butter - Find butterworth filter
call butter(hilo,cutoff,npoly,num,den)
>0 low-pass, <0 high-pass
cut-off frequency
number of poly
real(npoly)
numerator
real(npoly)
denominator
Find the numerator and denominator Z-transforms of the Butterworth filter. hilo={1.,-1.} for {low,high}-pass filter cutoff in Nyquist units, i.e. cutoff=1 for (1,-1,1,-1...)
Bandpass
sepfilterf90