% Digital Filter Specifications:
wp=0.2*pi;                     %   digital Passband freq in rad 
ws=0.3*pi;                     %   digital Stopband freq in rad
Rp=1;                          %   Passband ripple in dB
As=15;                         %   Stopband attenuation in dB
% Analog Prototype Specifications:Inverse mapping for frequencies
T=1;Fs=1/T;                    %      Set T=1
OmegaP=(2/T)*tan(wp/2);        %      Prewarp Prototype Passband freq
OmegaS=(2/T)*tan(ws/2);        %      Prewarp Prototype Stopband freq
% Analog Butterworth Prototype Filter Calculation:
[cs,ds]=afd_butt(OmegaP,OmegaS,Rp,As);
% ***Butterworth Filter Order=6
% Bilinear transformation:
[b,a]=bilinear(cs,ds,Fs);
[C,B,A]=dir2cas(b,a);
[db,mag,pha,grd,w]=freqz_m(b,a);