ref: c77b96333b471c7b8a89852ee941e6a8987751b4
dir: /silk/SKP_Silk_NLSF2A.c/
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/* conversion between prediction filter coefficients and LSFs */
/* order should be even */
/* a piecewise linear approximation maps LSF <-> cos(LSF) */
/* therefore the result is not accurate LSFs, but the two */
/* function are accurate inverses of each other */
#include "SKP_Silk_SigProc_FIX.h"
/* helper function for NLSF2A(..) */
SKP_INLINE void SKP_Silk_NLSF2A_find_poly(
SKP_int32 *out, /* o intermediate polynomial, Q20 */
const SKP_int32 *cLSF, /* i vector of interleaved 2*cos(LSFs), Q20 */
SKP_int dd /* i polynomial order (= 1/2 * filter order) */
)
{
SKP_int k, n;
SKP_int32 ftmp;
out[0] = SKP_LSHIFT( 1, 20 );
out[1] = -cLSF[0];
for( k = 1; k < dd; k++ ) {
ftmp = cLSF[2*k]; // Q20
out[k+1] = SKP_LSHIFT( out[k-1], 1 ) - (SKP_int32)SKP_RSHIFT_ROUND64( SKP_SMULL( ftmp, out[k] ), 20 );
for( n = k; n > 1; n-- ) {
out[n] += out[n-2] - (SKP_int32)SKP_RSHIFT_ROUND64( SKP_SMULL( ftmp, out[n-1] ), 20 );
}
out[1] -= ftmp;
}
}
/* compute whitening filter coefficients from normalized line spectral frequencies */
void SKP_Silk_NLSF2A(
SKP_int16 *a, /* o monic whitening filter coefficients in Q12, [d] */
const SKP_int16 *NLSF, /* i normalized line spectral frequencies in Q15, [d] */
const SKP_int d /* i filter order (should be even) */
)
{
SKP_int k, i, dd;
SKP_int32 cos_LSF_Q20[SKP_Silk_MAX_ORDER_LPC];
SKP_int32 P[SKP_Silk_MAX_ORDER_LPC/2+1], Q[SKP_Silk_MAX_ORDER_LPC/2+1];
SKP_int32 Ptmp, Qtmp;
SKP_int32 f_int;
SKP_int32 f_frac;
SKP_int32 cos_val, delta;
SKP_int32 a_int32[SKP_Silk_MAX_ORDER_LPC];
SKP_int32 maxabs, absval, idx=0, sc_Q16;
SKP_assert( LSF_COS_TAB_SZ_FIX == 128 );
/* convert LSFs to 2*cos(LSF(i)), using piecewise linear curve from table */
for( k = 0; k < d; k++ ) {
SKP_assert(NLSF[k] >= 0 );
SKP_assert(NLSF[k] <= 32767 );
/* f_int on a scale 0-127 (rounded down) */
f_int = SKP_RSHIFT( NLSF[k], 15 - 7 );
/* f_frac, range: 0..255 */
f_frac = NLSF[k] - SKP_LSHIFT( f_int, 15 - 7 );
SKP_assert(f_int >= 0);
SKP_assert(f_int < LSF_COS_TAB_SZ_FIX );
/* Read start and end value from table */
cos_val = SKP_Silk_LSFCosTab_FIX_Q12[ f_int ]; /* Q12 */
delta = SKP_Silk_LSFCosTab_FIX_Q12[ f_int + 1 ] - cos_val; /* Q12, with a range of 0..200 */
/* Linear interpolation */
cos_LSF_Q20[k] = SKP_LSHIFT( cos_val, 8 ) + SKP_MUL( delta, f_frac ); /* Q20 */
}
dd = SKP_RSHIFT( d, 1 );
/* generate even and odd polynomials using convolution */
SKP_Silk_NLSF2A_find_poly( P, &cos_LSF_Q20[0], dd );
SKP_Silk_NLSF2A_find_poly( Q, &cos_LSF_Q20[1], dd );
/* convert even and odd polynomials to SKP_int32 Q12 filter coefs */
for( k = 0; k < dd; k++ ) {
Ptmp = P[k+1] + P[k];
Qtmp = Q[k+1] - Q[k];
/* the Ptmp and Qtmp values at this stage need to fit in int32 */
a_int32[k] = -SKP_RSHIFT_ROUND( Ptmp + Qtmp, 9 ); /* Q20 -> Q12 */
a_int32[d-k-1] = SKP_RSHIFT_ROUND( Qtmp - Ptmp, 9 ); /* Q20 -> Q12 */
}
/* Limit the maximum absolute value of the prediction coefficients */
for( i = 0; i < 10; i++ ) {
/* Find maximum absolute value and its index */
maxabs = 0;
for( k = 0; k < d; k++ ) {
absval = SKP_abs( a_int32[k] );
if( absval > maxabs ) {
maxabs = absval;
idx = k;
}
}
if( maxabs > SKP_int16_MAX ) {
/* Reduce magnitude of prediction coefficients */
maxabs = SKP_min( maxabs, 98369 ); // ( SKP_int32_MAX / ( 65470 >> 2 ) ) + SKP_int16_MAX = 98369
sc_Q16 = 65470 - SKP_DIV32( SKP_MUL( 65470 >> 2, maxabs - SKP_int16_MAX ),
SKP_RSHIFT32( SKP_MUL( maxabs, idx + 1), 2 ) );
SKP_Silk_bwexpander_32( a_int32, d, sc_Q16 );
} else {
break;
}
}
/* Reached the last iteration */
if( i == 10 ) {
SKP_assert(0);
for( k = 0; k < d; k++ ) {
a_int32[k] = SKP_SAT16( a_int32[k] );
}
}
/* Return as SKP_int16 Q12 coefficients */
for( k = 0; k < d; k++ ) {
a[k] = (SKP_int16)a_int32[k];
}
}