[SciPy-user] filtering without phase shift

David Cournapeau david@ar.media.kyoto-u.ac...
Tue Jul 24 03:15:21 CDT 2007

Stef Mientki wrote:
> Bryan Cole wrote:
>> The easiest way to do non-phase-shifted filtering is to use FFTs i.e.
>> FFT -> Low Pass -> iFFT
>> The only reason to move to time-domain filters is if you must do
>> filtering in real-time where the complete dataset is (yet) unknown and
>> you must obey the laws of causality.
>> Using a frequency domain filter, you can have whatever cutoff you like.
> I'm a bit rusty, but isn't there a real danger by filtering in the 
> frequency domain,
> creating a rectangle cutoff in the frequency domain,
> leads to an infinite ringing signal in the time-domain.
I agree. It depends of course of the type of signals you have to filter 
and what you want to get from them, but I would not advise FFT filtering 
as THE method for filtering.
> The normal way of getting zero phaseshift,
> is to apply the filter twice, once normally and once in reversed order.
That's what I learnt too, but I am a bit rusty on those too :)


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