opus/silk/float/find_LPC_FLP.c

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#ifdef HAVE_CONFIG_H
#include "config.h"
#endif

#include "main_FLP.h"
#include "tuning_parameters.h"

void silk_find_LPC_FLP(
    opus_int16                      NLSF_Q15[],                         /* O    NLSFs                                       */
    opus_int8                       *interpIndex,                       /* O    NLSF interp. index for NLSF interp.         */
    const opus_int16                prev_NLSFq_Q15[],                   /* I    Previous NLSFs, for NLSF interpolation      */
    const opus_int                  useInterpNLSFs,                     /* I    Flag                                        */
    const opus_int                  firstFrameAfterReset,               /* I    Flag                                        */
    const opus_int                  LPC_order,                          /* I    LPC order                                   */
    const silk_float                x[],                                /* I    Input signal                                */
    const opus_int                  subfr_length,                       /* I    Subframe length incl preceeding samples     */
    const opus_int                  nb_subfr                            /* I    Number of subframes                         */
)
{
    opus_int     k;
    silk_float   a[ MAX_LPC_ORDER ];

    /* Used only for NLSF interpolation */
    double      res_nrg, res_nrg_2nd, res_nrg_interp;
    opus_int16   NLSF0_Q15[ MAX_LPC_ORDER ];
    silk_float   a_tmp[ MAX_LPC_ORDER ];
    silk_float   LPC_res[ ( MAX_FRAME_LENGTH + MAX_NB_SUBFR * MAX_LPC_ORDER ) / 2 ];

    /* Default: No interpolation */
    *interpIndex = 4;

    /* Burg AR analysis for the full frame */
    res_nrg = silk_burg_modified_FLP( a, x, subfr_length, nb_subfr, FIND_LPC_COND_FAC, LPC_order );

    if( firstFrameAfterReset ) {
        silk_bwexpander_FLP( a, LPC_order, FIND_LPC_CHIRP_FIRST_FRAME );
    } else {
        silk_bwexpander_FLP( a_tmp, LPC_order, FIND_LPC_CHIRP );
    }

    if( useInterpNLSFs && !firstFrameAfterReset && nb_subfr == MAX_NB_SUBFR ) {
        /* Optimal solution for last 10 ms; subtract residual energy here, as that's easier than        */
        /* adding it to the residual energy of the first 10 ms in each iteration of the search below    */
        res_nrg -= silk_burg_modified_FLP( a_tmp, x + ( MAX_NB_SUBFR / 2 ) * subfr_length,
            subfr_length, MAX_NB_SUBFR / 2, FIND_LPC_COND_FAC, LPC_order );

        silk_bwexpander_FLP( a_tmp, LPC_order, FIND_LPC_CHIRP );

        /* Convert to NLSFs */
        silk_A2NLSF_FLP( NLSF_Q15, a_tmp, LPC_order );

        /* Search over interpolation indices to find the one with lowest residual energy */
        res_nrg_2nd = silk_float_MAX;
        for( k = 3; k >= 0; k-- ) {
            /* Interpolate NLSFs for first half */
            silk_interpolate( NLSF0_Q15, prev_NLSFq_Q15, NLSF_Q15, k, LPC_order );

            /* Convert to LPC for residual energy evaluation */
            silk_NLSF2A_FLP( a_tmp, NLSF0_Q15, LPC_order );

            /* Calculate residual energy with LSF interpolation */
            silk_LPC_analysis_filter_FLP( LPC_res, a_tmp, x, 2 * subfr_length, LPC_order );
            res_nrg_interp =
                silk_energy_FLP( LPC_res + LPC_order,                subfr_length - LPC_order ) +
                silk_energy_FLP( LPC_res + LPC_order + subfr_length, subfr_length - LPC_order );

            /* Determine whether current interpolated NLSFs are best so far */
            if( res_nrg_interp < res_nrg ) {
                /* Interpolation has lower residual energy */
                res_nrg = res_nrg_interp;
                *interpIndex = (opus_int8)k;
            } else if( res_nrg_interp > res_nrg_2nd ) {
                /* No reason to continue iterating - residual energies will continue to climb */
                break;
            }
            res_nrg_2nd = res_nrg_interp;
        }
    }

    if( *interpIndex == 4 ) {
        /* NLSF interpolation is currently inactive, calculate NLSFs from full frame AR coefficients */
        silk_A2NLSF_FLP( NLSF_Q15, a, LPC_order );
    }

    silk_assert( *interpIndex == 4 || ( useInterpNLSFs && !firstFrameAfterReset && nb_subfr == MAX_NB_SUBFR ) );
}