%例程4－2 基于雙線性Z變換法設計數字濾波器

clear all;
fp=100;
fs=300;
Fs=1000;
rp=3;
rs=20;
%
wp=2*pi*fp/Fs;
ws=2*pi*fs/Fs;
Fs=Fs/Fs;
%Firstly to finish frequency prewarping;
wap=tan(wp/2);
was=tan(ws/2);
[n,wn]=buttord(wap,was,rp,rs,'s')
%Note:'s'!
[z,p,k]=buttap(n);
[bp,ap]=zp2tf(z,p,k)
[bs,as]=lp2lp(bp,ap,wap)
%Note:s=(2/Ts)(z-1)(z+1);Ts=1,that is 2fs=1,fs=0.5;
[bz,az]=bilinear(bs,as,Fs/2)
[h,w]=freqz(bz,az,256,Fs*1000);
plot(w,abs(h));
grid on;