FSK Hell
A variant of Hellschreiber that paints text using two tones instead of on-off keying.
What does it sound like?
A buzzy, rhythmic rasp with a two-tone texture.
12-second example. A clean example made with the free fldigi program (FSK Hell 105); signals on the air usually sound noisier.
Who uses it?
Amateur radio enthusiasts.
Where and when can I hear it?
Various ham frequencies.
The science
FSK Hell paints the same dot-matrix letters as Feld Hell, but instead of switching the carrier on and off it shifts between two frequencies, one for black and one for white. fldigi offers FSK Hell at 245 dots per second in about 490 Hz and FSK Hell 105 at 105 dots per second in about 210 Hz, both at about 2.5 characters per second. fldigi rates its average duty cycle at about 80 percent, compared with about 22 percent for Feld Hell.
Why use it?
As with all Hell modes, the eye does the decoding, so noise reduces readability rather than printing wrong letters. The 105-baud version, at about 210 Hz, fits in a narrower slice of spectrum than Feld Hell at about 350 Hz.
Lore
Although FSK Hell is often thought of as a modern amateur invention, frequency-shift Hellschreiber transmission was already being tested by the end of World War II, and later commercial Hell machines such as the Hell-80 also used FSK.
Easily confused with
- Hellschreiber (Feld Hell), from the same family