Hellschreiber
A facsimile teleprinter providing intelligible communication over poor links.
The Hellschreiber (also called the Feldhellschreiber or Typenbildfeldfernschreiber, and often shortened to Hell-Schreiber after its inventor, Rudolf Hell) is a teleprinter that works on a facsimile principle. At the time of its invention in the late 1920s, most teleprinters relied on complex and expensive typewriter mechanisms. The Hellschreiber was far simpler and more rugged, with many fewer moving parts. It also had a key advantage: it could produce readable text even over very poor radio or cable connections, where voice or other data transmissions would be unintelligible.
The device first saw use in the 1930s, mainly for landline press services. During World War II, the German military sometimes used it alongside the Enigma encryption system. After the war, it became increasingly common among newswire services and remained in that role well into the 1980s. Today, amateur radio operators use Hellschreiber with computers and sound cards; this modern implementation is called Hellschreiber, Feld-Hell, or simply Hell.
**Operation**
The Hellschreiber transmits a line of text as a series of vertical columns. Each column is divided into a grid of pixels, typically 7 by 7, to form characters. The data for a line is sent as a stream of on-off signals, usually at 112.5 baud, though the format varies by medium.
At the receiving end, a paper tape feeds at a constant speed over a roller. Above the roller, a spinning cylinder with small bumps arranged in a helix pattern sits. The received signal is amplified and drives a magnetic actuator that presses the cylinder down onto the roller, hammering a dot into the paper. The machine prints each column twice, one below the other. This compensates for slight timing errors common in the equipment, causing the text to slant. The result may appear as two identical lines of text stacked vertically, or as a single readable line with partial lines above and below—either way, at least one whole letter is always legible.
Because the original Hellschreiber was a mechanical device, it could send "half-pixels." For example, the right ends of loops in a letter like "B" could be shifted slightly to improve readability. However, any on-signal had to last at least 8 milliseconds, both to limit the occupied radio bandwidth and to accommodate the mechanical receiver.
- Inventor
- Rudolf Hell
- First developed
- late 1920s
- Saw use starting
- 1930s
- Baud rate
- 112.5 baud
- Pixel grid
- 7 by 7 pixels
- Minimum on-signal duration
- 8 ms
Lore & Background
The Hellschreiber was first developed in the late 1920s and saw use starting in the 1930s, chiefly being used for landline press services. During World War II it was sometimes used by the German military in conjunction with the Enigma encryption system. In the post-war era, it became increasingly common among newswire services, and was used in this role well into the 1980s. Today, the Hellschreiber is used as a means of communication by amateur radio operators using computers and sound cards; the resulting mode is referred to as Hellschreiber, Feld-Hell, or simply Hell.
Hellschreiber sends a line 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. Located above the roller is a spinning cylinder with small bumps in a helical pattern on the surface. The received signal is amplified and sent to a magnetic actuator that pulls the cylinder down onto the roller, hammering out a dot into the surface of the paper. A Hellschreiber will print each received column twice, one below the other, to compensate for slight timing errors that are often present in the equipment, causing the text to slant.
Reader's Guide
The Hellschreiber's significance lies in its robustness and simplicity. Its mechanical design had far fewer moving parts than contemporary typewriter-based teleprinters, making it more reliable. It could provide intelligible communication over very poor quality radio or cable links where voice or other teledata would be unintelligible. The original mechanical machine could send 'half-pixels', allowing slight shifts to improve readability. Software implementations later brought improvements such as depicting the received signal as shades of gray instead of monochrome, making weak signals much easier to read, and the ability to change to a different font. The mode is truly international and independent of character sets: anything that can be depicted as markings within a 7 pixels high grid can be transmitted over the air. Hellschreiber has spawned a number of variants, many due to radio amateur efforts in the 1990s, including PSK Hell, FM Hell (FSK Hell), Duplo Hell, C/MT Hell, and S/MT Hell. Slowfeld is an experimental narrow band communication program that uses the Hellschreiber principle, sending data at a very slow rate and received via a Fast Fourier Transform routine, giving a bandwidth of several Hz; it may be used when all other communication methods fail.
Did You Know?
- The original mechanical Hellschreiber could send 'half-pixels' to improve readability.
- During World War II the Hellschreiber was sometimes used by the German military with the Enigma encryption system.
- Slowfeld is an experimental narrow band mode using the Hellschreiber principle that may be used when all other communication methods fail.
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