wp=0.2*pi;ws=0.3*pi;fs=50;rp=1;rs=15;
t=1/fs;
op=wp*fs;
os=ws*fs;
[N,Wn]=buttord(op,os,rp,rs,'s');
[Z,P,K]=buttap(N);
[A,B,C,D]=zp2ss(Z,P,K);
[AT,BT,CT,DT]=lp2lp(A,B,C,D,Wn);
[num1,den1]=ss2tf(AT,BT,CT,DT);
[num2,den2]=bilinear(num1,den1,fs);
[H,W]=freqz(num2,den2,50);
stem(W*fs/pi,log10(abs(H)));
grid;
xlabel('fequency Hz');
ylabel('A');