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[what it is - and not]      [rasterization]      [transmission]      [printing]      [synchronization] 

[perfect for amateur radio]    [amateur radio Hellschreiber "firsts"]  [references]

©2004-2012 F. Dörenberg. All rights reserved worldwide. No part of this publication may be used without permission from the author. It has taken considerable effort to create these pages. If you "borrow" content from them, at least reference the source.

THE "HELL SYSTEM" - 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 (cf.  p. 2 and §10b in ref. 1):

 

 

Some non-German publications and websites persistently state that the word "Hellschreiber" actually means something like "bright writer", brightly writer", or worse, "bright writing". This is brought on by mis-use of dictionaries and online translation tools. Remember: "a fool with a tool is still a fool". If anything, the Hellschreiber is the opposite of a "bright writer": "Der Hellschreiber ist ein Dunkelschreiber!", as it prints dark on a light background.

 

 

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; "Bildtelegraphie"). I.e., a system for the telephotographic transmission of images.

	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) This required a bandwidth of 4700 Hz, and a 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). Cf. pp. 55, 56 in ref. 27.
	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.

	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 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 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  cause pulse duration is increased or reduced by as much as 60%, printed text remains readable. Cf. p. 57 in ref. 27.

 

 

The "Hell system" consists of three basic steps for tele-printing text character-images:

1. Rasterization: de-composition of text characters into pixels.

2. Transmission of selected characters as pixel streams.

3. 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 truely is a breakthrough of monumental importance. It can not be over-stated how important the invention of the pixelation is! You probably would not be reading this on your screen without Rudolf Hell's invention of the bitmap font. Though printing without the need for synchronization is not important anymore in our days,

 

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.

Rasterization of the scripted figure "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 stored in a memory chip.

 

TRANSMISSION OF SELECTED CHARACTERS

 

Transmission of text is a 2-step process by itself:

1. selection of the character that is to be transmitted, thereby selecting the associated character pixel map.

2. 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.)!

Serialization of the figure "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), 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:

 

No special transmitters or receivers are required - which is another advantage of the Hell system.

 

Pulse transmission via 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 f_c. When a single sinusoidal OOK audio tone is applied to an SSB modulator, the resulting RF-signal is identical to an OOK carrier f_c that is shifted by the modulation tone frequency f_m (to f_c + f_m or f_c - f_m, depending on selection of USB vs LSB). In other words: exactly the same spectrum as an OOK CW signal shifted by f_m. 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.

 

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.

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).

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: ".... not everyone could do it, it was hard to type, and only in a particular rhythm."

Here is a Youtube clip in which Arthur Bauer, PAØAOB, demonstrates 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.

 

Here is a 3-minute video clip that I made of the spinning character drum of the Feld-Hell machine:

 

PRINTING OF A PIXEL STREAM

 

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 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).

 

The combination of the spinning helix and the moving paper tape is illustrated below. Note that the spindle is installed across the paper tape, and just above it (there is a small gap).

 

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).

Video clip of a 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:

 

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.

 

  
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.

 

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! In fact , it is so simple, that you can build your own!

 

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 (US: gray-scale) printing of the received signals, which makes it even easier to recognize characters in the noise.

 

Note: the cartoon below is rather large (2MB dynamic gif file) and may take a while to load...

Complete send/print sequence for the character "3"

 

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 (synchronization).

 

As indicated above, the solenoid of the printer's electro-magnet is energized for any signal that exceeds the detection threshold: Hell-signals, noise, CW ("Morse"), etc. The figure below shows how Morse code is printed by the Feld-Hell (for a compatible telegraphy speed).

 

The word "SIEMENS" in Morse code, printed by a Hell Feldfernschreiber
(source: figure 668 in ref. 9)

 

Note that in those days, the four most generally used "recording" methods were photographic, ink, electrolytic, and carbon paper. The initial Hellschreiber prototype developments (mid 1920s) used an electrochemical printer (the principle of electrochemical telegraphs dates back to the mid-1840s by Scottish inventor Alexander Bain, 1811-1877). This method used moist, chemically impregnated paper and a set of styluses, aligned as a column and resting on the paper. This method quickly turned out to be impractical. 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 carbon paper approach was abandoned in favor of inking the spindle with a simple free-spinning ink roller made of felt.

 
Carbon-paper and dentilled spindle mechanism
(source: figure 3 in ref. 1)

 

SYNCHRONIZATION: "SEND ONCE - PRINT TWICE"

 

Characteristic of the Hellschreiber printer is the 2-turn printer spindle. But why does the thread of the spindle have to two turns (it is wound twice around the hub of the spindle)?

 

So that it prints two parallel, identical lines of text!

Two-turn spindle

(source: figure 2 in the original 1929 Hell patent 540849, ref. 12)

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 spindle has only one turn.

 

The Hellschreiber principle requires that transmitter and receiver 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 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:

Downward slant for receiver spindle that is slower than the transmitter
(figure 9 in ref. 1: 5% asynchrony; this corresponds to 3° slant angle, but slant in figure is about 6°)

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.

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.

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.

Double-line text remains completely legible despite split caused by phase difference

Text lines are slanted downward if the receiver's spindle is slower than transmitter's
(original 15 mm wide strip from original Hell Feldfernschreiber, courtesy 1992 Helmut, DL1OY)

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:

 

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:

 

Printing 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. 44). Toho Denki K.K. was a fax equipment manufacturer, and became part of Matsushita Graphic Communication Systems Inc. in 1962.

 

Print of a Chinese hellcast

(source: ref. 24; the text reads "Appointment of the post of the Republic of China [= Taiwan] is notified" or something to that effect - to be confirmed)

 

Print of an other Chinese hellcast

(source: ref. 25)

 

Of course, the design of the spindle can be adapted to increase the spacing between the printed paralell lines of text. Ref. 34, 35.

 

Asynchronous Hellschreiber machines such as T type 72b/c "GL" and 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.

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.

Slant of synchronous Hellschreiber                                Slant of 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)...