ft_preproc_highpassfilter

[Laurence Hunt]

Hi folks,

I'm trying to filter some MEG data sampled at 200Hz using ft_preproc_highpassfilter. It seems to be behaving slightly strangely. I'm calling it with
Fs = 200;
Fhp = 5;
N = 6;

and a stretch of single-channel data with ~50000 samples. However, the returned data doesn't seem to be filtered at 5Hz. 

When I go into the code and use fvtool to look at the frequency response of the filter, it doesn't have the shape I was expecting:



I wondered whether this was because of the GNU public license version of butter that was being called by fieldtrip (in preproc/private), so I took this out of my matlab path and let it find the Mathworks' butter.m, from the signal processing toolbox. This seemed to give a more sensible looking filter design in fvtool:



The Mathworks' butter.m also seemed to filter the data a bit more like I was expecting. Is this a problem with the GPL version, or am I calling the function wrong?

Cheers,
Laurence


===========================================
Laurence Hunt, DPhil Student
 Centre for Functional MRI of the Brain (FMRIB), 
University of Oxford
lhunt at fmrib.ox.ac.uk
Phone: (+44)1865-(2)22738
===========================================


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