silk/ana_filt_bank_1.c

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

#include "SigProc_FIX.h"

/* Coefficients for 2-band filter bank based on first-order allpass filters */
static opus_int16 A_fb1_20 = 5394 << 1;
static opus_int16 A_fb1_21 = -24290; /* (opus_int16)(20623 << 1) */

/* Split signal into two decimated bands using first-order allpass filters */
void silk_ana_filt_bank_1(
    const opus_int16            *in,                /* I    Input signal [N]                                            */
    opus_int32                  *S,                 /* I/O  State vector [2]                                            */
    opus_int16                  *outL,              /* O    Low band [N/2]                                              */
    opus_int16                  *outH,              /* O    High band [N/2]                                             */
    const opus_int32            N                   /* I    Number of input samples                                     */
)
{
    opus_int      k, N2 = silk_RSHIFT( N, 1 );
    opus_int32    in32, X, Y, out_1, out_2;

    /* Internal variables and state are in Q10 format */
    for( k = 0; k < N2; k++ ) {
        /* Convert to Q10 */
        in32 = silk_LSHIFT( (opus_int32)in[ 2 * k ], 10 );

        /* All-pass section for even input sample */
        Y      = silk_SUB32( in32, S[ 0 ] );
        X      = silk_SMLAWB( Y, Y, A_fb1_21 );
        out_1  = silk_ADD32( S[ 0 ], X );
        S[ 0 ] = silk_ADD32( in32, X );

        /* Convert to Q10 */
        in32 = silk_LSHIFT( (opus_int32)in[ 2 * k + 1 ], 10 );

        /* All-pass section for odd input sample, and add to output of previous section */
        Y      = silk_SUB32( in32, S[ 1 ] );
        X      = silk_SMULWB( Y, A_fb1_20 );
        out_2  = silk_ADD32( S[ 1 ], X );
        S[ 1 ] = silk_ADD32( in32, X );

        /* Add/subtract, convert back to int16 and store to output */
        outL[ k ] = (opus_int16)silk_SAT16( silk_RSHIFT_ROUND( silk_ADD32( out_2, out_1 ), 11 ) );
        outH[ k ] = (opus_int16)silk_SAT16( silk_RSHIFT_ROUND( silk_SUB32( out_2, out_1 ), 11 ) );
    }
}