DominoEX
A text mode that steps between tones in a way that tolerates poor tuning and fading.
What does it sound like?
A gentle, flowing sequence of tones, like a slow ripple of notes.
12-second example. A clean example made with the free fldigi program (DominoEX 4); signals on the air usually sound noisier.
Who uses it?
Amateur radio operators.
Where and when can I hear it?
Various ham frequencies.
The science
DominoEX is a single-tone MFSK mode using 18 tones, but the data lives in the difference between one tone and the next rather than in which tone is sent, a technique called incremental frequency keying (IFK). A fixed offset is added at each step (IFK+) so the same tone never repeats, which fights the echoes of multipath reception. Speeds range from DominoEX4 at about 4 tones per second in about 173 Hz to DominoEX88 in 1600 Hz, with DominoEX16 near 15.6 tones per second in about 355 Hz; it uses a Varicode alphabet and normally no error correction.
Why use it?
Because only tone differences matter, DominoEX shrugs off mistuning and drift that would stop other MFSK modes, up to about 200 Hz of offset by its designers' account, so it is easy for beginners to tune. It was designed for the lower HF bands, where near-vertical skywave paths cause heavy multipath, and it turns around quickly between stations because it avoids long error-correction delays.
Lore
DominoEX came out of work by Murray Greenman, ZL1BPU, who designed it, and Con Wassilieff, ZL2AFP, who coded it, in New Zealand. The underlying IFK idea was invented by Australian amateur Steve Olney, VK2XV, and an earlier experiment, DominoF, was the first use of IFK in an HF chat mode. When the typist pauses, DominoEX fills the gap by sending a secondary text channel, often used for station identification.
Easily confused with
- Thor: THOR adds error correction
- MFSK (MFSK16/32): MFSK16 uses fixed tone positions