Radio Teletype (RTTY) is a mode for transmitting printed messages over the air. The data coming out of the teleprinter is converted into tones, one tone called the mark and the other tone called the space. The conversion is done by a modem, that operates on the same principle as a phone modem or packet radio TNC. It simply modulates / demodulates the signal. The tones are transmitted using FSK
The mark frequency is 2125Hz and the space is 2275Hz. RTTY is commonly transmitted at speeds of 45.5 bits per second. RTTY is either transmitted in Baudot or ASCII code.