Recording microphone / audio signals on OpenBCI for use as a trigger channel
Hello,
I am interested in performing evoked response measurements using OpenBCI (auditory steady state response, p300, etc). I have looked into various methods for doing this, such as:
1. Using the lab streaming layer (http://openbci.com/forum/index.php?p=/discussion/862/simultaneous-eeg-and-audio-recording)
2. @qwer1304 mentioned of using a WAV Trigger board (http://openbci.com/forum/index.php?p=/discussion/296/auditory-steady-state-response-with-openbci)
I’m concerned about jitter in the trigger timing, so I’d prefer not to use the lab streaming layer if I can avoid it. I also I don’t have a WAV Trigger board.
The simplest solution I think would be to wire a microphone directly into one of the channels of the OpenBCI amp, to ensure that both the audio stimuli (e.g. the trigger) and the EEG data will be recorded in sync.
I tested a dry run of this (no EEG connected, only the mic). I connected the mic’s to leads to SRB2 and the bottom (N) pin of N1P, and disabled all other channels except Ch 1. I was able to pick up loud noises, such as if I clapped my hands. But the signal was quite weak. When I played auditory tracks (containing either click trains or pure sinusoidal tones), I could detect nothing.
A few hypotheses for why this didn’t work:
1) Something was wrong with my set up (e.g., is it okay that I did not have anything connected to bias or AGND, for that matter?)
2) The sampling rate of OpenBCI is too low to pick up the sounds. However, the audio clicks were square waves, so I would think they would have a low frequency component.
3) The amplitude of the microphone's signal is too low. However, I read elsewhere that mic signals should be on the order of millivolts (https://www.kfs.oeaw.ac.at/manual/3.8/html/userguide/461.htm; “a microphone level signal is more often in the range from 5 to 50 mV (millivolts)”). So I would think that this would be strong enough for OpenBCI to detect.
If anyone has recommendations or alternative suggestions, it’d be great to hear.
As a second attempt, I’m going to buy an audio cable splitter so as to split the audio line out of my laptop (running on battery). I’ll hook one end up to a pair of headphones that I'll wear, and the other end I’ll connect to the OpenBCI as above. I presume that directly capturing the line out will provide a stronger signal than what I had before from my Mic.
Is there any chance this could damage the OpenBCI? (Apparently the audio line out signals are on the order of volts). Does this sound like something that could feasibly work?
Thanks
Comments