ADPCM codec based on G.726 specifications

[bythesea]

Hi,

I'm trying to write a ADPCM codec following the G.726 specifications. I found the following codes written for the G.721 and though it is applicable, I had lotsa difficulties understanding the code. I would thus appreciate any help given.


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/*
* 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, 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 << 5, _fitab, 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, y); /* quantized diff. */

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

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

update(4, y, _witab << 5, _fitab, 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[], _witab[] and _fitab[] determined?