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.

Functions
static short	alaw2linear (unsigned char alawbyte)
void	ast_alaw_init (void)
	To init the alaw to slinear conversion stuff, this needs to be run.
static unsigned char	linear2alaw (short sample, int full_coding)
Variables
short	__ast_alaw [256]
unsigned char	__ast_lin2a [AST_ALAW_TAB_SIZE]
	converts signed linear to alaw

---

Detailed Description

a-Law to Signed linear conversion

Author:

Mark Spencer <markster@digium.com>

Definition in file alaw.c.

---

Function Documentation

static short alaw2linear

(

unsigned char

alawbyte

)

[inline, static]

Definition at line 128 of file alaw.c.

References AST_ALAW_AMI_MASK.

Referenced by ast_alaw_init().

{
   unsigned exponent, mantissa;
   short sample;

   alawbyte ^= AST_ALAW_AMI_MASK;
   exponent = (alawbyte & 0x70) >> 4;
   mantissa = alawbyte & 0x0f;
   sample = (mantissa << 4) + 8 /* rounding error */;
   if (exponent)
      sample = (sample + 0x100) << (exponent - 1);
   if (!(alawbyte & 0x80))
      sample = -sample;
   return sample;
}

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().

{
   int i;
   /*
    *  Set up mu-law conversion table
    */
#ifndef G711_NEW_ALGORITHM
   for (i = 0; i < 256; i++) {
      __ast_alaw[i] = alaw2linear(i);
   }
   /* set up the reverse (mu-law) conversion table */
   for (i = -32768; i < 32768; i++) {
      __ast_lin2a[((unsigned short)i) >> 3] = linear2alaw(i);
   }
#else
   for (i = 0; i < 256; i++) {
      __ast_alaw[i] = alaw2linear(i);
   }
   /* set up the reverse (a-law) conversion table */
   for (i = 0; i <= 32768; i += AST_ALAW_STEP) {
      AST_LIN2A_LOOKUP(i) = linear2alaw(i, 0 /* half-cooked */);
   }
#endif

#ifdef TEST_CODING_TABLES
   for (i = -32768; i < 32768; ++i) {
#ifndef G711_NEW_ALGORITHM
      unsigned char e1 = linear2alaw(i);
#else
      unsigned char e1 = linear2alaw(i, 1);
#endif
      short d1 = alaw2linear(e1);
      unsigned char e2 = AST_LIN2A(i);
      short d2 = alaw2linear(e2);
      short d3 = AST_ALAW(e1);

      if (e1 != e2 || d1 != d3 || d2 != d3) {
         ast_log(LOG_WARNING, "a-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, "a-Law coding tables test complete.\n");
#endif /* TEST_CODING_TABLES */

#ifdef TEST_TANDEM_TRANSCODING
   /* tandem transcoding test */
   for (i = -32768; i < 32768; ++i) {
      unsigned char e1 = AST_LIN2A(i);
      short d1 = AST_ALAW(e1);
      unsigned char e2 = AST_LIN2A(d1);
      short d2 = AST_ALAW(e2);
      unsigned char e3 = AST_LIN2A(d2);
      short d3 = AST_ALAW(e3);

      if (e1 != e2 || e2 != e3 || d1 != d2 || d2 != d3) {
         ast_log(LOG_WARNING, "a-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, "a-Law tandem transcoding test complete.\n");
#endif /* TEST_TANDEM_TRANSCODING */

}

static unsigned char linear2alaw	(	short	sample,
		int	full_coding
	)			[static]

Definition at line 70 of file alaw.c.

References AST_ALAW_AMI_MASK.

Referenced by ast_alaw_init().

{
   static const unsigned exp_lut[128] = {
      1,1,2,2,3,3,3,3,
      4,4,4,4,4,4,4,4,
      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,
      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 alawbyte;

   ast_alaw_get_sign_mag(sample, &sign, &mag);
   if (mag > 32767)
      mag = 32767;        /* clip the magnitude for -32768 */

   exponent = exp_lut[(mag >> 8) & 0x7f];
   mantissa = (mag >> (exponent + 3)) & 0x0f;
   if (mag < 0x100)
      exponent = 0;

   if (full_coding) {
      /* full encoding, with sign and xform */
      alawbyte = (unsigned char)(sign | (exponent << 4) | mantissa);
      alawbyte ^= AST_ALAW_AMI_MASK;
   } else {
      /* half-cooked coding -- mantissa+exponent only (for lookup tab) */
      alawbyte = (exponent << 4) | mantissa;
   }
   return alawbyte;
}

---

Variable Documentation

short __ast_alaw[256]

help

Definition at line 152 of file alaw.c.

unsigned char __ast_lin2a[AST_ALAW_TAB_SIZE]

converts signed linear to alaw

Definition at line 150 of file alaw.c.