IIRFilters.ButterFilter Method (integer, array of TSample, TFilterType, boolean, TMtx, TVec, TVec, TSample)

The resulting transfer function is returned in the state-space form with A,B,C,D variables.

public double ButterFilter(int Order, double[] CutoffFreq, TFilterType FilterType, bool Analog, TMtx A, TVec B, TVec C, ref double d);

C# Example

Design a digital bandstop filter with transition band between 0.2..0.3 and 0.6..0.7 Hz and with at least 50dB attenuation in the stopband. The passband should not have more then 0.2dB ripple. The sampling frequency is 2Hz.

using Dew.Math;
using Dew.Math.Editors;
using Dew.Math.Units;
using Dew.Signal;
using Dew.Signal.Units;
using Dew.Math.Tee;
using Dew.Signal.Tee;

private void button1_Click(object sender, EventArgs e)
{
    Vector z = new Vector(0);
    Vector p = new Vector(0);
    Vector num = new Vector(0);
    Vector den = new Vector(0);
    Vector Response = new Vector(0);
    Vector FreqFr = new Vector(0);
    double[] WcArray = new double[2];
    int Order; //design a fifth order filter.

    Order = IIRFilters.ButterOrder(new double[4] { 0.2, 0.3, 0.6, 0.7 }, 0.2, 50, TFilterType.ftBandStop, ref WcArray, true);  //design analog protype
    IIRFilters.ButterFilter(Order, WcArray, TFilterType.ftBandStop, false, num, den, TIirFrequencyTransform.ftStateSpaceAnalog);

    // Alternative 1. Specify the order and the 3 dB frequencies explicitely:
    //
    //     IIRFilters.ButterFilter(5, new double[2] {0.2,0.7}, TFilterType.ftBandStop,false,num,den,TIirFrequencyTransform.ftStateSpaceAnalog);


    // Alternative 2. Specifying the 3 dB frequencies explicitely
    // will result in 3 dB ripple (and not 0.2 as requested) in the passband,
    // but one coulde always move the 3 dB frequencies a little:
    //
    //     IIRFilters.ButterFilter(5, new double[2] { 0.22, 0.68 }, TFilterType.ftBandStop, false, num, den, TIirFrequencyTransform.ftStateSpaceAnalog);

    // Alternative 3. Specifying the order explicitely
    // will not ensure 50 dB attenuation in the edges of the stopband,
    // but one can increase filter order:
    //
    //     IIRFilters.ButterFilter(10, new double[2] { 0.22, 0.68 }, TFilterType.ftBandStop, false, num, den, TIirFrequencyTransform.ftStateSpaceAnalog);

    SignalUtils.FrequencyResponse(num, den, Response, 32, false, TSignalWindowType.wtRectangular, 0);
    TeeChart.DrawIt(Response, "Frequency response", false);