Tutorial RTTY

Tutorial: RTTY

What it is

RTTY is radioteletype: text sent as two alternating audio tones (mark and space) at 45.45 baud, the same technique as old teleprinters. It is still used in contests and on a few utility channels. OnAir decodes the standard amateur variant on upper sideband.

What you need

• An HF-capable receiver. An SDRplay or an Airspy HF+ tunes the HF bands directly. An RTL-SDR reaches HF only in direct-sampling mode (Radio settings, see Radio-RTL-SDR; the RTL-SDR Blog V4 uses its built-in up-converter). A HackRF or another radio that starts above the HF bands needs an up-converter such as Ham It Up or SpyVerter, which you pick in Radio settings so that the whole app shows the true frequency (see Supported-radios). The mode asks for 2 Msps and keeps working down to 250 ksps, so even a small radio is enough: the signals sit within about 3 kHz of the dial frequency.
• An HF antenna: a long wire of 10 m or more as far from household electronics as you can get it, a dipole cut for the band, or a magnetic loop for a noisy site. Keep the feed line away from switching power supplies and LED lighting, which are the usual source of S9 noise on a city balcony.
• Frequencies (USB dial): RTTY 3.58, 7.04, 14.08 (the default), 21.08 and 28.08 MHz. Contests use the CW/RTTY segments. The panel links to the RTTY QRG list of the RTTY Operators (rttyops.com), which opens your browser.

Before you start

Lower the gain until the ADC level in the status line is well below full scale: strong broadcast stations next to an amateur band overload a direct-sampling RTL-SDR easily, and the result is spurs that look like signals. Use Auto-tune or Optimise gain (see Your-first-reception) rather than guessing. Choose upper sideband for all dial frequencies in the lists below; the decoders listen from the dial frequency upwards. A DC spike in the middle of the spectrum is normal for some radios and is not a signal: leave a few kHz of offset if you can.

Step by step

1. Connect the radio and antenna and start OnAir. Open the group Amateur: digital and choose Teletype (the entry reads Teletype, 14.080 MHz).
2. Press Start. Until a signal is found the panel says searching for a signal; then it shows the tone frequencies, mark ... Hz space ... Hz.
3. Check Baud (45.45 for amateur RTTY) and Shift (170 Hz is the usual amateur shift).
4. Read the text in the pane. Use Copy to take it and Clear to empty it.
5. If the text is gibberish, tick Reverse (the mark tone is the upper one) and, if figures stay stuck after a space, Unshift on space.

What you should see

Readable text, usually contest exchanges (call, RST, zone or number). Quality shows how clean the signal is, Chars the characters decoded and Errors the characters dropped because the start or the stop bit was wrong.

Reading the screen

The text pane, the tone readout (mark/space), Quality, Chars, Errors. The spectrum shows the two tones 170 Hz apart.

Options and settings

• Baud: 45.45 is normal; 50 and 75 are used by commercial and weather stations.
• Shift: 170 Hz amateur, 425 and 850 Hz commercial.
• Reverse: swap mark and space.
• Unshift on space: a space switches the figures back to letters, which hides a lost shift character.
• RTTY QRG link and Band row.

Troubleshooting

• Garbage text. Wrong baud, wrong shift or reversed. Try Reverse first.
• Letters where figures should be. A lost shift code. Tick Unshift on space.
• Many Errors. Weak signal, selective fading or a drifting transmitter.
• No signal found. Look on the waterfall for two tones 170 Hz apart; contests are on weekends.
• Only contest chatter. Normal: the band is full of repeating exchanges.

Tips

• RTTY contests are the busiest time; the signal is easy to find.
• Keep a recording (Tutorial-Record-IQ-then-the-Signal-workbench) of a good opening and replay it to try the settings.

Legal and privacy notes

Receiving amateur signals is allowed in most countries; the text is public by design. Transmit works in OnAir and has been used on a real radio by the maintainer and in loop-back tests; you need an amateur licence to transmit (see Transmit and Legal). Some RTTY on the shortwave bands is commercial or press traffic; reading it is generally legal, but check your own country's rules.

Under the hood

RTTY uses the 5-bit ITA2 (Baudot) alphabet with a start bit and 1.5 or 2 stop bits at 45.45 baud, frequency-shift keyed with a shift of 170 Hz. The decoder finds the mark/space pair, tracks it and has a letter/figure shift state.

Related

Mode-RTTY, Tutorial-CW, Tutorial-PSK31, Radio-RTL-SDR, FAQ, Troubleshooting