[Coco] Coco Cassette interface help

[Dave Philipsen]

Well, if you want to use a tried & true circuit just do what the CoCo 
does for the incoming data from the cassette player.  The schematic is 
on page 68 and involves an op amp and a few other components.  Most 
circuits for reading data-on-tape that I've worked with have involved an 
op amp or two.  The idea is to amplify the signal so much that the data 
is effectively changed from a sine to a square.  You can also google 
"sine to square wave converter" and find a bunch of stuff.  The thing is 
that you want the square wave edge to correspond as exactly as possible 
with the zero crossing of the sine wave and that's easy to do with an op 
amp.

Dave


On 2/27/2017 9:21 AM, RETRO Innovations wrote:
> 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
>
>
>