G.726 ADPCM

Hi,

I'm trying to write an ADPCM G.726 specifications codec and I met with
some problems.

I found the following codes available from Sun Microsystems which is
written for G.721. However I had a lot of diffculties understanding
the codes and I would appreciate any help.

**********************************************************************
/*
 * This source code is a product of Sun Microsystems, Inc. and is provided
 * for unrestricted use.  Users may copy or modify this source code
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 * charge.
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 * INFRINGEMENT OF COPYRIGHTS, TRADE SECRETS OR ANY PATENTS BY THIS
SOFTWARE
 * OR ANY PART THEREOF.
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 * or profits or other special, indirect and consequential damages,
even if
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 *
 * Sun Microsystems, Inc.
 * 2550 Garcia Avenue
 * Mountain View, California  94043
 */

/*
 * g721.c
 *
 * Description:
 *
 * g721_encoder(), g721_decoder()
 *
 * These routines comprise an implementation of the CCITT G.721 ADPCM
 * coding algorithm.  Essentially, this implementation is identical to
 * the bit level description except for a few deviations which
 * take advantage of work station attributes, such as hardware 2's
 * complement arithmetic and large memory.  Specifically, certain time
 * consuming operations such as multiplications are replaced
 * with lookup tables and software 2's complement operations are
 * replaced with hardware 2's complement.
 *
 * The deviation from the bit level specification (lookup tables)
 * preserves the bit level performance specifications.
 *
 * As outlined in the G.721 Recommendation, the algorithm is broken
 * down into modules.  Each section of code below is preceded by
 * the name of the module which it is implementing.
 *
 */
#include "g72x.h"

static short qtab_721[7] = {-124, 80, 178, 246, 300, 349, 400};
/*
 * Maps G.721 code word to reconstructed scale factor normalized log
 * magnitude values.
 */
static short _dqlntab[16] = {-2048, 4, 135, 213, 273, 323, 373, 425,
425, 373, 323, 273, 213, 135, 4, -2048};

/* Maps G.721 code word to log of scale factor multiplier. */
static short _witab[16] = {-12, 18, 41, 64, 112, 198, 355, 1122, 1122,
355, 198, 112, 64, 41, 18, -12};
/*
 * Maps G.721 code words to a set of values whose long and short
 * term averages are computed and then compared to give an indication
 * how stationary (steady state) the signal is.
 */
static short _fitab[16] = {0, 0, 0, 0x200, 0x200, 0x200, 0x600, 0xE00,
0xE00, 0x600, 0x200, 0x200, 0x200, 0, 0, 0};

/*
 * g721_encoder()
 *
 * Encodes the input vale of linear PCM, A-law or u-law data sl and
returns
 * the resulting code. -1 is returned for unknown input coding value.
 */
int g721_encoder(int sl, int in_coding, struct g72x_state *state_ptr)
{
	short sezi, se, sez;/* ACCUM */
	short d;	/* SUBTA */
	short sr;	/* ADDB */
	short y;	/* MIX */
	short dqsez;/* ADDC */
	short dq, i;

	switch (in_coding) 
	{	/* linearize input sample to 14-bit PCM */
		case AUDIO_ENCODING_ALAW:
			sl = alaw2linear(sl) >> 2;
			break;
		case AUDIO_ENCODING_ULAW:
			sl = ulaw2linear(sl) >> 2;
			break;
		case AUDIO_ENCODING_LINEAR:
			sl >>= 2;			/* 14-bit dynamic range */
			break;
		default:
			return (-1);
	}

	sezi = predictor_zero(state_ptr);
	sez = sezi >> 1;
	se = (sezi + predictor_pole(state_ptr)) >> 1;	/* estimated signal */

	d = sl - se;				/* estimation difference */

	/* quantize the prediction difference */
	y = step_size(state_ptr);			/* quantizer step size */
	i = quantize(d, y, qtab_721, 7);	/* i = ADPCM code */

	dq = reconstruct(i & 8, _dqlntab[i], y);		/* quantized est diff */

	sr = (dq < 0) ? se - (dq & 0x3FFF) : se + dq;	/* reconst. signal */

	dqsez = sr + sez - se;				/* pole prediction diff. */

	update(4, y, _witab[i] << 5, _fitab[i], dq, sr, dqsez, state_ptr);

	return (i);
}

/*
 * g721_decoder()
 *
 * Description:
 *
 * Decodes a 4-bit code of G.721 encoded data of i and
 * returns the resulting linear PCM, A-law or u-law value.
 * return -1 for unknown out_coding value.
 */
int g721_decoder(int i, int	out_coding, struct g72x_state *state_ptr)
{
	short sezi, sei, sez, se;	/* ACCUM */
	short y;  /* MIX */
	short sr; /* ADDB */
	short dq;
	short dqsez;

	i &= 0x0f; /* mask to get proper bits */
	sezi = predictor_zero(state_ptr);
	sez = sezi >> 1;
	sei = sezi + predictor_pole(state_ptr);
	se = sei >> 1;			/* se = estimated signal */

	y = step_size(state_ptr);	/* dynamic quantizer step size */

	dq = reconstruct(i & 0x08, _dqlntab[i], y); /* quantized diff. */

	sr = (dq < 0) ? (se - (dq & 0x3FFF)) : se + dq;	/* reconst. signal */

	dqsez = sr - se + sez;			/* pole prediction diff. */

	update(4, y, _witab[i] << 5, _fitab[i], dq, sr, dqsez, state_ptr);

	switch (out_coding) 
	{
		case AUDIO_ENCODING_ALAW:
			return (tandem_adjust_alaw(sr, se, y, i, 8, qtab_721));
		case AUDIO_ENCODING_ULAW:
			return (tandem_adjust_ulaw(sr, se, y, i, 8, qtab_721));
		case AUDIO_ENCODING_LINEAR:
			return (sr << 2);	/* sr was 14-bit dynamic range */
		default:
			return (-1);
	}
}
**********************************************************************

Firstly, how are the array elements in qtab_721[], _dqlntab[16],
_witab[16] and _fitab[16] determined?