RTTY Modes


General Description

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 signal bandwidth.

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.

Protocol

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.

Operating Parameters

Mode Symbol Rate Typing Speed1 Duty Cycle2 Bandwidth3 ITU Designation4
RTTY 455 45.45 baud 6.0 cps (60 wpm) 100% 270 Hz 270HF1B
RTTY 50 50.0 baud 6.6 cps (66 wpm) 100% 270 Hz 270HF1B
RTTY 75 75.0 baud 10.0 cps (100 wpm) 100% 370 Hz 370HF1B

Notes:
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.