Say I have a digital butterworth lowpass filter with an order of 3 and a cutoff of 4Hz at -3dB. I understand that this filter should have a roll-off of 6*3 = 18dB/Octave.
If I now apply the filter a second time in the reverse direction (filtfilt in python scipy) in order to cancel the phase distortion, do I now have a filter with a roll-off 18*2 = 36dB/Octave? Is the cutoff now -6dB at 4Hz? Is there an easy way to find the cutoff at -3dB? Do I still call it a 3rd order filter?
do I now have a filter with a roll-off 18*2 = 36dB/Octave?
I believe so, but why not check and see?
Is the cutoff now -6dB at 4Hz?
Yes, but again, easy to confirm.
Is there an easy way to find the cutoff at -3dB?
Depends on what you consider easy. You need to find the the 1.5dB cutoff of the original filter, or measure.
Do I still call it a 3rd order filter?
It defies my intuition to talk about the order of this filter, but according to these docs for the equivalent matlab function, it would be a 6th order filter.
