From: Jim Meissner (jpmeissner_at_mindspring.com)
Date: 2002-01-14 18:54:35
Dear Andreas:
My reason for contributing to this list is not to get a cheap and dirty EEGmachine. I was hoping to hook up with people like you and Joerg ( and many others ) who have the digital and software know-how so we could put our heads together and build something valuable. Having an open design for the hardware and the software means that the machine can grow as more and more people contribute. Building something that works is the first step. Then there could be many branches from that. Either less expensive or more sophisticated depending on the group feedback.
Therefore I am interested in putting together modules ( as in Joerg's ModularEEG ) that can be hooked together to build systems of various complexities. The goal of building a $100 brain monitor has had a negative effect in my opinion and resulted in little or no progress. This is not to say that Joerg has not come up with some very clever and cheap designs.
This is where we are so far:
Andreas's input stage design is done and needs testing.
The opto isolator ( the expensive version ) is almost designed, needs some work and testing. It should be possible to reduce this price in the future.
The A/D converter and microprocessor is being considered at this time. It seem to me the minimum should be 10 bits such as in Joerg's processor. Twoother boards need to be offered, a 12 bit and a 16 bit. So that looks like 3 different versions. Maybe SPI to a separate A/D board.
I have worked with a 12 bit converter and getting the noise down to 1 or 2 counts is a challenge, so even though I would like the 14-16 bit, that should be on the back burner. I would like to see the implementation considered though.
Software for transmitting to the PC needs to be done.
Software for the PC needs to be looked at. Will the Electric Guru be able to be interfaced? Is the source code available to be modified if needed?
And last but not least, how about the brain connection? Is there an experthere who knows electrodes?
Juergen P. (Jim) Meissner
Check out my Website at www.MeissnerResearch.com
Read about the benefits of the Brain State Synchronizer sounds for improving your life and health.
----- Original Message -----
From: sleeper75se
To: buildcheapeeg_at_yahoogroups.com
Sent: Monday, January 14, 2002 12:00 PM
Subject: [buildcheapeeg] Analog-to-digital conversion
--- In buildcheapeeg_at_yahoogroups.com, "Jim Meissner" <jpmeissner_at_mindspring.com> wrote:
Hi Jim,
> I have used the LT1294, talking to it from the PC with Quick
Basic. Makes you wonder whether you need a micro processor at all?
>
> BUT, seriously, having written the PC program for the Com port,
> FFT, VGA display for Waterfall and waveform, etc there probably is
> not enough time to "handhold" the A/D converter.
Yes, that's another $4 we shouldn't save.
> My preference would be the HC11 ( $9 ) because I know the code and
> I use it in one of my products. The BrainMaster also uses it.
Ok, but AVR assembler is really easy and you personally won't have to
mess with that.
> Would compatibility with the BrainMaster be important?
Not for me. And as far as I know, the protocol is made for 8-bit
samples...(?)
> I would like to address the question of 8, 12, 16 bit converter
> choices.
>
> Having used an 8 bit converter for years, I can tell you the
> frustration with that limited range. I actually had to install a
> manual gain pot in order to get the signal within the range.
> During deep meditations I would have to increase the gain in order
> to see the signal.
That does it for me, I'll look for a good 14-to-16-bit ADC. Should be
around $20, but will probably need an external multiplexer for
another $7.
> At that gain, the eye blinks would overload the amp and cause
> clipping which causes havoc with the FFT analysis. Talk about data
> distortion! This is why I cautioned you about multiple AC coupling
> capacitors. These will create false frequency data to the FFT and
> take a long time to settle.
Yes, an overload takes about 15 seconds to settle.
> Since you and Joerg love to do simulations, look at the step
> response of an overload signal into an FFT. I don't know it your
> software can do transient analysis.
If I remember my signal-processing theory correctly, a fourier
transform of a step would pretty much be smeared all over the
frequency spectrum, with most of the power in the lower bands, so I
see what you mean.
> I would rather spend an extra $10 for the 12 bit converter.
>
> As a matter of fact I would be willing to spend an extra $50 for a
> 16 bit converter and have DC coupling throughout.
>
> Is your design flexible enough to give these options?
Yes. :-) Just change the gain and disable the AC coupling. The mixed-
signal part would have to be specifically designed, unless there are
12 and 16-bit ADC's that are pin-compatible.
I'm not sure a through hole PCB is good enough though. An all SMT
layout, would feel much better ...
/Andreas
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This archive was generated by hypermail 2.1.4 : 2002-07-27 12:28:36 BST