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     27 
     28 /*                                                                      *
     29  * silk_biquad_alt.c                                              *
     30  *                                                                      *
     31  * Second order ARMA filter                                             *
     32  * Can handle slowly varying filter coefficients                        *
     33  *                                                                      */
     34 
     35 #ifdef HAVE_CONFIG_H
     36 #include "config.h"
     37 #endif
     38 
     39 #include "SigProc_FIX.h"
     40 
     41 /* Second order ARMA filter, alternative implementation */
     42 void silk_biquad_alt(
     43     const opus_int16            *in,                /* I     input signal                                               */
     44     const opus_int32            *B_Q28,             /* I     MA coefficients [3]                                        */
     45     const opus_int32            *A_Q28,             /* I     AR coefficients [2]                                        */
     46     opus_int32                  *S,                 /* I/O   State vector [2]                                           */
     47     opus_int16                  *out,               /* O     output signal                                              */
     48     const opus_int32            len,                /* I     signal length (must be even)                               */
     49     opus_int                    stride              /* I     Operate on interleaved signal if > 1                       */
     50 )
     51 {
     52     /* DIRECT FORM II TRANSPOSED (uses 2 element state vector) */
     53     opus_int   k;
     54     opus_int32 inval, A0_U_Q28, A0_L_Q28, A1_U_Q28, A1_L_Q28, out32_Q14;
     55 
     56     /* Negate A_Q28 values and split in two parts */
     57     A0_L_Q28 = ( -A_Q28[ 0 ] ) & 0x00003FFF;        /* lower part */
     58     A0_U_Q28 = silk_RSHIFT( -A_Q28[ 0 ], 14 );      /* upper part */
     59     A1_L_Q28 = ( -A_Q28[ 1 ] ) & 0x00003FFF;        /* lower part */
     60     A1_U_Q28 = silk_RSHIFT( -A_Q28[ 1 ], 14 );      /* upper part */
     61 
     62     for( k = 0; k < len; k++ ) {
     63         /* S[ 0 ], S[ 1 ]: Q12 */
     64         inval = in[ k * stride ];
     65         out32_Q14 = silk_LSHIFT( silk_SMLAWB( S[ 0 ], B_Q28[ 0 ], inval ), 2 );
     66 
     67         S[ 0 ] = S[1] + silk_RSHIFT_ROUND( silk_SMULWB( out32_Q14, A0_L_Q28 ), 14 );
     68         S[ 0 ] = silk_SMLAWB( S[ 0 ], out32_Q14, A0_U_Q28 );
     69         S[ 0 ] = silk_SMLAWB( S[ 0 ], B_Q28[ 1 ], inval);
     70 
     71         S[ 1 ] = silk_RSHIFT_ROUND( silk_SMULWB( out32_Q14, A1_L_Q28 ), 14 );
     72         S[ 1 ] = silk_SMLAWB( S[ 1 ], out32_Q14, A1_U_Q28 );
     73         S[ 1 ] = silk_SMLAWB( S[ 1 ], B_Q28[ 2 ], inval );
     74 
     75         /* Scale back to Q0 and saturate */
     76         out[ k * stride ] = (opus_int16)silk_SAT16( silk_RSHIFT( out32_Q14 + (1<<14) - 1, 14 ) );
     77     }
     78 }
     79