%program p309
wp=2000*2*pi;
ws=3000*2*pi;
Rp=3;
Rs=15;
Nn=128;
Fs=10000;
[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]=bilinear(b,a,Fs)
freqz(bz,az,Nn,Fs)