[Eeglablist] about the filter setting
arno at salk.edu
Fri Feb 27 09:01:10 PST 2004
>Hi, I'm novice to the signal processing and trying to analyse my EEG data. I'd like to know what kind of filter it uses to filter the EEG data. I've tested with/without signal processing toolbox in MATLAB and the results came out to be different. I wanted to filter the data between 1 and 20 Hz but with signal processing toolbox, it won't let me set high frequency cutoff as 1, while I was able to do it without the tooolbox. It seemed that it uses different filter setting when I have signal processing toolbox than simple fft filter. I'll appreciate any help. Thnax!
Yes, when you do not have the signal processing toolbox, the process is
to apply a fourrier transform and remove the frequencies (set them to 0)
you want to filter out, then apply an inverse fourrier transform. This
type of filter have a smaller drop off in amplitude over filtered
regions (about 10dB) than standard filtering methods (more than 30dB).
If you use the signal processing toolbox, we have to define a transition
band (from the passband region to the stopband region). We set the
transition band by multiplying the cut-off frequency by a factor.
Sometimes, when the cutoff frequency is very low or very high, the
transition band does not fit in the frequency range or it becomes too
small and the signal processing toolbox function cannot implement it. We
did not find any rule to set transition bands, but if you (or someone
else) know any, we will be glad to implement it.
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*Arnaud Delorme, Ph.D.*
Computational Neurobiology Lab, Salk Institute
10010 North Torrey Pines Road
La Jolla, CA 92037 USA
*Tel* : /(+1)-858-458-1927 ext 15/
*Fax* : /(+1)-858-458-1847/
*Web page *: www.sccn.ucsd.edu/~arno <http://www.sccn.ucsd.edu/%7Earno>
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