[Coco] Coco Cassette interface help

[S Klammer]

I was investigating a cassette port idea a while back (rounded tuits are in
short supply) and this site (there are many others) has details you may
find useful...

http://labs.rakettitiede.com/12kbps-simple-audio-data-transfer-for-avr/

Shain
On Feb 27, 2017 12:19 PM, "Dave Philipsen" <dave at davebiz.com> wrote:

> 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
>>
>>
>>
>>
>
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