uses MtxExpr, Math387, MtxVec, MtxVecTee, MtxVecEdit,
         LinearSystems, IirFilters, SignalUtils;

    procedure TForm42.Button1Click(Sender: TObject);
    var z,p,num,den,Response: Vector;
        Order: integer;
        k,Wc: Double;
    begin
        Order := 4;//design a fourth order filter.
        EllipticAnalog(Order,0.1,30,z,p,k);  //design analog protype
        Bilinear(z,p,k,2); //bilinear with sampling frequency 2Hz.
        Wc := 0.5;   //desired cutoff at 0.5 Hz.

        LowpassToLowpassZ(z,p,k,Wc,BilinearUnwarp(1)); //move cutoff to 0.5 Hz.
        ZeroPoleToTransferFun(num,den,z,p,k);

    //Alternative:
    //       ...
    //      ZeroPoleToTransferFun(num,den,z,p,k);
    //      LowpassToLowpassZ(num,den,Wc,BilinearUnwarp(1)); //move cutoff to 0.5 Hz.

        FrequencyResponse(num,den,Response,64);
        DrawIt(Response);
    end;

    #include "MtxExpr.hpp"
    #include "MtxVecEdit.hpp"
    #include "MtxVecTee.hpp"
    #include "SignalUtils.hpp"
    #include "IirFilters.hpp"
    #include "LinearSystems.hpp"

    void __fastcall TForm1::BitBtn1Click(TObject *Sender)
    {
      sVector z,p,num,den,Response;
      int   Order;
      double k,Wc;

      Order = 4;//design a fourth order filter.
      EllipticAnalog(Order,0.1,30,z,p,k);  //design analog protype
      Bilinear(z,p,k,2); //bilinear with sampling frequency 2Hz.
      Wc = 0.5;   //desired cutoff at 0.5 Hz.

      LowpassToLowpassZ(z,p,k,Wc,BilinearUnwarp(1)); //move cutoff to 0.5 Hz.
      ZeroPoleToTransferFun(num,den,z,p,k);

    //Alternative:
    //       ...
    //      ZeroPoleToTransferFun(num,den,z,p,k);
    //      LowpassToLowpassZ(num,den,Wc,BilinearUnwarp(1)); //move cutoff to 0.5 Hz.

      FrequencyResponse(num,den,Response,64);
      DrawIt(Response);
    }