Hellschreiber

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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.

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.

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.

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.

Improvements that came as a result of software implementation:

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.

1 Variants
- 1.1 Slowfeld
2 Media
3 Reference

[edit] Variants

Hellschreiber has also spawned a number of variants over the years, many of them due to radio amateur efforts in the 1990s. Examples of them are:

PSK Hell encodes the pixels brightness in the carrier phase instead of the amplitude. Strictly speaking, it's encoded in the change of the phase (differential phase shift keying): an unchanged phase in the beginning of a pixel means white, and a reversed phase means black. It operates at 105 or 245 baud.
FM Hell uses frequency modulation with a careful control of phase, essentially minimum-shift keying.
Duplo Hell is a dual tone mode which sends two columns at a time at different frequencies (980 Hz and 1225/1470 Hz).
C/MT Hell or concurrent multitone Hell sends all rows at the same time using tones at different frequencies. The transmission can be read using an FFT display. It allows for high resolutions.
S/MT Hell or sequential multitone Hell is like C/MT but with a discrete number of tones (characters are restricted to 5x7 pixels).
Slowfeld is a very slow mode (2 characters per minute) intended for beacon use.

[edit] Slowfeld

Slowfeld is an experimental narrow band communication program that makes use of the Hellschreiber principle. Data is sent at a very slow rate and received via a Fast Fourier Transform routine giving a bandwidth of around 2Hz. As long as tuning is within 20Hz, the result will appear. The transmission rate is around 2, 1 and 0.5 {characters / second}. Slowfeld may be used when all other communication methods fail. A PC with a soundcard is required to use this mode.