Unexpected noise with Daisy
Hello!
Today I started trying the Daisy module for 16 channels.
The soldering went fine thanks to the tips here in the forum.
After assembly, I could check that the channels on board and daisy were working fine, with the expected drop in "actual sampling frequency" to half.
So the Nyquist Frequency (NF) would be 125/2=62.5.
However, I could verify that besides the expected 50 Hz and 75 Hz peaks [62.5 + (62.5-50) = 75], I also have the 25Hz peak you can see in the figure below. What would be the reason for this? I wasn't expecting to have "Subharmonic noise" being the orignal noise below the NF...
Tests with ECG signal
- With the Daisy module physically connected but 8 channels chosen: https://www.dropbox.com/s/2i5rovz261fa7qd/1.png?dl=0
- With the Daisy module physically connected and 16 channels chosen. (the bandpass applied is [1 40] and 2 notch filters: 50Hz and 75 Hz: https://www.dropbox.com/s/8ku2nf6752a5no5/2.png?dl=0
Thanks
Comments
It's possible that this is some side effect of the DSP filtering. Does the 25 hz peak change when you try various combinations of turning off/on the various bandpass / notch filter combinations?
Have you tried looking at your EEG or ECG signals with other interface programs such as neuromore, OpenViBE, etc.?
https://www.google.com/search?q=subharmonic+eeg
Your Dropbox images above have TONS more mains noise than those shown on the docs.openbci.com tutorial image (below). Why would that be? Where are your leads positioned, Ground / Bias, etc. Do you have any power supplies or AC cables running nearby? Try changing the location of your experiment. Sometimes AC conduits run through certain floor or wall areas.
http://docs.openbci.com/tutorials/01-GettingStarted#getting-started-w-openbci-v-connect-yourself-to-openbci-4-launch-the-gui-and-adjust-your-channel-settings
http://docs.openbci.com/tutorials/01-GettingStarted#getting-started-w-openbci-v-connect-yourself-to-openbci-5-minimizing-noise
William