Re: [buildcheapeeg] Re: Sound Card

From: Grant Bright, Ph.D. (gbright_at_b...)
Date: 2001-05-16 00:37:04


Hello Praven and Group,

I see a major problem in using sound cards...frequency response. In the medical technology world, eeg acquisition amplifiers are flat in their response from .5 hz to 40. Unless a sound card can have a digital smoothing technique applied through the use of FFT, FIR, or the like, the data is meaningless from a technical standpoint. A roll off of even 1db, unless compensatedfor in the software, will not be helpful. It is like the old expression ofcomparing apples and oranges. In order for the data to be useful, the response must be flat or be accompanied by correction table in the software.

Additionally, not all sound cards from the same manufacturer are equal. Creative originally had a wide dynamic range in their CT!745 hybrid. It is no longer made and has an equivalent, but the dynamic range is different...i.e., less, but the functions are the same.

It is also a big mistake to START at 5 (five) hz. That leaves out Delta .5 to 3.5 and the lower Theta. It is in these areas, significant data is taken. In the case of head injuries (Delta) and in the case of lower Theta (attention problems). In many circles it is now an acceptable practice to startDelta at 1.5 hz., but there are still many labs looking at .5hz upwards.

A sampling rate of 256 samples per second is used when looking for seizure activity--which is faster than normal eeg. 128 is the norm for eeg biofeedback. The higher the sampling rate, the greater the accuracy of detailing the waveform, but the files also become larger.

Praven, what is the moment to moment stability (in ppm) of your VCOs at the low frequencies of eeg?

This forum is most interesting at this point in time, as eeg biofeedback isreally only just beginning to have its usefulness discovered. It has only been about 30 years since anyone out of a laboratory did or knew anything about this non-drug, non-invasive approach to dealing with various symptoms.

Best wishes to all,

Grant Bright

----- Original Message -----
From: Praveen Kumar
To: buildcheapeeg_at_yahoogroups.com
Sent: Monday, May 14, 2001 12:44 PM
Subject: [buildcheapeeg] Re: Sound Card

Hi everyone,
I checked the group mail after a while a was
pleasantly surprised to see
that discussion on sound card has gained momentum. I
wish to set aside the
apprehension of Eric regrading the quality. Quality
will not be
compromised as its the proven hardware. Rest will be
determined by
software. Rob Sacks and Jiva have given good links
which can help in
development of software. I'm going to look at it as I
send this message.
Most of the discussion is centred around the low pas
cutoff frequency.
First we should freeze what freq are we looking for.
There are two possible
solutions:

1. As suggested by a member earlier, Changing the
input capacitor.
(or)
2. Amplified input analog signal can modulate a VCO
which are cheap and
readily available. I've designed my own for my work
and can be made
available to the group. Due to high sampling frequency
of the sound card,
the modulated signal can be easily acquired. Another
advantage of this
method is the signal can traverse long distance if
the patient is much away
from PC. However, the domdulation will have to be
done by PC based software
(don't know how to do it). The frequency response
down to DC (or anyother
low cutoff freq) is possible with this method.

Let us focus on the some more (if any) grey areas
before we commit ourselves
in the implementation.
Cheer,
Praveen
India

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