%program p310
wp=0.2*pi;
ws=0.3*pi;
Rp=1;
Rs=15;
Ts=0.01;
Nn=128;
Wp=wp/Ts;%convert digital freq. to analog freq.
Ws=ws/Ts;
[N,Wn]=buttord(Wp,Ws,Rp,Rs,'s')%compute order and cutoff freq.
[z,p,k]=buttap(N);%compute the analog filter
[Bp,Ap]=zp2tf(z,p,k);
[b,a]=lp2lp(Bp,Ap,Wn);
[bz,az]=impinvar(b,a,1/Ts)
freqz(bz,az,Nn,1/Ts)