This article has been cut from Ham Radio, December 1979. Click
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HELL schreibers
The Hellschreiber is a teleprinting machine on a principie entirely different from that of the RTTY teleprinter. The Hell sytem (named after its inventor, Dr. Rudolf Hell) could have been invented with the requirements of the Radio Amateur in mind, but strangely enough the Hell system has never been fully accepted by the Amateur fraternity. The reason may be that an enormous number of used RTTY machines flooded the market at low prices after World War II.
Hell and RTTY existed simultaneously for a long time for both military and commercial use. However Hellschreibers have now disappeared, mainly as a result of the introduction of protected RTTY systems with automatic-request and error-correcting circuits. Most hams have probably never heard of the Hell system as a means of communications.
What is the Hellschreiber? In contrast to the RTTY machine, in which received pulses determine the character to be printed, the Hellschreiber uses the transmitted pulses to directly write images of characters on paper tape. Thus, Hell writing could be considered a simple form of facsimile, covering seven image lines per character, with seven elements per line.
Not only has this system of printing character images some very important advantages to offer, but the simple way in which the Hell teleprinter works is extraordinarily elegant. The thread of a fast turning worm shaft wipes, with high speed, transversely across a slowly moving paper tape. This worm thread is wet with printing ink. Every time the paper is tapped against the turning worm shaft, little lines are formed across the paper tape. Several of these lines together form a character.
The Hellschreiber of the World War II Wehrmacht type we're using runs somewhat slower than the RTTY machine: 2 1/2 characters per second. Nevertheless, a respectable 25 words per minute is achieved. This CW terminology is not misplaced, as Hell and CW have much in common. In fact, given a certain bandwidth, the reliability of Hell communications approaches that of CW.
During World War II the Hellschreiber proved its reliability. Users recognized that a Hellschreiber could be the only link between an isolated military unit and its headquarters. When all other means of communications failed, often the Hellschreiber managed to get the message through, even when only barbed wire and an earth connection were available as a signal path.
Our Hell QSOs occur on 80 meters (over here, the official RTTY segment is between 3575-3625 kHz). It's difficult to think of a better part of the radio spectrum for putting the Hell system to the test because of the high QRM level in this portion of the band.
In this context I'd like to mention an interesting side effect. Our modest prrt, prrt, prrt Hell signals apparently tend to provoke fury among some hams, who seem to be convinced that the unusual sounds are caused by commercial stations. This turns our little Hell channel into the center of zero-beating and QRZ-blaring stations. This intentional interference does, however, provide us with an invaluable opporunity to test the communications system under highly adverse conditions and is, therefore, to some extent, not unwelcome.
Of course, the interfering transmitter determined to cause serious trouble by tuning carefully zero-beat with our Hell signals may eventually manage to temporarily destroy our communications. Provided, of course, that the signal is stronger than ours. By maneuvering with tuning, bandwidth, and threshold level it's possible to get through. We might lose contact for a moment; however, contact is restored through the foggy QRM clouds on our printouts, and we pick up the text as soon as the characters become distinguishable again. This sort of working on the threshold is possible with Hell: The text, even under the worst conditions, is never subject to errors of a substitution-of-characters type. The character may, however, be difficult to read because of mutilation.
Under certain circumstances the communications reliability of Hell can be even better than that of CW. The received Hell signal is printed in its original form. At the moment of reception no decision has to be made such as, "Did I hear correctly?" Thus wrong decisions are avoided. The Hell printer enables the reader to decide later on, at his ease, what was actually sent by the distant station.
Some examples are shown of radio Hell-communications in which the printer obviously has great trouble in keeping the text intelligible because of a high noise level or heavy QRM. The examples contain considerably more information than can be deciphered on first sight. If you really take the trouble to read the text, you immediately realize to what the Hellschreiber owes its superior qualities: it calls in the services of a Computer, i.e. , our human ability to recognize pictures in a chaos of little specks and lines.