Re: [Openeeg-list] Mains hum driving me crazy

[<so...@pk...>]

I'm not sure what your graph is showing. you say the x axis is time, what
is the y axis? What is a VEP event?

Even with the best setup, some 60 Hz is there. You have to manipulate the
data so that this is not a problem. Here is the output of my system with
eyes closed for two seconds.

http://www.pkfamily.com/users/solson/eeg/2seconds.gif

You see the "big" alpha of 1.5uV at 10 Hz. You also see about 0.7uV of 60
Hz. This is as low as my 60Hz ever gets. Sometimes it's more like 5uV. But
that doesn't cause a problem since I'm doing an FFT and only looking at
the lower frequencies. If you are averaging the entire signal before
filtering you will have a problem. Since the 60Hz is always there, then it
will always accumulate in the averaging process.

I actually like seeing the level of the 60 Hz. It is my best indication of
how well my electrodes are connected. If it is so large that it interfers
with my measurements, then my connections are not good, and my data is not
good anyway.

Scott O

>   I have a small 60hz wave which is making my EEG data worthless.
> Sometimes it's just in channel 1, sometimes channel 2, or sometimes
> both.  The fact that often one channel is clean suggests it's not a
> DRL problem.
>
>   You can see how it is killing my data:
>   http://jca3.freeshell.org/eeg/hum.html
>
>   I cleaned the electrodes (silver-based saline sponge type), checked
> the shielding and wires for any shorts or breaks, but no luck.
>
>   Is mains hum just something you can ever get rid of?  If so, what
> are the usual causes of this problem and how do you get rid of it?
>
>   Thanks,
>
>   John
>   http://jca3.freeshell.org