'beat pattern' noise in the signal ? [resolved]
amontserrat
mx
Hi,
there is something wrong with the signal acquired with the Ultracortex "Mark IV" EEG Headset. In the example attached, you can see that some electrodes has a noise like a rosary beads. Any idea about how to fix it?
Thanks in advanced.
Comments
Amont, hi.
What device is this? Do the channels look better in the OpenBCI_GUI, with the mains (50 or 60 Hz) notch filter turned on? My guess looking at this, is that you have mains noise, but you are not filtering it out. Is this being plotted with Brainflow? There are filters available with Brainflow:
https://brainflow.readthedocs.io/en/stable/Examples.html#python-signal-filtering
https://brainflow.readthedocs.io/en/stable/UserAPI.html#brainflow-data-filter ['notch' same as 'bandstop']
Regards, William
Thanks for your prompt response. The device I'm using is shown in this link.
I haven't filtered anything yet. It's the raw signal acquired with the CytonStopStreaming function provided by this GitHub link. The example is plotted with Matlab. What struck me was that in some electrodes there is not this noise, as in the attached image.
I will try the filters you suggest, thanks for your help.
You need to remove BOTH the DC offset, AND the mains noise. This is typically done with a combination of say a .5 Hz to 45 Hz bandpass, and a 50 or 60 Hz notch.
https://openbci.com/forum/index.php?p=/discussion/201/large-millivolt-data-values-fbeeg-full-band-eeg
That fixed it, thanks. Although I don't see the reason to use the notch filter once applied the bandpass.
@amontserrat With the Notch filter applied, you are free to try other Bandpass filter options and do A-B comparisons for your particular use case(s).
Richard and Amont, thanks.
Also see this diagram of the filter band edges, depending on filter order. The band edge can be quite wide.
https://en.wikipedia.org/wiki/Butterworth_filter#Transfer_function
Since the mains frequency is SO huge, the additional notch filter is frequently a good idea. Experiment around with the GUI to see how that works. However GUI does not let you experiment with filter orders. There are some online tools that let you visualize this.
William