noise at 20hz - unreliable signal ? [resolved]

lopezst1lopezst1 United States
edited August 2020 in Ganglion
I am unable to gather data due to persistent noise at 20hz. I have moved my laptop all over the apartment and the noise will not go away. When nothing is connected to my Ganglion, all of the channels are quiet, basically flatlined. It is also that way when I attach my lead to the reference using an alligator clip. The moment I have a lead attached to my Ganglion, there is a huge amount of noise, and when viewing the FFT plot there is a large spike at 20hz. This happens even when the electrode is not attached to me and I lay it on my bed. It also happens when I have no electrode attached. I have no idea what the cause could be, anybody have any ideas?

Comments

  • wjcroftwjcroft Mount Shasta, CA
    Lopez, hi.

    What is your mains frequency there, 50 or 60 Hz?

    Has this Ganglion EVER worked for you, in any situation, location, other laptops, other OS, etc.?

    On the time series graph, can you see the amplitude of the 20Hz in microvolts? Does that correspond the the 20Hz amplitude shown on the FFT?

    Have you tried switching your leads around? What paste / gel are you using? You did not mention your ground connection. Is reference on one ear, and ground on the other? Is the 20Hz present regardless of where you place the channel electrode?

    What happens when you follow the steps of the Ganglion tutorial exactly.

    Regards,

    William

  • lopezst1lopezst1 United States
    edited April 2019
    My mains is 60hz. This ganglion has sporadically worked for me but generally the signal has been noisy. There will be brief moments where the signal clears up before becoming noisy again. When I use conductive gel it seems to work more often. I have tried switching the leads around all different ways, nothing seems to work. Reference is on one ear, ground on the other ear. 20hz is there pretty much the entire time no matter what I do. I have followed the tutorial to the best of my ability.
  • wjcroftwjcroft Mount Shasta, CA
    How many microvolts is your 20 Hz signal?

    What electrodes are you using? Have you tried gold cups with paste? It sounds like you are using dry electrodes?
  • lopezst1lopezst1 United States
    My signal varies quite a bit, from 200 mv all the way up to 1000+. I am using dry electrodes most of the time.
  • wjcroftwjcroft Mount Shasta, CA
    Ok, just to clarify, the abbreviation for microvolts is uv (u like in greek mu), and millivolts is mv. So your 20 Hz noise ranges from 200 to 1000 microvolts. That is completely extreme. There must be something wrong with your electrodes, or skin interface.

    Are you using Ultracortex, or some other system? It sounds like you are not getting enough pressure. And the fact that when you use gel, the noise disappears. This means when you are using dry electrodes alone with no gel, the skin impedance is terribly high. Have you checked your impedance with the GUI widget?

    Explain what you are trying to measure, which 10-20 sites. Do you have gold cups and paste to compare with?

    Regards,
  • lopezst1lopezst1 United States
    Let me clarify, I am getting >200 uv without an electrode even on the lead wire, same when I do have an electrode on there. I will have it laying on my bed and the signal will be going nuts. I have tried to capture EMG on my arm, face, etc. but the signal still keeps going crazy. Until I can figure out what is causing all of the noise there doesn't seem to be any reason to start affixing it to my body. The impedance check doesn't work on my GUI, it just hangs.

    Do you think I should buy new lead wires/connectors? It's the only thing I can think of that could be causing this.
  • wjcroftwjcroft Mount Shasta, CA
    I'm not getting a picture of your electrode setup. EEG electrodes are guaranteed to produce erratic and unpredictable results when NOT attached correctly to the body. By correct, I mean at least three leads: channel, reference, and ground. This is because the wires, when disconnected, act as antennas for electromagnetic noise that is always present in the environment. Some environments have more noise than others. Your 20Hz noise is likely a subharmonic of the ubiquitous 60Hz.

    A wire attached to an amplifier acts like an 'antenna'.

    The reason you need a low impedance (gel or paste) connection to skin, is that this allows the differential amplifier, to remove the noise from the environment. Differential amps work by subtracting the signal between the + and - poles. Or in your case, the channel and reference.

    I would invest in a set of gold cups and paste. You never told me what type of dry electrodes, and how they are attached to the skin.
  • wjcroftwjcroft Mount Shasta, CA
    Current update on the Ganglion impedance issue, if you are using BLED112 dongle.


    If you have a Mac available, that can run WITHOUT a dongle.
  • lopezst1lopezst1 United States
    Sorry it has been awhile since I posted this. I am able to get a relatively clean signal with a gel but rarely without it. Whenever I try dry electrodes there is a consistent 20 hz noise. My friend has a new cyton and is able to get his signal without any 20hz noise using dry electrodes. I had him try my ganglion and he was also getting a 20hz noise, so I don't think it has anything to do with my computer setup/location/openbci version. Should I just assume this is a hardware issue? I'm considering buying a cyton, if I were to do so would I be able to return it if it proves defective like my ganglion is?
  • wjcroftwjcroft Mount Shasta, CA
    Lopez, hi.

    Have you emailed contact at openbci.com, asking some of the questions on your last post (today April 29?) 

    Ganglion and Cyton analog to digital converter front ends, do have different noise immunities. But your Ganglion sounds unusually bad. You never answered my questions asked on April 5, 3 posts above. When you have your Ganglion connected to your head with the dry electrodes, using at least on channel, reference, and ground -- what is then the amplitude of the 20 Hz noise? How many microvolts? You mentioned ">200uV", but that figure was misleading, as you did not have all leads connected to the skin.

    What dry connections are you using for the reference and ground? Are these the OpenBCI or FRI ear clips? It's possible your reference electrode or clip is bad. Is this Ultracortex or something else?

    Thanks,

    William

  • lopezst1lopezst1 United States
    I will fire up the ganglion tonight and report back here with some actual figures. Thanks for your help! I will say that I have the alligator ear clips from FRI with the silver discs. I usually keep my electrodes in place using a headband. The connection is pretty secure and the electrodes don't have room to wiggle around.
  • wjcroftwjcroft Mount Shasta, CA
    One or more of the ear clips may be creating a bad connection. Did you check your impedance? Try swapping the ear clips, or measuring them with a digital voltmeter / ohmmeter. The plating on dry electrodes can also degrade, especially if they get wet with gel. The electrodes are plastic with a very thin silver plate, that silver is then chlorided. If any of the 2 coatings wear down, they will not perform correctly. 

    If the plating is ok it should look matte dark grey. If you see silvery or white, that means the chloride has gone, or even the silver.
  • lopezst1lopezst1 United States
    The plating looks ok and my friend who has a working cyton used all his equipment on my ganglion and was also seeing the 20hz noise. It does appear to be an issue with the hardware itself as far as I can tell.
  • wjcroftwjcroft Mount Shasta, CA
    Please quantize the 20 Hz noise. You might take a screenshot of the GUI, showing both FFT and time series. Assuming you have solid head connections.

    Also note, particularly with dry electrodes, the pressure with which they contact the skin, has a huge impact on the skin-electrode impedance. Experiment with applying different pressures. Also try this simultaneously with measuring the impedance values with the impedance widget.
  • lopezst1lopezst1 United States
    The impedance widget does not work for me, but I will post screenshots of the gui as soon as I can.
  • lopezst1lopezst1 United States
    Here is a picture from a session I had recently, this one is especially bad. Channel one had an electrode firmly attached to my head, the rest of the channels had no electrodes attached. Usually when no electrode is attached the line will be flat, for whatever reason this time it was bugging out. As you can see the 20hz FFT reading is off the charts:

  • wjcroftwjcroft Mount Shasta, CA
    Have you emailed contact at openbci.com? ALL the channels have bizarrely distorted waveforms. Why would there be square waves of hundreds of microvolts in amplitude on your channels 2, 3, and 4??  Your channel one noise is in the thousands of microvolts.

    If it has been like this since you've received it, there is clearly something bad / intermittent about your Ganglion. I would ask for an exchange. Your friend's Cyton works fine, so that rules out any weird interference that might be in your area.

    Mentioning @openbci.

  • lopezst1lopezst1 United States
    Oh that'd be great. I've reached out to them and they said they are reviewing, thanks for your help!
  • wjcroftwjcroft Mount Shasta, CA
    Send the image link you just posted to that same email.

    It's remotely possible your Simblee firmware could be corrupted (flash memory loses some bits). But not if it intermittently works, then not. Flash does not repair itself. But an intermittent solder joint someplace could be behind this. Have never seen such odd square waves.
  • lopezst1lopezst1 United States
    They are honoring an exchange for a new ganglion. Very happy I was able to get this resolved, thanks again for the help.
  • bciryanbciryan USA

    Chiming in for those who end up on this thread. I was also having 20HZ noise with ganglion, dry electrodes and the headband set up. The peak in the spectrum around 20 Hz was 1-2 orders of magnitude above the standard 1/f PSD shape. As wjcroft said, floating eletrodes (not connected to anything) can act as antennas. So I figured I might not have a good connection. I tightened the headband and the 20 Hz noise went away. Its pretty tight and clean signals are still a bit intermittent in the comb electrodes back (as compared to the flat electrodes on my forehead).

    TLDR: Check your connections. Make sure your electrodes are making good contact with your skin.

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