[Mne_analysis] Low pass filtering questions

[Matti Hamalainen]

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
mhamalainen at partners.org






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