Hellschreiber, or Hell, is a method of sending and receiving text using facsimile technology. It is unique in that the characters are not decoded, but "painted" or printed on a screen. There are several modes of Hellschreiber, the most popular being a single-tone version call Feld-Hell, an on-off keyed system with 122.5 dots/second, or about a 35 WPM text rate. FH has a narrow bandwidth of about 75 Hz. Feld-Hell also has the advantage of having a low duty cycle meaning your transmitter will run much cooler with this mode.
How does Hellschreiber work (from Wikipedia)
Hellschreiber sends a page of text as a series of vertical columns. Each column is broken down vertically into a series of pixels, normally using a 7 by 7 pixel grid to represent characters. The data for a line is then sent as a series of on-off signals to the receiver, using a variety of formats depending on the medium, but normally at a rate of 112.5 baud.
At the receiver end, a paper tape is fed at a constant speed over a roller. Above the roller was a spinning cylinder with small bumps on the surface. The received signal was amplified and sent to a magnetic actuator that pulled the cylinder down onto the roller, hammering a dot into the surface of the paper. 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 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.
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.
Suggested further reading: How the Hellschreiber works
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