silk/silk_resampler_private_ARMA4.c

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

#include "silk_SigProc_FIX.h"
#include "silk_resampler_private.h"

/* Fourth order ARMA filter                                             */
/* Internally operates as two biquad filters in sequence.               */

/* Coeffients are stored in a packed format:                                                        */
/*    { B1_Q14[1], B2_Q14[1], -A1_Q14[1], -A1_Q14[2], -A2_Q14[1], -A2_Q14[2], gain_Q16 }            */
/* where it is assumed that B*_Q14[0], B*_Q14[2], A*_Q14[0] are all 16384                           */

void silk_resampler_private_ARMA4(
    opus_int32                        S[],            /* I/O: State vector [ 4 ]                        */
    opus_int16                        out[],            /* O:    Output signal                            */
    const opus_int16                    in[],            /* I:    Input signal                            */
    const opus_int16                    Coef[],            /* I:    ARMA coefficients [ 7 ]                 */
    opus_int32                        len                /* I:    Signal length                            */
)
{
    opus_int32 k;
    opus_int32 in_Q8, out1_Q8, out2_Q8, X;

    for( k = 0; k < len; k++ ) {
        in_Q8  = silk_LSHIFT32( (opus_int32)in[ k ], 8 );

        /* Outputs of first and second biquad */
        out1_Q8 = silk_ADD_LSHIFT32( in_Q8,   S[ 0 ], 2 );
        out2_Q8 = silk_ADD_LSHIFT32( out1_Q8, S[ 2 ], 2 );

        /* Update states, which are stored in Q6. Coefficients are in Q14 here */
        X      = silk_SMLAWB( S[ 1 ], in_Q8,   Coef[ 0 ] );
        S[ 0 ] = silk_SMLAWB( X,      out1_Q8, Coef[ 2 ] );

        X      = silk_SMLAWB( S[ 3 ], out1_Q8, Coef[ 1 ] );
        S[ 2 ] = silk_SMLAWB( X,      out2_Q8, Coef[ 4 ] );

        S[ 1 ] = silk_SMLAWB( silk_RSHIFT32( in_Q8,   2 ), out1_Q8, Coef[ 3 ] );
        S[ 3 ] = silk_SMLAWB( silk_RSHIFT32( out1_Q8, 2 ), out2_Q8, Coef[ 5 ] );

        /* Apply gain and store to output. The coefficient is in Q16 */
        out[ k ] = (opus_int16)silk_SAT16( silk_RSHIFT32( silk_SMLAWB( 128, out2_Q8, Coef[ 6 ] ), 8 ) );
    }
}