Re: [Openeeg-list] soundcard EEG simplification?

[<Gra...@ao...>]

Hi Dan,
 
As you've noticed, there are many things to consider when designing an EEG 
unit to interface to the PC. I've also modified some sound cards and have 
been able to lower the input frequency response of the card, although not as 
successful as you have. If you are able to lower the input response to 0.2 Hz 
then one does not need a modulator for a single or dual channel unit. You 
can scale the gain accordingly (one usually limits the level to about 1 volt 
pp). However, how do you expand the unit for 4, 8, or 16 channels of 
operation? The AM modulation technique allows for 16 (8 channels on each stereo 
side) EEG channels at a 256 Hz sampling rate. In general, I don't expect people 
to be able (or want to) to pry open their laptops and modify the input 
sound card section. Its tricky and dangerous.
 
The second issue would be the isolation of the unit to the computer. The 
sound card designs use a 1100 volt isolation transformer (a 600 ohm MODEM 
transformer). This isolation provides good protection for the individual. It has 
the limitation that low frequency response falls off rapidly below 50-100 
Hz. No problem if you have a modulation frequency much higher. In general, 
small audio transformers have poor low frequency response. I've investigated 
using opto-isolated amplifiers but they don't have the needed +80 dB of 
dynamic range for good EEG work. They also consume power. 
 
The latest amplifier design on the web site does not use a modulator. It is 
used to amplifier the EMG signals for cursor control and voice recognition 
applications. Using NeuroProbe, I limit the response with high and low pass 
filters to between 70 and 500 Hz. Most of the EMG energy falls within that 
frequency range. 
 
The 4 and 8 channel units have DC to DC converters in their power supply 
designs. Just remove the pre-requlators and they can be used with the USB +5 
volt output. 
 
Hope that provides you some insight into the design trade-offs for the 
sound card units.
 
Grant
 
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Hi Grant-
Thanks for your work on the soundcard EEG project. I'm interested in
building an EEG device for the purpose of monitoring sleep, and would
like to ask a few questions to see if I'm on the right track.

Based on what I read on the open-eeg list and a few web sites
(including http://openeeg.sourceforge.net/doc/hw/sceeg/ and
http://www.hotamateurprograms.com/eeg.htm), it appears to me that the
soundcard eeg device needs to do the following three things:
1) isolation, for safety of the person connected to the electrodes
2) amplification, to work within the 0-5V input range of the soundcard's ADC
3) modulation, to work within the 20Hz-20kHz range of the soundcard's
microphone input circuit

If those 3 are correct and cover all the bases, then I have a
suggestion that could make things simpler. Now that there are
relatively inexpensive USB sound cards available (eg
http://www.newegg.com/Product/Product.aspx?Item=N82E16812186035 ), we
can consider modifying the sound card itself to remove the need for
modulation. For example, with this sound card I was able to increase
the value of the capacitor buffering the microphone input from 1mF to
100mF and thereby reduce the high pass filter's cutoff from 20Hz to
roughly 0.2Hz. It took a bit of careful soldering on the small PCB,
but wasn't too bad. Would you agree that this change removes the need
for modulation?

If yes, then can you suggest how the proposed soundcard EEG circuits
should be adapted to work with this low-frequency capable soundcard?
For example, on the single-channel FM design here:
http://www.hotamateurprograms.com/EEG%20Data/Hardware%20Design/Single%20Chan
nel,%20Low%20Power,%20FM%20Design.jpg
My inexperienced guess would be to take the output of the AD8221 as
the signal and feed it straight into the soundcard. Would this produce
the 0-5V range we would need? Are there other issues to consider?

And can you suggest a way to use the 5V USB power source instead of
the 9V battery, that safely isolate the person attached to the
electrodes?

Thanks!
Dan

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