Ganglion noise abnormally high amplitude very often

Radha_openbciRadha_openbci United Kingdom
edited July 3 in Ganglion
I have been using Open BCI Ganglion for quite a long time now(7 months). I purchased a new board and started using it a month ago. I am using Gold cup electrodes. Since a week it has started giving more amplifier saturations and sometimes it samples very slow. Typically, now the data can look like:


The first 3 are raw from the time it was sampling slow. The last one is filtered, and I am referring to the double spike on the left that comes frequently  now. I am using bled112 dongle. What can be the issue  here?

Comments

  • wjcroftwjcroft Mount Shasta, CA
    Radha, your 1st three images appear to show lots of mains frequency noise on primarily the CP3 electrode, but other channels are also picking it up.

    Have you tried swapping your electrodes around or using new electrodes? If the gold plate gets worn or chipped, it can distort the signal quality. 

    Cleaning the paste from gold cups in a particular way, can prolong life -- by reducing any rubbing.


    Regards, 

    William

  • Radha_openbciRadha_openbci United Kingdom
    edited July 6
    Well to troubleshoot the problem, we tried various things:
    1. Changing electrodes doesn't help
    2. Streaming LSL so could be a memory issue causing packets to loose, but NO, I have 32 GB RAM
    3. Read on another forum that it can be a charge accumulation issue since our encasing is a plastic box. Our encasing is plastic but 1 side is metal (near the pins). We keep it on a plastic chair when we record. When the chair is touched the peaks definitely occur, but not when it is frequently touched. Otherwise peaks occur randomly. So we removed the box and checked, it still happens. 
    4. Connected the G pin to the metal but no luck
    5. Tried in 4 different environments, still happens

    Do not know what else to try.
  • wjcroftwjcroft Mount Shasta, CA
    In rare circumstances, the flash memory in the processor (Simblee), has a corruption / bits altered. Which can affect operation. You can reflash using the instructions here,


    The images you showed look primarily like mains noise. What types of filters are using using? Notch / bandpass?

    William

  • Radha_openbciRadha_openbci United Kingdom
    Both notch 50 Hz and band pass 2-35 . But the raw signals reflect it and despite the 50Hz notch on Open BCI GUI, it is evident always. I know the notch there is only for viewing .

    Radha
  • Radha_openbciRadha_openbci United Kingdom
    Now at least it is not sampling slow frequently, but the dual peaked high amplitude is my concern that happens abrubtly.
  • wjcroftwjcroft Mount Shasta, CA
    Does the time series graph in the OpenBCI_GUI (with notch and bandpass turned on), look similar to what you are seeing in Matlab / EEGLAB? Or does it look cleaner in the GUI? If it looks ok in the GUI but not Matlab, that could indicate something is amiss with your filtering in Matlab.

    Does the FFT graph in the GUI (without notch) show a large frequency component near your mains frequency? Is it EXACTLY 50 or 60 hertz, or some multiple (called harmonic)? Harmonics can also be subharmonics.

  • Radha_openbciRadha_openbci United Kingdom
    I can share here the raw files of what I am referring to. No in the FFT everything goes higher during those dual peaks, actually sways up and then comes back to normal. In the GUI I don't switch on the band pass. Yes, it is fairly similar.
  • wjcroftwjcroft Mount Shasta, CA
    Please share the GUI screen, WITH bandpass and notch turned on.
  • Radha_openbciRadha_openbci United Kingdom
    Now I remember the BP is always on. Nevertheless here it is:
  • wjcroftwjcroft Mount Shasta, CA
    OK, so with notch filter turned on, the GUI shows ZERO mains noise. Yet still shows spikes occurring periodically. In a previous post you said:

    "When the chair is touched the peaks definitely occur, but not when it is frequently touched. Otherwise peaks occur randomly."

    So this tends to confirm that the spikes are related to some type of electrostatic buildup.

    Your Ganglion when originally received worked fine for at least a month, according to your first post on this thread. Then last week the spikes began. What did you change? Can you replicate your original circumstances?

    I'm not sure where your spikes are coming from, it could be related to clothing, hair products, etc. In clinical settings it is advised to wash the hair before EEG sessions, but NOT to use any hair conditioners or other products. These can cause unusual distortions.

    Regards,

    William

  • Radha_openbciRadha_openbci United Kingdom
    Hi!

    Thanks for the feedback. But the same happens in now 4 different environments on all people. I don't know why the electrostatic buildup would happen even after opening the encasing.
    Before a month i was using another Ganglion and it seemed to work fine for many months. I will post here if i figure out something.

    Radha
  • wjcroftwjcroft Mount Shasta, CA
    Can you please run some tests with the old Ganglion to see if the issue disappears with your current experiment setup? That would then confirm that something has gone wrong with your new board. If that is the case then I suggest reflashing. 

    Where is the Ganglion Ground pin (D_G) connected on the head? Don't connect anything to GNDA. Some clinical EEG connects Ground to a site on the midline, for example AFz with Electro-cap. Or any unused midline such as Pz CPz Cz CFz, etc.
  • Radha_openbciRadha_openbci United Kingdom
    I connect it on forehead. Unfortunately it is not possible to test both simultaneously now as both are in different countries. I can test both in a month when i have them together.
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