src/G72x/g723_24.c

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/*
 * g723_24.c
 *
 * Description:
 *
 * g723_24_encoder (), g723_24_decoder ()
 *
 * These routines comprise an implementation of the CCITT G.723 24 Kbps
 * ADPCM coding algorithm.  Essentially, this implementation is identical to
 * the bit level description except for a few deviations which take advantage
 * of workstation attributes, such as hardware 2's complement arithmetic.
 *
 */

#include "g72x.h"
#include "g72x_priv.h"

/*
 * Maps G.723_24 code word to reconstructed scale factor normalized log
 * magnitude values.
 */
static short _dqlntab [8] = { -2048, 135, 273, 373, 373, 273, 135, -2048 } ;

/* Maps G.723_24 code word to log of scale factor multiplier. */
static short _witab [8] = { -128, 960, 4384, 18624, 18624, 4384, 960, -128 } ;

/*
 * Maps G.723_24 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 [8] = { 0, 0x200, 0x400, 0xE00, 0xE00, 0x400, 0x200, 0 } ;

static short qtab_723_24 [3] = { 8, 218, 331 } ;

/*
 * g723_24_encoder ()
 *
 * Encodes a linear PCM, A-law or u-law input sample and returns its 3-bit code.
 * Returns -1 if invalid input coding value.
 */
int
g723_24_encoder (
	int		sl,
	G72x_STATE *state_ptr)
{
	short		sei, sezi, se, sez ;	/* ACCUM */
	short		d ;			/* SUBTA */
	short		y ;			/* MIX */
	short		sr ;			/* ADDB */
	short		dqsez ;			/* ADDC */
	short		dq, i ;

	/* linearize input sample to 14-bit PCM */
	sl >>= 2 ;		/* sl of 14-bit dynamic range */

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

	d = sl - se ;			/* d = estimation diff. */

	/* quantize prediction difference d */
	y = step_size (state_ptr) ;	/* quantizer step size */
	i = quantize (d, y, qtab_723_24, 3) ;	/* i = ADPCM code */
	dq = reconstruct (i & 4, _dqlntab [i], y) ; /* quantized diff. */

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

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

	update (3, y, _witab [i], _fitab [i], dq, sr, dqsez, state_ptr) ;

	return i ;
}

/*
 * g723_24_decoder ()
 *
 * Decodes a 3-bit CCITT G.723_24 ADPCM code and returns
 * the resulting 16-bit linear PCM, A-law or u-law sample value.
 * -1 is returned if the output coding is unknown.
 */
int
g723_24_decoder (
	int		i,
	G72x_STATE *state_ptr)
{
	short		sezi, sei, sez, se ;	/* ACCUM */
	short		y ;			/* MIX */
	short		sr ;			/* ADDB */
	short		dq ;
	short		dqsez ;

	i &= 0x07 ;			/* 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) ;	/* adaptive quantizer step size */
	dq = reconstruct (i & 0x04, _dqlntab [i], y) ; /* unquantize pred diff */

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

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

	update (3, y, _witab [i], _fitab [i], dq, sr, dqsez, state_ptr) ;

	return arith_shift_left (sr, 2) ;	/* sr was of 14-bit dynamic range */
}