- [What it is - and not]
- [Perfect for amateur radio]
- [Amateur radio Hellschreiber "firsts"]
Latest page update: 19 April 2019
©2004-2019 F. Dörenberg, unless stated otherwise. All rights reserved worldwide. No part of this publication may be used without permission from the author.
WHAT IT IS - AND WHAT IT IS NOT
The Hellschreiber is a Typenbildfernschreiber "System Hell", that is: a "character-image tele-writer" or "character-image printing telegraph", based on the Hell-system. It was customary in the technology industry through WW2 to append the name of the inventor or developer as "system name" to the generic name of a novel transmitter or receiver, radio tube (valve), etc. Here: Rudolf Hell, who invented the system in 1929. Rudolf Hell explained the purpose of the Hell-Schreiber as follows (ref. p. 2 and §10b in ref. 1):
"Das Entwicklungsziel, ein für Presseempfang
brauchbares Gerät zu schaffen, konnte nur mit einem denkbar
einfachen Schreibgerät erreicht werden."
"The objective of the development was a
practical device for the reception of messages from news agencies.
This could only be achieved with a very simple teleprinter."
Sometimes it is easier to describe something by what it is not. E.g., "if you throw an object, and it does not come back, it is not a boomerang"...
The Hellschreiber is not a facsimile system ("fax" for short; D: "Bildtelegraphie"). I.e., a system for the telephotographic transmission of images. Characteristics:
- In those days (late 1920s, early 1930s), telecopying required photochemical development of the image after its complete transmission.
- partial images are not available in real-time, i.e., during transmission; so the receiver and printer cannot be adjusted during image reception.
- transmission requires relatively large signaling bandwidth. In the late 1920s, there were fax systems that transferred single-line text on paper tape (e.g., "Schmalstreifen-Faksimiletelegrafen" [narrow tape telefax], ref. 2). However, this required a large bandwidth of 4700 Hz, and post-reception photo development time of about 10 sec.
- it requires synchronization between scanner/sender and receiver/printer.
- unsuitable for transmission of telegrams: too cumbersome, and low information transfer-rate for the required telegraphy speed (bandwidth). Ref. pp. 55, 56 in ref. 27.
- does have a high level of data redundancy, such that omitted or suppressed pulses may reduce readability (subjective interpretation by the human operator/reader), but generally do not cause incorrect text to be printed.
The Hellschreiber is also not a teleprinter system ("Fernschreiber", "Drucktelegraphen"). Such systems transmit text characters in the form of symbols. The receiver prints them onto paper tape or sheets. Characteristics:
- the symbols are coded (Baudot code, ITA, etc.). These codes do not contain any redundancy (additional bits for error detection and correction).
- each completely received symbol is decoded (interpreted) by the receiver/printer machine. This makes the transmission sensitive to noise, interference, and fading: if one or more bits of the received symbol code are distorted (inverted), then a wrong character is printed.
- upon reception of the complete symbol, the symbol is decoded symbol and printed immediately onto a paper tape or sheet ("direct printing").
- requires synchronization between sender and receiver/printer. This is typically done by including a sync pulse (start bit) at the beginning of the symbol codes:
- this requires mechanically complicated start-stop mechanisms.
- upon false detection of the start bit (due to noise, interference), a random character is printed, even though no character was transmitted. This will also cause a real start bit to be missed, if it was sent during the symbol-duration after the false start bit.
- upon missing a start bit (due to noise, interference, or fading) no character is printed, even though one was transmitted.
So, then what is the Hellschreiber system? Well, it is a telegram system in between fax and teleprinter. But mechanically (much) simpler than either of these, and without their basic technical or operational disadvantages (besides cost):
- no encoding:
- noise, interference, and fading can cause a graphically distorted character to be printed (extra or lost back pixels), but never a wrong character, or a character that was not transmitted at all.
- no en-coding at the transmitter means no de-coding at the receiver/printer. This simplifies the receiver/printer. This also means that it is (much) less expensive.
- no synchronization:
- no special start-stop mechanism required in the receiver/printer.
- since no sync pulse (e.g., start bit) is used, there cannot be false or missed detections.
- direct-printing onto paper tape:
- no photochemical development.
- characters are not printed the way it is done in conventional teleprinters. The printer has no type-wheel or typewriter type-bars (i.e., a disk or arm that has one or more character types on it).
- characters are transmitted as streams of black and white pixels (similar to fax). Each individual pixel is printed immediately, as it is being received. This allows the receiver/printer to be adjusted for best print quality - in real-time, during reception.
- narrow signaling bandwidth - much less than what is required for "voice" communication:
- can be used over all regular phone lines and standard "voice" transmitters/receivers, even if the transmission quality of the communication channel no longer suffices for voice.
- can also be used with regular CW-transmitters/receivers ("Morse" telegraphy).
- insensitive to many transmission-path effects:
- even if pulse duration is increased or reduced by as much as 60%, printed text remains readable. Ref. p. 57 in ref. 27.
The "Hell system" consists of three basic steps for tele-printing text character-images:
- Rasterization: de-composition of text characters into pixels.
- Transmission of selected characters as pixel streams.
- Printing: re-composition of received pixels streams, to reconstruct the transmitted character - without the need for synchronizing the printer to the received pixel stream.
These three steps are discussed and illustrated below.
The Hell-system truly is a breakthrough of monumental importance. It can not be over-stated how important this invention of the pixelation is! You probably would not be reading this on a computer screen without Rudolf Hell's invention of the bitmap font. In our days, the advantage of tele-printing without the need for synchronization is not important anymore. However, in the 1920s-30s, it meant dependable and inexpensive world-wide communication.
See ref. 36 for a 4-page article that I wrote to summarize "Hellschreiber".
RASTERIZATION OF THE CHARACTER SET
Decomposition and rasterization of images has become the recurring theme in the life of Rudolf Hell - be it the Hellschreiber, his co-invention of the TV camera, or his revolutionary inventions in the typesetting and graphics industry (see the Rudolf Hell History page).
Hell wanted to transmit the images of the alphabet characters (letters, numbers, punctuation marks) by means of telegraphy. He decomposed each characters in black and white pixels (picture elements). This is done by overlaying each scripted character with a fixed-size raster. See the "Hellschreiber fonts" page for details. The result is a simple bitmap font, with a fixed number of rows and columns (2-dimensional array or matrix). Hell did edit the bitmap of several rasterized characters, to make them more distinct from other characters or to make them more legible.
Fig. 1: Rasterization of the scripted character "3"
The entire font is permanently stored in the mechanical, magnetic, or optical "font memory" of the Hellschreiber-sender. From there, the bitmap of each selected character can be retrieved for transmission. Of course, in the computer age, the font is simply stored in a memory chip or on a disk drive.
TRANSMISSION OF HELLSCHREIBER SIGNALS
Transmission of text is a 2-step process by itself:
- selection of the character that is to be transmitted, thereby selecting the associated character pixel map.
- conversion of the selected pixel map into a stream of black and white pixels. The stream is transmitted to the receiver/printer via "wired" or "wireless" means.
Let's start with the easy part: conversion of the 2-dimensional bitmap of the selected character into a 1-dimensional pixel stream. This is done by "scanning" the bitmap matrix column-by-column. Effectively, this arranges the columns in a head-to-tail manner. The result is a series of black and white pixels or bits ("binary digits"). Obviously, one could also scan row-by-row. But this makes (mechanical) printing more complicated. Note that, other than the sequencing, there is no encoding whatsoever in the pixel map or in the pixel stream (unlike telex/RTTY, etc.)!
Fig. 2: Serialization of the rasterized character "3"
The character is now in a form that can easily be transmitted by any "wired" or "wireless" communication channel - be it public or private: public telephone systems, military field telephone systems, utility networks (railroads), private networks (banks, news agencies, police), teleprinter networks, radio, electrical power lines (60 kV and 100 kV high-voltage power lines in the late 1930s, ref. 3, 26), etc.
In all cases, the pixel sequence is simply transmitted as a series of pulses - be it modulated or un-modulated:
- DC-current pulses over a suitable phone line, i.e., a line without transformers or blocking capacitors (black pixel = "current", white pixel = "no current"),
- tone-pulses over any phone or teleprinter line (black pixel = "tone on", white pixel = "tone off"),
- tone-pulses via a "voice" radio transmitter (black pixel = "tone on", white pixel = "tone off"; amplitude-, frequency-, or phase-modulation),
- with a telegraphy (CW, Morse) transmitter (black pixel = "transmitter on", white pixel = "transmitter off"; on-off-keyed unmodulated carrier).
No special transmitters or receivers are required - which is another advantage of the Hell system.
Fig. 3: Pulse transmission via ordinary phone-lines
In the old days, standard options for Hellschreiber radio operation were CW-telegraphy and Amplitude Modulation (AM - "Double-Sideband Unsuppressed Carrier"). Today, we prefer to use "Single-Sideband Suppressed Carrier" (SSB) modulation instead of AM: it has half the bandwidth of an equivalent AM signal, and the transmission energy is concentrated into a single sideband, instead of two mirror-image sidebands and a carrier. The RF-spectrum of a CW signal is an On-Off-Keyed (OOK) carrier at the frequency fc. When a single sinusoidal OOK audio tone is applied to an SSB modulator, the resulting RF-signal is identical to an OOK carrier fc that is shifted by the modulation tone frequency fm (to fc + fm or fc - fm, depending on selection of USB vs. LSB). In other words: exactly the same spectrum as an OOK CW signal shifted by fm. Feld-Hell transmissions look just like fast Morse telegraphy.
In principle, the pulses can also be sent optically. E.g., the Wehrmacht used a "light telephone": Lichtsprechgerät Li.Spr.80 (starting in 1935, ref. 4), with optics from the famous Carl Zeiss factory in Jena. Here is a 7-minute video by Helge Fykse, LA6NCA, demonstrating his Li.Spr.80 for voice communication. Given the limited range (several km), this particular system was not used for Hell-communication. Today, modern telephone systems are also based on optics: fiber optic "wiring" and modulated laser light pulses of various wavelengths.
Now let's go back to selecting a character and retrieving it from the "font memory". We will limit ourselves here to the mechanical memories of Hellschreiber systems from the 1930s-50s. The selection is done with a keyboard that is mechanically linked to the memory. Alternatively, a punch-tape reader can be used, especially for transmissions that are faster than regular human typing. Note that there have also been Hell systems that perform real-time scan of text that is handwritten on paper strips (ref. 5, the RCA tape fax (WW2), and the Hell ZETFAX).
The mechanical memories consist of a cylinder. The pixel sequence of each character is captured as a ring of conducting patches on the surface of the cylinder (combined with a slip-contact), or as a notched disk (combined with a contact that is actuated by the notches). See the "font memory" of the Hellschreiber-sender on the "Hellschreiber fonts" page. The cylinder turns continuously, and each character takes one full revolution. Note that Rudolf Hell also patented the "revolving drum" idea in the USA in 1940 (US patent nr. 2,335,410) .
Transmission of a character must always start at the first pixel of the sequence (typ. the bottom of the first column). Hence, the keyboard (or punch-tape reader) must only be allowed to start retrieving the selected pixel sequence, during the brief time window when the position of the cylinder is just before the start of that sequence. This is done with a lock-out/enable mechanism, as shown below for the keyboard interface of the Hell Feldfernschreiber ("Feld Hell").
When a key is depressed, the associated spring-loaded slip-contact flips, and makes contact with the associated track of the drum. At the end of the revolution of the drum, the slip-contact flips back and no longer touches the drum. At the same time, the keyboard is once again briefly enabled for selection of the next character to be transmitted.
While the pixel sequence of the selected character is being output, no other character can be selected: the keyboard is disabled (no "type ahead"). A very similar mechanism is used with a mechanical font memory that consists of a stack of notched disks.
Fig. 4: Illustration of the key locking/enabling and slip-contact (dis)engaging mechanism
(the vertically oriented black lever is the slip-contact associated with the key)
Note that when you keep a key depressed for more than one revolution of the drum, the slip-contact is still released after one revolution and the character is only sent once. To send the same character multiple times in a row, you have to push the key multiple times (section II.a) in ref. 6).
Fig. 5: Details of the Feld-Hell mechanism for key locking/enabling and slip contact (dis)engagement
(source: figure 11 in ref. 7)
Clearly, it is impossible to simply guess when the tiny key-enabling time window occurs, and press a key just at that very moment. A simple technique is used to deal with this: lightly depress the desired key until it hits the stop (down about 6-7 mm). Keep light pressure on the key, until it drops down when all keys are enabled by keyboard mechanism. Then let go of the key. This does take a tad of Fingerspitzengefühl, and unlike old mechanical typewriters and Glockenspiele, there is no tapping or hammering on the keys here! The operator has to develop a special typing rhythm to avoid having lots of blank spaces in the transmitted text. The quote below shows that it was not necessarily an easily acquired skill (ref. 8):
"Also in Bar-le-Duc [Lothringen] hab' ich ja nur am Feldfernschreiber geschrieben, da gab es nur einen Feldfernschreiber, (...) das ist ein riesen kasten und da konnte man nur - also das konnten nicht alle, es war schwierig zu schreiben, es ging nur ganz nach einem bestimmten Takt, sie konnten also nicht, wie bei einer Schreibmaschine losschreiben, sondern es musste im Takt immer geschrieben werden..."
"So, in Bar-le-Duc [small town in Lorraine/France] I only typed on a [Hell] Feldfernschreiber. There was only such field teleprinter, ... it was a huge box, and you could only type on it in a very particular rhythm - it was hard to type like that and not everyone could do it, it was not like you could just type away like on a regular typewriter, it had to be the right rhythm...
Nachrichtenhelferin [Wehrmacht Signal Corps Aide]
1941-1945 in Ukraine, France, and Germany
The video clip below shows Arthur Bauer (PAØAOB) demonstrating the required typing rhythm on a Feld-Hell machine. The fact that the keyboard mechanism enables all keys at the same time, means that multiple characters can be transmitted at the same time. The result is a combination (logical OR-ing) of the selected character patterns. Arthur, uses this technique to combine "0" and "/" to get Ø.
Arthur Bauer typing on a Hell Feldfernschreiber
Here is a 3-minute video clip that I made of the spinning character drum of the Feld-Hell machine:
Character drum of a Hell Feldfernschreiber
(the noise you hear during the slow-motion part, is an electric screwdriver that I used to slowly turn the drum)
PRINTING HELLSCHREIBER SIGNALS
The Hellschreiber printer must do a couple of things, to print the incoming pixel (pulse) sequence from the sender:
- mimic the scanning action of the sender. Remember that the sender basically takes the bitmap of the selected character, and creates the pixel stream by scanning this bitmap, column-by-column.
- a very simple and elegant way to mimic the column-scanning action, is with a spindle (helix, worm gear, screw, D: "endlose Schnecke", F: "vis sans fin") that turns continuously - exactly like the mechanical font-memory of the Hellschreiber sender.
- the printer must print onto some form of medium:
- as text is printed in the form of lines, paper tape is a good candidate. This has also been used for Morse printers, ever since the mid-1800s. This is very simple, mature technology. The paper tape is slightly wider than the hub of the printer spindle.
- columns must be printed, one next to the other:
- Option 1: move the spindle with respect to the paper, with a constant speed. This is rather complicated!
- Option 2: move the paper with respect to the spindle, with a constant speed. This is very simple! A paper tape transportation mechanism pulls the paper tape by the spindle. This mechanism consists of two rollers that grab the paper. One of the rollers is motorized. The second roller (the "pinch" roller) pushes the tape down onto the motorized roller. This is low risk, mature technology that also dates back to the Morse telegraphy printers of the 1830s. The pinch roller only touches the edges of the paper tape, so as not to smudge the printed text (ref. 33).
Fig. 6: Two-turn helix sweeps two (inked) points across the paper tape
Looking at a turning spindle from the paper tape point of view, the spindle thread appears as a point that sweeps ( = scans) across the width of the paper. The image above shows a helix with a thread that has two turns. We'll get back to that further below (synchronization).
Hellschreiber printer spindle
Notice that the spindle is turning clockwise when looking at its tip. This direction of rotation is chosen on purpose: where the paper meets the helix, the circumference of the helix moves in the same direction as the paper tape. As we will see shortly, during the printing process, the paper touches the spindle. If the spindle were to turn counter-clockwise, then it would push against the movement of the paper... This would vary the load on the motor of the printer, and make it more difficult to keep the spindle speed and the paper transport speed constant.
The turn direction of the helix thread is also chosen on purpose. Combined with the spinning direction of the helix, it creates a bottom-to-top scanning action. The spindle of some of the initial Hellschreiber models had a right-hand thread, and a top-to-bottom scanning action (as did the associated senders). If the opposite scanning direction is used (at the sender or the printer), then the printed text is upside down and mirror-image:
Fig. 7: Opposite scan direction - upside-down & mirror-image
We're not quite done yet! Obviously we will be printing with the spinning helix. Part of any printing process, is to get ink onto the paper. So we must get ink onto the spindle, and keep it inked all the time. Rudolf Hell eventually settled on a simple felt ring, impregnated with ink. It is mounted at the top of a spring-loaded lever, and lightly rests onto the spindle (ref. 32). The ring is mounted onto a holder that can spin freely.
Fig. 8: Felt ink rollers for Hellschreibers
The ink must be transferred from the spindle to the paper, at the right time:
- Option 1: move the inked spindle down, against the paper, when a pixel must be printed. This is rather complicated.
- Option 2: move the paper tape up, against the inked spindle, when a pixel must be printed This is very simple! Take an electromagnetic relay, and use its armature (the hinged part that moves) to push the paper up against the spindle as soon as - and as long as - a black pixel signal is received.
Fig. 9: Electromagnet of a Hellschreiber printer
When the paper is tapped against the spindle, the ink is transferred to the paper at the spots where the paper touches the spindle. If the paper is tapped against the spindle in the rhythm of the received pixel sequence, then the sent column is recreated. Very simple and ingenious! The paper tape moves at a speed such that the next revolution of the spindle creates a new column, closely to the right of the preceding column.
There you have it: the Hellschreiber printer! With very few moving parts: besides a small motor, it has a spindle, rollers of the paper tape transportation, and an electro-magnet with armature. It is small, simple, robust, and inexpensive. In fact, it is so simple, that you can build your own! I have done so myself (1984), and so have many others.
Note that this clever printer is stupid! It prints all sufficiently strong signals that it receives – be it the intended pixels, noise or interference. It has no notion of fonts, bits, pixels, encoding, speed, or synchronization. It does no decoding or interpretation, other than deciding "signal (not) present". Signal distortion may add unintended pixels, and may cause omission of intended pixels. This only makes the printed text harder to read. But unlike teleprinters, a Hell-printer can never print a wrong character! The human reader, with its impressive pattern-recognition capability, can read messages that are well "down in the noise":
Fig. 10: Print-out of a Hell signal with lots of noise - it reads "THE OVERALL SPEED IS ABO"
(source: adapted from Fig. 11.1.d in ref. 40, courtesy RSGB; used with permission)
Under the same noisy conditions, the same message received via a "telex" teleprinter would have resulted in a completely garbled print out!
As stated above, the solenoid of the Hellschreiber printer's electro-magnet is energized for any signal that exceeds the detection threshold: be it Hell-signals, noise, CW ("Morse"), etc. The figure below shows how Morse code is printed by the Feld-Hell machine (for a compatible telegraphy speed).
Fig. 11: The word "SIEMENS" in Morse code, printed by a Hellschreiber
(source: Figure 668 in ref. 9)
If the pixel pulses are received as DC-current pulses, they can be used directly to energize the solenoid of the printer's electromagnet. A simple amplifier may be needed to get the required current-drive. However, in most cases, the pixel pulses are received as audio tone pulses. Now we need a detector circuit, to convert the tone pulses back to DC pulses. The detector is a simple diode rectifier (typically a full-wave rectifier) and a simple capacitor smoothing-filter ("Glättungsfilter"). Again, a solenoid driver-amplifier is required.
Note that early Hellschreiber models used a very fast telegraph relay that was used directly with tone pulses - without rectification! It was developed by Siemens-Halske in 1933 (ref. p. 3 in ref. 1, and ref. 31). The relay comprises two solenoids, as often depicted in older literature:
Fig. 12: Dual-solenoid printer magnet
The solenoids (M1 and M2 in the figure below) are connected in parallel. Capacitor C2 creates a phase difference of 90º between the currents through the solenoids M1 and M2. When excited with an AC current, the combined solenoids exert a constant pull on the armature S.
Fig. 13: Resonant telegraphy relay and control circuit
(source: figure 4 in ref. 31)
The design of this circuit can be tuned, either with the armature at the fully released, fully attracted, or the average position - depending on whether it is desired to have rapid armature response, or strong armature attraction force, or both. The relays were fast enough to cleanly print tone pulses of 1.2 msec (6 cps, 12x12 pixels)!
Choke D and capacitor C1 in the anode circuit are there to block DC current from reaching the solenoids. C1 (and C4) causes filtering of frequencies below the resonance frequency. If desired, filtering of higher frequencies can be obtained by adding C5 across the choke.
Clearly, the detector/driver energizes the solenoid for all input signals that exceed the solenoid actuation threshold. That is: the received Hell-signals and all noise/interference that is not suppressed by the filter preceding the detector. This is not a problem, as the Hell-system does not in any way interpret received signals to determine which character was received and needs to be printed - the human operator does this, by looking at the print out.
Fig. 14: The moving paper tape is tapped against the inked spindle in the rhythm of the received signal
Modern PC-based Hellschreiber "printers" do not apply a binary detection threshold, but use the PC's soundcard to digitize the received signal. This allows grey-scale (or even color-scale) printing of the received signals, which makes it even easier to recognize characters in the noise.
Fig. 15: Complete send/print sequence for the character "3"
(Note: this cartoon is rather large and may take a while to load...)
The image above shows a helix with a thread that has two turns, and prints two identical lines of text. The reason for this is explained further below (see synchronization).
Please compare the simplicity of the Hellschreiber printer mechanism above, with that of a "regular" teleprinter of the era:
Fig. 16: Teletype Model 15 teleprinter
Note that in those days, the four most generally used "recording" methods were photographic, ink, electrolytic, and carbon paper. The initial Hellschreiber prototype developments (late 1920s) used an electrochemical printer. Such printers - with a single stylus - were used for "Morse" telegraphy since the 1830s. The Scottish inventor Alexander Bain (1811-1877) patented an electro-chemical copying-telegraph in 1843. A century later, in the early 1930s, an electro-chemical paper tape audio recorder/player was invented (ref. 37, 38; the patent includes a "recipe" for the chemical mixture).
Hell's initial printer method used moist, chemically impregnated paper and a set of 14 styluses, aligned as a column and resting on the paper. This method quickly turned out to be impractical (and smelly), ref. 39. In 1931 Dr. Hell replaced it with the printer-spindle mechanism, which has been characteristic of Hellschreiber printers ever since.
The early spindle designs used carbon paper tape, and the spindle thread was dentilled (toothed). The mechanisms to ensure proper (un)winding of the carbon tape and for rubbing the carbon onto the spindle, were rather complicated. The transfer of the carbon particles required significant pressure on the paper, i.e., a relatively strong motor and printer-solenoid (= heavier, more expensive). Also, special carbon paper tape was expensive (considerations not unlike those for ink cartridges of today's PC-printers!). The spindle turned at 3600 rpm = 60 rps: 5 characters/sec, and 12 columns per character (ref. p. 9 in ref. 28). The carbon paper approach was abandoned in favor of inking the spindle with a simple free-spinning ink roller made of felt. Years later, during the late 1930s, the French company L.M.T. developed a very large and complicated Hellschreiber sender/printer system, and also found out that carbon paper is not the way to go...
Fig. 17: Carbon-paper and dentilled spindle mechanism
(source: Fig. 3 in ref. 1)
Fig. 18: Arrangement of magnet armature, paper tape, printing spindle, and ink roller
(source: Figure 2 and 3 in Rudolph Hell's US patent 2731322, ref. 10)
As with all inventions and patents, it includes elements of so-called "prior art". E.g., paper tape Morse telegraphy printers were developed during the 1830s. The earliest patents related to "image telegraphy" date back to the period of 1843-1860. In particular: Bain (Scotland), Blakewell (England), and Caselli (Italy).
Fig. 19: Paper tape printing telegraph by Samuel Morse (ca. 1837), with pencil stylus
(source: ref. 25)
Fig. 20: Electrochemical paper tape telegraphy printer by Alexander Bain (ca. 1846) - up to 1000 WPM
Fig. 21: Electrochemical paper tape telegraphy printer by Du Moncel (ca. 1850)
Inventions of "type printing" telegraphs such as "ticker tape" teleprinters with a type-wheel, also date back to the 1840s. E.g., R.E. House and D.E. Hughes. There is a French patent, awarded to Bernhard Meyer in 1865, entitled "Helical text reproduction"; a large spindle was used in his autographic telegraph. Ref. ref. 11, 28. It is sometimes referred to as the "Meyer blade-edge" ("Meyersche Schneide"), ref. C. Lorenz AG patent nr. 744883). This telegraph used wide with paper tape and a helical spindle. The sender part of this telegraph "scanned" a metal tablet with a platinum stylus. Text was written on the tablet with non-conductive ink. The printer was electro-chemical. However, Meyer used a 1-turn spindle. Rudolf Hell's clever idea was to use a 2-turn spindle!
Fig. 22: "Copying telegraph" of Bernhard Meyer (1864)
(source: ref. 21, ref. 11)
SYNCHRONIZATION: SEND ONCE - PRINT TWICE
Characteristic of the Hellschreiber printer is the 2-turn printer spindle. But why must the thread of the spindle have two turns (it is wound twice around the hub of the spindle)?
So that it prints two identical, parallel lines of text!
Fig. 23: The two-turn helix printer spindle, patented by Rudolf Hell
(source: Figure 2 of the 1929 Hell patent)
Text characters are only sent once! So why print them twice?
To be able to read the printed text, without needing to synchronize sender and receiver/printer!
This requires some further explanation. The easiest way to do this, is to see what happens if the thread of the spindle has makes one turn.
The Hellschreiber principle requires that transmitter (sender) and receiver (printer) use the same column-scan speed. Mechanical Hellschreibers senders and printers are motor driven. There will always be differences in motor speed between two Hellschreibers (just like no two PCs have 100% the same clock speed or exactly the same soundcard sampling rate).
Without some form of synchronization, the speed difference causes the spindle of the receiver to either lead or lag the scan speed of the transmitter. A leading spindle prints a received pixel at a higher line number of the character matrix than where it is supposed to be printed (or at a lower line number in the next column). In other words: the printed text line is slanted upward and runs off the paper. Conversely, if the receiver spindle turns slowly with respect to the transmission (i.e., it lags), then the printed text line is slanted downward. If only a single text line is printed, such asynchrony makes it hard to read the text:
Fig. 24: Upward slant for receiver spindle that is faster than the transmitter
Now imagine always printing two identical lines of text - one right above the other. As the text lines are identical and parallel, they will both be slanted upward or downward. When the top text line runs off the top of the paper tape, the bottom line is completely visible and readable. Conversely, when the bottom line runs off the bottom of the paper, the upper line is completely visible and readable.
Fig. 25: Double-line text remains completely legible despite slant caused by speed difference
Clearly, a 2-turn spindle fixes the slant problem. But that's not all! Even if the two motors are running at exactly the same speed, there will be a phase difference between them. With a 1-turn spindle, this causes the text line to be split: the upper part of the line is printed below the lower part.
Fig. 26: Split text line due to phase difference between sending and receiving motor
(motor speeds identical - text lines are horizontal)
Again, the 2-turn spindle fixes this problem. One of the two parallel lines will still be split, but the second line will remain whole, and perfectly legible.
Fig. 27: Double-line text remains completely legible despite split caused by phase difference
Fig. 28: Text lines are slanted downward if the receiver's spindle turns slower than transmitter
(original 15 mm wide strip from original Hell Feldfernschreiber, courtesy 1992 Helmut Liebich, DL1OY)
The above illustrations clearly show that:
with a 2-turn spindle, there is no need for a mechanism
that makes the motors iso-synchronous (same speed and same phase)!
This is what is done in Hellschreibers such as the "Presse Hell" and Hell Feldfernschreiber. Very ingenious indeed! This method takes care of speed differences as much as 5%. This type of Hellschreiber is referred to as "synchronous" or "quasi-synchronous", as the motor at the sending and the receiving station must have the same speed. Note that the term "asynchronous" is only used for Hellschreiber models that use a "start-stop" method to... synchronize. Yes, this terminology may be confusing, but is has been used like this since before the 1930s.
Note that no matter at what speed the characters are sent to this printer, a 2-turn spindle will always print at least two identical lines. The 7x14 Hell font has 2+2=4 white pixels above and below the 10 character pixels (total 14). Reducing the character transmit speed (without changing the printer's speed) will indeed increase the height of the printed characters. But the height of the printable space is fixed mechanically: it is equal to the pitch of the thread of the spindle. The character speed can be reduced by up to ≈30% (4/14) before the top of both printed lines is cut off:
Fig. 29: Printing characters that are transmitted at normal, 1/2, and 1/4 speed
In principle, a Hellschreiber printer can also print characters that are sent faster than normal - unless the pixel pulses are too short for the printer solenoid to follow:
Fig. 30: Printing of characters that are transmitted at normal and double speed
Note that the Chinese news and meteorological services, and Japanese news services did use a Hellschreiber system for transmission of pictographic characters. However, given the thousands of characters in use, this was note done with a keyboard, but with a system that optically scanned hand-written text. combined with a Hell-printer, similar to the ZETFAX of the Hell company, the RCA Tapefax, and the RC-58B system of the US Army (WWII). The Toho Denki Kabushikigaisha company (Eastern Electric Ltd.) in Japan made such Hellschreiber systems for the Japanese and Chinese markets (ref. 24). Toho Denki K.K. was a fax equipment manufacturer, and became part of Matsushita Graphic Communication Systems Inc. in 1962.
Fig. 31: Hellcast in Chinese: "... was appointed as government official of the Republic of China [ = Taiwan]"
(source: ref. 24)
Fig. 32: Hellcast in Chinese: "...formal diplomatic relations between two countries. The Government of the Republic of China..."
(source: ref. 45)
Fig. 33: Hellcast in Chinese: "The destination is important. The maintenance manual shall be visible"
(source: ref. 18)
Fig. 34: Hellcast in Chinese - recorded from a Beijing station on 14040 kHz (late 1970s/early 1980s)
(source: Fig. 11.1.f in ref. 40, courtesy RSGB; used with permission)
Of course, the design of the spindle can be adapted to increase the spacing between the printed parallel lines of text. Ref. 34A/B.
Asynchronous Hellschreiber machines such as model T type 72b/c "GL" and model Hell-80, use "start-stop" synchronization. The Hell-80 still has a 2-turn spindle, because it can be switched between (high-speed) asynchronous mode and start-stop mode. Ref. 13, 14, 15.
In start-stop machines, the motor in the receiving station also turns continuously. However, the drive-train to the spindle and the paper transportation mechanism is connected via an electromagnetic clutch. When a start-pulse is received, the clutch is engaged for a fixed amount of time (or a fixed number of turns of the drive shaft). As the first and the last column of the Hell-font contain no pixels, the start-pulse has conveniently been "hidden" there. As the first column is now not printed anymore, the space between characters is reduced from two, to one column widths.
Fig. 35: Start pulse "hidden" in the first column of the font in "Start-stop Hellschreibers"
With this method, the spindle always starts at the correct angle ( = phase). The motors still have different speeds, but the amount of slant that accumulates during the transmission of a single character is completely negligible.
Fig. 36: Slant of a synchronous Hellschreiber & slant of a start-stop Hellschreiber
(same motor speed-difference in both cases)
Hence, only a 1-turn spindle is needed, and narrower paper tape can be used (9.5 mm wide instead of standard 15 mm). As the paper tape is only transported when a character is being printed, no paper is wasted when no text is being received (unlike Feldfernschreibers). This enables unattended operation, without the need for a remote control system. Clearly, the required detection of the start-pulse makes this method sensitive to noise/interference (just like telex/RTTY)...
PERFECT FOR AMATEUR RADIO
Based on the Hell-system characteristics described above, it is obvious that it is perfectly suited for amateur radio communication, in particular the classic - and classy - "Feld Hell" mode (modern Hellschreiber software-modes have quite different characteristics):
- signal characteristics are similar to high-speed CW "Morse"
- insensitive to interference or poor signal quality, due to: 1) high level of pixel redundancy, 2) special Hell-font that maximizes legibility while minimizing misinterpretation, combined with 3) the human's excellent pattern recognition capability.
- insensitive to path delays (unlike RTTY, PSK, Throb, Domino).
- insensitive to polar flutter (40-160 m; unlike PSK and other phase-modulated modes).
- exact frequency tuning and drift (e.g., tube transceiver) are not critical (unlike, e.g., PSK and many other modern digi-modes.)
- Pleasant character transmission rate:
- 2.5 characters/sec corresponds to a decent average typing speed.
- suitable for real conversation QSOs (unlike PSK and other fast digi-modes, where only pre-programmed, impersonal messages can be exchanged).
- Narrow bandwidth:
- Feld-Hell qualifies as a "narrow bandwidth digital mode", as the required bandwidth of ≈300 Hz is less than 500 Hz. So it may be used in narrow-bandwidth band segments (check the bandplan of your IARU region and country for definition and details). Note that the occupied bandwidth may be much larger than the required bandwidth if you are using non-Hell PC-fonts or over-modulating your transmitter.
- compatible with narrow CW IF-filters in the receiver.
- Low duty-cycle:
- average about 25% (min 6%, max 39%) is easy on the transmitter, unlike PSK, FSK and other 100% duty-cycle modes.
- no special requirements for transmitter or receiver.
- can be used with a very simple CW transceiver. No other digi-mode can do this!
- printers can even be home-built.
- original machines are rare, but all it takes is a PC (need not be powerful), free software, and a simple interface between the PC and the transceiver! See the Hell software & PC-interfacing page.
Ref. 16, 17, 18, 19.
AMATEUR RADIO - HELLSCHREIBER "FIRSTS"
The table below captures Amateur Radio Hellschreiber "First" milestones. Possibly, Hellschreiber QSOs have also already been made via aurora propagation, aircraft/tropo/rain-scatter, and with a Space Shuttle and/or the International Space Station (ISS). However, I have no documented info about that. If you do, please contact me.
|Amateur Radio QSO||March 1959||On HF, between Hans Horn (DL1GP), who was a Luftwaffe radio technician in WWII (SK 1998), and DM3KG. Hans obtained a special RTTY and Hellschreiber permit from the German Bundespostministerium (BPM) in February of 1959. Ref. 35.|
|Hell Meeting||1977||Took place in The Netherlands. This is now the Annual Hell Meeting.|
|Weekly Hell Net||1979||Originally on 40m, since many years on 80m. Then, as it is now, the Net Leader is Arthur Bauer (PA0AOB), using one of his Feld-Hell machines. Stations from northwestern Europe. Check here for time & frequency.|
|Contest||Sept 1980||Organized by the DAFG (Deutsche Amateur Fernschreib Gruppe). Since 1982, this is the annual DARC KW-Hell-Contest in October.|
|Moonbounce (Earth-Moon-Earth, EME)||1980||By Jan Ottens (PA0SSB), with his 6 m (20 ft) dish antenna. Ref. 20, 43. The main challenge with moonbounce is the 273 dB (!) round-trip signal damping. In April of 1981, he also made the first transatlantic EME QSO (though in CW rather than in Hell mode) on 2.3 GHz, with W6YFK in California/USA, and in 1983 on 1296 MHz with ZL3AAD in New Zealand.|
|Satellite relay||May 1999||Attributed to Peter Klein (KD7MW), via the AMSAT OSCAR10 (AO-10, uplink on 70 cm, downlink on 2 m) amateur radio satellite (ref. 23). Shortly thereafter, he had a number of Hell QSOs via AO-10 with Tony Bombardiere AB2CJ/K2MO. Tony also experimented via AMSAT Fuji-OSCAR 20 (FO-20, incl. QSO with Mark Gamber (KB3CWS/W3MRG) and FO-29, as well as the soviet Radio-Sputnik 13 (RS-13). Ref. 44.|
|QSO via meteor scatter||February 2001||Attributed to Dave Greer (N4KZ) and Randy Tipton (WA5UFH). During the initial QSO on 80 meters, they learned that they both enjoyed meteor scatter on VHF. Their Hell-software [probably an early version of IZ8BLY] had a 9X speed setting, suitable for meteor scatter. They switched to 6 m (50 MHz) and got enough good random meteor bursts to exchange the required QSO/QSL info. The distance was about 900 miles (1450 km). Dave was running 100 watts out to a 5-element 6 meter yagi up at 40 feet. Ref. 42.|
|Web-cam ("Hell-cam")||May 2008||Created by me, Frank (N4SPP). Whenever I am operating in Hell-mode, a screen-shot of my Hell-software receiver window is uploaded automatically every 2 minutes to my Hell-Cam page.|
|Optical QSO||August 2013||Made by Barry Chambers (G8AGN) and Richard Hanes (G0RPH), with powerful red LEDs ("Phlatlights") and IZ8BLY software (ref. 41). The distance was 66 km (≈ 41 miles).|
|Air-Mobile QSO||August 2015||Made by myself, Frank (N4SPP), operating as HB/N4SPP/AM/QRP, and Rolf Buese (DF7XH). So it was an international QSO as well - and multiband (VHF + UHF). I called CQ from the passenger seat of a small airplane (an old Falke C2000 motorized glider, courtesy Christian B., pilot). We flew along the southern ( = Swiss) shore of Lake Constance, on the Swiss-German border. I used a small old laptop with IZ8BLY software, a simple homebrew interface, and a small handheld transceiver (Baofeng UV-5R). Rolf happened to be scanning the bands and was kind enough to answer my CQ calls. See Fig. 35 & 36 below.|
Fig. 37: The sequence of 6 screenshots, covering the entire 20 min air-mobile QSO
Fig. 38: A selfie of the author, crammed in the cockpit during the first air-mobile Hell QSO
- Ref. 1: "Die Entwicklung des Hell-Schreibers" by the inventor himself: Rudolf Hell; pp. 2-11 in "Gerätentwicklungen aus den Jahren 1929-1939", Hell - Technische Mitteilungen der Firma Dr.-Ing. Rudolf Hell, Nr. 1, May 1940.
- Ref. 2: p. 456 in Fritz Schröter "Fortschritte in der Bildtelegrafie", Elektrische Nachrichten-Technik (Telefunken E.N.T.) Band 5, Heft 11, November 1928, pp. 449-458. Source: www.cdvandt.org.
- Ref. 3: "Der Betrieb von Siemens-Hell-Schreibern auf Hochspannungsleitungen im Elektrizitätswerk-Nachrichtendienst" [operating Hellschreibers on 60 kV and 100 kV power lines], Fernmeldetechnik, Siemens & Halske, Berlin-Siemensstadt, 1939, SH. 7802, 1,5 9. 39, 4 pp.
- Ref. 4: "Optical communications - 1935 style", D.W. Rollema (PA0SE), pp. 46, 47, 49 in "Electronics and Wireless World", August 1985
- Ref. 5: "Nog een hell-systeem", A. van Ooijen, PE1AQB, Electron, 8/1983, p. 417.
- Ref. 6: "Der Siemens-Hell-Feldschreiber", by Rudolf Hell's co-workers G. Ege and H. Promnitz, pp. 11-20 in "Gerätentwicklungen aus den Jahren 1929-1939", Hell - Technische Mitteilungen der Firma Dr.-Ing. Rudolf Hell, Nr. 1, May 1940.
- Ref. 7: "Der Feldfernschreiber", document D 758/1 of the Oberkommando des Heeres, Heereswaffenamt, Amtsgruppe für Entwicklung und Prüfung, Berlin, 1 April 1941.
- Ref. 8: "Take 29" in transcript of "Täterinnen? - Frauen im 2. Weltkrieg" [TV documentary "Perpetrators - Women in the second World War"], Anja Schrum, Sonja Striegel (ed.), Südwestrundfunk (SWR2) "Wissen", 7-March-2001.
- Ref. 9: "Abtast-Telegrafen", chapter IV in "Taschenbuch für Fernmeldetechniker", H. W. Goetsch, Oldenbourg Verlag, 1940, pp. 411-427 of 787.
- Ref. 10: "Schreiber für Morsezeichen und Bildtelegrafenimpulse" [printer for Morse characters and image telegraph pulses], Reichspatentamt Patentschrift Nr. 694437, Dipl.-Ing. Dr.-Ing. Rudolf Hell, filed: 13-August-1937.
- Ref. 11: "Autographic telegraph of Bernhard Meyer", pp. 151-152 in "The Worldwide History of Telecommunications", Anton A. Huurdeman, Wiley-IEEE Press, 2003, 638 pp., ISBN 0471205052.
- Ref. 12: "Vorrichtung zur elektrischer Übertragung von Schriftzeichen" [Device for the electrical transmission of text characters], Dr.-Ing. Rudolf Hell, Reichspatentamt, Patentschrift 540849, filed: 3-April-1929.
- Ref. 13: "Grundlagen der Springschreibertechnik" (Part 1-5) [start-stop synchronization], F. Schiweck, Telegraphen-, und Fernsprech-Technik, Jg. 25, Nr. 3, March 1936, pp. 53-57, Nr. 4, April 1936, pp. 91-97, Nr. 6, June 1936, pp. 139-144, Nr. 9, September 1936, pp. 245-250, Nr. 11, November 1936, pp. 307-313 – 31 pp. total; also: Fernmelde-Technik, 1937, Siemens & Halske A. G., Wernerwerk, Berlin-Siemensstadt, 26 pp.; also: SH. 6623, 1939, 26 pp.
- Ref. 14: 158-165 in "Die Erde wird kleiner: vom Fackelzeichen zum Bildtelegramm" [brief description of start-stop synchronization and Hellschreiber], Johannes Sigleur, Franckh Publ., 1954, ASIN: B0028N8VXK.
- Ref. 15: "The Creed No. 10 Tape Teleprinter" [start-stop synchronization], A. E. Thompson, Electrical Communication (Int’l Western Electric Co.), Vol. 16, Nr. 4, April 1938, pp. 289-297.
- Ref. 16: "Hellschreiber - a rediscovery", Hans Evers (PAØCX / DJØSA), Ham Radio, 12/1979.
- Ref. 16A: "Hellschreiber" series of comments to the above 12/1979 Ham Radio article "Hellschreiber - a rediscovery" article, by E. Concklin, K6KA, Henry Plant, W6DKZ, and Stanley Cook (G5XB), Ham Radio, 3/1980 and 9/1980.
- Ref. 17: "Hellschreiber - what it is and how it works", S. Cook (GB5XB), pp. 320-323 in "Radio Communication", 4/1981
- Ref. 18: "Hellschreiber - Nostalgie oder Realität?", Helmut Liebich (DL1OY), pp. 58-61 in "Funkschau", 11/1990
- Ref. 19: "Hellschreiber, An old mode still has performance that's hard to beat; Part I - History and description; Part II - getting on the air; Part III - Signal analysis and digital philosophy; Part IV - Frequency domain", Murray Greenman (ZL1BPU), Break-In Amateur Radio Magazine (NZART), Vol. 71, October 1998, pp. 7-8, Vol. 71, November 1998, pp. 4-6, Vol. 71, December 1998, pp. 6-8, Vol. 72, Jan/Feb 1999, p. 6.
- Ref. 20: "Hell naar de maan en terug" [moon bounce], Dick Rollema (PA0SE), pp. 182-183 in "Electron" (VERON), nr. 4, April 1982
- Ref. 21: "Geschichte der elektrischen Telegraphie", Karl Eduard Zetzsche, Vol. 1 of "Handbuch der elektrischen Telegraphie", Springer Verlag, Berlin, 1877, 579 pp.
- Ref. 22: "Communications: an international history of the formative years", R. W. Burns, The Institution of Engineering and Technology (IET), 2004, ISBN: 978-0-86341-330-8, 656 pp.
- Ref. 23: "Hellschreiber in Space KD7MW Seattle", Peter A. Klein (KD7MW), message posted on ONElist reflector on 18 May 1999, (ONElist merged with eGroups early 2000, which merged with Yahoo Groups later that year).
- Ref. 24: "Oriental approach to transpacific transmission", Donald K. deNeuf (WA1SPM, SK), pp. 16, 18 in "Proceedings of The Radio Club of America, Inc.", Vol. 51, Nr. 1, March 1977.
- Ref. 25: p. 409 in "Electric Telegraphs", A.P. Deschanel, pp. 400-419 in "Popular Science", Vol. 3, No. 26, August 1873 [pdf]
- Ref. 26: pp. 94-96, 162, 163, 170-172 in "Hochfrequenz-Nachrichtentrechnik für Elektrizitätswerke", 2nd ed., Gerhard Dreßler, Heinrich-Karl Podszeck, Springer Verlag, 1952, 183 pp.
- Ref. 27: pp. 56-58 in "Verringerung der Fehleranfälligkeit drahtloser Telegrafiewege durch Maßnahmen im Niederfrequenzteil der Überttragungssysteme", K. Reche, A. Arzmaier, R. Zimmermann, in "Telefunken-Hausmitteilungen", Jg. 20, Heft 80, March 1939, pp. 53-62.
- Ref. 28: "Der Siemens-Hell-Schreiber", H. Stahl, Siemens & Halske A. G., Wernerwerk, Berlin-Siemensstadt, 1934, 15 pp. SH5358, 1.34.1,5 T. reprint from Telegraphen- und Fernsprechtechnik, Jg. 22, Heft 11, November 1933, pp. 291-295.
- Ref. 29: "Meyer's mehrfacher Telegraph" [with transcript], pp. 310-325 in "Dingler's Polytechnische Journal", Vol. 215, 1875
- Ref. 30: p. K137 and Fig. 15 in "Zur Vorgeschichte des Internationalen Telegraphenalphabetes Nr. 2", Volker Aschoff, pp. K134-K138 in "NTZ-Kurier" (Nachrichtentechnische Zeitschrift), Vol. 28, Issue 4, 1975.
- Ref. 31: "An arrangement for the Electro-magnetic Recording of Letter and Like Signals", Siemens & Halske A.G., Great Britain patent 427257, application date 24 March 1933.
- Ref. 32: "Fernschreiber mit einer an einem Hebel angeordneten Farbrolle", Rudolf Hell, Reichspatentamt Patentschrift Nr. 513715, filing date 3 June 1939.
- Ref. 33: "Vorschubeinrichtung für Papierstreifen an Telegrafengeräten" [paper-tape transport device for telegraph equipment], Kurt Winkelmann, Siemens & Halske AG, Reichspatentamt, Patentschrift Nr. 736240, 9 June 1940 [Hell printer-head, paper tape pinch-roller with only one flange]
- Ref. 34: selected patents
- Ref. 34A: "Schreibspindel zur bildpunktweisen Aufzeichnung von Telegrafiezeichen" [printer-spindle for pixel-wise printing of telegraphy characters], Gebhard Ege, Dr.-Ing Rudolf Hell company, Reichspatentamt, Patenstschrift Nr. 723243, 8 August 1939.
- Ref. 34B: "Schreibsprindel zur bildpunktweisen Aufzeichnung" [printer-spindle for pixel-by-pixel printing], Gebhard Ege, Dr.-Ing Rudolf Hell company, Reichspatentamt, Patenstschrift Nr. 723244, 8 August 1939.
- Ref. 35: "Unser Start mit RTTY", Hans Horn, pp. 71-73. in "DL-QTC", Heft 8, 1960
- Ref. 36: "Hell: Been There, Done That, Love It! Part 1-3", Frank Dörenberg (N4SPP), in "K9AY Telegraph", Vol. 12, Issue 1-3, January-March 2015; ©2015 Robert F. Heytow Memorial Radio Club, posted with permission.
- Ref. 37: "Method Of And Apparatus For Electrically Recording And Reproducing Sound And Other Vibrations", Merle Duston, United States Patent 2,030,973, filed: 13-Aug-1931, awarded: 18-Feb-1936
- Ref. 38: "Record Of Voice Now Made On Moving Paper Tape", Popular Science, Volume 124, Nr. 2, February 1934, p. 40
- Ref. 39: "Anwendung von Faksimile-Hellschreibern für den Boden-Bord-Verkehr", Rudolf Hell, pp. 67-75 in "Flugsicherungs-Verfahren und -Technik: Grundsätzliches über die Technik für Flugsicherung und Betrachtung der wichtigsten technischen Hilfsmittel außer Radar", Teil IIIB in "Flugnavigation und Flugsicherung", Band 7 of "Bücherei der Funkortung", proceedings of the "Internationale Jahrestagung des Ausschuß für Funkortung", Berlin; Verkehrs- und Wirtschafts-Verlag, 1958, 107 pp.
- Ref. 40: "The Hellschreiber System", pp. 11.1-11.3 in "Teleprinter Handbook", A. G. Hobbs (G8GOJ), E. W. Yeomanson (G3IIR), Arthur C. Gee (G2UK), Radio Society of Great Britain, 2nd ed., 1983, 350 pp., ISBN 0900612592
- Ref. 41: "More fun and games", message by Barry Chambers (G8AGN) in the Yahoo "UK Nanowave Group" on 28-Aug-2013
- Ref. 42: personal email exchange with David Greer (N4KZ), 2-Oct-2016.
- Ref. 43: personal email exchange with Jan Ottens (PA0SSB), Nov-Dec 2016.
- Ref. 44: "Hellschreiber - an old mode revisited" [Hellschreiber via satellite], by Clive Wallis (G3CWV) in "AMSAT-SM Info-Bladet", Nr. 3, 2000, pp. 4-5 [pdf]
- Ref. 45: "Tall Tales of Tape", pp. 12, 32 in "Monitoring Times", Vol. 3, Nr. 6, June 1984.
External links last checked: October 2015