[Eeglablist] Problem understanding my EEG recorder filters

[Enrico Fratto]

Good afternoon,
First, thank you the EEGLAB developers and the community all for the
precious opportunity of gathering expert advice.

I am a clinical clinical neurologist, and in the last few weeks I am
beginning to study EEG spectral analysis. My unit uses CADWEL ARC Apollo 32
channels amplifier for EEG recording. I know from the producer that the
amplifier bandpass can be set as wide as 0.16 to 100 Hz. Specifically, I
have been working with EEGs originally recorded with a 0.5-70 bandpass, 256
Hz sampling rate and notch filter ON. I have been told these recording
settings to correspond to the actual hardware filters affecting the
recorded EEG, so heavily cutting frequencies below 0.5 and above 70.
However, when I extract the full edf from the software and I analyse it
with EEGLAB or EDFBrowser, the spectrum seems to contain the whole array of
frequencies below 0.5 and above 70 without any true slope or filter effect.
For instance, 50 Hz peak and its 100 Hz harmonic are dominant in the
spectrum and the actual EEG is unreadable at its raw state.

For these reasons it seems to me that the filters that I set during the
recording are only active at the post-recording state (I.e. digital
filter).
I know that digital systems often use wide bandpass (i.e. 0.1 -100 Hz)
allowing more signal in prior to filter it digitally.

My concern is that I do not know what HF and LF physical filters my
amplifier uses at the acquisition stage and so what  frequency array truly
is allowed, i.e. what is the passband prior to digital filtering.

Does Anyone have an idea on how I might proceed? Am I overlooking some
other issues?

I sincerely apologise if I have said anything wrong, or if I asked a stupid
question; I am new to signal analysis and possibly I need to build some
basic knowledge, but I have not been able to solve this doubt by myself. I
basically have no informatic/engineering skills.

I thank whoever will answer in advance.
Enrico Fratto, MD
Institute of Neurology,
Department of Medical and Surgical sciences
Università Magna Graecia, Catanzaro, Italy