OpenBCI_GUI scale factor / "no BP" setting

MikeMike
edited August 2016 in OpenBCI_GUI
Hi,

I was experimenting with the OpenBCI client I have few questions. 

1. When selecting "no BP" for filtering in the UI data doesn't show up in the graph on the left, is there some reason for this behavior?
2. I was checking the way that the scaling factor is computed in the client - "float scale_fac_uVolts_per_count = ADS1299_Vref / ((float)(pow(2,23)-1)) / ADS1299_gain  * 1000000.f;
It is different than how it is described in your document, that is,  your document doesn't mention multiplying by 10^6

Thanks, 
Mike

Comments

  • chipaudettechipaudette
    Hi,

    Those two questions are squarely in my lap, as I wrote those parts of the Processing GUI.

    "No BP":  The "no BP" filtering means "No Bandpass", which means that there is no bandpass filtering being applied.  When there is a bandpass filter (any of them), it removes both very high frequencies and very low frequencies.  Since EEG data often has a large DC component, one of the benefits of the BP filter is that it removes the DC component.  With the DC removed, all of the data traces are centered around 0 uV and fit nicely on the plot.  With the DC still in each EEG signal, the data traces are centered around other values (+10000 uV? -200000 uV?  It's ever changing...) they get plotted way off the screen.  That's not too useful, is it?  Nope.  That's why I hardly ever use this mode.

    Note: the EEG data written to the log file on the harddrive is the raw EEG signal, as received from the OpenBCI board.  So, the data in the logfile contains any such DC offset in the EEG data.

    Scale Factor: The equation that you copied into your post converts the data into microvolts.  The equation in the documentation converts it into volts.  The difference between volts and microvolts is the 10^6.  I generally find it easier to work in microvolts, so I included 10^6 in the equation in the Processing GUI.

    I hope that this helps!

    Chip



  • MikeMike
    edited February 2015
    Thanks for the clarifications Chip!
    Yeah, I understood the purpose of "No BP". But when disabling it I was actually expecting to see the original EEG signal with everything included and not 0 signals :) so it made me wonder... 
    About the scale factor, I assumed that's probably the case, however, I found the comment about Table 7 in the ADS datasheet kinda confusing... now it makes more sense though.



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