Also known as: RTTY, Radioteletype
RTTY (Radioteletype) is the classic way of sending typed text over radio: each character is a 5-bit Baudot code shifted onto the carrier by frequency-shift keying, toggling between two tones called mark and space. In its standard amateur form it runs at 45.45 baud with a 170 Hz shift, a specification inherited almost unchanged from the mechanical teleprinters of the mid-20th century.1 Simple, robust, and instantly recognizable by its two-tone warble, RTTY remains in daily use on the HF bands nearly a century after its introduction.
Overview
RTTY has no error correction and no handshaking — it is a raw, one-way stream of framed characters. A start bit, five data bits, and a 1.5-bit stop period make up each character, and the receiver simply tracks whether the incoming tone is mark or space. Because Baudot’s 5-bit alphabet only has 32 codes, two “shift” characters (LTRS and FIGS) switch the decoder between letters and figures/punctuation, doubling the usable symbol set.
Technical characteristics
| Property | Value |
|---|---|
| Alphabet | 5-bit Baudot / ITA2, with LTRS/FIGS shift |
| Modulation | FSK — mark and space tones |
| Shift | 170 Hz (amateur standard; 425/850 Hz also used) |
| Rate | 45.45 baud (amateur), 50 / 75 baud (commercial) |
| Framing | 1 start + 5 data + 1.5 stop bits |
| Error control | None |
History
Radioteletype grew out of landline teleprinter networks; on-air use expanded during and after World War II for military, press, and weather traffic, and amateurs adopted surplus mechanical machines in the 1950s and 60s. The 45.45-baud/170 Hz amateur convention dates to that era of Teletype hardware and has persisted into the software-decoding age.2 Commercial and diplomatic services used related speeds and shifts, and RTTY-style FSK long carried weather and news via services such as NAVTEX.
Deployment
On the amateur HF bands RTTY is still common for ragchews, DX, and especially contests, where its speed and simplicity remain competitive. Utility and maritime FSK services historically used the same signaling, and much surviving off-air RTTY on shortwave is weather, news-agency, or navigational traffic.
Decoding it with GopherTrunk
GopherTrunk does not decode RTTY — it is a trunked land-mobile scanner, not an HF text-mode decoder. RTTY is received with an SSB receiver or SDR feeding audio into a multimode program such as fldigi, MMTTY, or MultiPSK, which tracks the mark/space tones and reassembles Baudot characters. Any SDR that provides a clean audio slice on the right frequency and sideband will serve as the front end.
Sources
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Radioteletype — Wikipedia, for RTTY’s mark/space FSK signaling, standard amateur speed and shift, framing, and history. ↩
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Baudot code — Wikipedia, for the 5-bit ITA2/Baudot alphabet and the LTRS/FIGS shift mechanism RTTY uses. ↩