RTTY is one of the oldest automated modes and traditionally uses the Lower Side Band, whilst almost all the other digital modes use the USB. The typing speed is usually 45.45 or 50 baud corresponding to about 66 words per minute. The signal is digitised by switching between two audio frequencies. There is no error correction so that when the propagation conditions are poor a certain amount of guesswork is required when reading the message.

Spectrum of a RTTY transmission, 45 baud, 850 Hz frequency shift