[Coco] Cassette Program Loading Help Needed

RETRO Innovations go4retro at go4retro.com
Sun Mar 19 21:04:03 EDT 2017

On 3/18/2017 11:12 PM, Dave Philipsen wrote:
> In theory, a square wave should work.  The only reason for the sine 
> wave was to be more accommodating to the analog circuitry of a 
> cassette recorder.  You might try a square wave with some sort of 
> attenuator on it to control the amplitude of the signal going into the 
> CoCo.  As I recall, the CoCo was somewhat sensitive to level and you 
> had to tweak the volume control on the cassette player to get it to 
> work reliably.  You can always put a 'scope probe on the output of the 
> comparator to see what it's passing (pin 13 of U14 or pin 2 of U4).
Did that today, and rewrote the code to output a 1200/2400Hz square 
wave, with 68ohm resistor on output, fed into a 1uF cap to the cassette 
audio in.

Looking at my output versus pin 2 of the 6821, the results look 
identical, so I think I am sending the right voltage levels and such.

I am concerned the problem is the last bit of the last byte sent in a 
set, the 0x55.  since the last half of the cycle has the output low, the 
junction between the end of the last bit and the rest period is lost.  I 
wonder if that is the issue.

At this point, it appears I need to find a way to load an actual audio 
file into the coco to see what the comparator output looks like.


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