[Eeglablist] Filtering trouble

[Jakub Szewczyk]

Hello Erin and Andreas, hello everybody,

>> Additionally, it seems I am unable to use 0.1 Hz as the lower edge of
>> my accepted band -- when I try to do the highpass step, I get the
>> following error:
>> Filter error: continuous data portion too narrow (DC removed if
highpass only)
>> Processing continuous data (307201:450306)
>> eegfilt() - performing 7680-point highpass filtering.
>> eegfilt() - highpass transition band width is 0.01 Hz.

> 0.1 Hz is an extreme filter. The shorter the transition band the higher
> the filter order/length, and the EEGLAB built-in fir filter requires the
> epoch to be longer than some multiples of filter length. Do you really
> need the cutoff frequency to be so low? Using 0.5 Hz might already help.

I seem to have the same trouble.
I can't succesfully apply highpass filters of satystyingly low
magnitude (like 0.01 Hz) in EEGLAB.
* FIR filter breaks because of memory problems (I have 2gb): at 256hz
sampling rate the lowest limit for the filter seems 0.5hz; for 128Hz
it works for 0.1hz, but not for 0.05hz.
* FFT linear filter works but introduces a very large signal distortion
* IIR filter again seems to be sensitive to sampling rate: for 256hz
breaks at 0.1hz, for 128hz at 0.05hz - in such a case it computes a
new empty dataset (filled with NaNs), reporting many "Matrix is close
to singular or badly scaled." warnings.

Is there any way to bypass this? I work on data acquired by Biosemi
amplifier, having practically no hardware highpass filters, so I need
a software highpass filter to clean the data, but with a reasonably
low threshold (0.01)
And yes, that low (0.01 Hz) filter is a must for me, since higher
highpass thresholds distort ERPs and are no longer happily accepted by
reviewers (at least in my field).

with best regards
Jakub Szewczyk
Psychophysiology Lab
Jagiellonian University, Cracow, Poland