Cyton+Daisy sample rate / counter; MNE other apps?
Posting on behalf of a customer:
Hello,
I’m somewhat of a novice when it comes to EEG but have experience with data processing.
I’m using a Cyton board with an extension to 16 channels and a sample rate of 125hz. I was wondering why the raw data (recorded as a txt file using the OpenBCI GUI) shows a cycle to run from 0-127 as opposed to 125 steps. Also, looking at the system timestamp the cycles aren’t quite exactly a second each, which would create a problem when segmenting the data. Is there any advice you could give me regarding the matter? Please find attached a sample of some test data.
Also, I’ve found a Python library called MNE, which seems fairly comprehensive. Are there any other useful/standard tools for processing EEG that you could recommend?
Comments
The sample counter is a seven bit counter that repeats, 0-127, then back to 0. It has nothing to do with the 125 hz sample frequency. Similarly if you were running without the Daisy, sample counter would cycle 0-255, then repeat. Even though in that case the sample rate is 250 Hz.
The clock oscillator is not crystal controlled on Cyton. Hence it may differ slightly from exactly 125.000 Hz. For example it might be 125.01 Hz. The technical term for this is 'drift', and it is normal for EEG equipment of various types. The system timestamp you are looking at is assigned by the GUI/Hub when the packets are received at your laptop. Hence, they reflect some minor delays caused by OS scheduling, other processes, etc. Just assume you have a fixed sample rate at 125.xx or 124.xx and you will be fine.
See, the Software / Third Party section on the docs server for more recommended apps. MNE is also good, but not currently listed.
https://docs.openbci.com/docs/Welcome.html
Regards, William
Here's an example of MNE receiving an LSL stream. OpenBCI GUI can output LSL stream from Cyton data.
https://mne.tools/mne-realtime/auto_examples/lslclient_rt_.html
Also search for string LSL on this page:
https://mne.tools/dev/whats_new.html