opus/silk/LPC_analysis_filter.c

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

#include "SigProc_FIX.h"

/*******************************************/
/* LPC analysis filter                     */
/* NB! State is kept internally and the    */
/* filter always starts with zero state    */
/* first d output samples are set to zero  */
/*******************************************/

void silk_LPC_analysis_filter(
    opus_int16                  *out,               /* O    Output signal                                               */
    const opus_int16            *in,                /* I    Input signal                                                */
    const opus_int16            *B,                 /* I    MA prediction coefficients, Q12 [order]                     */
    const opus_int32            len,                /* I    Signal length                                               */
    const opus_int32            d                   /* I    Filter order                                                */
)
{
    opus_int         ix, j;
    opus_int32       out32_Q12, out32;
    const opus_int16 *in_ptr;

    silk_assert( d >= 6 );
    silk_assert( (d & 1) == 0 );
    silk_assert( d <= len );

    for( ix = d; ix < len; ix++ ) {
        in_ptr = &in[ ix - 1 ];

        out32_Q12 = silk_SMULBB( in_ptr[  0 ], B[ 0 ] );
        /* Allowing wrap around so that two wraps can cancel each other. The rare
           cases where the result wraps around can only be triggered by invalid streams*/
        out32_Q12 = silk_SMLABB_ovflw( out32_Q12, in_ptr[ -1 ], B[ 1 ] );
        out32_Q12 = silk_SMLABB_ovflw( out32_Q12, in_ptr[ -2 ], B[ 2 ] );
        out32_Q12 = silk_SMLABB_ovflw( out32_Q12, in_ptr[ -3 ], B[ 3 ] );
        out32_Q12 = silk_SMLABB_ovflw( out32_Q12, in_ptr[ -4 ], B[ 4 ] );
        out32_Q12 = silk_SMLABB_ovflw( out32_Q12, in_ptr[ -5 ], B[ 5 ] );
        for( j = 6; j < d; j += 2 ) {
            out32_Q12 = silk_SMLABB_ovflw( out32_Q12, in_ptr[ -j     ], B[ j     ] );
            out32_Q12 = silk_SMLABB_ovflw( out32_Q12, in_ptr[ -j - 1 ], B[ j + 1 ] );
        }

        /* Subtract prediction */
        out32_Q12 = silk_SUB32_ovflw( silk_LSHIFT( (opus_int32)in_ptr[ 1 ], 12 ), out32_Q12 );

        /* Scale to Q0 */
        out32 = silk_RSHIFT_ROUND( out32_Q12, 12 );

        /* Saturate output */
        out[ ix ] = (opus_int16)silk_SAT16( out32 );
    }

    /* Set first d output samples to zero */
    silk_memset( out, 0, d * sizeof( opus_int16 ) );
}