%wp=0.2*pi; ws=0.3*pi; ap=1; as=15; T=1;
function IIR(wp,ws,ap,as,T)
x=[-4,-2,0,-4,-6,-4,-2,-4,-6,-6,-4,-4,-6,-6,-2,6,12,8,0,-16,-38,-60,-38,-60,-84,-90,-66,-32,-4,-2,-4,8,12,12,10,6,6,6,4,0,0,0,0,0,-2,-4,0,0,0,-2,-2,0,0,-2,-2,0,0,-2,-2,-2,-2,0];

[a,b]=butt(wp,ws,ap,as,T);
[h,w]=freqz(b,a);
mag=abs(h);
db=20*log10((mag+eps)/max(mag));
subplot(2,2,1);
plot(w/pi,db);
grid on
xlabel('w/pi');
ylabel('20log|H(esp(jw))|');
title('IIR濾波器的幅頻曲線');

m=length(x);
n=1:m;
subplot(2,2,2);
plot(n,x);
grid on
xlabel('n');
ylabel('x(n)');
title('濾波前心電圖x(n)');

y=filter(b,a,x);
subplot(2,2,4);
plot(n,y,'r');
grid on
xlabel('n');
ylabel('y(n)');
title('經IIR濾波后心電圖y(n)');