
     */
#ifndef G711_NEW_ALGORITHM
    for (i = 0;i < 256;i++) {
        short mu,e,f,y;
        static const short etab[]={0,132,396,924,1980,4092,8316,16764};
 
        mu = 255-i;
        e = (mu & 0x70)/16;
        f = mu & 0x0f;
        y = f * (1 << (e + 3));
        y += etab[e];
        if (mu & 0x80) y = -y;
        __ast_mulaw[i] = y;
    }
    /* set up the reverse (mu-law) conversion table */
    for (i = -32768; i < 32768; i++) {
        __ast_lin2mu[((unsigned short)i) >> 2] = linear2ulaw(i);
    }
#else
 
    for (i = 0; i < 256; i++) {
        __ast_mulaw[i] = ulaw2linear(i);
    }
    /* set up the reverse (mu-law) conversion table */
    for (i = 0; i <= 32768; i += AST_ULAW_STEP) {
        AST_LIN2MU_LOOKUP(i) = linear2ulaw(i, 0 /* half-cooked */);
    }
#endif
 
#ifdef TEST_CODING_TABLES
    for (i = -32768; i < 32768; ++i) {
#ifndef G711_NEW_ALGORITHM
        unsigned char e1 = linear2ulaw(i);
#else
        unsigned char e1 = linear2ulaw(i, 1);
#endif
        short d1 = ulaw2linear(e1);
        unsigned char e2 = AST_LIN2MU(i);
        short d2 = ulaw2linear(e2);
        short d3 = AST_MULAW(e1);
 
        if (e1 != e2 || d1 != d3 || d2 != d3) {
            ast_log(LOG_WARNING, "u-Law coding tables test failed on %d: e1=%u, e2=%u, d1=%d, d2=%d\n",
                    i, (unsigned)e1, (unsigned)e2, (int)d1, (int)d2);
        }
    }
    ast_log(LOG_NOTICE, "u-Law coding table test complete.\n");
#endif /* TEST_CODING_TABLES */
 
#ifdef TEST_TANDEM_TRANSCODING
    /* tandem transcoding test */
    for (i = -32768; i < 32768; ++i) {
        unsigned char e1 = AST_LIN2MU(i);
        short d1 = AST_MULAW(e1);
        unsigned char e2 = AST_LIN2MU(d1);
        short d2 = AST_MULAW(e2);
        unsigned char e3 = AST_LIN2MU(d2);
        short d3 = AST_MULAW(e3);
 
        if (i < 0 && e1 == 0x7f && e2 == 0xff && e3 == 0xff)
            continue; /* known and normal negative 0 case */
 
        if (e1 != e2 || e2 != e3 || d1 != d2 || d2 != d3) {
            ast_log(LOG_WARNING, "u-Law tandem transcoding test failed on %d: e1=%u, e2=%u, d1=%d, d2=%d, d3=%d\n",
                    i, (unsigned)e1, (unsigned)e2, (int)d1, (int)d2, (int)d3);
        }
    }
    ast_log(LOG_NOTICE, "u-Law tandem transcoding test complete.\n");
#endif /* TEST_TANDEM_TRANSCODING */
}

References __ast_lin2mu, __ast_mulaw, AST_LIN2MU, ast_log, AST_MULAW, AST_ULAW_STEP, linear2ulaw(), LOG_NOTICE, and LOG_WARNING.

Referenced by asterisk_daemon().

linear2ulaw()

static unsigned char linear2ulaw ( short  sample )

static

Definition at line 52 of file ulaw.c.

{
    static int exp_lut[256] = {
        0,0,1,1,2,2,2,2,3,3,3,3,3,3,3,3,
        4,4,4,4,4,4,4,4,4,4,4,4,4,4,4,4,
        5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,
        5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,
        6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,
        6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,
        6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,
        6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,
        7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,
        7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,
        7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,
        7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,
        7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,
        7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,
        7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,
        7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7 };
    int sign, exponent, mantissa;
    unsigned char ulawbyte;
 
    /* Get the sample into sign-magnitude. */
    sign = (sample >> 8) & 0x80;          /* set aside the sign */
    if (sign != 0)
        sample = -sample;              /* get magnitude */
    if (sample > CLIP)
        sample = CLIP;             /* clip the magnitude */
 
    /* Convert from 16 bit linear to ulaw. */
    sample = sample + BIAS;
    exponent = exp_lut[(sample >> 7) & 0xFF];
    mantissa = (sample >> (exponent + 3)) & 0x0F;
    ulawbyte = ~(sign | (exponent << 4) | mantissa);
 
#ifdef ZEROTRAP
    if (ulawbyte == 0)
        ulawbyte = 0x02;   /* optional CCITT trap */
#endif
 
    return ulawbyte;
}

References BIAS, and CLIP.

Referenced by ast_ulaw_init().

Variable Documentation

__ast_lin2mu

unsigned char __ast_lin2mu[16384]

converts signed linear to mulaw

Definition at line 49 of file ulaw.c.

Referenced by ast_ulaw_init().

__ast_mulaw

short __ast_mulaw[256]

help

Definition at line 50 of file ulaw.c.

Referenced by ast_ulaw_init().