RTTY uses a 5 bit code transmitted at 45 or 50 baud. It is a bit like asynchronous or serial data. There are one and a half stop bits and the half is very important. It uses a shift code to get all
the letters and numbers out of the 32 code values 5 bits give you.
It is transmitted as two audio tones one for mark and the other for space. It should be possible to do this with an arduino but I don't know anyone who has.