From: sleeper75se (sleeper75se_at_yahoo.se)
Date: 2002-07-11 10:49:39
Hi Chris,
great work!
I think it is really nice that you have built the ModularEEG already!
> After the downloads (eagle-cad, avr-gcc, SP12), printouts, PCB,
> orderings (I got most of the parts here in vienna, 1 capacitor 0,3u
> and 4 'Spulen' 0,22uH have slightly different values) I am finished
> with my
Do you mean 22uH?
> I have problems with 50-Hz humm (I can reject it to an amount that I
> actually see brainwaves only when i connect myself to GND, can
> somebody help here ?
Do you mean ModularEEG ground or ground as in "earth" (eg a
radiator....)?
> I also get this noise when I use battery-power. Could shielding of
the
> Electrode-cables help ?
What kind of noise is it? Could you describe it more in detail?
Try connecting a large (>100uF), low-impedance capacitor across the
battery (to lower its impedance) and see if that helps. Does anyone
else on the list have any suggestions? Joerg, Jim-M?
> I don't fully understand the concept of the right-leg-driver -
> is it ok when i use the average-signal from the earlobes ?)
Actually, the DRL is an output and replaces the regular passive
ground lead. It is part of a regulator circuit that tries to suppress
low-frequency common mode signals such as 50Hz hum about 100 times.
The DRL should work anywhere (as far as I know). Try putting it
farther away from the head (on the arm for example) and see if there
is a difference in how well (or poorly) it works.
Hum caused by electrode-impedance mismatch will remain unaffected,
and may only be reduced by more careful cleaning and abrasion of the
scalp. However, if the hum does not saturate the amplifiers at any
time, you can filter the signals digitally with a 50Hz notch filter.
I can get you some source code for you to try, if you like.
Another thing you can try, to reduce the noise:
Put the amplifier board in a metal box and connect the box to virtual
ground with a short wire, near the amplifier inputs. Drill holes just
large enough for the electrode wires and ribbon cable to pass through.
Ok, to sum it up, the things you can try are:
* Place a large low-impedance capacitor (>100uF) at the battery.
* Place DRL electrode farther away from the head
* Put the amplifier in a metal box (and shield the electrode wires)
* Add a digital 50Hz notch filter.
* Clean the scalp more carefully.
Please let us know what you do, and what results you get, we'd really
appreciate it!
Btw, consider signing up to the new list at
http://lists.sourceforge.net/lists/listinfo/openeeg-list so that we
can continue the discussion over there... :o)
Regards,
Andreas
This archive was generated by hypermail 2.1.4 : 2002-07-27 12:28:44 BST