g726_16.c

优化的G.726编码代码冲G.726到ADPCM

/*
 * g726_16.c
 *
 * Description:
 *
 * g726_16_encoder(), g726_16_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"

/*
 * Maps G.726_16 code word to reconstructed scale factor normalized log
 * magnitude values.
 */
static short	_dqlntab[4] = {-116, 365, 365, -116};

/* Maps G.723_16 code word to log of scale factor multiplier. */
static short	_witab[4] = {-127, 14, 14, -127};

/*
 * Maps G.723_16 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[4] = {0, 0xE00, 0xE00, 0};

static short qtab_726_16[1] = {261};

/*
 * g726_16_encoder()
 *
 * Encodes a linear PCM, A-law or u-law input sample and returns its 2-bit code.
 * Returns -1 if invalid input coding value.
 */
int
g726_16_encoder(
	int		sl,
	int		in_coding,
	struct 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;

	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;		/* sl of 14-bit dynamic range */
		break;
	default:
		return (-1);
	}

	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_726_16, 1);	/* i = ADPCM code */
	dq = reconstruct(i & 2, _dqlntab[i], y); /* quantized diff. */

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

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

	update(1, y, _witab[i], _fitab[i], dq, sr, dqsez, state_ptr);

	return (i);
}

/*
 * g726_16_decoder()
 *
 * Decodes a 2-bit CCITT G.726_16 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
g726_16_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 &= 0x03;			/* 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 & 0x02, _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);

	switch (out_coding) {
	case AUDIO_ENCODING_ALAW:
		return (tandem_adjust_alaw(sr, se, y, i, 4, qtab_726_16));
	case AUDIO_ENCODING_ULAW:
		return (tandem_adjust_ulaw(sr, se, y, i, 4, qtab_726_16));
	case AUDIO_ENCODING_LINEAR:
		return (sr << 2);	/* sr was of 14-bit dynamic range */
	default:
		return (-1);
	}
}