maltman23 at hotmail.com
Sat Mar 28 11:02:01 PDT 2009
Noise is everywhere in our crazy modern world (electrical, social, psychic, etc., etc. -- and of course, TV, which is all of the above -- but I digress).
Sensors act as antennas for all sorts of noise, 60Hz from the AC power all around us being particularly strong. The signals from our brains running down wires from a passive electrode are on the order of 10 microvolts, which is way (WAY) lower amplitude than the noise the wires pick up. To deal with the noise that the wires pick up as an antenna, the analog board of the OpenEEG has a "Differential Input" on the it's first input stage (which is a high-gain, low-noise op-amp). It works quite well.
But if the signal from the electrode were amplified at the source (like, putting an amp at the electrode, right next to your head), then the analog board would not have to get rid of so much noise, since the amount of noise compared to the amount of useful signal is a much better ratio (this is called Signal-To-Noise Ratio -- the higher number for this, the better). This is the idea behind the active electrodes. But designing and using the high-gain, low-noise amplifiers required for these active electrodes is not easy. As stated earlier, me and Tracy tried building and using "Pedro's Active Electrodes" from the OpenEEG project website. But we got lots of other interesting noise that would oscillate at a very low (about 0.02Hz, as a guess). We never did figure out what was going wrong with them. Some people, including Pedro, have great results with them, and others have problems like me and Tracy had. There's probably issues that are beyond my expertise in analog design (which is not my realm).
Creating a Bluetooth interface would also theoretically get rid of the issue of noise picked up on the wires, since it gets rid of the wires. But then we have some similar problems to the active electrodes: of creating an amplifier at the electrode (and this signal would go to the Bluetooth transmitter). Probably a non-trivial problem to solve. But it would be a way cool way to use an EEG, without wires hanging off of your head.
> Date: Sat, 28 Mar 2009 10:19:19 -0700
> From: rachel at xtreme.com
> To: kinetical at comcast.net
> Subject: Re: [EEG] wireless?
> CC: eeg at lists.noisebridge.net
> Huh, I had the impression that the noise source was local radiation
> being picked up by the wires, which were acting as antennas. So I guess
> removing the wires would reduce that, at the cost of adding more local
> radiation sources. What about the wires to the sensors though, or am I
> completely wrong about the sources of noise?
> Tracy Jacobs wrote:
> > Hi,
> > Thanks to everyone who came to the meeting last night, I'm really
> > excited about the group. After everyone went home I was talking to
> > Michael Kan, who had some interesting ideas about engineering the EEG.
> > I encouraged him to join us next time. He suggested we make the
> > connection between the EEG and the computer wireless, using a bluetooth
> > device, to reduce noise and also to ensure safety by having the circuits
> > completely separate from one another. I don't have any bluetooth
> > device, but if any of you do, bring it along next time. I will bring a
> > box for the circuit boards.
> > Tracy
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