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Latest update: 6-Mar-2010 (added click on any 3D image  to get full-size/high-resolution image)
 

Update: 12 Feb 2010 (added ref .12, cf. new); Update: 17 Jan 2010 (added ref 9-11); Update: 3 Jan 2010 (added ref. 8,cf. new); Update: 29 Dec 2009 (added document, cf. new); Update: 17 Dec 2009 (added manuals and schematics); Update: 28 July 2009

©2007-2009 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.

 [Description]        [Characteristics]        ["regular" photos]        [3D/Stereoscopic photos]

The Hellschreiber "GL“ was introduced ca. 1952. It combines the simplicity of the Hell Feldfernschreiber with a start-stop mechanism ("Geh-Steh-Verfahren", "Springschreiber") similar to that of a telex system. As this mechanism provides synchronization between the sending and receiving Hellschreiber, "GL" (for Gleichlauf - synchronization) was appended to the type number.

The Feld-Hell machine has a character-drum with a pixel-sequence ring for each character; the pixel bits are formed by conductive patches.  In the "GL" there also is a character-drum, but it consists of a set of disks that are stacked onto the shaft of the drum. Each character-disk has notches that represent the pixels.

      
Siemens-Hell-Schreiber T type 72c "GL“

The "GL" basically uses the same typeface as the Feldfernschreiber, without the pause-character, but expanded with . , ' = ( ) : and some modifications to the E, K, Q, = and ?. The font is illustrated below (thanks to Heinz, DC4GL). The binary form of the "GL" font is here. To hear what the "GL" character set sounds like: click here.

The font of the Hell 72 "GL" (start pulse in first column)

In the Feld-Hell font, the first column is always blank (does not contain any data). The "GL" includes a start-pulse in this first column. It has a duration of 8 pixels (out of the 14 pixel column), but is not printed.

This form of transmitter-receiver synchronization at character level dates back to 1869 (start-pulse patent by  Goyot d'Arlincourt in France) and the early 1900s (e.g., patents by Howard Krum).

The receiver of the "GL" contains a start-pulse tone-detector that drives an electro-magnetic clutch. This clutch couples the continuously running electro-motor to the spindle and paper transport mechanism. Using a clutch avoids the need for a motor with a very high starting-torque (very short spool-up time). After a fixed delay time (6 spindle revolutions), the clutch disengages: both the spindle and the paper tape stop. This is referred to as a start-stop mode. Other start-stop methods actually use a stop-bit or stop-pulse.

As with the Feld-Hell, the motor of a "GL" transmitter and receiver are completely asynchronous and their speed will always be different. With the Feld-Hell, these speed differences cause printed text lines to be slanted upward or downward. As the "GL" has synchronization at character-level, accumulated slant is reset for each received character. Text lines of a "GL" are never slanted, only characters.

Slant of printed text: asynchronous (left) vs. start-stop (right)
(for same speed difference between transmitter and receiver)

Hence, there is no need for the receiver to print two parallel text lines. So, the thread of the printer spindle only has a single turn (vs. two in the Feld-Hell), and the paper tape is narrower (9.5 mm vs 15 mm in the Feld-Hell).

           
Hell "GL": 1-turn spindle                                Feld-Hell: 2-turn spindle

The start-stop mechanism significantly reduces paper tape consumption: the paper tape only moves when text is transmitted or received. This also means that the "GL" can be used for unattended operation. Note that the paper tape of the Feld-Hell moves continuously, independent of whether or not text is transmitted or received.

Obviously, having a start-stop mechanism implies that noise can cause false starts, and missing of real starts. This characteristic has led the amateur radio use of "GL" to primarily take place on VHF. The "GL" was used for teleprinting via regular telecom networks (unlike telex). E.g., railway systems (ref. 12), police and fire department, banks, stock- and commodity-market exchanges, etc.

Print-out of the "GL" character set
(ruler scale is in cm)

Principle diagram of the "GL" character-drum and keyboard mechanism
(source: Figure 3 in ref. 4)

The figures immediately above and below show that there is a separate notched disk for the start-pulse.

Notched disks for the letter "E" and for the start-pulse
(source: page 7 in ref. 8)

 
Pulse sequence of the character "8" (left)                    Contact-bounce at the start of a pulse

	Ref. 1: "Hell op de twee-meter-band in alle modes", by Hans, PEØHGD, Electron, February 1978, pp. 82-83
	Ref. 2: "Hellschreiben: Nostalgie oder Realität?", by Helmut, DL1OY, Funkschau, Sonderheft Spezial, November 1990, pp. 58-61
	Ref. 3: "RTTY-Funkfernschreiben nach dem System-Hell", Peter Stolzenberg, DC9XD, QRV - Amateur Radio, Vol. 29, 1975, pp.  562-571 [courtesy Gerard Wolthuis, PA3BCB]
	Ref. 4: "Der Siemens-Hell-Schreiber", Siemens & Halske A.G., Siemens Fernmelde Technik, SH 8354. 443. TT1, 1943 [courtesy Siemens Corporate Archives, München]

	Ref. 5: “Siemens-Hell-Schreiber „GL““ T typ 72 c – Beschreibung“, St Bs 1211/2, October 1955, 31 pp. + schematics , Siemens & Halske AG, Wernerwerk für Telegrafen- und Signaltechnik, T Werb 2880 R GN, 956.0,2 (courtesy Heinz Blumberg, DC4GL)  [52 MB].
	Ref. 6: "Siemens-Hell-Schreiber „GL“" T typ 72 c – Betriebs- und Bedienungsanleitung mit Teileverzeichnis", St  Ba 1211/2, December 1956, 56 pp. + schematics, Siemens & Halske AG, Wernerwerk für Telegrafen- und Signaltechnik (courtesy Heinz Blumberg, DC4GL)  [48 MB]
	Ref. 7: “Siemens-Hell-Schreiber „GL“  T typ 72 c” [brief description & characteristics], Siemens & Halske AG, Wernerwerk für Telegrafen- und Signaltechnik, SH 4057, KB T App 15, 7591. T FS 3, 2 pp., 1975? (courtesy Arie van Oijen, PE1AQB)     new
	Ref. 8: "Hell-72 "GL" Start-Stop-Sender, Start-Stop-Empfänger", Siemens Hell-Technik, T Ausbildung S-Hell, Änderung 1, pp. 7-8, SuW 2957 R, 559.0,2 (courtesy Arie van Ooijen PE1AQB)     new
	Ref. 9:“Hell”, Reflecties door PAoSE, Dick Rollema (PA0SE), Electron, nr. 7, July 1977, p. 349 (courtesy Gerard Wolthuis, PA3BCB)     new
	Ref. 10:“Hellschrijven met autostart” , Reflecties door PA0SE, Dick Rollema (PA0SE), Electron, nr. 5, May 1979, p. 313 (courtesy Gerard Wolthuis, PA3BCB)    new
	Ref. 11: "Hellschrijven wint terrein", Dick Rollema (PA0SE), Electron, nr. 2, February 1983, pp. 73-76  (courtesy Gerard Wolthuis, PA3BCB)
	Ref. 12: "Die Fernmeldetechnik als Hilfe bei der Rationalisierung der Eisenbahn", W. Leitenberger [Hellschreiber use in German railroad system is described in Section 3: "Modernisierung des Eisenbahnbetriebes"], ETR: Eisenbahntechnische Rundschau, Nr. 4, April 1966, pp. 139-146    new

The detailed circuitry schematic of the Hell 72c "GL" is here (with some annotations by Hans, PAØCX).

Here is a summary of the characteristics of the Hell "GL": 

	Keyboard: as for the Feld-Hell, the "GL" keyboard is also upper-case only. However, it is expanded with additional punctuation marks  . , : ' ) ( as well as = . Instead of a space-key, there is a conventional space-bar. There is no pause-character key.
	Font: same 7x14 as Feld-Hell, hence, compliant with minimum 2 pixel height. First column contains a start-pulse (line numbers 5-12). Size of the printed characters is 4½ x 2½ mm (max).
	Transmission rate: 6.13 characters per second (vs. 2.5 cps in Feld-Hell), i.e., 368 per minute.
	Shortest pulse (for Hell font): 3 1/3 msec (300 baud) vs. 8.16 msec (122.5 baud) in Feld-Hell).
	Duty cycle: worst-case 47% (character "8", with a total of 46 bits out of 7x14=98); note that although the font is basically the same as that of the Feld-Hell, each character has a start-pulse of 8-bits (out of a 14-bit column). The comma and quote have the lowest duty cycle: 15% (14 bits total). The file with the binary format of this font includes the bit-count for each character.
	Modulation: single-tone ASK. Keyed output tone (and center frequency of the associated input bandpass filter) is switchable between 1000 and 3000 Hz tone. Line bandwidth permitting, 3000 Hz could be used at the same time as voice channel on the same line (limited to standard 2400 Hz).
	Size of printed characters: 2.5 x 4.5 mm (WxH).
	Paper tape width: 9.5 mm.
	Printer helix: single-turn.
	Ink roller: same as Feldfernschreiber and Hell-80.
	Motor: 220 Vac (vs. 12 Vdc in Feld-Hell).
	Nominal motor speed: 3000 rpm (vs. 3600 of the Feld-Hell).
	Input bandpass filter: center frequency selectable between 1000 and 3000 Hz (switches at the same time as the tone frequency). Filter bandwidth is 700 Hz.
	Line impedance: 600 ohm (standard POTS telephone land-line).
	Communication channel bandwidth: 450 Hz recommended (300 Hz minimum)
	Allowed signal path attenuation: 35 dB / 4 neper (vs. 43 dB / 5 Np for Feld-Hell); note that reliable voice communication is not possible at this level of attenuation.
	Minimum input signal: 23.4 mV into 600 ohm ( -30 dB / -3.5 Np)
	Output signal: I measured ≈75 Vpp at the "Mh" ("Mithören") output (left side of the unit).
	Character drum: separate notched ring for each character (vs. rings of embedded conductive patches in Feld-Hell).
	Tubes (valves): EF80 (pre-amp), EL83 (final amp / solenoid driver), and ECC81 (double triode, one used for remote-control tone oscillator, the other as tone-detector). Rectifier and suppression diodes are solid-state.
	Current-loop: 18-45 mA.Detectors for tone and start-pulse: separate 4-diode full wave rectifiers (vs. one 2-diode full wave rectifier tone detector in the Feld-Hell).
	Tolerance to transmitter-receiver motor speed difference: ±2% (vs. ±5% for Feld-Hell)
	Paper tape speed: 1.1 m per minute (vs. 0.47 m/min for Feld-Hell).
	Dimensions: 43x43x26 cm (WxDxH, ≈ 17x17x10").
	Weight: 26 kg (just under 58 lbs: about the same as the Feld-Hell machine in its case).
	Power: 220 Vac, 40-60 Hz, 100 W.

I have also captured the above list and the schematic into a single document "Characteristics and Circuit Diagram of the Hellschreiber Model 72b/c "GL". It is available here. It includes this detailed circuit diagram.

A slight variation of the Siemens-Hell-Schreiber 72b/c "GL" is the T typ 73 "AGL". The "AGL" is a "GL", expanded with a punch tape reader. As you can imagine, full utilization of the 368 characters per minute capability is only possible via punch tape operation.

The Siemens-Hell-Schreiber T typ 73 "AGL"
(source:  “Technical Collection from Dr. Rudolf Hell” in Kiel e. V.)

These photos have been down-sampled. Full size, print quality, jpg files are available from Frank N4SPP

The AC-motor of the "GL"

The 3D/stereoscopic photos below are anaglyphic: you will need a pair of red/green glasses to get the 3D effect. I am not aware of the existence of any other 3D photos (historical or new, like mine) of Hellschreibers. If you know of any, please let me know.

Click on any image to get full size & high-resolution!

Above photos ©2008 F. Dörenberg N4SPP

©2004-2009 F. Dörenberg N4SPP

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