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.

Defines
#define	BIAS   0x84
#define	CLIP   32635
Functions
void	ast_ulaw_init (void)
	Set up mu-law conversion table.
static unsigned char	linear2ulaw (short sample, int full_coding)
static short	ulaw2linear (unsigned char ulawbyte)
Variables
unsigned char	__ast_lin2mu [AST_ULAW_TAB_SIZE]
	converts signed linear to mulaw
short	__ast_mulaw [256]

---

Detailed Description

u-Law to Signed linear conversion

Author:

Mark Spencer <markster@digium.com>

Definition in file ulaw.c.

---

Define Documentation

#define BIAS   0x84

define the add-in bias for 16 bit samples

Definition at line 46 of file ulaw.c.

Referenced by linear2ulaw().

#define CLIP   32635

Definition at line 47 of file ulaw.c.

Referenced by linear2ulaw().

---

Function Documentation

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 175 of file ulaw.c.

References AST_LIN2MU(), AST_LIN2MU_LOOKUP, ast_log(), AST_MULAW, AST_ULAW_STEP, f, linear2ulaw(), LOG_NOTICE, LOG_WARNING, and ulaw2linear().

Referenced by load_module(), and main().

{
   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 */
}

static unsigned char linear2ulaw	(	short	sample,
		int	full_coding
	)			[static]

Definition at line 102 of file ulaw.c.

References ast_ulaw_get_sign_mag(), BIAS, and CLIP.

Referenced by ast_ulaw_init().

{
   static const unsigned 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 };
   unsigned sign, exponent, mantissa, mag;
   unsigned char ulawbyte;
   
   /* Get the sample into sign-magnitude. */
   ast_ulaw_get_sign_mag(sample, &sign, &mag);
   if (mag > CLIP)
      mag = CLIP;                /* clip the 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. */
   mag += BIAS;
   exponent = exp_lut[(mag >> 7) & 0xFF];
   mantissa = (mag >> (exponent + 3)) & 0x0F;
   
   if (full_coding) {
      /* full encoding, with sign and xform */
      ulawbyte = ~(sign | (exponent << 4) | mantissa);
#ifdef ZEROTRAP
      if (ulawbyte == 0)
         ulawbyte = 0x02;   /* optional CCITT trap */
#endif
   } else {
      /* half-cooked coding -- mantissa+exponent only (for lookup tab) */
      ulawbyte = (exponent << 4) | mantissa;
   }

   return ulawbyte;
}

static short ulaw2linear

(

unsigned char

ulawbyte

)

[inline, static]

Definition at line 155 of file ulaw.c.

Referenced by ast_ulaw_init().

{
   unsigned exponent, mantissa;
   short sample;
   static const short etab[]={0,132,396,924,1980,4092,8316,16764};
   
   ulawbyte = ~ulawbyte;
   exponent = (ulawbyte & 0x70) >> 4;
   mantissa = ulawbyte & 0x0f;
   sample = mantissa << (exponent + 3);
   sample += etab[exponent];
   if (ulawbyte & 0x80)
      sample = -sample;
   return sample;
}

---

Variable Documentation

unsigned char __ast_lin2mu[AST_ULAW_TAB_SIZE]

converts signed linear to mulaw

Definition at line 99 of file ulaw.c.

short __ast_mulaw[256]

help

Definition at line 100 of file ulaw.c.