RTTY
From Wikiham, the ham radio free encyclopedia
Radioteletype (RTTY) sends text using teleprinter-style coding. In common amateur operation, two frequency states called mark and space represent the serial data. The mode is used for keyboard contacts and contests.
A common amateur format
| Parameter | Typical value |
|---|---|
| Symbol rate | 45.45 baud |
| Frequency shift | 170 Hz |
| Character code | 5-bit ITA2 |
| Start interval | 1 bit |
| Stop interval | 1.5 bits |
These are common settings, not the only RTTY formats. ITA2 uses letters and figures shift codes because five data bits alone cannot represent the full character repertoire at once.
Time per symbol ≈ 1 / 45.45 ≈ 22 ms Intervals per character = 1 + 5 + 1.5 = 7.5 Maximum character rate ≈ 45.45 / 7.5 ≈ 6.06/s
Shift characters and other overhead reduce the useful text rate. 170 Hz shift describes the frequency difference between mark and space; it is not an exact occupied-bandwidth specification.
FSK and AFSK
With direct FSK, a suitable transmitter changes its RF frequency through a keying input. With audio frequency-shift keying (AFSK), a modem or computer generates two audio tones and an SSB transmitter translates them to RF.
The combination of sideband, tone assignment and any reverse setting must give the correct mark/space polarity. Selecting USB rather than LSB without adjusting the remaining conventions may invert the data.
Practical limitations
Traditional RTTY has no strong forward error correction or automatic retransmission for a keyboard conversation. Fading and interference can produce incorrect letters or leave the receiver in the wrong letters/figures state.
Its sustained transmissions also require attention to transmitter duty cycle and cooling. In AFSK operation, audio overdrive and clipping can degrade the signal even when the receiver still appears to decode it locally.