[Eeglablist] EEGLAB, BioSemi, BioSig, and means

[Tero Hakala]

Hi,

> See the images "raw-spectrum.jpg" and "raw-spectrumZoomed.jpg" for
> examples (these data are from different subjects, with the "zoomed"
> version consisting of a different number of time points than the
> non-zoomed version so you can see the saw-toothed shape).
>
> http://darb.ketyov.com/files/raw-spectrum.jpg
> http://darb.ketyov.com/files/raw-spectrumZoomed.jpg
>
> We suspected several possible causes, but in the end tracked it back
> to an issue with means (once again). More specifically, somewhere
> along the way there were some rounding errors being introduced,
> resulting in some strange spectra.

Seems rather familiar. I always get the same kind of results when 
importing raw biosemi data. However, after band-pass filtering the data 
(using standard fir-filter to cut 0-1 Hz and also very high frequencies) 
seems to remove the "effect". As this is what we always do anyway, I 
haven't worried it too much. I first thought it might be due to some 
weird stuff happening at 0-1hz range and saw-tooth spectra would be the 
higher order harmonics..

-t