how to calculate impedance (in other software)?

khofstadterkhofstadter Colchester, U.K.
in Software
hello! 

in an other software (SuperCollider) to calculate impedance we 

1. send the following message to the board (based on the tutorial):
~openbci.impedance(channel: 1, pchan: 1, nchan: 0);

2. then we see the spike in the fft plot; 

Now, what is the next step, how do I calculate the kOhms for the channels? 

Thanks! k

P.s: 

1) should I check impedance one by one on each channel or can I check them simultaneously? 
2) when do I use P and when N channels? 

Comments

  • wjcroftwjcroft Mount Shasta, CA
    Kris, it looks like the code is here, line 145


    The default electrode setup for EEG uses only the IN1N through IN8N pins (closest to board). SRB2 (also closest to board) is then bussed to all the IN1P through IN8P pins.

    You could check all at once or one at a time. For comparison try the same operation with the GUI, turning on impedance check for multiple channels does not seem to create cross talk on others; compared to turning on individually.

    William

  • khofstadterkhofstadter Colchester, U.K.
    Thanks William. I'll try... 
  • khofstadterkhofstadter Colchester, U.K.
    Hi William, 

    I think I might need some more guidance. I made a short video which might help understand what I struggle with: 


    Based on how the messages 'z 1 1 0 Z' and 'z 1 0 1 Z' effect the plot in another software (see pictures below) I think the OpenBCI_GUI when pressing the impedance button sends the later message, 'z 1 0 1 Z', correct? 

    https://www.dropbox.com/s/fzo7f4zmi67g5iz/20190731-sc-impedance01.jpg?dl=0
    https://www.dropbox.com/s/3r1crhp7yezch63/20190731-sc-impedance02.jpg?dl=0

    Thanks! k



  • khofstadterkhofstadter Colchester, U.K.
    quick update: 

    now when i measure impedance on one channel, only one channel shows the peak in the fft plot: 


    i will investigate a bit .... 

  • khofstadterkhofstadter Colchester, U.K.
    ... now, when pressing the impedance button in the gui, the 31.5hz signal is only introduced to the relevant channel. Good. 


    My understanding at the moment from the other software is that 

    message 'z 1 1 0 Z' adds the 31.5hz signal to all channels (perhaps all active channels) and 

    message 'z 1 0 1 Z' adds the 31.5hz signal to only channel 1. Am I correct? 

    Maybe I remove the wifi shield from the cyton as these tests are all done on the serial communication. 

    Thanks, sorry for all the replies. k
  • wjcroftwjcroft Mount Shasta, CA
    Kris, hi.

    The SDK doc states:


    z (CHANNEL, PCHAN, NCHAN) Z

    So it only operates on one channel at a time. With the default Cyton settings, all the P channel pins are bussed together on SRB2. And the N channel pins go to the scalp. So it only makes sense to use a "z * 0 1 Z". (Where star is the channel.) To measure that particular channel. 

    While it might seem that since the P channel pins are bussed to SRB2, that placing the 31.5 Hz signal on a P pin, would propagate to all other P pins (connected to SRB2 reference.) I don't believe that will give you the effect you are looking for. You want to inject the 31.5 signal only on the scalp pins.

    Regards,

    William

  • khofstadterkhofstadter Colchester, U.K.
    Thanks for clarifying this.. I think I am getting there ... :) 
Sign In or Register to comment.

© Vanilla Keystone Theme 2026