I'm having a hell of a time getting this to work. I've got a Casio fx-720p + FA-3 cassette interface, a TRS-80 PC-3 with its cassette interface and printer dock, and a Tandy 102 with a cassette cable.
I've tried hooking all three of these to both a modern PC running Audacity, and a small Philips digital audio recorder that's equipped with a mic input and records as WAV (i.e. no mp3 compression artifacts). On all of the computers, I can SAVE/CSAVE a program, and get an audio file that sounds like I would expect, i.e. lead-in tones, and separate bursts of data that sound like a file header and program data separated by a brief pause.
I've also checked continuity of audio cables with a multimeter.
Try as I might, I can't get any of these computers to successfully load or verify the program that was just saved. I've tried a wide range of volume settings (the Philips recorder has 15 discrete volume levels, and I tried every single one with the Casio). The Tandy 102 passes the audio input through to its internal modem speaker when the input volume is above a certain threshold, but it juts sits there with the "cload?" command on the display, never reporting any success or failure.
Do I need to be concerned about the recording volume level? The Philips recorder just has "high" and "low" sensitivity settings, which I'm guessing act as a coarse mic gain (either that or "high" sensitivity just compresses the dynamic range by boosting the low end; I haven't tested extensively, though I definitely hear a slight loudness difference between the two settings). What should the graph look like in Audacity? Just shy of clipping at +/-1.0? Hovering around +/-0.5?
I even tried hooking the Casio to my record player/cassette deck stereo system, using a sealed new-old-stock cassette tape, with similar (i.e. unsuccessful) results. I could hear the recorded data, but not actually load it.
What are the tricks to making this work? I would expect these computers to be pretty forgiving about audio, given the immense variability in quality and speed/pitch regulation of the average 1980s tape recorder, but even near-flawless digital recordings are getting me nowhere. Does the fact that these are all mono plugs being used in stereo jacks have anything to do with it?
FOLLOWUP: I acquired an adapter to split a 1/8" stereo jack into independent left/right 1/8" mono audio jacks. Still nothing from the Tandy 102; I've yet to test the other machines with it.