src/libosmo-mgcp/g711common.h

/*
 *  PCM - A-Law conversion
 *  Copyright (c) 2000 by Abramo Bagnara <abramo@alsa-project.org>
 *
 *  Wrapper for linphone Codec class by Simon Morlat <simon.morlat@linphone.org>
 *
 *
 *  This program is free software; you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation; either version 2 of the License, or
 *  (at your option) any later version.
 *
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 */

static inline int val_seg(int val)
{
	int r = 0;
	val >>= 7; /*7 = 4 + 3*/
	if (val & 0xf0) {
		val >>= 4;
		r += 4;
	}
	if (val & 0x0c) {
		val >>= 2;
		r += 2;
	}
	if (val & 0x02)
		r += 1;
	return r;
}

/*
 * s16_to_alaw() - Convert a 16-bit linear PCM value to 8-bit A-law
 *
 * s16_to_alaw() accepts an 16-bit integer and encodes it as A-law data.
 *
 *		Linear Input Code	Compressed Code
 *	------------------------	---------------
 *	0000000wxyza			000wxyz
 *	0000001wxyza			001wxyz
 *	000001wxyzab			010wxyz
 *	00001wxyzabc			011wxyz
 *	0001wxyzabcd			100wxyz
 *	001wxyzabcde			101wxyz
 *	01wxyzabcdef			110wxyz
 *	1wxyzabcdefg			111wxyz
 *
 * For further information see John C. Bellamy's Digital Telephony, 1982,
 * John Wiley & Sons, pps 98-111 and 472-476.
 * G711 is designed for 13 bits input signal, this function add extra shifting to take this into account.
 */

static inline unsigned char s16_to_alaw(int pcm_val)
{
	int		mask;
	int		seg;
	unsigned char	aval;

	if (pcm_val >= 0) {
		mask = 0xD5;
	} else {
		mask = 0x55;
		pcm_val = -pcm_val;
		if (pcm_val > 0x7fff)
			pcm_val = 0x7fff;
	}

	if (pcm_val < 256) /*256 = 32 << 3*/
		aval = pcm_val >> 4; /*4 = 1 + 3*/
	else {
		/* Convert the scaled magnitude to segment number. */
		seg = val_seg(pcm_val);
		aval = (seg << 4) | ((pcm_val >> (seg + 3)) & 0x0f);
	}
	return aval ^ mask;
}

/*
 * alaw_to_s16() - Convert an A-law value to 16-bit linear PCM
 *
 */
static inline int alaw_to_s16(unsigned char a_val)
{
	int		t;
	int		seg;

	a_val ^= 0x55;
	t = a_val & 0x7f;
	if (t < 16)
		t = (t << 4) + 8;
	else {
		seg = (t >> 4) & 0x07;
		t = ((t & 0x0f) << 4) + 0x108;
		t <<= seg -1;
	}
	return ((a_val & 0x80) ? t : -t);
}
/*
 * s16_to_ulaw() - Convert a linear PCM value to u-law
 *
 * In order to simplify the encoding process, the original linear magnitude
 * is biased by adding 33 which shifts the encoding range from (0 - 8158) to
 * (33 - 8191). The result can be seen in the following encoding table:
 *
 *	Biased Linear Input Code	Compressed Code
 *	------------------------	---------------
 *	00000001wxyza			000wxyz
 *	0000001wxyzab			001wxyz
 *	000001wxyzabc			010wxyz
 *	00001wxyzabcd			011wxyz
 *	0001wxyzabcde			100wxyz
 *	001wxyzabcdef			101wxyz
 *	01wxyzabcdefg			110wxyz
 *	1wxyzabcdefgh			111wxyz
 *
 * Each biased linear code has a leading 1 which identifies the segment
 * number. The value of the segment number is equal to 7 minus the number
 * of leading 0's. The quantization interval is directly available as the
 * four bits wxyz.  * The trailing bits (a - h) are ignored.
 *
 * Ordinarily the complement of the resulting code word is used for
 * transmission, and so the code word is complemented before it is returned.
 *
 * For further information see John C. Bellamy's Digital Telephony, 1982,
 * John Wiley & Sons, pps 98-111 and 472-476.
 */

static inline unsigned char s16_to_ulaw(int pcm_val)	/* 2's complement (16-bit range) */
{
	int mask;
	int seg;
	unsigned char uval;

	if (pcm_val < 0) {
		pcm_val = 0x84 - pcm_val;
		mask = 0x7f;
	} else {
		pcm_val += 0x84;
		mask = 0xff;
	}
	if (pcm_val > 0x7fff)
		pcm_val = 0x7fff;

	/* Convert the scaled magnitude to segment number. */
	seg = val_seg(pcm_val);

	/*
	 * Combine the sign, segment, quantization bits;
	 * and complement the code word.
	 */
	uval = (seg << 4) | ((pcm_val >> (seg + 3)) & 0x0f);
	return uval ^ mask;
}

/*
 * ulaw_to_s16() - Convert a u-law value to 16-bit linear PCM
 *
 * First, a biased linear code is derived from the code word. An unbiased
 * output can then be obtained by subtracting 33 from the biased code.
 *
 * Note that this function expects to be passed the complement of the
 * original code word. This is in keeping with ISDN conventions.
 */
static inline int ulaw_to_s16(unsigned char u_val)
{
	int t;

	/* Complement to obtain normal u-law value. */
	u_val = ~u_val;

	/*
	 * Extract and bias the quantization bits. Then
	 * shift up by the segment number and subtract out the bias.
	 */
	t = ((u_val & 0x0f) << 3) + 0x84;
	t <<= (u_val & 0x70) >> 4;

	return ((u_val & 0x80) ? (0x84 - t) : (t - 0x84));
}