wp=0.2*pi;Rp=1;ws=0.3*pi;As=15;
[n,wn]=buttord(wp,ws,Rp,As)
[b,a]=butter(n,wn);
[db,mag,pha,grd,w]=freqz_m(b,a,0.5*pi);
figure(3);
subplot(2,2,1),plot(w/pi,mag);title('幅度');
subplot(2,2,2),plot(w/pi,db);title('幅度 in db');
subplot(2,2,3),plot(w/pi,pha/pi);title('相位');
subplot(2,2,4),plot(w/pi,grd),title('群延遲');
figure(4)
freqz(b,a);