%  Name: dfre.m
%  The program to draw the frequency response of discrete system
A=input('Type in the coefficient vector of denominator polynomial:');
B=input('Type in the coefficient vector of numerator polynomial:');
k=512;r=1;
p=roots(A);                     %求極點
q=roots(B);						%求零點
%===================================================================
p=p';									       %將極點列向量轉置為行向量
q=q';								    	   %將零點列向量轉置為行向量
x=max(abs([p q 1]));							%確定縱坐標范圍
x=x+0.1;
y=x;											%確定橫坐標范圍
clf
subplot(2,2,1)
hold on
axis([-x x -y y])								%確定坐標軸顯示范圍
w=0:pi/300:2*pi;
t=exp(i*w);
plot(t)											%畫單位園
axis('square')
plot([-x x],[0 0])								%畫橫坐標軸
plot([0 0],[-y y])								%畫縱坐標軸
text(0.1,x,'jIm[z]')
text(y,1/10,'Re[z]')
plot(real(p),imag(p),'x')						%畫極點
plot(real(q),imag(q),'o')						%畫零點
title('The pole-zero diagram for discrete system')	%標注標題
hold off
%===================================================================
[H,w]=freqz(B,A);
fai=angle(H);

subplot(2,2,2)
plot(w,abs(H));					%繪制幅頻特性曲線

%axis([0,0.5,0,2])

title('The amplitude reponse curve')
xlabel('Angle frequency')
ylabel('Amplitude')
subplot(2,2,4)
plot(w,fai);

%axis([0,0.5,-2,2])
axis([-max(w),max(w),min(fai),max(fai)])
title('The phase response curve')
xlabel('Angle frequency'),ylabel('Phase')