[Eeglablist] Question about filter order

[Andreas Widmann]

Hi,

the order of high- and lowpass filtering should basically not make any difference apart from rounding errors. Please post your EEGLAB version and the output of the eegh command (typed on the command line) after replicating your problem.

Bandpass filtering will give different results as filter order/transition band width is then estimated from the lower highpass cutoff frequency. Bandpass filtering can reduce passband ripple, but effective lowpass transition band width can be shorter than necessary in bandpass filtering (possibly resulting in stronger filter effects/artifacts). So, depends on your particular application/preference.

In case you use the default automatic estimation of FIR filter order in the Basic FIR filter, please note that the applied algorithm for the automatic estimation of filter order is conceptually flawed for both firls and fir1 filters. I do not recommend using the automatic estimation of FIR filter order. In any case, you should always check the filter's frequency response (checking the "Plot the filter frequency response" checkbox) whether it matches your expectations.

Best,
Andreas

Am 26.11.2012 um 18:53 schrieb Makoto Miyakoshi <mmiyakoshi at ucsd.edu>:

> Dear Shirly,
> 
> Actually I don't know what difference it would make if I do it
> highpass first or lowpass first.
> 
> If I remember correctly, when you apply bandpass filter the function
> tells you that if you can't run it because of memory issue, high-pass
> filter first and then low-pass filter second. Since then I usually
> apply highpass first.
> 
> Does anybody in the list know about this?
> 
> Makoto
> 
> 2012/11/20 shirly xu <shirlyxulin at gmail.com>:
>> Hello, I used Tools->Filter the data->Basic FIR filter to filter my EEG
>> data.
>> 
>> I found that, if I exchange the order of the pass band (use Lower edge of
>> the frequency pass band first, then use Higher edge of the frequency pass
>> band,  or  use Higher edge of the frequency pass band first,  then use
>> Lower edge of the frequency pass band), the results are different from each
>> other.
>> 
>> Which order is better? Or may I use low pass and high pass simultaneously?
>> 
>> 
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> 
> 
> -- 
> Makoto Miyakoshi
> JSPS Postdoctral Fellow for Research Abroad
> Swartz Center for Computational Neuroscience
> Institute for Neural Computation, University of California San Diego
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