[Mne_analysis] Low pass filtering questions

[Krieger, Donald N.]

Thanks very much.

Don

Don Krieger, Ph.D.
Department of Neurological Surgery
University of Pittsburgh
(412)648-9654 Office
(412)521-4431 Cell/Text
________________________________
From: Matti Hamalainen [msh at nmr.mgh.harvard.edu]
Sent: Thursday, March 14, 2013 8:45 AM
To: Alexandre Gramfort
Cc: Krieger, Donald N.; mne_analysis at nmr.mgh.harvard.edu
Subject: Re: [Mne_analysis] Low pass filtering questions


On Mar 14, 2013, at 8:30 AM, Alexandre Gramfort wrote:

hi Don,

Please bear with a further question:
If the low-pass is set at 40 Hz, then the cos^2 drop off is applied beginning at the 40 hz components and falling to zero at 45 ?  Or is it applied beginning at something like 38 hz so that the fourier coefficients are attenuated by a fact of 2 at 40 ?

I am not 100% about the C code but I bet for 40 to 45 with 0 at 45 Hz

And pardon this stupid question: Does this produce a gain of 0.0 for all higher frequencies in the filtered signal above 45?  Or is there some kind of ringing which occurs?

it does remove all frequencies about 45Hz up to some numerical errors
and if there is some ringing it will be in time. The less the stop
band (here 5) the more ringing in time domain.

Hi Don,

In the C code the default width of the lowpass transition is 5 Hz. For 40 Hz lowpass this means that the falloff starts at 37.5 Hz and ends at 42.5 Hz. You can adjust this value in the mne_browse_raw or with the --lowpassw option in mne_process_raw and mne_browse_raw.

The manual tells this in sections 4.5.1 and 4.2.1.

- Matti




---------

Matti Hamalainen, Ph.D.
Athinoula A. Martinos Center for Biomedical Imaging
Massachusetts General Hospital

msh at nmr.mgh.harvard.edu<mailto:msh at nmr.mgh.harvard.edu>
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