RTTY has been used by radio amateurs since the 1950s. Initially an electromechanical system designed for use on
telephone wires, it was not conceived as a radio system, and could not be used by radio until the development of
the Ratio Detector during the 1939-1945 war. RTTY (the name means simply Radio Teletype) uses FSK to avoid
noise on the transmission path, but requires high power and is still prone to propagation effects, especially
selective fading and multi-path timing.
Early RTTY equipment used separate oscillators for each of the tones, and so could produce very wide key clicks,
requiring extra filters. Modern software uses phase coherent switching between tones, which somewhat improves the
With no error correction, and a start-stop system that is prone to false starts on noise, RTTY is not the best
mode for amateur use. However, it is easy to use, easy to tune, fast, tolerant of drift,
and is widely used for contesting for these reasons alone. A linear transmitter is not required.
RTTY is an unconnected, manually controlled message asynchronous character asynchronous simplex chat mode, used
without Forward Error Correction. The most widely used and default calling mode is RTTY 45 (45.45 baud).
Other less common speeds are 50 baud and 75 baud. The shift is usually 170 Hz, with the upper tone used for idle
condition (MARK). Commercial systems operate 425 or 850Hz shift.
Coding and Character Set
The ITA-2 character set is used. This has two 'shifts', one for letters, the other for figures and punctuation,
a total of 60 characters. There is no lower case. Modulation is direct 2-FSK, one data bit per symbol. Each character
is transmitted serially, preceded by an equal length start bit and
followed by a stop bit of the opposite sense at least 1.5 data bits long. Receiver synchronism is from the leading
edge of the start bit and independently timed from there.
||6.0 cps (60 wpm)
||6.6 cps (66 wpm)
||10.0 cps (100 wpm)
1. WPM is based on an average 5 characters per word, plus word space.
2. Transmitter average power output relative to a constant carrier of the same PEP value.
3. This is the "Necessary Bandwidth" as defined by the ITU.
4. A summary of the ITU Designation system can be found at http://en.wikipedia.org/wiki/Types_of_radio_emissions
5. Default and normal calling mode.