Throb
An unusual text mode that sends pairs of tones in slow pulses.
What does it sound like?
A slow, pulsing, throbbing tone.
12-second example. A clean example made with the free fldigi program (Throb 1); signals on the air usually sound noisier.
Who uses it?
Amateur radio experimenters.
Where and when can I hear it?
Various ham frequencies; rarely heard.
The science
Throb sends text as pairs of tones played at the same time, chosen from nine closely spaced tones (8 or 16 Hz apart), with some characters using a single tone. Each symbol is shaped with a raised-cosine swell in loudness, which gives the mode its pulsing sound. It runs at 1, 2 or 4 symbols per second (about 10 to 40 words per minute) in just 72 to 144 Hz; the ThrobX variant uses 11 tones. There is no error correction.
Why use it?
The slow, narrow signal is fairly sensitive and moderately robust for keyboard chat, and it fits in very little spectrum. The costs are demanding tuning, since tones sit only about 8 Hz apart, and the need for a very linear transmitter because two tones are sent at once.
Lore
Throb and its successor ThrobX were created by English amateur Lionel Sear, G3PPT. fldigi's manual says flatly that you will never mistake Throb for any other mode.
Easily confused with
- Olivia, from the same family
- Contestia, from the same family
- MFSK (MFSK16/32), from the same family