[Coco] Coco Cassette interface help

[RETRO Innovations]

On 2/27/2017 11:19 AM, Dave Philipsen 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.
Looking at the schematic, it is unclear how it detects them.  The output 
of the comparator goes into PA0, but that's not a special pin.  Does the 
cassette code spin on watching PA0 and count clock pulses, or is there 
something I am missing in the schematic?  I see where you can pipe 
cassette sound to the speakers, but nothing else.

Jim