Cassette Interface Assistance

[Jim Brain]

On 2/28/2017 2:34 PM, Brent Hilpert wrote:
> In answer to the final point, and the interests of some analog education, looking at your reffed pdf, page 75, the reason (or one reason)
> your duplicated Coco input circuit is not working is the gross impedance mismatch between the coco tape output and input circuits.
To get to the point, this advice was golden.  After stripping away the 
220, 8K2,6K8 and the 56K to Vcc, the unit started functioning. A nasty 
60Hz blip was removed by slightly biasing the negative input with a 1M 
resistor to GND.  The 1M drive to ground pulled the input over so much 
lower, making the center around .7V, so I used Tony's idea to create a 
.7V reference on the + end with a forward biased diode and a week pullup 
to Vcc, which made things ever so much better.

Obviously, given the LM339 issues, this is unworkable in the end, but it 
did get me to realizing some outputs on the LM339, so thanks to both of 
you.

I can plainly see the oscillation on the comparator transitions, so it 
is obvious I need a better comparator.  I'll have to source a few 
different comparator options from Digikey to fill my parts box. 
Suggestions are welcome.
> Using the internal AVR comparator sounds like a better final solution (fewer components), but in devising an
> input circuit for the AVR you may be running into the same issue of loading a high-impedance source.
Though it plays into Tonly's notes about my analog understanding (or 
lack thereof), I was so disillusioned by the analog uncertainties that I 
initial focused on the AVR comparator.  By correctly biasing the 
comparator, I was able to get much better results tonight, but the lack 
of any ability to add hysteresis into the design hindered my success.  
At transition, the signal simply bounces badly.  I tried to construct a 
low pass filter (22pF and 5K6), and some variations, but I was not able 
to overcome.  I then tried to deal with it in SW, but I have not yet 
been successful.  I'll try again, but lack of hysteresis is a big issue.
>
> However, considering you have a known and fixed source circuit (the coco tape output) you might try something even simpler for the
> AVR comparator input, such as just a 1K series R followed by a 220K to 470K R to GND. The comparator should then be seeing essentially the coco output
> wave shape and levels, without you having to adjust the software detection for biasing introduced by the input circuit.
I will try this tomorrow.

Jim


-- 
Jim Brain
brain at jbrain.com
www.jbrain.com