Radioteletype

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RadioTeleType (RTTY) is a telecommunications system consisting of two teleprinters linked by a radio link.

Early RTTY operators used equipment built for Bell System wire-linked TeleType(tm) systems. When ham radio operators began using this equipment for RTTY, they were required to sign agreements not to use the equipment for commercial purposes. This equipment was expensive and required periodic maintenance.

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[edit] How it works

RTTY uses a variety of different modulation methods, of which frequency shift keying is the most common.

The FSK RTTY signal is composed of an audio signal which alternates between two tone pitches. It is this "shift" which tells the receiver what character is being transmitted, as the length of each shift varies. These signals can be heard on a normal shortwave radio, and have a "beedle-eeeedle-eedle-eee" sound, usually starting and ending on the high-pitched tone.

Experienced RTTY operators are able to recognize some FSK patterns by ear. The most common test/idle signal is a series of "RY" characters, as these are the widest characters in the code set, and are an easily-recognized pattern of tones.

  • Coding used is typically 5-bit ITA2 code (also known as the Baudot code), which is used asynchronously with start and stop bits.
  • At least one common RTTY system uses 6-bit ITA2 character codes.
  • More modern systems use 7-bit or 8-bit bytes.

Many RTTY operators had equipment which featured paper punch-tape readers. The operator would type the message on the TTY keyboard, which would punch the code into the tape. The tape could be edited as desired, then transmitted at a steady, high rate, without typing errors. A tape could be reused, and in some cases might be made of plastic or even metal in order to be reused many times.

[edit] Slow by modern standards

RTTY is extremely slow by modern standards; a typical baud rate for RTTY operation was 45.45 baud (approximately 60 words per minute). This is one reason that RTTY has declined in popularity, as faster, computerized transmission modes were developed, using less-expensive equipment.

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. Part of this is due to the fact that FSK, like FM, always operates at maximum power. FSK is the single most demanding mode for transmitter equipment.

[edit] Spectrum efficiency


RTTY and Packet spectrum efficiency compared with respect to {bits/second} thruput
RTTY and Packet spectrum efficiency compared with respect to {bits/second} thruput


[edit] Primary users

Principally users that need robust shortwave communications

  • Various navies of the world
  • Various armies of the world
  • Diplomats, especially in Africa and parts of Asia
  • Weather reports are transmitted by the US Coast Guard nearly continuously
  • RTTY systems are also fielded by amateur radio operators, and are popular for long-distance contacts

[edit] Pronunciation

The pronunciation of RTTY is disputed

  • In very few applications, notably the U.S. military, radio teletype is known by the acronym RATT rather than RTTY.
  • A more common term is "TWX" (pronounced "twix"), for "TeleType Wireless Exchange."
  • Those who are actively engaged in amateur RTTY communications pronounce RTTY not by its initials but as "ritty".

[edit] Spectrum usage

Comparisons of MT63, RTTY and HF Packet spectrum
Comparisons of MT63, RTTY and HF Packet spectrum


[edit] Media

[edit] See also

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