RTTY uses a variety of different modulation methods, of which frequency shift keying are the most common. The coding used is the 5-bit ITA2 code (also known as the Baudot code), which is used asychronously with start and stop bits.
RTTY is not fast by modern standards; a typical baud rate for RTTY operation was 45 baud (approximately 60 words per minute). If upgraded to a "100 wpm wheel" one could achieve a rate of 75 baud, but HAMs generally did not use this faster speed.
The combination of low baud rate with robust FSK modulation makes RTTY highly resistant to most forms of radio interference, second only to Morse code.
I initially operated RTTY using AFSK with a Rigblaster Nomic interface to an Icom IC-756 Pro transceiver.
Using AFSK meant that I could not use the Icom's RTTY filters and QRM during contesting meant that I generally operated rather high in the RTTY sub-bands.
Running FSK I was able at long last to use the Icom's RTTY filters (5 of them - 1 kHz, 500Hz, 350 Hz, 300 Hz and 250 Hz) along with the wonderful Twin APF filter which literally brought RTTY signals out of the noise.
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