Radioteletype (Baudot/ITA2)

Baudot RTTY

RTTY, the old version anyway, uses a 5-bit, digital code called Baudot, for its original inventor. This is also where we get the Baud, a unit of digital information. HF baudot is usually sent by frequency-shift keying, as we see here. I set the shift to 850 Hz so it would show up better. Hams and many other users prefer a 170-Hz shift, which fits a CW channel.

The two reddish-brown lines are the FSK tones, and everything else is keying transients, phase weirdness, software "dog-boning," and some instrument artifacting, all due to the instant frequency shift. While FSK can be a pretty clean mode, all this stuff increases with baud rate, so higher speeds require more bandwidth. Note how the two tones settle down to sine-wave purity between keyings. The duty cycle of a perfect, phase-coherent, RTTY signal approaches 100%, requiring that transmitters be operated at much lower power than for CW and SSB.

Baudot is asynchronous, and uses pulses of varying lengths, accounting for the gaps we see here and also for its intermittent sound on the air. A practiced ear can distinguish Baudot from such synchronous modes as SITOR-B, which gurgle along more steadily. Baudot bits have no redundancy, and degraded propagation causes missed characters, wrong characters, or whole screens full of gibberish. Just about all newer teleprinting schemes incorporate error correction to partially solve this problem.

All plots made with GRAM.EXE.