A kind of radio facsimile that was prevalent in the first half of the 20th century, in particular in Germany, but is now used exclusively by radio amateurs.

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 (pixels per second). Since the text was printed on continuous rolls, the number of columns is indefinite.

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.

All implementations of Hellschreiber print all received columns twice, one below the other (but they are not transmitted twice). This is to compensate for slight timing errors that are often present in the equipment, and causes the text to slant. The received text can look like two identical texts coming out one below the other, or a line of text coming out in the middle, with chopped-off lines above and below. In either case, at least one whole letter can be read at all times.

The digital revolution has made Hellschreiber available as a "sound card mode" for radio amateurs, and it is steadily gaining popularity. The software implementations can also do things that the electromechanical teletypes never could:

  • Depicting the received signal as shades of gray instead of monochrome, thereby making it much easier to read weak signals.
  • Changing to a different font. Here is one mode that is truly international and independent of character sets: any thing that can be depicted as markings within a 7 pixels high grid, can be transmitted over the air.

Hellschreiber has also spawned a number of variants over the years, many of them due to radio amateur efforths in the 90s. Examples of them are: PSK Hell, FM Hell, Duplo Hell, C/MT Hell, S/MT Hell, and Slowfeld.

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