The Hellschreiber, Feldhellschreiber or Typenbildfeldfernschreiber (also Hell-Schreiber named after its inventor Rudolf Hell) is a

facsimile A facsimile (from Latin ''fac simile'', "to make alike") is a copy or reproduction of an old book, manuscript, map, art print, or other item of historical value that is as true to the original source as possible. It differs from other forms of r ...

-based

teleprinter A teleprinter (teletypewriter, teletype or TTY) is an electromechanical device that can be used to send and receive typed messages through various communications channels, in both point-to-point (telecommunications), point-to-point and point- ...

invented by

Rudolf Hell Rudolf Hell (19 December 1901 – 11 March 2002) was a German inventor and engineer. Career Hell was born in Eggmühl. From 1919 to 1923, he studied electrical engineering in Munich. He worked there from 1923 to 1929 as assistant of Prof. M ...

. Compared to contemporary teleprinters that were based on

typewriter A typewriter is a Machine, mechanical or electromechanical machine for typing characters. Typically, a typewriter has an array of Button (control), keys, and each one causes a different single character to be produced on paper by striking an i ...

systems and were mechanically complex and expensive, the Hellschreiber was much simpler and more robust, with far fewer moving parts. It has the added advantage of being capable of providing intelligible communication even over very poor quality radio or cable links, where voice or other teledata would be unintelligible. The device 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 In Cryptography law, cryptography, encryption (more specifically, Code, encoding) is the process of transforming information in a way that, ideally, only authorized parties can decode. This process converts the original representation of the inf ...

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 operator An amateur radio operator is someone who uses equipment at an amateur radio station to engage in two-way personal communications with other amateur operators on radio frequencies assigned to the amateur radio service. Amateur radio operators ...

s using computers and sound cards; the resulting mode is referred to as Hellschreiber, Feld-Hell, or simply Hell.

Operation

Hellschreiber sends a line of text as a series of vertical columns. Each column is broken down vertically into a series of

pixel In digital imaging, a pixel (abbreviated px), pel, or picture element is the smallest addressable element in a Raster graphics, raster image, or the smallest addressable element in a dot matrix display device. In most digital display devices, p ...

s, 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 In telecommunications and electronics, baud (; symbol: Bd) is a common unit of measurement of symbol rate, which is one of the components that determine the speed of communication over a data channel. It is the unit for symbol rate or modulat ...

. 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. 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 Bandwidth commonly refers to: * Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range * Bandwidth (computing), the rate of data transfer, bit rate or thr ...

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.

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 a pixel's brightness in the carrier

phase Phase or phases may refer to: Science *State of matter, or phase, one of the distinct forms in which matter can exist *Phase (matter), a region of space throughout which all physical properties are essentially uniform *Phase space, a mathematica ...

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 (or FSK Hell) uses

frequency modulation Frequency modulation (FM) is a signal modulation technique used in electronic communication, originally for transmitting messages with a radio wave. In frequency modulation a carrier wave is varied in its instantaneous frequency in proporti ...

with a careful control of phase, essentially

minimum-shift keying In digital modulation, minimum-shift keying (MSK) is a type of continuous-phase frequency-shift keying that was developed in the late 1950s by Collins Radio employees Melvin L. Doelz and Earl T. Heald. Similar to OQPSK, MSK is encoded with bit ...

. The most common variant is FSK Hell-105. * 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 or a waterfall plot. It allows for high resolutions. * S/MT Hell or sequential multitone Hell is like C/MT but it sends only one tone (for one row) at a time. As a result, characters received have a bit of slant, they look like an oblique font.

Slowfeld

Slowfeld is an experimental narrow band communication program that makes use of the Hellschreiber principle requiring that the transmitter and receiver both use the same column-scan speed. Data is sent at a very slow rate and received via a

Fast Fourier Transform A fast Fourier transform (FFT) is an algorithm that computes the discrete Fourier transform (DFT) of a sequence, or its inverse (IDFT). A Fourier transform converts a signal from its original domain (often time or space) to a representation in ...

routine giving a bandwidth of several Hz. As long as tuning is within several signal bandwidths, the result will appear. The transmission rate is around 3, 1.5 and 0.75 characters per second. Slowfeld, along with similar modes such as very slow QRSS Morse code, may be used when all other communication methods fail.

Media

Image:Hellschreiber-machine-at-bletchley-park.jpg, Hellschreiber on display at Bletchley Park, United Kingdom (March 2010) Image:Feld hell spectrum.jpg,

Spectrogram A spectrogram is a visual representation of the spectrum of frequencies of a signal as it varies with time. When applied to an audio signal, spectrograms are sometimes called sonographs, voiceprints, or voicegrams. When the data are represen ...

of a Feld-Hell transmission centered on 7072.0

kHz The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), often described as being equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose formal expression in terms of SI base uni ...

(at ~200 Hz bandwidth) File:QSL SV8QG (2018).jpg , QSL card confirming an

amateur radio Amateur radio, also known as ham radio, is the use of the radio frequency radio spectrum, spectrum for purposes of non-commercial exchange of messages, wireless experimentation, self-training, private recreation, radiosport, contesting, and emer ...

FeldHell contact (2018)

References

External links