From: Waldemar Neto (wpneto_at_o...)
Date: 2001-05-19 02:07:41
FOR "DATA ACQUISITION PEOPLE"
I've done ( 10 years ago) an analog-device converter using the simple ADC808 ( for 8 inputs-MUX ) and it worked well ,
saving other kinds of processes (PLL or VCo) - the cost was less than $ 30( higher than sound card ).
It can use the exact frequency one wants - ( taking a proper crystal clock , or dividing the absolute value ) .
I shall have the pcb yet . What do you think ?
----- Original Message -----
From: jiva_at_humboldt1.com
To: buildcheapeeg_at_yahoogroups.com
Sent: Friday, May 18, 2001 9:47 PM
Subject: [buildcheapeeg] Re: Sound Card-Phase II
> ...
> > I'm still not following Joerg's explanation all that well.
> > (my lack of tech skills, not his lack of explanation)
I've gathered that in simpler terms you are doing a
variation on FM modulation/demodulation. I see how that
skirts the low frequency issue.
> >
> > Why he opted to put two VCOs on one channel further
> > confuses me.
> ...
>
> One reason is to eliminate a second optocoupler for the second
> channel and another reason is to reduce the amount of data
> that is sampled into the PC - and has to be processed.
I've no idea how much cpu horsepower is consumed in the process.
What little I've read and understood suggests the sampler
does it's thing until a buffer is full then notifies.
> On the other hand frequency demodulation could be made
> very simple if only one frequency per physical channel were used.
> The sinewave could be transformed into a rectangle wave
> with 2 thesholds (for hysteresis) and then the number of samples
> between two low to high transitions are counted. The reciproce
> of the count gives the frequency. Subtract the center frequency
> and then multiply with dV/df of the modulator VCO.
> Because this demodulation is very simple, a very high soundcard
> sample rate (BTW. what is the maximum sample rate available on
> normal sound cards ?) could be used to increase the resolution
> (number of bits).
>
the low end ($20 street) turtle beach mentioned way back specs as:
Sampling Rates: up to 48kHz in 1Hz steps.
Audio Converters: 18-bit ADC and DAC with sophisticated sample rate
conversion for high quality digital mixing.
--------
http://www.voyetra-turtle-beach.com/site/products/montego2/specificati
ons.asp (sorry about the split line ... it's a hassle)
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This archive was generated by hypermail 2.1.4 : 2002-07-27 12:28:30 BST