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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. "Hellschreiber" translates into English as "Light Writer," "Bright Writer," or "Clear Writer," and is a pun on the name of its inventor (Hell (the adjective) is German for "light," the adverb is "bright.")
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.
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.
Feld-Hell time averaged spectrum (soft keying)The original Hellschreiber machine was a mechanical device, so therefore it was possible to send "half-pixels". The right ends of the loops in B, for instance, could be shifted a little, so as to improve the readability. Any on-signal could in any case last no shorter than 8 ms, however, both because of having to restrict the occupied bandwidth on the radio, but also for reasons having to do with the mechanical makeup of the receiving machinery.
