Advice on connecting multiple people using a single OBCI

Winslow_StrongWinslow_Strong
edited August 2016 in Hardware
I am part of a project that is going to study neurocorrelates of groups of people getting into states of flow together.  We are interested in connecting up to 4 people, but we can start with solving it for 2 at the moment.

I'd like to use a single OBCI board to connect multiple people, so that the sampling is synced, so that phase-dependent measurements can be made up into gamma (up to 50Hz).  We will use the mods in my other thread ( http://openbci.com/forum/index.php?p=/discussion/712/prospects-for-higher-sample-rates )  to connect OBCI via wired USB + USB isolator to allow upping the sample rate to 500Hz for the 16 channel board and 1khz for the 8 channel. 

To connect multiple people would canonically require a separate ground (Bias) and reference for each person.  

Using multiple references on a single board seems easy - Use bipolar mode, i.e. the reference for a pin is on the 'P' (top) pin and the signal on the 'N' (bottom) pin.  

I don't know if multiple Bias on one board is doable though.  Even just for 2 people using the 16channel daisy board, the Bias pins of the main and daisy are connect together, I believe.  Would there be reasonable workarounds to make it possible?  E.g. What would the consequences be for signal quality if a bias from each subject were connected together to form a common bias across them, before sending it to OBCI's single Bias pin?  

Or would a better approach be just to use a single OBCI board for each person, and somehow sync the clocks?  As far as I can tell, the OBCI data packets don't contain a timestamp, which poses a challenge for syncing the packets post hoc after they get to the PC.  It would be sensitive to variations in packet deliveries across the different USB connections (far better than the Rfduino / bluetooth packet variations, but possibly still problematic?)

Thanks for any assistance!

Comments

  • wjcroftwjcroft Mount Shasta, CA
    Winslow, hi.

    Checkout Chip @chipaudette 's description on his blog page,

    http://eeghacker.blogspot.com/2015/03/brain-controlled-shark-attack.html

    Chip mentions using the extra Bias pins on the mainboard and daisy as well as AGND. But all the Bias pins are really the same signal. So you can make a little adapter running one wire from one Bias pin to a "star" connection to all the heads in your experiment. (Such as one of their ear lobes, or mastoid, etc.)

    Yes, the way to get synced samples is with a single mainboard, plus possibly a daisy extension. Multiple mainboards, it would be hard to infer coherence because they are all running slightly different sample times and drift rates.

    The main thing the Bias pin does is to center the differential amplifier so it stays within range. Plus it injects a 180 degree out of phase very small signal to help in common mode noise cancellation. (Typically 60 hz power mains noise). But since you likely have a software notch filter at 60 hz that's handled for the most part. Bias does help reduce the amplitude of it somewhat.

    William

  • Winslow_StrongWinslow_Strong
    Thanks for the info, William.  

    So to see if I understand: Even though there are 4 Bias pins, they are all connected to each other within the 16 channel OBCI board.  So connecting an electrode from each of 4 subjects to a Bias pin will result in the same single Bias used in the processing of each subject's signals.  This single bias signal will be something resembling the average of the bias signals from each person.  This would be worse than having each subject's signals biased to their unique bias signal, since the signal from e.g. their earlobe is more relevant to the signals on their head than to the signals on their neighbors' heads.  But it might not be that big a deal, especially if we are protected from 60Hz noise.  

    Thanks a lot!
  • wjcroftwjcroft Mount Shasta, CA
    On many other amps, the Bias wire is simply called "Ground". That is it's primary purpose, to just reference the differential amplifiers + and - signals to some real world voltage potential. In many cases an EEG amp will even operate ok without the Ground line, but it's still recommended. You can try a test with your own head, connecting and disconnecting the Bias/Ground lead, valid signal still comes in.

    So it's primary purpose is as an incoming 'signal'. But many amps also inject an out of phase common mode waveform back out to the subject on the Ground wire, in an attempt to cancel some of the power mains noise. Since you already have a notch at 60 hz, this injection is slightly helpful but not critical.

    Yes, running a single Bias pin out to a 'star' ground is really equivalent to using the individual onboard 4 pins, since they are internally all connected.
  • biomurphbiomurph Brooklyn, NY
    One thing you should also make sure to try while you work on this is to 'remove' the separate channels from BIAS generation. You can do this through the GUI. It may or may not make the signal better.
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