Cassette Interface Assistance
ajp166 at verizon.net
Tue Feb 28 10:21:45 CST 2017
On 2/28/17 10:55 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
The 339 is a very strange beast and can behave very badly. The 741 is
slow but should work as an amplifier and as a comparator
with minimal grief. Trying to use 339 or 714 circuits for each other
will usually fail as they behave differently.
> 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))
The tape recorders used had no agc. They were simple portables and used
the mic or line input and headphone output.
The big thing is most of those circuits took advantage of the audio out
of the recorder (low level and fairly low impedance)
easily but larger they would overload (sometimes badly).
The output was for a low level input and loading it usually lowers the
output more (high impedance source).
> 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.
Why not just go inside and grab the signal at it source before any of
the analog? it starts life at TTL waveforms (not 5V).
Why are you trying to get to 5V in the first place?
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