Cassette Interface Assistance

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: http://labs.rakettitiede.com/12kbps-simple-audio-data-transfer-for-avr/ 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 <http://www.colorcomputerarchive.com/coco/Documents/Manuals/Hardware/Color%20Computer%20Technical%20Reference%20Manual%20%28Tandy%29.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 -- Jim Brain brain at jbrain.com www.jbrain.com