Compute butterworth filter order and cutoff for the desired response
characteristics. Rp is the allowable decibels of ripple in the pass
band. Rs is the minimum attenuation in the stop band.

[n, Wc] = buttord(Wp, Ws, Rp, Rs)
Low pass (WpWs) filter design.  Wp is the
pass band edge and Ws is the stop band edge.  Frequencies are
normalized to [0,1], corresponding to the range [0,Fs/2].

[n, Wc] = buttord([Wp1, Wp2], [Ws1, Ws2], Rp, Rs)
Band pass (Ws1