[FieldTrip] Low-pass filter before resampling?

Wunderle, Thomas thomas.wunderle at esi-frankfurt.de
Thu Dec 12 10:46:49 CET 2013

Hi all,
I was also thinking about resampling data recently.
The reason is that I saw (as Eelke mentioned) that the Matlab "resample" function applies a Kaiser window function to lowpass filter the data before (down/resampling).
Now, I was not sure if this can create a problem for directed connectivity analysis (like Granger connectivity), because you apply a non-causal kernel which "contaminates" a
given moment in time with the future.
I'm not an expert enough to see through if this is a problem for methods like Granger which rely on predicting a signal given the data in the past.
If so, one should be careful in using non-causal filters e.g. for resampling. 
At the moment I'm using the Matlab function "resample" with the option "0" ( like: y = resample(data,D,N,0)  ), which does then nearest neighbor interpolation.
What's your opinions about that topic?


Dr. Thomas Wunderle
Ernst Strüngmann Institute (ESI) for Neuroscience
in Cooperation with Max Planck Society 
Deutschordenstrasse 46
60528 Frankfurt am Main, Germany
thomas.wunderle at esi-frankfurt.de
Tel:   +49 69 96769 519
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-----Ursprüngliche Nachricht-----
Von: fieldtrip-bounces at science.ru.nl [mailto:fieldtrip-bounces at science.ru.nl] Im Auftrag von Eelke Spaak
Gesendet: Donnerstag, 12. Dezember 2013 09:48
An: Email discussion list for the FieldTrip project
Betreff: Re: [FieldTrip] Low-pass filter before resampling?

Oops, I was looking in the wrong part of the code for ft_resampledata (usetime = 1, while I should have been looking at usefsample = 1).
There, it is clear that Matlab's resample() is being used, which tells me in the documentation "resample applies an anti-aliasing (lowpass) FIR filter to x during the resampling process. It designs the filter using firls with a Kaiser window." So, it is definitely not needed.

Sorry for filling up your inboxes :)

On 12 December 2013 09:45, Eelke Spaak <eelke.spaak at donders.ru.nl> wrote:
> Hi all,
> A quick question: in your experience, is it necessary (or advisable) 
> to explicitly apply a low-pass filter (using e.g. ft_preprocessing) 
> before calling ft_resampledata? I previously assumed ft_resampledata 
> (or the low-level implementation, pchip) was taking care of this, but 
> looking in the code I cannot find evidence for or against my 
> assumption.
> Thanks,
> Eelke
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