[Coco] It's a small win, but a win nonetheless

Gene Heskett gheskett at shentel.net
Sat Mar 11 17:18:20 EST 2017

On Saturday 11 March 2017 15:33:45 RETRO Innovations wrote:

> OK, the circuit I have now looks to be relatively solid:
> 1uF cap from input, other side goes to - comparator input
> - comparator input held to ground via 10M resistor
> 100K/10K voltage divider (.45V on positive pin)
> Hysteris from output to + pin, of 200K (pretty wide gap)
> I got it working with a comparator IC, then switched to use the
> comparator in the Atmel AVR (lower parts count).
> Notes:
> Coco3 scope on - comparator input.  Hovers at .12V when idle (I would
> think the cap would remove all DC bias, but something is leaving
> something there)
Do not forget that if the input r on the comparator's input is 10 megs, 
your taking a look at it withe a 10x scope probe, or a dvm lead, either 
the scope or the dvm is another 10 meg r, so the resting voltage you 
measure is really twice that.

If that 1uf is a small electrolytic, its leakage could account for that. 
Also the comparators input current has to flow thru that r so you might 
want to switch comparator types from one with a starved current pnp 
input stage, to one with depletion mode fet inputs, which have femtoamps 
for input currents. Neither of us has gear to measure that low a 

> When active, signal swings from .12 to .8V.  Median is .46, which is
> where voltage divider is set.  With hysteresis, the voltage swings
> from .43 to .65V
Which input is getting the hysteresis feedback?

> Coco2 is .18 to .88V.  Medium is .5, within band
> Coco1 is -.28 to .88V.  Median is .3, quite a bit lower than my
> comparator's center, but the data seems to come in correctly.
> Every once in a while, the detector messes up on bits, and I get AA AA
> AA instead of 55 55 55, but I am hopeful that my crappy prototype is
> the cause, and a solid PCB will address the issue.

Needs 2 stop bits?  More positive framing sync, but will the music 
tolerate that?  I dunno.

> Jim

Cheers, Gene Heskett
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