INTERNATIONAL  TELECOMMUNICATION  UNION

Γενικά
 

•                          Η σύσταση G.723 είναι μια από τις συστάσεις του ITU-T που ορίζουν την κωδικοποίηση  SB-ADPCM για κανάλι 24kbit/s . Επειδή έχει γίνει εκτενής αναφορά στον τόπο λειτουργίας του συγκεκριμένου τρόπου κωδικοποίησης στην σύσταση G.722  δεν θα περιγραφεί ξανά.
•                    Παρέχεται μόνο κώδικας C που εξομοιώνει την λειτουργία του κωδικοποιητή /αποκωδικοποιητή  από την SUN Microsystems inc.
•  

/*
 * 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,
 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_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,
 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 &= 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);

 switch (out_coding) {
 case AUDIO_ENCODING_ALAW:
  return (tandem_adjust_alaw(sr, se, y, i, 4, qtab_723_24));
 case AUDIO_ENCODING_ULAW:
  return (tandem_adjust_ulaw(sr, se, y, i, 4, qtab_723_24));
 case AUDIO_ENCODING_LINEAR:
  return (sr << 2); /* sr was of 14-bit dynamic range */
 default:
  return (-1);
 }
}