Re: Facial muscles and EEG-ECG-EMG

Grant and further down my msg, for Daniel

Any EEG signal is full of artefacts.  If your name is Emotiv and you use
shitty saline electrodes with an EEG front-end that only has a 1 mega-ohm
input impedance, then the signal quality will be 10x worse than what the
professionals get with prepped and conductive paste applied Ag/AgCl
electrodes.

The Emotiv EPOC is a toy, intended for gamers, doesn't matter what anybody
says, publishes or writes.  The low input impedance is just beyond
laughable.

Rather than try and wish the artefacts away, Emotiv turned this around and
made it a selling point for their so called expressive detection software.

Here is your problem: Emotive only uses electrodes on the scalp.  If you
want to reliably pick up facial muscles, it stands to reason that you need
to apply electrodes to the face (EMG) and around the eyes (EOG).

What do you mean with a "simple" ECG or EMG circuit?  I hope you don't mean
simpler than openeeg.  Well, actually you could build a very good device
with a much reduced component count, using modern ADCs such as the ADS1xxx
family from TI.  But doing such will not be lighter on the pocket, in fact,
it's likely to cost more.  Not sure if I equate your simple as also meaning
low cost.

If we ignore the possible requirement of many channels, the monopolar
versus bipolar input design and ignore the possible concurrent sampling
requirement for EMG (the latter is not applicable for you Grant, but to our
other friend (Daniel) looking for a ECG/EMG/EEG circuit it might be), then
the circuit will look the same.

Seeing that the EEG is the biometric signal with the lowest amplitude, a
circuit such as openeeg could very easily be modified to work with ECG or
EMG - you simply reduce the gain.  In fact, an adjustment on that gain
pre-set might be enough.

Granted, the EEG has the lowest bandwidth requirement, while ECG and EMG
are higher.  But this is really only applicable to professional equipment
used in a professional application.  The typical sampling rate of 256SPS is
enough for general ECG monitoring.

EMG has a VERY, VERY wide possible application range, so there it is more
difficult to comment.  You could argue that EOG is a subset of the EMG and
for EOG a professional would need to go down to DC, i.e. 0Hz.  For general
muscle flexing pickup such as Grant wants to do, the 256SPS is no problem.
If on the other hand, you want to check nerve impulse propagation over a
specific area, you would want concurrent sampling at a minimum of say, 4Khz
sampling, for 16/32/64/ etc. channels, making such an EMG device rather
expensive, in comparison.

Because 256SPS is SOOOO pedestrian with todays technology, a modern
amplifier that samples at 4 or 8 KHz is absolutely no problem to build and
design, and that would then be enough for any and all EEG, ECG and EMG
work.  The very small disadvantage with a wideband amplifier would be that
for EEG, where this wider bandwidth is not required, it would increase the
noise floor slightly.  But even then, you could filter it out in the
digital domain.

That brings us to the different gain/sensitivity requirements of EEG, ECG
and EMG.  If you use a modern 24-bit low noise ADC, you can expect a real
world resolution of perhaps in the order of 18 or so bits.  That should be
enough dynamic range for the amplifier to handle all type of signals.
Obviously, if you go cheap and don't even use a separate ADC, but instead
use the internal ADC of some micro (as openeeg does), well then forget it.
Most micros have 10-bit ADCs and some manage 12-bits.  That is not good
enough.  In that case one has no option but to implement gain switching.

For the benefit of the Daniel crowd over here; gain switching would best be
done using CMOS switches sitting in the negative feedback loop of an
opamp.  Preferably a bit further up the signal path, after the signal has
already been amplified somewhat.  I did such on my own EXG2 design, see
here: *sites.google.com/site/ <http://sites.google.com/site/>openexg/*

At the end of the day, whatever you want to do, you don't only need a
reasonable performing amplifier, but a good electrode setup.  If you use a
shitty pickup such as is used in the Emotiv EPOC, then it is a matter of
crap in, crap out.  No amplifier can create a good signal from garbage.
Ask any RF engineer.  You start with a GOOD aerial if you want good RF
reception.  Ditto with biometric signals.

Finally, as much as I have been bashing the Emotiv EPOC for the last few
years, considering the cost restrain, the design is not that bad at all.
Patent applications show that the Emotiv crowd wanted to implement active
electrodes.  Emotiv's bean-counters must have stopped the engineers from
implementing the extra 14x unity gain opamp buffers due to costs.  This is
the decision that torpedoed that product into a heap of junk.  Just my
view.

Regards
Stefan

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