Butterworth filter order selection.
Return the order of the lowest order digital Butterworth filter that loses no more than gpass dB in the passband and has at least gstop dB attenuation in the stopband.
Parameters:  wp, ws – Passband and stopband edge frequencies, normalized from 0 :
gpass – The maximum loss in the passband (dB). : gstop – The minimum attenuation in the stopband (dB). : analog – Nonzero to design an analog filter (in this case wp and :


Returns:  ord – The lowest order for a Butterworth filter which meets specs. : Wn – The Butterworth natural frequency (i.e. the “3dB frequency”). :
