[Coco] Cassette Program Loading Help Needed

[Dave Philipsen]

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).

Even a sawtooth wave could work but might be kind of finicky on the 
timing of the zero crossings generated.

Another thing you might experiment with is some sort of decoupling 
capacitor inline in case you've got too much DC bias coming out of the AVR.


Dave


On 3/18/2017 10:56 PM, RETRO Innovations wrote:
> On 3/18/2017 10:36 PM, Dave Philipsen wrote:
>> What does the output of the AVR look like going to the CoCo?  If the 
>> signal is coming out with a high frequency component 
> Not really a HF component, but the decay on the cap is making the sine 
> wave very sawtooth shaped.
>
> I tried doing a square wave, but the Coco3 did not like it at all (the 
> S does not even change to inverse), and so I decided to try the Coco 
> method of a R DAC.  I have it looking good on the scope, using the 
> exact values in the Coco1 schematic, but when I connect it to the Coco 
> input, it just drags the entire thing down to ground.  I am using the 
> output pins on the AVR, which can source quite a bit of current, so I 
> am not sure what is going on.
>> due to the way the PWM works it will trigger undesirable 
>> zero-crossings in the comparator input on the CoCo. You could try 
>> increasing the value of the resistor.
> I will try that.
>>
>> You could go about it in a completely different manner and not 
>> generate the audio using PWM.  Just generate a straight square wave 
>> and feed that to the CoCo.
> As noted above, I thought of that, but early results were not 
> encouraging.
>
> I thought for sure the resistor DAC would work, and I could get back 
> to laying out the board, but I am back to diagnosing again.
>
> Jim
>