avoiding noise with external trigger ? [resolved]
Hello,
I was using Ultracortex Mark IV to measure EEG while a person clicks a button. Every time a button is clicked, I send a digital HIGH to the Cyton digital input from Arduino, this gives me a way to mark events. I encountered two major issues that maybe you guys can help me resolve @wjcroft, @retiutut.
1) When I plot the raw data the signal looks like it's mostly noise, see Figure 3 and Figure 5 (zoomed in on two of the EEG channels, think they look better), and Figure 6 (they look much worse). After filtering the channels on Figure 5, they look like Figure 7 and zoomed in Figure 8. I thought I had a good connection and the different sessions I tried look slightly different from one another for better or worse. I can keep trying to have a better one but I first wanted to see if anyone had a similar issue and where to start solving it.
2) I was having an interference from the digital signal (see Figure 4, but after extensive search on the forum, I think using a coax and an optoisolator may have resolved it, please let me know if there are other methods I can use for measuring a clean EEG with push button triggers.
Thank you for your time. 
Comments
MCM, hi.
Yes, optoisolator is the way to go. Glad you got it working. No need to post all the ways it "did not work" before you used the optoisolator.
William
Hi @wjcroft,
Thanks for reaching out.
All the other pictures except the last one is after the optoisolator, I had it there for comparison.
So are the 3rd and 4th signals good EEG signals, because the 3rd one especially look likes like it's just noise (see the oscillation)? And is the baseline wander typical?
The 3rd image has TONS of 60 Hz mains noise, can you see that by counting the peaks? Are you using a 60 Hz notch filter?
Re: DC offset, read:
https://openbci.com/forum/index.php?p=/discussion/201/large-millivolt-data-values-fbeeg-full-band-eeg
So typical filtering will do say a bandpass from .5 Hz to 45 Hz, then a notch at your mains frequency. The .5 high pass removes the DC offset.
Yeah that makes sense.
How about the 4th one? I filtered that with a bandpass filter (0.5Hz - 50Hz) and got 5th one? Is that good?
You should always still use a notch filter. Harmonics and subharmonics of the mains frequency will otherwise contaminate your EEG. Bandpass filters, filters in general have a "filter order" term. This determines sharpness of the bandpass cutoff skirts. As well as how much suppression happens outside the band. A notch filter will always surpass what is possible with lowpass or bandpass in mains filtering situations.
Any time you send a trigger from an external device (with power) to the Cyton, it's imperative to use an OptoIsolator.
In the Video Experiment Doc, a button is used. Though, it draws electricity from the Cyton itself, so there is no need to use an optoisolator for this specific kind of use case.
https://docs.openbci.com/Examples/VideoExperiment/
Take Care,
RW