% Name:Ha2Hz_im
% This program is used to compute the system function from
% an analog lowpass filter using the impusle invariance transformation
% b:The numerator coefficient vector of Ha(s)
% a:The denominator coefficient vec
clear,close all
b=input('Type in the numerator coefficient vector of analog filter:');
a=input('Type in the denomintor coefficient vector of analog filter:');
T=input('Type in the sampling period :');
% omega=input('Type in the maximum analog frequency :');
[bz,az]=impinvar(b,a,1/T);
bz
az
%==========================================================================
[H,w]=freqz(bz,az);
%=================================================================
[Ha,wa]=freqs(b,a);
H0=max(abs(H));Ha0=max(abs(Ha));

subplot(222)
plot(w/pi,abs(H)/H0);
title('Amplitude response of digital filter'),
ylabel('|H|'),axis([0,1,0,1.1*max(abs(H)/H0)]),grid on
subplot(224)
plot(w/pi,20*log(abs(H)/H0));
ylabel('dB'),axis([0,1,-80,0]),
xlabel('Frequency (pi)'),grid on
subplot(221)
plot(wa/(2*pi),abs(Ha)/Ha0);
title('Amplitude response of analog filter'),
ylabel('|Ha|'),axis([0,1,0,1.1*max(abs(Ha)/Ha0)]),grid on
subplot(223)
plot(wa/(2*pi),20*log(abs(Ha)/Ha0));
ylabel('dB'),axis([0,1,-80,0]),
xlabel('Frequency (Hz)'),grid on