% Program P7_1
% Design of a Butterworth Bandstop Digital Filter
Ws = [0.4 0.6]; Wp = [0.2 0.8]; Rp = 0.4; Rs = 50;
% Estimate the Filter Order
[N1, Wn1] = buttord(Wp, Ws, Rp, Rs); 
% Design the Filter
[num,den] = butter(N1,Wn1,'stop'); 
% Display the transfer function
disp('Numerator Coefficients are ');disp(num);
disp('Denominator Coefficients are ');disp(den);
% Compute the gain response
[g, w] = gain(num,den); 
% Plot the gain response
plot(w/pi,g);grid 
axis([0 1 -60 5]);
xlabel('\omega /\pi'); ylabel('Gain in dB');
title('Gain Response of a Butterworth Bandstop Filter');
