Basic questions: saving data and data format?

KCDTylerKCDTyler Melbourne, Australia
in Software
Hi there,

I have recently received my Cyton board, so am fairly new to it...

I can't for the life of my find any information regarding recording data and saved data format. 

The OpenBCI GUI has an obvious "Start Streaming Now' button. But I don't see any obvious 'Start Recording Now" button, or see anything about this on the forum.
Does data just record to PC by default when ever the Cyton is streaming data live?

If so, where and what format is the data saved as? 
I see the txt files in my saved folder, e.g. 'OpenBCI-RAW-2017-08-07_20-07-21.txt'.
But surely data is not saved as a txt file?

Ultimately I will be wanting to import saved data into Matlab, but for now, I just want to figure out when, how and where it is saved in the first place.

Thanks in advance!



Comments

  • mark.o.smark.o.s
    edited May 2019
    Ya, it saves as a txt.file.
    When you open the GUI, and start System control panel.
    You will see the Data log file, etc. This is the name theat your data is saved under.
    It's a 13 column text file. First column is the sample number, columns 2-9 are the 8 channels then (I think). 
    If you open it up in matlab, its easy to see.
    The link below is from the OpenBCI tutorila where it mentions playback and logging.


    [expand the section by clicking on the arrow]

    I'm currently a bit puzzled myself as to the metric which each EEG channel is stored though.
    Is the txt file the EEG after filtering on the GUI? i.e. if I have the 50Hz notch filter selected in the GUI, will my txt file data also be filtered?
    Furthermore, I can't figure out what units are used? When I look at my data in Matlab, is it the ADC value I'm getting or the converted Volt or microVolt value?
    Looking at my data, its in the range of -9000 to 7000. For example, one channel fluctuates around 4000 (I assume if this is voltage values, that my EEG is riding on top of a DC offset). And another channel of EEG recorded simultaneously, fluctuates around -7000.

    Can someone please explain what unit this is?

    Thanks
  • wjcroftwjcroft Mount Shasta, CA
    Mark and KCD, hi.

    The text file is in microvolts (stated in the text file comment) and before any of the onscreen filtering. So the DC offset (typically tens of millivolts, in your case -9 to 7 mV) needs to be removed with your Matlab or other filters. Can done with a hipass filter at .5 hz or bandpass .5 to 50 Hz for example. You can also apply your own notch at the mains frequency; or just lop it off via the bandpass.


    William

  • ajebulonajebulon South Korea
    Dear @wjcroft,

    If I set the gain to be 24x in the OpenBCI GUI, should I divide it by 24 after Notch Filtering (at 50/60Hz) and Bandpass Filtering (0.5~50Hz)?
    What is the relation between the gain I set in the GUI and the saved CSV data?
    If I set the gain to smaller value, would the saved data become smaller compared to if I set the gain at 24x?

    Thank you.
  • wjcroftwjcroft Mount Shasta, CA
    The CSV file should always show the data in microvolts, regardless of changes in gain on the CHAN SET tab.
  • SangeethaSangeetha Chennai
    Hello,
             I have troubled in exporting the text data which is saved in the folder .where i cant get the signal which i was seeing in the GUI by plotting it in xlsx formal by selecting the column . My doubt , When the data is converted from text to xlsx formal , by plotting i can visualize it as a pulse like waveform as like ADC values get plotted. Is there any means to overcome this , or else is this an ADC value through converting we can get proper plot of it or we need to change any settings before recording the EEG signal? Kindly get me through this soon as possible.

    thank you
  • KCDTylerKCDTyler Melbourne, Australia
    Hi Sangeetha,

    I am not 100% sure what you are asking.
    Are you asking how to export the .txt file into another format?

    What format are you wanting to export it too?
    What software platform are you using to do this?

    Regards,
    Kaelasha 
  • SangeethaSangeetha Chennai
    Hai Kaelasha,
                
    Actual thing is , the saved  data is either digital values or analog values. Because when i plot the values in matlab i was getting pulsed output. Is it correct ? or else i need to do any conversion to get an analog plot.

    Regards,
    sangeetha
  • KCDTylerKCDTyler Melbourne, Australia
    I'm sorry I don't understand. 
    The data is all digitalised. Everything in Matlab is digitalised. I can explain how to plot the EEG data recorded with the OpenBCI system in Matlab if that is what you are asking?




  • wjcroftwjcroft Mount Shasta, CA
    KCDTyler, thanks for your comments.

    Sangeetha, the values in the txt file are digital samples in microvolts. The header (in comments) of the file states that as well. Do note that NO FILTERS have been applied in the samples in the text file. Whereas the GUI user interface screen DOES have filters applied. An easy filter to apply to the data would be a bandpass filter, from say .5 Hz to say 45 Hz. You could also apply a notch filter at your mains frequency, if that helps.

    I don't understand your distinction between analog and digital. All EEG input is analog, then gets converted to digital in the ADC, Analog to Digital Converter chip.

    Do note that the unfiltered values also include a DC offset, (in the range of millivolts) which your bandpass will remove.

    Regards,

    William
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