Hellschreiber
Encyclopedia
Facsimile
A facsimile 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 reproduction by attempting to replicate the source as accurately as possible in terms of scale,...
-based teleprinter
Teleprinter
A teleprinter is a electromechanical typewriter that can be used to communicate typed messages from point to point and point to multipoint over a variety of communication channels that range from a simple electrical connection, such as a pair of wires, to the use of radio and microwave as the...
invented by Rudolf Hell
Rudolf Hell
Rudolf Hell was a German inventor. He was born in Eggmühl, Germany.From 1919 to 1923 he studied electrical engineering in Munich....
. Compared to contemporary teleprinters that were based on typewriter
Typewriter
A typewriter is a mechanical or electromechanical device with keys that, when pressed, cause characters to be printed on a medium, usually paper. Typically one character is printed per keypress, and the machine prints the characters by making ink impressions of type elements similar to the pieces...
systems, the Hellschreiber was much simpler and more robust, with only two 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. During WW2 it was sometimes used by the German military
Wehrmacht
The Wehrmacht – from , to defend and , the might/power) were the unified armed forces of Nazi Germany from 1935 to 1945. It consisted of the Heer , the Kriegsmarine and the Luftwaffe .-Origin and use of the term:...
in conjunction with the Enigma
Enigma machine
An Enigma machine is any of a family of related electro-mechanical rotor cipher machines used for the encryption and decryption of secret messages. Enigma was invented by German engineer Arthur Scherbius at the end of World War I...
encryption
Encryption
In cryptography, encryption is the process of transforming information using an algorithm to make it unreadable to anyone except those possessing special knowledge, usually referred to as a key. The result of the process is encrypted information...
system. In the post-war era, it became increasingly common among newswire services, and was used in this role well into the 1980s. In modern times Hellschreiber is used as a communication mode by amateur radio operators using computers and sound cards; the resulting mode is referred to as Hellschreiber, Feld-Hell, or simply Hell.
"Hellschreiber" translates into English as "Light Writer," "Bright Writer," or "Clear Writer," and is a pun on the name of its inventor. Hell (the adjective) is German for "light," the adverb is "bright."
Operation
Hellschreiber sends a page of text as a series of vertical columns. Each column is broken down vertically into a series of pixelPixel
In digital imaging, a pixel, or pel, is a single point in a raster image, or the smallest addressable screen element in a display device; it is the smallest unit of picture that can be represented or controlled....
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.
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.
- 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 phasePhase (waves)Phase in waves is the fraction of a wave cycle which has elapsed relative to an arbitrary point.-Formula:The phase of an oscillation or wave refers to a sinusoidal function such as the following:...
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 modulationFrequency modulationIn telecommunications and signal processing, frequency modulation conveys information over a carrier wave by varying its instantaneous frequency. This contrasts with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant...
with a careful control of phase, essentially minimum-shift keyingMinimum-shift keyingIn digital modulation, minimum-shift keying is a type of continuous-phase frequency-shift keying that was developed in the late 1950s and 1960s. Similar to OQPSK, MSK is encoded with bits alternating between quadrature components, with the Q component delayed by half the symbol period. However,...
. - 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 second) intended for beacon use.
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 2 Hz. As long as tuning is within 20 Hz, 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.Media
Digital HF radio communications systems
- ACARS, used by commercial aviation – packet based
- CLOVER2000CLOVER2000CLOVER is the name of a series or class of modem modulation techniques specifically designed for use over high frequency radio systems....
developed by HAL company, USA, for Radio Amateur application - HellschreiberHellschreiberThe Hellschreiber or Feldhellschreiber is a facsimile-based teleprinter invented by Rudolf Hell. Compared to contemporary teleprinters that were based on typewriter systems, the Hellschreiber was much simpler and more robust, with only two moving parts...
, a FAX-RTTY hybrid, very old system from the 1930s - MFSK including COQUELET and PICCOLO, also referred to generically as Polytone
- MT63MT63MT63 is a digital radio modulation mode for transmission in high-noise situations developed by Pawel Jalocha SP9VRC. MT63 is designed for keyboard-to-keyboard conversation modes, on HF HAM bands.- Features and Attributes :...
, developed and used by Radio Amateurs and some government agencies - NavtexNavtexNAVTEX is an international automated medium frequency direct-printing service for delivery of navigational and meteorological warnings and forecasts, as well as urgent marine safety information to ships...
, used for maritime weather reports, with FEC error control code, - Olivia MFSKOlivia MFSKOlivia MFSK is an amateur radioteletype protocol designed to work in difficult conditions on shortwave bands. The signal can still be properly copied when it is buried 10 dB below the noise floor...
from the creator of MT63 - PACTORPACTORPACTOR is a radio modulation mode used by amateur radio operators, marine radio stations, and radio stations in isolated areas to send and receive digital information via radio. A robust network of PACTOR stations has been established to relay data between radio stations and the Internet,...
, a packet SITOR variant, developed by Radio Amateurs in Germany - PSK31PSK31PSK31 or "Phase Shift Keying, 31 Baud" is a digital radio modulation mode, used primarily in the amateur radio field to conduct real-time keyboard-to-keyboard informal text chat between amateur radio operators.- History :...
& PSK63PSK63PSK63 or Phase Shift Keying, 63 Baud, is a digital radio modulation mode used primarily in the amateur radio field to conduct real-time keyboard-to-keyboard informal text chat between amateur radio operators.- History :...
developed and used by Radio Amateurs - Q15X25Q15X25Q15X25 is a communications protocol for sending data over a radio link. It was designed by amateur radio operator Pawel Jalocha, SP9VRC, to be an open communications standard. Like all amateur radio communications modes, this protocol uses open transmissions which can be received and decoded by...
, a Radio Amateur created packet format(AX25), similar to the commercial X25 standard - SailmailSailmailSailMail is radio based e-mail system designed for yacht owners operating outside of areas of internet service. Much of its underlying technology is built upon the Winlink software originally developed by amateur radio enthusiasts....
, a commercial HF mail system - SITORSITORSITOR is a system for transmitting text messages. Although it uses the same frequency-shift keying modulation used by regular radioteletype , SITOR uses error detection, redundancy, and/or retransmission to improve reliability.There are two SITOR modes:* SITOR-A is used for point to point links...
, (SImplex Teleprinting Over Radio) a commercial RTTY variant with error control (the Radio Amateur version is called "AMTOR")