Source

asterisk / ulaw.c

/*
 * Asterisk -- An open source telephony toolkit.
 *
 * Copyright (C) 1999 - 2005, Digium, Inc.
 *
 * Mark Spencer <markster@digium.com>
 *
 * See http://www.asterisk.org for more information about
 * the Asterisk project. Please do not directly contact
 * any of the maintainers of this project for assistance;
 * the project provides a web site, mailing lists and IRC
 * channels for your use.
 *
 * This program is free software, distributed under the terms of
 * the GNU General Public License Version 2. See the LICENSE file
 * at the top of the source tree.
 */

/*! \file
 *
 * \brief u-Law to Signed linear conversion
 * 
 */

#include "asterisk.h"

ASTERISK_FILE_VERSION(__FILE__, "$Revision$")

#include "asterisk/ulaw.h"

#define ZEROTRAP    /*!< turn on the trap as per the MIL-STD */
#define BIAS 0x84   /*!< define the add-in bias for 16 bit samples */
#define CLIP 32635

unsigned char __ast_lin2mu[16384];
short __ast_mulaw[256];


static unsigned char linear2ulaw(short sample)
{
	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;
}

/*!
 * \brief  Set up mu-law conversion table
 */
void ast_ulaw_init(void)
{
	int i;
	for(i = 0;i < 256;i++) {
		short mu,e,f,y;
		static 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);
	}

}
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