Yes of course current loop, hadnt thought of that, should have as they use that for some of the instruments where I work, 4-20mA is the industrial standard.
What I have set up is I spose to an extent a current loop, transmission is done by dumping current through one resistor for a 1 and another for a 0, the receiver discriminator detects current my measuring voltage across a resistor inline, thinking about it the only thing I need to do to improve noise immunity is lower the value of the resistors to make current mod higher, so long as I dont create too much ringing, its a 2.5mm straight run cable.
I have some experience of telex and rtty, what I'm doing is similar only 8 bit instead of 5, an digital instead of tone.
I have a test jig on breadboard held together with elastic bands and luck, I'll have another trial session with greater current mod this weekend.