The Hellschreiber or Feldhellschreiber was a facsimile-based teleprinter invented by Rudolf Hell. It has since been emulated on computer sound cards by amateur radio operators; the resulting mode is referred to as Hellschreiber, Feld-Hell, or simply Hell.

It was developed at the end of the 1920s, and has the advantage of being capable of providing intelligible communication even over very poor quality radio or cable links.

During WW2 it was sometimes used by the German military in conjunction with the Enigma encryption system. The name is a pun on the inventor's surname, which approximates to 'clear' in English.

Hellschreiber transmits text by dividing each column into 7 pixels, and transmitting them sequentially, starting at the lowest pixel. A black pixel is transmitted as a signal, and a white pixel is transmitted as silence. This takes place at a rate of 122.5 baud. Since the text was printed on continuous rolls, the number of columns is indefinite.


Frequencies:

10-25 WPM. 21% Duty cycle. 3.57500 (Region 1) 3559.00 (Region3), 3.580, 7.037, 7030.00 - 7040.00, 10.137, 10135.00 - 10145.00, 14.0635, 14063.00 - 14070.00 (many monitor 14063.00), 18100.00 - 18105.00, 21.063, 21063.00 - 21070.00, 28.120, 28063.00 - 28070.00 and 28100.00 - 28110.00 (Novice).