void _celt_lpc(
opus_val16 *_lpc, /* out: [0...p-1] LPC coefficients */
const opus_val32 *ac, /* in: [0...p] autocorrelation values */
int p
)
{
int i, j;
opus_val32 r;
opus_val32 error = ac[0];
#ifdef OPUS_FIXED_POINT
opus_val32 lpc[LPC_ORDER];
#else
float *lpc = _lpc;
#endif
for (i = 0; i < p; i++)
lpc[i] = 0;
if (ac[0] != 0)
{
for (i = 0; i < p; i++) {
/* Sum up this iteration's reflection coefficient */
opus_val32 rr = 0;
for (j = 0; j < i; j++)
rr += MULT32_32_Q31(lpc[j],ac[i - j]);
rr += SHR32(ac[i + 1],3);
r = -frac_div32(SHL32(rr,3), error);
/* Update LPC coefficients and total error */
lpc[i] = SHR32(r,3);
for (j = 0; j < (i+1)>>1; j++)
{
opus_val32 tmp1, tmp2;
tmp1 = lpc[j];
tmp2 = lpc[i-1-j];
lpc[j] = tmp1 + MULT32_32_Q31(r,tmp2);
lpc[i-1-j] = tmp2 + MULT32_32_Q31(r,tmp1);
}
error = error - MULT32_32_Q31(MULT32_32_Q31(r,r),error);
/* Bail out once we get 30 dB gain */
#ifdef OPUS_FIXED_POINT
if (error<SHR32(ac[0],10))
break;
#else
if (error<.001f*ac[0])
break;
#endif
}
}
#ifdef OPUS_FIXED_POINT
for (i=0;i<p;i++)
_lpc[i] = ROUND16(lpc[i],16);
#endif
}
void autoCorrelation2LP(word32_t autoCorrelationCoefficients[], word16_t LPCoefficientsQ12[], word32_t reflectionCoefficients[], word32_t *residualEnergy) {
/*********************************************************************************/
/* Compute the LP Coefficient using Levinson-Durbin algo spec 3.2.2 */
/*********************************************************************************/
/* start a iteration i=2, init values as after iteration i=1 : */
/* a[0] = 1 */
/* a[1] = -r1/r0 */
/* E = r0(1 - a[1]^2) */
/* */
/* iterations i = 2..10 */
/* sum = r[i] + ∑ a[j]*r[i-j] with j = 1..i-1 (a[0] is always 1) */
/* a[i] = -sum/E */
/* iterations j = 1..i-1 */
/* a[j] += a[i]*a{i-1}[i-j] use a{i-1}: from previous iteration */
/* E *=(1-a[i]^2) */
/* */
/* r in Q31 (normalised) stored in array autoCorrelationCoefficients */
/* E in Q31 (can't be > 1) */
/* sum in Q27 (sum can't be > 1 but intermediate accumulation can) */
/* a in Q4.27 with full range possible */
/* Note: during iteration, current a[i] is in Q31 (can't be >1) and is */
/* set to Q27 at the end of current iteration */
/* */
/*********************************************************************************/
word32_t previousIterationLPCoefficients[NB_LSP_COEFF+1]; /* to compute a[]*/
word32_t LPCoefficients[NB_LSP_COEFF+1]; /* in Q4.27 */
word32_t E = 0; /* in Q31 */
word32_t sum = 0; /* in Q27 */
int i,j;
/* init */
LPCoefficients[0] = ONE_IN_Q27;
LPCoefficients[1] = -DIV32_32_Q27(autoCorrelationCoefficients[1], autoCorrelationCoefficients[0]); /* result in Q27(but<1) */
reflectionCoefficients[0] = SHL(LPCoefficients[1],4); /* k[0] is -r1/r0 in Q31 */
/* E = r0(1 - a[1]^2) in Q31 */
E = MULT32_32_Q31(autoCorrelationCoefficients[0], SUB32(ONE_IN_Q31, MULT32_32_Q23(LPCoefficients[1], LPCoefficients[1]))); /* LPCoefficient[1] is in Q27, using a Q23 operation will result in a Q31 variable */
for (i=2; i<NB_LSP_COEFF+1; i++) {
/* update the previousIterationLPCoefficients needed for this one */
for (j=1; j<i; j++) {
previousIterationLPCoefficients[j] = LPCoefficients[j];
}
/* sum = r[i] + ∑ a[j]*r[i-j] with j = 1..i-1 (a[0] is always 1) */
sum = 0;
for (j=1; j<i; j++) {
sum = MAC32_32_Q31(sum, LPCoefficients[j], autoCorrelationCoefficients[i-j]);/* LPCoefficients in Q27, autoCorrelation in Q31 -> result in Q27 -> sum in Q27 */
}
sum = ADD32(SHL(sum, 4), autoCorrelationCoefficients[i]); /* set sum in Q31 and add r[0] */
/* a[i] = -sum/E */
LPCoefficients[i] = -DIV32_32_Q31(sum,E); /* LPCoefficient of current iteration is in Q31 for now, it will be set to Q27 at the end of this iteration */
reflectionCoefficients[i-1] = LPCoefficients[i]; /* k[1] is needed by VAD others by RFC3389 RTP payload for Comfort Noise spectral information encoding */
/* iterations j = 1..i-1 */
/* a[j] += a[i]*a[i-j] */
for (j=1; j<i; j++) {
LPCoefficients[j] = MAC32_32_Q31(LPCoefficients[j], LPCoefficients[i], previousIterationLPCoefficients[i-j]); /*LPCoefficients in Q27 except for LPCoefficients[i] in Q31 */
}
/* E *=(1-a[i]^2) */
E = MULT32_32_Q31(E, SUB32(ONE_IN_Q31, MULT32_32_Q31(LPCoefficients[i], LPCoefficients[i]))); /* all in Q31 */
/* set LPCoefficients[i] from Q31 to Q27 */
LPCoefficients[i] = SHR(LPCoefficients[i], 4);
}
*residualEnergy = E;
/* convert with rounding the LP Coefficients form Q27 to Q12, ignore first coefficient which is always 1 */
for (i=0; i<NB_LSP_COEFF; i++) {
LPCoefficientsQ12[i] = (word16_t)SATURATE(PSHR(LPCoefficients[i+1], 15), MAXINT16);
}
}
