Cassette Interface Assistance

Paul Berger phb.hfx at
Tue Feb 28 12:06:11 CST 2017

Now that I read your whole email I would like to ask what you plan to do 
with this boosted signal?   I have never had a CoCo, but most cassette 
interfaces on these old computers pretty much all worked like a modem 
but you where recording the signal instead of sending it down a phone line.

 From what I read the CoCo uses   1200Hz for zero and 2400 Hz for one, 
what I was reading also went on to claim that it is a single cycle of 
the tone to represent a bit.  If you intend to feed this into a computer 
converting the signal to a square wave would be ok as long as the 
software monitoring the port had a high enough sampling rate to be able 
to differentiate the width of the pulses, which should not be hard as 
one is twice as long as the other.  A comparator would be the best way 
to produce a square wave representation, as they normally have a sharp 
threshold which you would want to adjust so that it switches over at 
about the 50% level of the input signal.  If you have a dual channel 
scope you can compare the input signal to the output to see if that is 
what is happening.  That might be hard to see on an analog scope, this 
is where a storage scope or a digital scope comes in handy because you 
can  freeze the signal.


On 2017-02-28 11:55 AM, Jim Brain via cctalk wrote:
> Analog, which is my nemesis, curses me again.
> I have a cute idea for a cassette port project for the Tandy line of 
> computers (the ones with the cassette port).  I have a Coco 3 on the 
> bench, so I scoped the output line while doing 'csave "jim"'.  The 
> signal looks to be just under 1V PtP (0-1V on the scope), and rests at 
> about .3V when not sending data.
> I have tried 6 different ways to boost the signal to 5V digital, to no 
> avail, and so I ask humbly if someone with analog knowledge might be 
> able to assist.
> I first tried to boost the signal with a transistor (with variations 
> using a N channel FET as well). Arguably, that was foolhardy, and it 
> did not work.
> My second attempt was based on this link that was shared with me:
> The output from the Coco3 does not appear to be "loud" enough to work 
> with this circuit.
> So, I finally decided a comparator solution would be required.
> First, I tried a design using a 741 op-amp, which failed miserably, 
> but probably would have worked, but I tried to merge the design from 
> the Coco1, and replace the LM339 in the Coco 1 design with the 741, 
> and I feel I did not merge the designs well :-)
> I then tried using the comparator in an Atmel AVR, and had minimal 
> success.  By biasing one input via a variable resistor to around .8V, 
> I was able to get a digital stream, but it did not look like the data 
> stream of the cassette format.
> I then pried an LM339 out of my Coco1 and replicated the circuit int 
> the Coco 1, as noted in the tech manual:
> Color Computer Technical Reference Manual (Tandy).pdf 
> <> 
> I was shocked that I had no success with that design at all.  I 
> assumed (wrongly, it appears) that the Coco cassette input circuit 
> would read the output of it's output circuit.  Beyond the possibility 
> that my components are defective or I wired it up wrongly, I can only 
> theorize that Tandy assumed that all tape recorders would AGC the 
> output and then feed a 2V PtP signal back to the Coco (the Coco 1 
> circuit looks to bias the comparator at 1.05V (not sure about the 
> feedback resistor's impact))
> I can fiddle around with the AVR solution, which might work if I can 
> smooth out the spikes and bias the comparator right, but it just 
> bothers me that the Coco 1 circuit does not work, as I assumed I would 
> at least have success by copying a working design.
> Jim

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