[Coco] Coco Cassette interface help

[RETRO Innovations]

On 2/27/2017 1:10 AM, Dave Philipsen wrote:
> Are you primarily looking for information on how to deal with the analog signals and convert them between digital and analog or how to encode/decode?
I see the format (and I figure, once I can see the bitstream in the AVR, 
I can easily figure any lingering questions out), so I am not asking 
about that.

However, as I scope the cassette output from the Coco, I am seeing a 
very small sine wave signal (as expected), with a DC offset of ~1.1V, 
that looks to be .3V PtP (I had to eyeball it on the scope, so those are 
approximate).
> The best way to deal with this type of data is to detect zero crossings and their timings.
I saw the discussions about zero crossing, which is fine, but I first 
need to amplify the signal high enough that the AVR can detect 
anything.  If nothing else presents itself, I figured I'd copy the LM339 
op-amp circuitry in the Coco1 to feed the signal into the AVR, but if a 
simpler option presents itself, that would be great.  I tried the 
electronics "duct tape" option, a small signal transistor, but that just 
pushed the entire signal up, and did not create something I could use.  
A FET was a total disaster :-)

Right now, if I can just get the signal from the Coco into my AVR, I'd 
be happy.  Writing a signal back to the Coco seems much easier from an 
electrical perspective.

Jim