You can get MIDI on a 3.5mm plug and lookie here... my modules are covered in 3.5mm jacks that aren't labeled MIDI. From what I've been looking up, MIDI transmits at 3.5V to 5V through the port... doesn't it? I'm asking.
I'm thinking it's an audio rate square signal that bursts on note on and note off, if you're sending other data, like CC, or pressure, I'm sure it's continuous. Anyone messed around with this?
I haven't but I do believe MIDI is indeed TTL (5V). This sorta thing kinda reminds me of the PC tracker days when someone (myself included) would load up say config.sys as a wave file and use that to make music :) Could be really neat to try! One thing you will want to consider is that at least DIN MIDI connections use opto-isolators to prevent damage and that might be something you may want to look at (or perhaps use say a cheap USB to MIDI device). It probably won't break anything but I'd be worth taking a look at those circuits. There's several Arduino MIDI boards and things so it should be an easy circuit to track down.
http://www.gweep.net/~prefect/eng/reference/protocol/midispec.html
It's actually a bit more like what we have in ham radio with packet radio...digital bursts as specific "packets" with distinct destinations, other SYSEX data, and the note on/off/# that comprises most of it. And m00dawg's quite right about the optoisolators...they do a good job of compensating for voltage changes so that most any signal still gets converted back to a digital stream on "wire". And if you do something like, say, plugging the MIDI into a synth and a power outlet (not sure HOW, but I'm sure someone's tried), it'll blow the opto to kingdom come, but everything else stays more or less safe.
