[Mne_analysis] Low pass filtering questions

[Krieger, Donald N.]

Hi Everyone,

I use the low pass filtering build into mne_process_raw().
By default, it appears to use a window of 2048 points and a taper of 5 Hz.

I would like to make sure I understand what the filter is doing.
I assume that these mean the following:

2048 points at a time are run through an FFT.
A cosine^2 window is applied to the fourier coefficients beginning at a point where, close to the selected ½ amplitude point, for the entire set of coefficients?
Or perhaps the fourier coefficients are tapered down to zero over a 5 hz interval?
I'm definitely hazy on this part.

The resulting "filtered" fourier coefficients are then run through an inverse fourier transform to replace the original time domain signal with a low pass filtered one.

What is the correct understanding of this?
And is there information about the transfer function of the filter?

For instance, what is the fall off per octave depending on the selected ½ amplitude point and width of the taper?

Thanks for any help you can provide.

Regards,

Don

Don Krieger, Ph.D.
Department of Neurological Surgery
University of Pittsburgh

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