From: Jim Meissner (jpmeissner_at_mindspring.com)
Date: 2002-02-14 22:36:09
Dear Jim Peters:
> As you can see, because this is optimized for off-line analysis, I'm
> getting farther and farther away from what is needed for real-time
> responsiveness and biofeedback.
>
> However, this will give some really good results and allow us to see
> things buried in EEG recordings that we might never have seen before
I have been putting in my 2 cents worth to Andreas during the design of hisinput stage. I have pleaded with him not to "mess up" the data with fancyhigh pass and low pass filters. Although he does not agree with me, he was kind enough to put some test points on the board that would allow me to bypass the filters when I want to use his boards.
So long as the raw EEG data is spooled to the hard drive without adulteration then I have no argument with you and your filtering ideas. You could grab the data from the buffer and filter to your hearts content without destroying the original data. Then you could go back multiple times and try different algorithms.
> buried in EEG recordings that we might never have seen before
I spent 10 years going down that path, thinking like you, there "must" be some magic frequency in there somewhere. That is what drove me to spend thetime building my own EEG because I was able to see the flaws in all the other approaches. In some ways that itch is still there. I am quite willingto have another go at it. If your post filtering finds some magic data, Iwill not be unhappy just because I could not find it. My main caution is to run a clean system that does not create artifacts. White noise and impulse step functions run through sharp filters will show data that is not really there. Normal beta brain waves are just random noise.
I see that you have been able to run my files on your system. Take a look at the Test.dat file more carefully. I purposely modulated a sine wave tone various ways to see what the FFT did with it. Look at that with your voice print and or filter scheme. Make sure your system does not "create" things that are not there.
The data that I would like to go over with a fine toothed comb is the file "Downey.dat" on my webpage. At some point during that session the medium in trance stated: "Look at these frequencies that are coming through right now, those are the ones you are looking for!" I will tell you more about that session when you are ready.
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: Jim Peters
To: buildcheapeeg_at_yahoogroups.com
Sent: Wednesday, February 13, 2002 5:19 AM
Subject: Re: [buildcheapeeg] Loose ends: Power
Jim Meissner wrote:
> Please don't feel insulted! There is absolutely no insult meant.
I am not insulted, don't worry! Some things you say are obviously
really key pieces of information, and others I simply can't judge or
argue with without trying them out for myself. I read what you write,
and mostly I'm chuckling. I don't know how everyone else takes it,
but it's fine with me.
> I would love the option of looking at a portion of the file and play
> it, reverse and replay. How wonderful that would be!!!!!!!
I think this is the best way for me to put something in at the moment.
> How about starting with the linear frequency bins FFT like the Mind
> Mirror, Hal-4, ElectricGuru, etc. Then it would be compatible with
> existing display conventions. You can always add the log display
> later as an option. I also like the voice print display very much.
According to their web-site, the Mind Mirror III uses a series of
carefully matched filters, and they are not on a linear scale. The
frequencies are: 0.75Hz, 1.5Hz, 3Hz, 4.5Hz, 6Hz, 7.5Hz, 9Hz, 10.5Hz,
12.5Hz, 15Hz, 19Hz, 24Hz, 30Hz and 38Hz. So, not everyone uses FFT
and equally-spaced bins.
To me using an FFT and a linear scale is like going down a dead-end
street. Anyway, I already know how I'm going to do the analysis, so
taking a different approach to everyone else isn't going to slow me
down.
It will look something like the voice-print display, only the
frequency scale will be different on the left.
> Have you gotten the "Awakened Mind" by Maxwell Cade yet?
I haven't got an answer from the phone number on the web-site, and
I've been quoted 4-6 weeks from a bookseller because it has to be
ordered from the States ! I'll try again.
> I have read in some of your posts that you are concerned with the
> time delay of your filters to do biofeedback. Again, I believe you
> are listening to some information that I do not find to be credible.
Well, when I have a working EEG unit, then I can judge this. What I'm
really learning about here are the mathematical limits of the signal
processing techniques, nothing special about brainwaves.
There are different kinds of techniques that have different advantages
and disadvantages. These are the ones I've come across so far:
- Bank of IIR filters: small amount of work on each sample, provides
an immediate answer after each sample, unlimited flexibility in how
the bands are arranged, some blurring into the recent past.
- FFT of windowed data: medium-sized chunk of work every few samples,
limited to linear scales (equally-spaced bins), answers available
only every few samples, loses detail at low frequencies, too much
frequency detail and time-blurring at high frequencies.
- FFT-accelerated convolution of signal and a set of complex wavelets:
large chunk of work every few seconds, unlimited flexibility in
choosing frequency scale and amount of blurring.
This last one is what I plan to use. I've made it sound really
clever, but all it is really is the signal amplitude-modulated with a
carrier (to shift the interesting frequency to 0Hz), and low-pass
filtered with a Blackman window. In fact I'm combining the carrier
and the window to make something that looks like a wavelet, and the
FFT convolution is just a way to accelerate applying this to a large
run of data all in one go.
As you can see, because this is optimised for off-line analysis, I'm
getting farther and farther away from what is needed for real-time
responsiveness and biofeedback.
However, this will give some really good results and allow us to see
things buried in EEG recordings that we might never have seen before.
Jim
-- Jim Peters (_)/=\~/_(_) jim_at_uazu.net (_) /=\ ~/_ (_) Uazú (_) /=\ ~/_ (_) http:// B'ham, UK (_) ____ /=\ ____ ~/_ ____ (_) uazu.netYahoo! Groups Sponsor ADVERTISEMENT
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This archive was generated by hypermail 2.1.4 : 2002-07-27 12:28:38 BST