From: sleeper75se (sleeper75se_at_yahoo.se)
Date: 2001-12-28 17:02:56
Hi Jim,
I know this topic is a bit off track, but I think it is interesting
to discuss these kind of things, so bear with me and let me explain
my thinking.
> Why filter at 9kHz ? We're talking about sampling at 256Hz in the
> modular EEG design, so I reckon a 1kHz carrier would be fine. If
> making fancy filters is difficult, use a 4kHz carrier. This leaves
> plenty of space between 128Hz and 4kHz for the filter to operate.
I chose 9kHz because I hoped the demodulation would be easier to do
with a high carrier frequency. Filtering and downsampling from 40kHz
to 256Hz in software after demodulation is fairly cheap, I think, but
perhaps you are right, that 1kHz would work well too.
It's not difficult to make fancy filters (with the right synthesis-
software), but they cost more money. Also, if you know nothing of
filters, but need one, you can buy 8th order switched-capacitor
lowpass filters for $9. Just add a clock-signal - the frequency
decides what cutoff frequency you get. Can't get any easier than
that. :-)
> To my simple-minded way of looking at it, you get a data point for
> every carrier cycle. All you have to do is look for a maximum (with
> maybe some fiddling due to the inversions for -ve signal levels).
[snip]
> trouble. Or is this too `cheap' an approach ?
I think it would be hard to find the actual peaks using this method.
In most cases it will fall between two samples, so we would need some
way of extrapolating their actual position and height.
> > * Modulate using a switch, flipping it back and forth
[snip]
> I'm cautious about this approach. For a slow 1Hz wave, for example,
> this means that we would be switching between a +ve voltage and 0V
for
> 0.5 sec, then -ve voltage and 0V for the next 0.5sec.
It's actually the other way around. The slow signal is chopped up by
a kHz square wave that is in turn filtered (by the 9th order filter)
to be identical to sine-wave modulated signal. The square wave does
not have to be balanced - the high pass filter in the sound card
would do it for us.
> I'll not mention AM again !
Me neither. :-)
Unless someone asks ...
See you later,
Andreas
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