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mono/stereo biquadratic filter with funny filter coefficient array ```sbfunction BIQUAD input[], output[], filterCoefficients[] ``` nicole drawing !https://smilebasicsource.com/api/File/raw/23626 reimplementation ``` DEF BIQUAD2 _OU,_IN,F VAR L = LEN(_IN) 'make temporary arrays with 2 extra elements at the start DIM IN[L+2],OU[L+2] COPY IN,2,_IN 'fill first 2 elements of temp arrays with old values from F IN[0] = F[6] IN[1] = F[5] OU[0] = F[8] OU[1] = F[7] ' VAR I FOR I=2 TO L+2-1 OU[I] = F[0]*IN[I] + F[1]*IN[I-1] + F[2]*IN[I-2] - F[3]*OU[I-1] - F[4]*OU[I-2] NEXT 'store last 2 elements of arrays in F F[6] = IN[I-2] F[5] = IN[I-1] F[8] = OU[I-2] F[7] = OU[I-1] 'return output COPY _OU,OU,2,L END ``` note: even if the input/output arrays are the same array, it creates a copy before doing the operations, so the values in the input array will never be updated during the calculations F[0..4] - filter parameters F[5..8] - last values of previous calculation F[9..12] - (same as 5-8, for stereo mode) ** Examples (assumes no OPTION DEFINT): "OOH I BET YOU CAN USE BIQUAD TO GENERATE THE FIBONACCI SEQUENCE LOL" (I think this skips the first 2 values though, starting 1 2 3 5 8 ...) ``` DIM F[13] F[4]=-1:F[3]=-1 'output[n] = 0 - -1*output[n-1] - -1*output[n-2] DEF FIB(N) DIM A[N] F[8]=1 'output[-2] = 1 F[7]=0 'output[-1] = 0 BIQUAD A,A,F RETURN A END ``` generating a list from 0 to n-1: ``` DIM F[13] F[4]=1:F[3]=-2 'output[n] = -2*output[n-1] - 1*output[n-2] DEF RANGE(N) DIM A[N] F[8]=-2 'output[-2] = -2 F[7]=-1 'output[-1] = -2 BIQUAD A,A,F RETURN A END ``` array sum: ``` DIM F[13] : :F[0]=1 :F[3]=-1 'output[n] = 1*input[n] - -1*output[n-1] DEF SUM(A) DIM TEMP[LEN(A)] F[7]=0 'output[-1] = 0 BIQUAD TEMP,A,F RETURN TEMP[LEN(A)-1] END ``` low pass filter 8000Hz on 48kHz stereo data ```sb4 DIM OU[2,1000],IN[2,1000],FP[13] BQPARAM FP,#BQLPF,48000,8000,1/SQR(2) BIQUAD OU,IN,FP 'equivalent to setting biquad filter parameters: DIM OU[2,1000],IN[2,1000],FP[13] 'FP[6]=0:FP[5]=0 FP[2]=0.15505102:FP[1]=0.31010205:FP[0]=0.15505102 FP[4]=0.24040818:FP[3]=-0.62020403 'FP[8]=0:FP[7]=0 BIQUAD OU,IN,FP ```

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