freeswitch/libs/silk/src/SKP_Silk_resample_1_3.c

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/*                                                                      *
 * SKP_Silk_resample_1_3.c                                            *
 *                                                                      *
 * Downsamples by a factor 3                                            *
 *                                                                      *
 * Copyright 2008 (c), Skype Limited                                    *
 * Date: 081113                                                         *
 *                                                                      */
#include "SKP_Silk_SigProc_FIX.h"

#define OUT_SUBFR_LEN        80

/* Downsamples by a factor 3 */
void SKP_Silk_resample_1_3(
    SKP_int16            *out,       /* O:   Fs_low signal  [inLen/3]                */
    SKP_int32            *S,         /* I/O: State vector   [7]                      */





    const SKP_int16      *in,        /* I:   Fs_high signal [inLen]                  */
    const SKP_int32      inLen       /* I:   Input length, must be a multiple of 3   */
)
{
    SKP_int      k, outLen, LSubFrameIn, LSubFrameOut;
    SKP_int32    out_tmp, limit = 102258000; // (102258000 + 1560) * 21 * 2^(-16) = 32767.5
    SKP_int32    scratch0[ 3 * OUT_SUBFR_LEN ];
    SKP_int32    scratch10[ OUT_SUBFR_LEN ], scratch11[ OUT_SUBFR_LEN ], scratch12[ OUT_SUBFR_LEN ];
    /* coefficients for 3-fold resampling */
    const SKP_int16 A30[ 2 ] = {  1773, 17818 };
    const SKP_int16 A31[ 2 ] = {  4942, 25677 };
    const SKP_int16 A32[ 2 ] = { 11786, 29304 };

    /* Check that input is multiple of 3 */
    SKP_assert( inLen % 3 == 0 );

    outLen = SKP_DIV32_16( inLen, 3 );
    while( outLen > 0 ) {
        LSubFrameOut = SKP_min_int( OUT_SUBFR_LEN, outLen );
        LSubFrameIn  = SKP_SMULBB( 3, LSubFrameOut );

        /* Low-pass filter, Q15 -> Q25 */
        SKP_Silk_lowpass_short( in, S, scratch0, LSubFrameIn );

        /* De-interleave three allpass inputs */
        for( k = 0; k < LSubFrameOut; k++ ) {
            scratch10[ k ] = scratch0[ 3 * k     ];
            scratch11[ k ] = scratch0[ 3 * k + 1 ];
            scratch12[ k ] = scratch0[ 3 * k + 2 ];
        }

        /* Allpass filters */
        SKP_Silk_allpass_int( scratch10, S + 1, A32[ 0 ], scratch0,  LSubFrameOut );
        SKP_Silk_allpass_int( scratch0,  S + 2, A32[ 1 ], scratch10, LSubFrameOut );

        SKP_Silk_allpass_int( scratch11, S + 3, A31[ 0 ], scratch0,  LSubFrameOut );
        SKP_Silk_allpass_int( scratch0,  S + 4, A31[ 1 ], scratch11, LSubFrameOut );

        SKP_Silk_allpass_int( scratch12, S + 5, A30[ 0 ], scratch0,  LSubFrameOut );
        SKP_Silk_allpass_int( scratch0,  S + 6, A30[ 1 ], scratch12, LSubFrameOut );

        /* Add three allpass outputs */
        for( k = 0; k < LSubFrameOut; k++ ) {
            out_tmp = scratch10[ k ] + scratch11[ k ] + scratch12[ k ];
            if( out_tmp - limit > 0 ) {
                out[ k ] = SKP_int16_MAX;
            } else if( out_tmp + limit < 0 ) {
                out[ k ] = SKP_int16_MIN;





            } else {
                out[ k ] = (SKP_int16) SKP_SMULWB( out_tmp + 1560, 21 );
            }
        }

        in     += LSubFrameIn;
        out    += LSubFrameOut;
        outLen -= LSubFrameOut;
    }
}