Use F11 on your keyboard to switch between "normal view" and "full screen" (without browser toolbars)

 

 

    

  Most recent updates on this page:

	14-Apr-2010: added downloadable Feld-Hell TrueType fonts.
	17-Dec-2009: added references for start-stop sync, added photo of slip-contacts, added text about character-drum manufacturing, added text about turning direction of the spindle-thread and of the spindle shaft, add comparison to RTTY speed.
	4-Nov-2009: added pictures of re-inking device.

    

    

[raster scan]      [printer]      [synchronization]      [drive mechanism]

[speed control]      [interfaces and controls]      [modulation]      [operation]

Note: this page contains a number of photos and several dynamic cartoons that may take a while to download

©2007-2010 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.

HELLSCHREIBER FONT RASTER SCAN

In a nutshell, the Hellschreiber is a teleprinting system based on low-resolution character-level facsimile, sometimes referred to as "mosaic telegraphy", or even "remote dot-matrix printer".

	Text characters are decomposed by raster-scan into a series of pixels. In mechanical Hellschreibers, these binary series are captured mechanically as bands of conductive patches or notches on a cylindrical drum that turns continuously (see photo page).
	The sender selects the character to be transmitted via a typewriter-style keyboard or punch tape (no type-ahead). There are optical Hell systems that perform real-time scan of text that is handwritten on paper strips (ref. 1)
	The associated series of binary pixels is transmitted to a remote receiver via electrical signaling (telephone line) or via radio. Other than this sequencing, there is no encoding whatsoever (unlike telex/RTTY, etc.).
	The printing mechanism of the receiver comprises an inked spindle. It prints the pixel stream in real-time on a moving paper strip.
	There is no decoding: each individual pixel is printed as soon as it is received. There is no process to "collect all pixels of a character, then interpret and decide which character to print". In fact, the receiver prints all received signals that exceed the detection threshold, including noise/interference.
	The receiver can never print a wrong character, only a graphically distorted character (extra pixels or lost pixels). It is the human operator who interprets the received print-out, and decides which text characters were received. The pattern-recognition capability of the human brain can even recognize text below the noise level!

Initial Hell designs (1933/34) worked with a 12x12 pixel character raster: 12 columns of 12 pixels. With a transmission rate of 5 characters per second, this resulted in an equivalent telegraphy speed of 12 x 12 x 5 = 720 baud. The corresponding signal bandwidth was considered too large: 2147 Hz (assuming F1 modulation).

  
12x12 rasterization
(source: p. 6 in ref. 2)

The final design settled on a 7x14 matrix: 7 columns of 14 pixels. The first and the last column of the 7x14 matrix is not used. These blank columns effectively create a two-column wide space between subsequent characters. The top and bottom lines are also not used (exceptions are the "tails" of the Feld-Hell font characters Q, 0, 3-9,  ?, and / ). This provides vertical separation when two identical text lines are printed, one above the other, as is done in the Feld-Hellschreiber (explained further below).

Two improve readability and to minimize the required bandwidth, the Hell-font applies several rules:

	Only upper-case (capital) letters.
	Each column-element is at least 2 pixels high. This may sound like a 7x7 matrix, rather than 7x14. It is, but only for signalling speed ("baud rate") and signal bandwidth calculation purposes! Beyond that, it would be a "7x7 matrix in which pixels can be shifted by half a pixel within a column ".
	Note that the two-pixel rule also applies to "white pixels", as well as to the transition between the top of one column and the bottom of the adjacent column to the right.
	Special font design to preclude misinterpretation (mistaking one character for an other) in the presence of noise/interference in the received signal.

At a rate of 2.5 characters per second (cps), the corresponding telegraphy speed is 7 x 7 x 2.5 = 122.5 Bd. This compares to 6 or 6.6 cps of RTTY (traditional US speed is 45.45 Bd / 6 cps / 60 wpm, whereas the traditional European speed is 50 Bd, 6.6 cps / 66 wpm).

The shortest pulse is 2 pixels or 8.16 msec ( 1 pixel = 1000 ms/sec ÷ 2.5 characters/sec  / (7x14) pixels/char = 4.08 msec). The corresponding "necessary bandwidth" is 3 x 122.5 = 367 Hz (see ref. 3, and the ITU standard formula for non-fading transmission paths; the minimum Nyquist factor of 2 only applies to an ideal, noiseless transmission medium). Note that this is not the same as "occupied bandwidth"! The latter depends on the shape of the pulse envelope, resulting from the (CW) transmitter's keying characteristics or applied by DSP/software processing ("raised-cosine comes close to the ideal shape). Improper pulse shaping may cause the actual bandwidth to be several 10s of kilohertz under good propagation conditions! Obviously overdriving/over-modulating the transmitter will also result in (much) higher actual signal bandwidth (many kHz!). Unfortunately this is not at all uncommon for operators with PC-Hellschreibers (or digi-modes in general) that use a PC-soundcard...

Of course today's PC software implementations of the Hellschreiber can use any font set, and the Hell-system as such has no limitations in this respect. However, not abiding by the 2-pixel rule raises the baud-rate and thereby unnecessarily increases the occupied bandwidth of the transmitted signal (as much as 4 times!). This is why it is highly recommended to use the original Hell font with PC-Feld-Hellschreibers.

 
Quasi-7x7 rasterization
(source: figure 8, p. 6 in ref. 2)
 

Character rasterization is done column-by-column, bottom-to-top, left-to-right. See figure below.

Rasterization of the character "3"

The raster-scan has been done for all characters on the keyboard. Each one of the resulting strips of 98 black-and-white pixels must somehow be captured and memorized, so they can be recalled during character transmission. The Hellschreiber pre-dates magnetic-core and semiconductor memory chips. The strips are captured mechanically, by wrapping each pixel strips into a ring. This is done for all characters. The rings are then combined into a cylinder: the character-drum ("Geberwalze"). 

The "black" pixels are captured in the form of conductive metal patches that are embedded into the surface of the drum (later mechanical models, such as the T type 72b/c "GL", have a notched drum). All patches are interconnected inside the drum, and are also connected to a continuous "common" track around the drum. The "white" pixels are simply formed by the dark, non-conductive material of the drum. Per ref. 8 (below), the metal patches are made of nickel. They are actually the tips of notched metal disks that are stacked to form the drum. The space around and between the notches is filled in with an insulating  material/mass. The surface is turned to its final diameter on a lathe, and smoothed.

The circumference of the character-drum

The Feld-Hell font can be reconstructed from the above photo of the character drum circumference. This is what I did on a rainy afternoon early June of 2009. I first determined the column boundaries (horizontal red lines in the photo immediately below), and then added the (light blue) pixel boundary lines. Note the small notch for the start of the first column: it is marked "start" at the top right hand corner of the photo above.

The tracks of the character-drum divided into individual "pixels"

After some minor adjustments, it was fairly easy to capture the pixel/bit pattern. See the pixel-patter in binary form below (the binary file is available here). There are 41 characters of 7 x 14 bits each. Hence, the Feld-Hell's character drum is actually a 4 kilobit Non-Volatile Read-Only Memory!

The binary version of the character-drum (7 columns of 14 pixels per character)
(click on the image to get a text file with the same data)

From the above bit pattern, I then reconstituted the actual font. Note the odd shape of the ? and the pause-character (listed below after the? /). The latter is unique to the Feld-Hell. See description further below.

 

 
The character set of the Feld-Hell

 

I have used FontStruct™ to capture the above Feld-Hell character set as a "TrueType" font that can be used in regular Windows^® and MacOS^® programs (e.g., Word^®, PowerPoint^®). Note that only the 41 characters above are defined; use the # key for the pause character. Lower-case A-Z map to upper-case.

	Click here for the single-character high font
	Click here for the two-character high font (requires larger font size setting than single-height)

Compare the 1930s Hell-font to a "modern" PC font on an LCD screen (OK, OK, it is the close-up of a small-size true-type font, so vector based, rather than bitmap or raster):

The font used in the Hell 72 "GL" is presented here, and that of the Hell-80 machine here.

The drum turns continuously and each character takes one full revolution of the drum. The drum is combined with the keyboard mechanism; the latter resembles a conventional mechanical typewriter. Each key of the keyboard has a slip contact that is dedicated to the drum-track of the particular character. This slip contact only makes contact with the drum when its key is engaged. All character slip contacts are also interconnected. The "common" track of the drum is always connected - via a carbon brush, rather than a slip contact. This construction effectively creates a switch that opens and closes in the rhythm of the pixel pattern of the selected character (if any).

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)

There is a notch on the shaft of the character drum, and for each key of the keyboard there is a spring-loaded lever mechanism. The drum turns continuously. Once per revolution, towards the end of column 7 of the tracks on the drum, the notch moves the key levers such that all keys are enabled. At that point, the desired key can be depressed. Shortly thereafter, the notch has passed, and the key levers resume their rest position again. The selected key is locked in the down position (no need to keep pushing down on it), and all other keys are locked out.  At the end of the new drum revolution, once again the notch enables all keys: the key that was selected is released and pops back up by itself, and the next desired key can be depressed. When a key is depressed, its spring-loaded slip-contact flips, and makes contact with the associated track of the drum. When the key is released at the end of the revolution of the drum, the slip-contact flips back and no longer touches the drum. 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.

Details of the key locking/enabling and slip contact (dis)engaging mechanism
(figure 11 in ref. 4)

Clearly, it is impossible to simply guess when the short key-enabling time window occurs, and press a key just at that 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 the drum notch. Then let go of the key - it should stay down until it is released a ¼ sec later (2.5 cps). This does take a tad of Fingerspitzengefühl, and unlike old mechanical typewriters and Glockenspiele, there is no tapping or hammering on the keys! The operator has to develop some sort of typing rhythm to avoid having lots of spaces in the transmitted text. This sounds complicated and awkward, but in practice it is actually easy to get used to and not at all uncomfortable.

Close-up of the slip-contacts of the Hell Feldfernschreiber - one contact in the engaged position

The fact that the notch on the drum enables all keys at the same time, means that multiple characters can be transmitted at the same time. The result is an OR-ing of the two character matrices. Arthur, PAØAOB, uses this technique to combine "0" and "/" to get Ø.

Keyboard of the Hell Feldfernschreiber

The keyboard obviously has a European QWERTZ layout rather than QWERTY (US). It only has capital letters A-Z, the figures 0-9, and the characters + - ? / but there is no period or comma.

Note the Hell-unique "pause" character key to the right of the "P" key. It is unique to the Feld-Hell, and you won't find this nice feature in any of the software implemented Hellschreibers! Contrary to the other keys, once the pause-key is depressed, it remains depressed until another key is depressed. A special cam wheel in the gearbox of the Feld-Hell (located below the Motor-Generator) has three notches that actuate a contact such that only one in four pause-characters are transmitted. That is: once per 4 x 1 / 2.5 sec, or 80 pause characters per 50 sec. This is used to signal to the switch operator or opposite station that the line (or frequency) is occupied.

A blank/space key is located at the bottom right of the keyboard; it does not have a corresponding track on the character drum, but it is subject to the keyboard enabling/locking mechanism described above. A telegraphy "Morse" key is to the left of the Q and A keys. It is marked with a green dot.

Telewriter Tbs/24a-32

17½"x15½"x9½"-57 lbs

Sends figures 1 to 9 and 0, the characters+, -, /, ?. and the 26 letters of the alphabet. Works directly into a telephone line. Field telephone can be plugged in for speech working. A 900-cycles filter can be switched in to reduce interference. When the interference is too great, 900/c/s morse code can be sent and received on headphones. 12-pt socket on panel is for connection to radio set through an intermediate unit. Tube system 900 c/s sender oscillator ─Rec amplifier─Rec rectifier speed control. All tubes are type RV12P4000.  Power supply 12-volt storage battery to motor, coupled to dynamo for H.T. for tube.

The mechanism of the set is simple but precision made, and the keyboard is continental type.

Description of the Hell Feldfernschreiber in ref. 6
 

Ref. 1: "Nog een hell-systeem", A. van Ooijen, PE1AQB, Electron, 8/1983, p. 417
Ref. 2: "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, Mai 1940   [in German]
Ref. 3: "Hellschreiber - what it is and how it works", S. Cook GB5XB, Radio Communication, 4/1981
Ref. 4: "Der Feldfernschreiber", document D 758/1 of the Oberkommando des Heeres, Heereswaffenamt, Amtsgruppe für Entwicklung und Prüfung, Berlin, 1 April 1941
Ref. 5: "The Feldfernschreiber", translation  by Frank Dörenberg, N4SPP, of "Der Feldfernschreiber" (updated: 2 May 2009)
Ref. 6: Chapter VIII, Fig. 52, p. 459 in "Handbook on German Military Forces", US War Department Technical Manual TM-E 30-451, 15 March 1945

Ref. 7: “Abtast-Telegrafen“, chapter IV in “Taschenbuch für Fernmeldetechniker“, Goetsch, H. W., Oldenbourg Verlag, 1940, pp. 411-427 of 787

Ref. 8: bottom of page 154 "Siemens-Hell-Schreibers", pp.  149-166 in “Fernmeldetechnik“, Band 9 of “Lehrbücher der Feinwerktechnik“, Fritz Schiweck, 1st ed., 1942, 526 pp.,  C. F. Winter'sche Verlagsbuchandlung   new

 

 

HELLSCHREIBER PRINTER

The initial Hellschreiber 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 wet-paper electrochemical method was impractical. In 1931 Dr. Hell replaced it with a printer-spindle mechanism. To get "ink" on the paper, the spindle needed to be "inked". The early spindle designs used carbon paper tape and the spindle thread was dentilled. The mechanisms to ensure proper (un)winding of the carbon tape and for rubbing the carbon onto the spindle, were rather complicated. Also, special carbon paper tape was expensive (considerations not unlike those for ink cartridges of today's PC-printers!).

Carbon-paper and dentilled spindle mechanism
(source: figure 3, p. 4 in ref. 2)

The carbon paper approach was abandoned in favor of inking the spindle with a very simple free-spinning ink roller made of felt. The same ink roller was later used on Siemens-Halske type "GL" (72b/c), Hell-80 Hellschreibers, and others. The manual recommends using ink that is a "colloidal solution of pigment in oil", to avoid both drying out of the felt roller and collecting deposits of ink on the spindle. Water-based ink is not recommended: it causes the thin steel bushing on which the felt ring is installed to rust. Also, the capillary action of the paper causes such ink to be dispersed, making the printed text fuzzy shortly after  being printed. I keep inked felt rings (including the one normally installed on the printer) wrapped in kitchen cling-wrap, to prevent them from drying out.

  
The Hellschreiber ink roller spare felt rings

The Hellschreiber transmitter sends text in the form of serialized raster-scanned characters. The printer must take the received sequence of pixels, and reconstitute the sent characters pixel-stream in real-time. That is, it must also perform a raster scan, at the same scanning speed as the transmitter. This can be done by continuously repeating a column-scan with a printer-head. Printing is then done onto a paper tape that passes by the printer-head at a constant speed (47 cm/min or ≈0.3"/sec in the Hell Feldfernschreiber). A very elegant way to do this is with a worm spindle that is installed across the paper tape.

Two-turn helix spindle installed just above and across the paper
(paper transport rollers at the far left; the top roller only touches the edges of the paper, so as not to smear the printed text; the ink roller is swung out of view)

Looking at a turning spindle from the paper tape point of view, a spindle winding appears as a point that sweeps ( = scan) across the width of the paper.

Two-turn helix sweeps two (inked) points across the paper tape

The Feld-Hell character images comprise 7 columns per character, and 2.5 characters are sent/printed per second. That is: 7 x 2.5 x 60 = 1050 columns per minute. If the spindle is single-thread (a.k.a. single-flight, single-start, 1-start, "eingängige Spirale”), a spindle speed of 1050 rpm would be required. However, the Feld-Hell spindle is a double-thread worm (double-helix, "Doppelspirale", "zweigängige Schnecke"): it has two intertwined sets of windings. With 525 rpm, it produces the same scan speed as a single-thread worm at 1050 rpm; the latter is typically used in home-built Hellschreiber printers like my own. Note that the number of threads is not related to the number of turns of the threads.

Cross-sections of single-, double-, and triple-thread worms

The Feld-Hell printer spindle on its shaft

The spindle is kept wet with ink from the ink roller that rests on top of the spindle, and spins freely. 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, the sent column is recreated. Very ingenious indeed! The paper tape moves at a speed such that the next revolution of the spindle creates a new column right next to the preceding one.

Note that the spindle is right-turning, i.e., clockwise when looking at its tip, whereas the spindle has a left-turning thread. This is often stated and illustrated incorrectly. This is done on purpose: where the paper meets the helix, the circumference of the helix moves in the same direction as the paper tape. This minimizes the drag on the helix and on the paper. In turn, this helps keep spindle speed and the paper transport speed constant.

As stated above, the spindle is 12 mm wide, whereas the paper tape is 15 mm wide paper tape. This leaves 1.5 mm along both edges of the tape,  for the paper transport rollers to "grab" the tape - without smudging the printed text.

The moving paper tape is tapped against the inked spindle in the rhythm of the received signal

The paper is tapped against the turning spindle by a modified electromagnetic relay. The armature is the hinged, moving part of a relay. It does the actual tapping. It is as wide as the spindle thread is long. The magnet-solenoid is (de)energized by a detector-driver circuit. The detector is a full-wave rectifier. The Feld-Hell uses two cuprous-oxide-on-copper diodes for this purpose: they have an excellent low forward-voltage (0.2 volt, similar to germanium diodes); germanium and silicon diodes did not yet exist in those days. Solid-state implementations of the driver circuit may need a diode (installed with reversed polarity) across the magnet solenoid, to avoid damage due to the inductive voltage spike upon de-energizing the solenoid. This is referred to as a flyback, snubber, freewheeling, catch, or suppressor diode.

Clearly, the detector/driver energizes the solenoid for all input signals that exceed the 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.

The paper tape moves at 47 cm/min (≈1½ ft/min), 150 characters are sent per minute (2.5 cps), and only 5 out of the 7 matrix columns are actually used for the font. Hence, the printed characters are close to 3 mm wide (1/8 "). A standard 300 m roll of paper is good for about 11 hours of operation.

            

 

 

 

    1   Ink roller

    2   Ink roller lever

    3   Spring (to "flatten" paper tape)

    4   Two-turn printer spindle

    5   Electro-magnet and "tapper"

    6   Paper transport rollers
         (only lower roller is powered)

    7   Paper transport (dis)engage lever
 

 

 

The printer mechanism of the Feldfernschreiber (solenoid box cover removed)

     
Other illustration of the Feld-Hell printer mechanism
(figure 12 in ref. 4 above)

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"

 

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 speed).

 

The word "SIEMENS" in Morse code, printed by a Hell Feldfernschreiber
(figure 668 in ref. 7 above)

 
SYNCHRONIZATION

The Hellschreiber principle requires that transmitter and receiver use the same raster-scan speed. Mechanical Hellschreibers are motor driven. There will always be differences in motor speed between two Hellschreibers (just like no two PCs have the same clock speed or 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, then the printed text line is slanted downward.  If only a single text line is printed, then the asynchronism may make it hard to read the text.

Downward slant for receiver spindle that is slower than the transmitter
(p. 19, figure 9 in ref. 2: 5% asynchronism; 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:

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, and the top half of the line to be printed below the bottom half.

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.

Split text line due to phase difference between sending and receiving motor

Text lines are slanted downward if the receiver's spindle is slower than transmitter's
(original 15 mm wide strip from WWII German Wehrmacht 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 asynchronous Hellschreibers such as the Hell Feldfernschreiber. Very ingenious indeed! This method takes care of speed differences as much as 5%. This type of Hellschreiber is also referred to as "quasi-synchronous".  

1944 Feld-Hell Fernschreiben (telegram) from General Wenck to Major-General Wisch, congratulating the latter with his promotion *
(courtesy and © M. Lippl; click on image or here to get full size. * General Wenck was Chief of Staff of the 1st Panzer Army and youngest general in the German Army during WWII;  Major-General Wisch was Commander of 1st SS Division "LSSAH")

An other solution is used in later models Hellschreiber. Here, the motor in the receiving station also turns continuously, but the drivetrain 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. As the first and the last column of the Hell-font contain no pixels, the start-pulse has been "hidden" there.

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 asynchronous 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 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. Clearly, the required detection of the start-pulse makes this method sensitive to noise/interference (just like telex/RTTY)...

This "start-stop" mechanism is used in Hellschreibers such as T type 72b/c "GL" (the "GL" part stands for "Gleichlauf" = synchronism), and Hell-80. The latter still has a 2-turn spindle, and can be switched between (high-speed) asynchronous mode and start-stop mode.

	"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.    new
	pp. 158-165 in "Die Erde wird kleiner: vom Fackelzeichen zum Bildtelegramm" [brief description of start-stop sync and Hellschreiber], Johannes Sigleur, Franckh Publ., 1954, ASIN: B0028N8VXK.    new
	"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    new

DRIVE MECHANISM

In mechanical Hellschreibers, a single motor drives the spindle, the character-drum, and the paper transport mechanism. A gearing system is used to reduce the motor rpm: down to 150 rpm for the drum (1 revolution per character, 2.5 characters per sec, 150 cps), 525 rpm for the spindle, and 12.5 rpm for the paper transport roller (the powered roller has a diameter of 12 mm; 47 cm paper per minute).

a. Stiftkupplung – pin coupling
    (of main shaft to electric motor)

b. Hauptachse – main shaft

c. Schreibspindel – printer helix

d. Geberwalze – transmitter drum

e. Papiervorschubrolle – paper
    transport roller

s. Schnecke – worm drive

 

Schematic representation of the gearing mechanism
(source: p. 14 in ref. 2)

The two spur gears driven by the helical gears are made of Turbax® (from Jaroslaw's Erste Glimmer-Waren Fabrik of Berlin-Weißensee). This is a many-layer cotton-reinforced laminate, impregnated with thermosetting "Phenolharz", a phenol-formaldehyde resin (PF), similar to what is used in Bakelite®.

The gear box is located directly below the motor-generator and behind the printer mechanism. The motor/generator-printer unit and the keyboard-drum unit are actually a single unit. The spur gear on the drum shaft is part of the keyboard-drum unit (outlined on blue in the above diagram). This allows these two units to be quickly and easily separated for maintenance purposes, without the need for an additional shaft coupler.

Peeking into the gear box from the back of the unit

The 3-notch cam wheel in the above photo (lower right-hand corner) is mounted on the same driveshaft as the paper transport roller, so it turns at 12½ rpm. The character-drum turns at 150 rpm (150 characters per minute): 12 times as fast. The notches actuate a contact that is placed in series with the slip-contact of the pause-character key. The contact closes three times per revolution of the cam wheel. Each notch covers 1/12th of the circumference of the wheel. Hence, 3-out-of-12 = 1-out-of-4 pause characters is actually sent.

Hellschreibers not only print received text, but also the text that is being sent (the keyed 900 Hz tone is transformer-coupled to the telephone line port, which is coupled to the input filter and amplifier). Due to the single-motor mechanism, the drum and the printer spindle are synchronous: the machine's own text lines are always perfectly horizontal. However, it is desirable that both of these text lines are fully visible, without one of them being split between the top and the bottom of the paper. That is: the spindle and drum must by iso-synchronous: same speed and same phase. There must not be an offset between the starting point of the drum tracks, and the corresponding angular position of the spindle. If the printer-mechanism or the drum/keyboard unit is disconnected from the gearbox, the spindle shaft or the drum  shaft must be rotated before re-assembly so as to re-establish the correct relative position (this may take a couple of tries, but is not too complicated).

The gearbox is located below                             The character drum with its spur gear
the motor/generator                                                is located behind  the keyboard 
 

MOTOR SPEED REGULATION AND CONTROL

The Hellschreiber contains a motor-generator. The motor is a 12 V DC shunt motor. When the current through the shunt (governor) field windings is decreased, the field flux is decreased, and the motor's armature (rotor) must turn faster in order to produce the same amount of back-EMF. Conversely, increasing the shunt field current causes the motor speed to decrease. This property is used to regulate the motor rpm. In the Hellschreiber, the motor's field current is the anode current of the motor regulator RV12P4000 tube. The tube's grid voltage is switched between two levels, depending on whether the rpm is below or above the rpm set-point. This switching is done by a centrifugal switch.

 
Centrifugal switch of the motor-generator
(figure 13 of ref. 4)

As the motor speed increases, the centrifugal force moves the spring-loaded governor weight of the switch farther away from the motor shaft. A lever attached to the weight causes the distance between the switch contacts (C1) to decrease. When the contacts close, the tube's grid voltage increases, the anode current flows, the field flux increases, and the motor slows down rapidly until the switch contact opens up again; the anode current is shut off, the field flux drops, and the motor speeds up again. This cycle is repeated continuously. Such switching between two limit values is referred to as "bang-bang control" in control engineering parlance. The regulator maintains the rpm to within 0.1 % (ref. 1) of the rpm set-point.

Rudolf Hell patented this electrical centrifugal governor ("Elektrischer Fliehkraftregler") in 1951 (German patent nr. 803577)

A filter is installed across the switch contacts; it fully suppresses EMI from LF to VHF. There is no arcing across the contacts. Likewise, the motor's brushes are fully filtered.

"Bang-bang control" motor-generator rpm regulation

By design, the unregulated rpm (at least 9000 rpm!) is much higher than the required nominal rpm (3600 rpm). A protection is built in, to avoid reaching this rpm in case of failure of the regulator tube. A second contact of the centrifugal switch (C2) closes at a speed somewhat higher than the normal speed. When this safety contact closes, the regulator tube is completely bypassed, and the motor's field is tied directly to +12 V. Unlike the regular contact, there is no arcing or EMI suppression circuitry for this contact.

To transmit and print straight text lines, the motor speed must be constant and at the correct value. Motor speed is kept constant with the regulator (governor). It is kept at the right value by manually controlling the set-point of the speed regulator. The set-point is adjusted until the received text lines are horizontal (text printed during transmission is always horizontal!). The adjustment is done by manually turning the cap of the motor. The (stationary) switch contact assembly is fixed to this cap. As the cap is turned, it moves up or down, and the contacts with it. Hence, more or less centrifugal force ( = motor speed) is required to close the contacts of the centrifugal switch. The set-point can be adjusted over a range of ±5%.

The cap of the motor housing (black cap + band with numbers on it) can be turned manually to
adjust the motor speed set-point and thereby adjust the slant of received text lines

The output voltage of the generator depends on the back-EMF, rather than the regulator field flux. Hence, this voltage is basically independent of the current pulses through the field windings that occur continuously as part of speed regulation. When the small blue button of the voltmeter on the front of the electronics unit is pushed, the voltmeter shows the generator voltage. It must be in the blue range (150-190 volt, 165 nominal).


HELL FELDFERNSCHREIBER INTERFACES AND CONTROLS

The Hell Feldfernschreiber has several controls that are located on front of the electronics unit:

	a small light blue pushbutton on the voltmeter
	900 Hz bandpass filter on/off ("Tonsieb mit/ohne")
	audio gain ("Verstärkung")
	3-position main switch ("Aus-Bereit-Ein")

The momentary pushbutton is marked "200 V", and switches the voltmeter between the 12 Vdc and the output of the generator. The latter must be in the blue range of the meter (150-190 volt). The nominal 12 Vdc must be in the red range (11-13 volt).

The bandpass filter on/off switch does what the name suggests. It switches in a resistor between the taper of the audio gain potentiometer and input of the 900 Hz bandpass filter. This effectively kills the bandpass selectivity.

The audio gain potentiometer is used to adjust the volume of the input signals (coming from the phone line or radio receiver, and the keyed 900 Hz tone from the own Hellschreiber). In part, this variable gain was added to complement the RF-AGC of receivers.

The main switch has three positions: Off, Standby (ready), and On. Off and on are intuitively obvious. In the standby mode, only the heater filaments of the tubes are powered. The motor-generator is off. The (optional) red indicator light just above this switch only illuminates when the switch is in the standby position.

The set-point of the motor speed regulator is controlled by the manually rotating the top part of the motor/generator assembly. It has a scale from 0 - 10. If received text lines are slanted downward, the speed setting number is reduced to increase the local motor rpm set point. The nominal setting is 5.

There is one more switch: for selecting operation from a storage battery (accumulator or "Sammler") vs. AC-transformer-rectifier power supply ("Netzgleichrichter"). This switch is located on top of the electronics unit, rather than on the front panel. Per the 1941 manual (see the literature page),  the switch should always be in the "Sammler" position, as it is only intended for operation with an older model of the Feldfernschreiber power supply. The switch puts a 5 ohm wirewound resistor in series with the 12 volt power input, but only when the main switch is in the standby ("Bereit") position. In standby, only the heater filaments of the tubes are powered - (the motor-generator and printer are not). Maybe the old power supplies had problems regulating their output at low current draws... The nominal 12 Vdc can varied by at least +/- 10% without any impact on motor rpm.

In the lower left hand corner of the front panel is a bolt with a red circle around it. This bolt goes through the electronics unit, to a nut in the back of the carrying case. It fixes this unit into place. All bolts that are used in normal operation and line maintenance, are marked with such a red circle.

 
The battery/power-supply switch and fuse are located on top of the electronics unit

The power and signal connectors are all located on the right hand half of the front panel of the electronics unit. At the top of the connector block are two phone-line banana-plug binding posts. They are marked "La" and "Lb/E", short  for "Leitung-a" and "Leitung-b/Erde". I.e., Line-a and Line-b/earth (ground). This transformer coupled interface has an impedance of 800 ohms (the same as a standard standard 600 ohm POTS telephone land-line at 900 Hz).

Phone line connector La - Lb/ground ("Leitung a, b/Erde")

Right below the binding posts, between the La and Lb/E markings, is a jack for a 2-wire field phone. It is connected in parallel with the above binding posts. It requires a plug that is longer and larger than a standard (US) 6.3 mm plug. I have not found the spec for this plug yet...

Below the phone jack is a terminal with two jacks for inputting the audio from a radio receiver ("Empfänger"). Banana plugs fit just fine. This input has an impedance of 4000 ohms. AF input voltage range is 50 mV to 5 V. Below the receiver audio input is a terminal with two jacks for listening in ("Mithören") with a high impedance headset to the line or radio audio that has been passed through the 900 Hz bandpass filter.  In some models, this port is connected in parallel with the "Empfänger" port. Both ports have the standard 19 mm distance between pins (dual banana-plugs, European 220 V power plugs).

At the bottom of the connector block is the socket for the 12 volt power connector. This connector is keyed against inadvertent upside-down insertion: one socket pin has 5 mm diameter, the other 4 mm. Fortunately, my Hellschreiber came with an original 12 V power plug. Current settles around 0.78 A in "Bereit" Standby mode (vs. 0.9 A per the manual); the in-rush current is several amps for several seconds. During normal operation I measured a steady 3.13 A (vs. 2.75 A per the manual).

There is a binding post between the connector block and the main switch, for ground ("earth" in UK English).

                              Original power plug                                               Ground lug

This particular Feld-Hell has the optional large round connector on the front of the electronics unit (lower right hand corner).

To contact the Hellschreiber receiving station, a regular field telephone ("Feldfernsprecher", FF) can be connected in parallel with the Feld-Hell line terminal (using the same La-Lb/E binding post connectors, or the phone jack). As the Feld-Hell is capable of communicating across distances much larger than a regular FF, a special Hell-FF was developed. It is connected to the round connector just below the connector block described above), rather than to the phone line binding posts or jack. The amplifier tube of this Hell-FF receives 12 Vdc anode voltage (!) via that same connector. The same Hell-FF allowed remote control of the receiver Feld-Hell via the latter's own Hell-FF: the Hell-FF contains a 900 Hz tone-controlled relay that could bypass the receiver Feld-Hell's main switch to "Ein"/"On" (but only if the latter is in "Bereit"/"Standby" position).

An other accessory that can be connected to the round connector is a Hell Co. transmitter keying relay unit (for transmitters with direct keying of anode or cathode current).  

My transceiver (an Alinco DX70TH) does not have a VOX function (yes, I could build one and connect it to the PTT input of the transmitter). So I cannot use the keyed-tone from Hellschreiber to drive the transmitter via VOX. Instead, I have to connect the keying contacts (from the drum and the Morse key on the Hellschreiber keyboard) directly to the telegraphy key input of the transceiver. These contacts are accessible at pins 3 and 4 of the 12-pin round connector on the front panel of the Hellschreiber's Amplifier & Interconnect Unit. When using this connector, a contact behind the connector is opened. This interrupts the path of the keyed 900 Hz tone such that it is no longer coupled to the "Leitung" ("line") I/O transformer. Also, own-text is no longer printed during transmission, unless the side-tone of the CW transmission is fed back to the "Empfänger" ("receiver") or "Leitung" input of the Hellschreiber. More details and options are described on the interfaces page.

HELL FELDFERNSCHREIBER MODULATION

A Feldfernschreiber can be used with a radio transmitter in several ways:

use the PTT output to key a CW transmitter. I.e., switch the carrier on/off, exactly as is done with on-off keyed (OOK) Morse telegraphy. This is amplitude shift keying (AFSK). In this case, the Hellschreiber's tone output is not used. Note that the transmitter's internal keying circuitry must have rise and fall times that are compatible with the 8.16 msec minimum pulse width (using the original Hell-font).

feed the keyed 900 Hz tone to the microphone input of an SSB transmitter, and use the transmitter's VOX (Voice Operated Transmitter) function to automatically key the transmitter. The setting of the on- and off-delay of the VOX must be selected appropriately. Note that the 2.5 Vpp tone amplitude must be appropriately reduced via a fixed voltage divider or potentiometer.

Obviously a Hellschreiber can also be used with an AM transmitter, as was done during the 1930s-40s.

The only connection to the keying contacts is via the (optional) round 12-pin connector on the front panel of the electronics unit. See the "Interfaces to transceiver" page. When these contacts are used, the keyed 900 Hz tone is no longer output. Also, the keying is at character pixel level, rather then at character level. In other words, the Hellschreiber does not output a "PTT" keying contact that is closed for the duration of each character. So it is not an option to key the transmitter with such a keying signal, and connect the keyed tone to the transmitter's microphone input. 

So, the options are OOK CW and single-tone SSB. Both are Amplitude Shift Modulation (ASK). The RF of the CW signal is an OOK carrier at frequency carrier f_c. When a single sinusoidal audio tone is applied to an SSB modulator, the resulting "single-sideband suppressed carrier" signal is identical to an unmodulated 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 as the CW signal shifted by f_m. Feldhell transmissions have all the characteristics of fast Morse telegraphy.

Feld-Hell is typically reported as having a duty cycle of about 22%, which would make it relatively easy on the transmitter. My binary file of the Feld-Hell font shows that the character "8" has the highest duty-cycle: 39% (it consists of 38 out of the 7x14=98 possible pixel-bits). The "period" (a.k.a. "full stop") has the lowest duty-cycle: 6% (6 pixels total out of 98). I have no idea what mix of characters was assumed by the people who claim the average is22% (or those who quote it without verification).

HELL FELDSCHREIBER OPERATION

The dynamotor-printer unit          The keyboard/character-drum unit        The electronics unit
(paper tray drawers pulled out)

Here are the operating instructions for Hell Feldfernschreiber type a₂ (see copy of original at the bottom of this page). Additional detail (including from other manuals) has been added in italics.

Put equipment in operating position. Release the latch at the lower left by pulling the latch lever to the left until it stops. While holding the lever at this position, pull out the printer/keyboard unit 1 cm out of the carrying case, and let go of the latch lever.  Then pull out the equipment until the latch snaps back to the right.

"Riegel lösen" = "release latch"

In order to remove the printer/keyboard unit completely from the carrying case while it is in the operating position, slide and hold the lever to the left as far as it will go, then push the printer/keyboard unit back into the carrying case until the lever can be slid all the way to the left. Then pull the unit out completely. If the unit was still in the fully pushed-in position, slide and hold the lever to the left as far as it will go, pull the unit out until the lever can be slid all the way to the left. Then pull the unit out completely.

    
Left: latch track at the bottom of the case.   Right: bottom of the printer, with latch lever

Ground the equipment. Use the ground/earth binding post on the front panel of the electronics unit.

Verify that the needle of voltmeter is at zero.

Connect 12 V battery or rectifier. Ensure correct polarity! Lead-acid batteries and most amateur radio "12 volt power supplies" actually provide 13.8 volt. In this case, the "Sammler" / "Netzgleichrichter" switch ("Battery" / AC-transformer-rectifier power supply") must be switched to the "Sammler" position.

Connect phone line to jacks La and Lb/E.

Connect field telephone in parallel to phone line.

Select main switch from "Aus' ("off") to "Bereit" ("ready/standby"). The red indicator light above the switch will illuminate. Steady-state current is about 750 mA. Note that the circuit for the red light passes through the two carbon brushes and the commutator of the motor. If the red light does not come on, this may be a hint for the cause...

Wait 1 minute, until the equipment is ready for operation. In standby mode, only the heater filament of the tubes is powered. The voltmeter reads about 10 volts; this is consistent with a 3.8 volt drop from the 13.8 volt power supply due to the 0.75 A standby current through the 5 ohm resistor that is in series with the supply, when the power source switch on top of the electronics unit is in the "Sammler"/"Battery" position.

Establish connection with receiving party via field telephone.

Select main switch to "Ein" ("on"). The indicator light is turned off; the battery voltage is in the red range of the voltmeter (11-13 volt). The motor/generator will be turning. If the blue button is depressed, the anode voltage is in the blue range of the voltmeter (150-190 volt). Steady-state current varies between 3.5 and 4 A.

	While writing, hold finger lightly on key without applying pressure, until key descends. Immediately release the key and actuate the next key.
	Adjust reception for best print quality with "Verstärkung" ("Gain") knob.

In case of poor text printing, select 900 Hz filter from "Ohne" ("without/off") to "Mit" ("with", "on").

Adjust for straight text lines. If text lines are slanted downward, adjust the speed control cap on top of the motor-generator assembly to higher number. If text lines are slanted upward, adjust speed control to lower number.

Check paper supply. Open the paper tray cover by pushing down on the small black pushbutton at the middle of the base of keyboard (between the V and B keys).

Insert paper. Separate the end of the paper tape from the paper roll, and place it on the paper platform such that the roll unwinds clockwise. Feed the paper tape through the guides in the box, twist it 90º (if using gummed paper, the gummy side will be down), feed it through the slit in the left hand side of the paper tray box, while inserting the tray.

Insert paper from roll into the system. Pull up the ink roller lever, pull the paper tape under the printer helix, and feed it between the paper transport rollers. Briefly pull the upper paper transport roller lever to the left; the ink roller will drop back onto the printer helix again.

Replace ink roller. Pull ink roller lever upward and hold; pull off the used ink roller; insert a replacement roller from the spares box. If only replacement felt rings are available, pull the used felt ring off the steel bushing of the ink roller, put an inked replacement felt ring on, and push the roller back on its spinner.

Immediately re-ink removed ink roller.

A condensed Feld-Hell operating manual

Condensed manual for Model a2 Feld-Hell, from the inside of the lid of the carrying case

©2004-2009 F. Dörenberg N4SPP

   top of page