• [Introduction]
• [Concept & evolution]
• [Synchronization]
• [Variations of the concept]
• [Patents]
• [References]

©2004-2019 F. Dörenberg, unless stated otherwise. All rights reserved worldwide. No part of this publication may be used without permission from the author.

Latest page update: 25 March 2019

INTRODUCTION

Tape-printers, such as the military Siemens Feld-Hell and the traditional Presse-Hell machines, print on paper tape. The printed tapes were either transcribed with a typewriter, or cut up into segments and glued onto telegram forms. This is a disadvantage, especially compared to the competing "telex" type teleprinters that printed on single sheet, roll or fanfold paper.

Obviously, Rudolf Hell was well aware of this disadvantage. He actually already foresaw a Blattschreiber based on the Hell-principle some 15+ years earlier, in his 1933 patent 668821. In 1947, he re-established his war-torn company for telecommunications and electronic image reproduction equipment. However, now in Kiel (port city in the far north of Germany), rather than the original locations in the then-Soviet-occupied part of Berlin. Early 1948, the company built a batch of five prototype sheet-printers ("Blattschreiber"), based on the Hellschreiber principle. They were quickly sold to newspapers in the region. Ref. 1. Siemens started series production of Blattschreiber "P" around 1949. The full designator of this machine is 9 T empf 1b (ref. 2A).

Fig. 1: The Siemens-Hell-Blattschreiber

(source: Fig. 8 in ref. 3,  Fig. 56a in ref. 2B, ref. 9)

One identified user of this Blattschreiber was the Norsk Telegrambyro (NTB, Norwegian Telegram Bureau). They used the "Presse Hell" system" since the German occupation during WW2, and later switched over to Hell Blattschreiber (see p. 114 in ref. 4). The Blattschreiber was publicly demonstrated at the headquarters of the German news agency DENA in 1948 (ref. 10).

Fig. 2: The Siemens-Hell-Blattschreiber with a Funk.empf.61 longwave receiver and shortwave down-converter

(source: ref. 8)

In 1956, the Foreign Broadcast Information Service (FBIS) of the CIA placed Blattschreiber equipment in operation at the their Okinawa Bureau, "to cover NCNA numeral code from Peking, with good results". Ref. 11. Note: NCNA is the New China News Agency (Hsin Hua Tung-hsün, Xinhua), the government news agency of the People's Republic of China, founded 1931. It was named Red China News Agency until 1937.

CONCEPT AND EVOLUTION

As the name suggests, the "Blattschreiber" is a Hell page-printer instead of a Hell tape-printer. Tape printers have the inconvenience that the printed tape has to be cut up and glued onto telegram forms or other sheets, or be transcribed manually or with a typewriter. The competition (regular teleprinters) had already adopted the page-printer approach decades earlier. Also, the teleprinters print single lines of text, instead of two identical lines, as typical of the pre-1950s Hell tape printers.

In principle, one can conceive a page-printer that uses the Hell column-printing concept: the printer-spindle is mounted on a carriage, and the carriage is moved across the width of the paper sheet. The sheet of paper is nothing more than series of paper tape segments. This is what Rudolf Hell actually proposed in his patent 825277 (filed in 1948).

Fig. 3: The Hell-Blattschreiber as proposed in patent 82577 - item 6 is the printer helix

(source: Fig. 1 & 2 in the patent)

The main components of the proposed printer are:

• A carriage (item 5 in the figure above) with printer helix (item 6) and felt ink roller (item 10). The thread of helix only makes one turn, as only a single line of text is to be printed - not two identical parallel lines.
• A T-shape printer bar ("Magnetleiste", item 4). The bar is as long as the width of the paper sheet, and as wide as the height of the characters that are to be printed. It is actuated by "one or more" printer-magnets (item 9). A paper-guiding rail ("Führungsschiene", items 2 & 3) in front of and behind the T-bar ensures that the paper is flat and taught, when and where it is printed.
• A motor that drives the printer helix via a drive shaft (item 7); the coupling gear-wheel (item 19) turns with the drive shaft, but can slide freely along the length of that shaft. The same motor also drives a long spindle shaft (jack screw, item 16); the carriage has a small lever arm (item 7) that rests in the groove of this drive shaft.
• An electromagnet (item 23) that can decouple the carriage lever arm from its drive shaft. When decoupled, the carriage is immediately pulled back to its starting position at the left-hand edge of the paper, by a spring-loaded pull cord (item 25, 26).
• A chain-form or roll of paper (item 1). The paper is either transported continuously, or a line-feed is performed at the end of each printed line of text (i.e., a combined carriage-return/line-feed, CR/LF). Rudolf Hell proposed to somehow execute the carriage return during a space between characters or words (claim 3 in patent 825277), or use a special carriage-return signal ("Zeilenwechselzeichen" pulse with the duration of a single character, claims 5 & 6 in patent 825277). The same pulse could also be used to enable the printer.

As stated above, this is the originally proposed embodiment of the Hell-Blattschreiber. It was never turned into a working commercial product. In a follow-on patent (848970) from 1951, Hell recognizes difficulties with the mechanical construction, due to the relatively large dimensions of the printer-head, combined with the very short time allowed for carriage-return. Solution of this problem requires a special control character for commanding a carriage-return/line-feed.

In his 1961 patent nr. 1157258 he actually states (column 3, lines 46-59) that all efforts to make a page-printer with a helix-carriage have so far been without success: "... sind alle Versuche, das Schreibsystem des Streifenschreibers in der geschilderten Weise für einen Blattschreiber zu verwenden, bisher gescheitert." Primary difficulties (besides designing a sufficiently fast printing relay) are:

• The inertia of the carriage (esp. noticeable during carriage-returns)
• Feeding the advancing paper sheet correctly between the spindle and printer-hammer while the carriage moves across the paper (effectively, the paper simultaneously moves in two perpendicular directions with respect to the printer-head).

So: back to the drawing board and back to basics. Recall that the Hell tape-printers only use a helix, because it is the simplest mechanical means to obtain a small striking-surface that scans across a paper tape. As the helix turns continuously, the scanning movement is repeated continuously. At the same time, the paper tape continuously moves by the spindle. It is this basic concept of continuous scanning action and moving the paper relative to the striking surface (or vice versa), that is essential - and not necessarily the implementation with a helix.

This is exactly what Hell did! An essential part of this approach is the platen: a large rubber-covered steel cylinder, as used in standard typewriters and in some types of conventional teleprinters. The rubber of the platen cushions the striking force of the printing hammer, type wheel, or typewriter type - as the case may be. This provides a cleaner print and reduces wear. The platen has a fairly large diameter to provide a local striking surface that is relatively flat.

But there is a critical difference: in typewriters and teleprinters, the paper sheet is wrapped around the platen and in constant contact with it. That is, the platen is part of the paper transport mechanism. In the Hell Blattschreiber, this is not the case! Recall that in Hell tape printers, the paper moves freely between the printer hammer and only touches the rapidly spinning printer helix when the hammer is actuated. The paper is slowly advanced with motorized pinch rollers. The exact same concept is applied in the Hell Blattschreiber.

Fig. 4: Blattschreiber as proposed in a Hell patent from 1933

(source: Fig. 4 in patent 668821)

In the 1933 patent, a standard Hell printer head (magnet + armature + hammer) is moved across the paper sheet. The hammer is oriented tangentially to the platen, aligned with the direction of the paper. See item 7 in the figure above. An inked ribbon or carbon tape is fed between the hammer and the paper.

Fig. 5: Hammer blade (item 8) and ribs

(source: Fig. 2 in patent 848970)

The intent at the time was to reproduce the tape-printer on a sheet. That is: print two identical parallel lines of text. This requires that two pixels be printed simultaneously. This was easy with a printer helix. Without a helix, one could consider using a hammer with two prongs. A better, and much more elegant solution is the converse: use a normal hammer with a straight blade, and add longitudinal ribs ("Rippen", "Stege", "Zahnleisten") to the platen. The ribs are spaced evenly along the circumference of the platen. Their spacing is equal to the desired distance between same-pixels in the two parallel lines of text. That is, equal to the pitch of the thread of an equivalent printer helix. The height of the hammer blade is slightly larger than this distance, so as to always fully strike two ribs. The platen spins with the same speed as the helix of a Hell tape printer. This creates the desired column-scanning action in front of the printer hammer. This makes the Blattschreiber in principle compatible with Hell-systems that operate at the same speed.

In the "real" Blattschreiber, we do not want to print two identical lines of text. Not a problem, just make the height of the hammer blade less than the distance between the ribs! Now the hammer can only strike one rib at a time.

At this point, we still need to solve the mechanical problems with the carriage-return. Hell's patent number 848970 from 1948 provides the solution. The continuous scanning action across the paper is obtained by mounting the hammer blade ("Schreibfähnchen", "Schreibschneide") on a continuous belt or chain. The belt moves in a race-track pattern. The length of the straight sections of this pattern is slightly smaller than the width of the paper sheet. The pattern is placed across the paper sheet, parallel to both the sheet and the ribbed platen. The speed of the belt is the same as the paper transport speed of Hell tape printers.

Fig. 6: Blattschreiber as proposed in the Hell patent from 1948

(source: Fig. 1 in patent 848970)

If we use a belt with only a single hammer (Hell patent 818515), then the hammer will still have to do a "carriage return" at the end of a scan, and instantaneously fly back from the right-hand edge of the paper to the left-hand edge. This is impossible. On the belt, the blade actually has to move all the way around in the pattern, to end up at the far left of the paper again. No problem: just add a second hammer to the belt and place it opposite the first hammer. When the first hammer is at the end of its scan, at the far right of the paper, the second hammer is ready to start the next scan at the far left of the paper. Well.... no! The overall length of the race-track pattern is always larger than the two straight segments of the pattern combined. In other words: the belt is longer than twice the printable width of the paper sheet. But the distance between successive blades must be equal to that printable width. This problem is solved by using three blades. This is what has been done in the Blattschreiber (see items 7, 8, 9 in the figure above). Of course, we could use four blades (ref. 1949 Hell patent 832444 and 1950 US patent 2656240), but then the belt becomes unnecessarily long (the distance between blades is fixed), and more difficult to fit inside a printer. Note that the belt speed is independent of the number of blades.

Fig. 7: Blattschreiber with four hammer blades

(source: Fig.4 in Hell's US patent 2656240; similar to Fig. 1 in Hell patent 832444 and Fig. 2 in 847024)

The printer blade has to be tapped against the inked ribbon and the paper, in the rhythm of the pixels that are to be printed. We could mount three solenoids on the belt, one behind each blade. But the wires to the solenoids would get twisted and destroyed within several turns of the belt, unless a cumbersome and unreliable slip-ring were used. It is much easier to mount two solenoids in a fixed position, one at each end of the scan pattern. Via simple levers, they actuate a hammer bar. The bar is installed along the belt, behind the scan-path of the blades. Voilà, the essence of the Hell Blattschreiber printer!

Fig. 8: Simplified mechanism of the page-printer - with 3 hammers

There are two options for advancing the paper sheet:

• Advance in one step, at the end of a printed line of text. The step size is the height of the font (including blank pixel-rows for line spacing).
• Slowly advance continuously. The speed is such that the above step height is achieved at the end of the text line.

In the first option, the printed line of text will be straight across the paper, perpendicular to the side of the paper sheet. Fine. However, this approach requires an additional actuator, such as a stepper motor. This violates the "single motor" approach... Furthermore, the printer hammer moves sideways while the platen rib sweeps upward. This causes the pixel-columns of the printed character to "lean" slightly to the right. See the Figure below.

Fig. 9: Leaning + slant

The same effect happens in Hell tape-printers, as the paper tape moves to the left while the helix sweeps across the tape. In the Blattschreiber, this is corrected by slightly slanting the ribs downward from left to right, giving the ribbed platen a spiraled appearance:

Fig. 10: Slanted ribs

(source: adapted from Fig. 6 in Hell patent 668821)

In the second option, the paper advances while a printer hammer sweeps across the paper. This causes the entire printed line of text to slant downward to the right (in addition to the "leaning" described above). This problem is easily fixed by installing the printer belt and hammer actuation bar at an opposite angle, i.e., slanting upward:

Fig. 11: The slanted installation of the printer mechanism is clearly visible

(source: Fig. 9 in ref. 3)

Fig. 12. The slanted installation of the printer mechanism is clearly visible

(source: Fig. 56b in ref. 2B)

Fig. 13: Close-up of the printer mechanism

Fig. 14: Right-hand view of the opened Siemens-Hell-Blattschreiber

(original photo: courtesy Jan Smeets (ON4ASZ); used with permission)

Fig. 15: Left-hand view of the opened Siemens-Hell-Blattschreiber

(original photo: courtesy Jan Smeets (ON4ASZ); used with permission)

Fig. 16: Bottom view of the opened Siemens-Hell-Blattschreiber

(original photo: courtesy Jan Smeets (ON4ASZ); used with permission)

SYNCHRONIZATION

As stated above, the Blattschreiber printing mechanism is - in principle - compatible with the standard Hell-system (at 5 chars/sec). However, only a single line of text is printed, not two identical parallel lines. To be able to use the Blattschreiber with existing standard Lochstreifensender ( = punch tape reader + Hell-format sender), the Blattschreiber must somehow synchronize itself to the received pulse stream. If not, the text line will be slanted an become illegible, as with a Hell tape printer with a single-turn spindle:

Fig. 17: Upward slant for receiver that is faster than the transmitter

Contrary to some publications, the Blattschreiber does not use start-stop synchronization at character level, e.g. via a an explicit start-bit or start-pulse. However, it does use the pixel-pulses of received characters to detect a phase difference between the printer and the sender. Recall that the top and bottom rows of the Hell-font are blank. The Blattschreiber "knows" the momentary scan position of the rib that is in front of the printer hammer. If this position corresponds to a pixel in the top or bottom row of a character, then no pixel-pulse should be received. If, however, a pixel-pulse is received during this time, then there is a phase difference between the sender and the Blattschreiber. The phase difference is eliminated by slightly increasing or decreasing the printer's motor speed. Ref. Hell patent 863358.

Two cam wheels are used to detect if the rib in front of the hammer is at the bottom or top pixel-position of the current column scan. This is indicated by notches N₃ and N₄. The cams are on a shaft in the platen drive-train. The shaft makes one revolution per column scan (5 char/sec at 7 columns/char = 28.57 msec/rev :: 2100 rpm). Each notch closes a contact for about half the duration of a single pixel (4.08 msec). If a pixel-pulse is detected while N₃ is closed, then the local motor is ahead of the sender's motor. If, instead, a pulse is detected while N₄ is closed, then the local motor is lagging the sender's motor. In other words, notches N₃ and N₄ are used to determine the direction of the required phase correction.

Fig. 18: Cam wheels indicate the rib position

There is a second pair of cam wheels on the same shaft, with notches N₁ and N₂. The associated contacts close for 10 msec (≈2½ pixel durations) per revolution. N₁ closes just after N₄ opens, whereas N₂ opens just before N₃ closes. When N₁ is closed, a capacitor (C₃ in the diagram below) is positively charged during pixel pulses that arrive during that closure. Conversely, that capacitor is negatively charged during pixel pulses that arrive during closure of N₂. The resulting capacitor voltage reflects the (vertical) symmetry of the received characters. When the voltage is sufficiently positive or negative ( = significant asymmetry), it energizes a bi-directional relay (G in the diagram below). This relay enables the "direction" contacts of the notches N₃ and N₄. The printer pulses that arrive during closure of N₃ and N₄ now energize a second bi-directional relay (F). Its contacts temporarily alter the speed set-point of the motor by plus 1.4% or minus 1.4% (via relays L ("langsam" = slow) and S ("schnell" = fast). The (filtered) control pulses are also shown on a lead/lag-indicating needle instrument in the front of the Blattschreiber. An equivalent method is proposed in Siemens-Halske patent 922538. Hell patented a method with unformly distributed sync-pixel pulses in 1950 in the USA (US patent 267465).

Fig. 19: Relays and cam wheel contacts determine the need for, and direction of the motor speed correction

(source: Fig. 293 in ref. 5B)

So much for synchronization at character level. A bit complicated, as it does not involve explicit sync-pulses or sync-characters sent by the Hell-sender. It does meet the stated objective of being able to use existing Hell-senders - without modification.

The Blattschreiber printer is dimensioned to print lines of 69 characters. Without additional measures, the 69^th character of a line would typically be printed partially at the end of one line, and partially at the beginning of the next line. One way around this, is to reduce the distance between the printer hammers by the width of one character (ref. lines 23-30 on p. 2 in Hell patent 848970 and 15-20 on p. 2 of 866052). The 69^th character will then always be printed in full at the beginning of the next line, even if it is only printed partially at the end of the preceding line. Two hammers are actuated simultaneously. Note that this trick is also applied by all software-implemented Hellschreibers - many decades after the Blattschreiber!

Clearly, partial characters and/or repeated characters do not look professional on a printed sheet, and are disturbing. This cannot be fixed without some form of synchronization at text-line level ("Zeilensynchronisierung"). To this end, a special "start/stop" character is used. It is simply a long pulse, with the duration of an entire Hell character. The leading edge of the start/stop pulse is used to start the motor. In the Blattschreiber, the trailing edge is used to engage the electromagnetic clutch that couples the belt with the printer hammers (and the platen), to the drive shaft. In the sender, the trailing edge is used to start transmission of a new text line. Upon receipt of the stop-pulse, the clutch is disengaged when a printer hammer is exactly at the far left edge of the next text line.

Fig. 20: Complete printer of the Siemens-Hell-Blattschreiber

(source: Fig. 291, p. 369 in ref. 5B; also Fig. 2 in ref. 3)

This line-sync approach only works if the sender asserts the start/stop pulses, and sends strings of no more than 69 characters. Existing Siemens-Halkse Hell punch-tape senders ("Lochstreifensender") did not have this feature. So a special line-control unit was developed: Zeilensteuergerät Stg 15b. It interacts with a Lochstreifensender, in order to ensure the correct maximum string length, and to insert the start/stop pulses. This control unit also provided remote on/off control of the receiving Blattschreiber, and can generate the Hell Pause-character. Ref. 2A.

The standard Siemens-Halske punch-tape reader/transmitter for use with Hellschreibers is the T.send.62a. It does not have an input for a external control unit. This control input was added in model T.send.62c. Whereas 62a has a built-in tone generator with three selectable tone-frequencies for the output pulses (900, 1000, 1500 Hz), model 62c only uses 1000 Hz.

As the control unit can generate a small number of characters, it has a motor and several notched character-disks. The motor of the T.send.62c punch-tape reader/sender and the control unit must be synchronized. Both are synchronized to a signal from a tuning-fork clock signal generator unit: Stimmgabel-Tonfrequenzgenerator Sg.61.a.The combination of T.send.62c, Stg.15b, and Sg.61a is referred to as Siemens-Halske Sendeanlage "Z" (ref. 2A).

Fig. 21: "Zeilensteuergerät" for the Blattschreiber

(source: Fig. 58 in ref. 2B)

Fig. 22: Print-out without line synchronization

(source: Fig. 6 in ref. 3)

Fig. 23: Print-out with line synchronization

(source: Fig. 7 in ref. 3)

Fig. 24: Print-out  with line synchronization of a Hell-cast from the German news agency DPA

(source: ref. 9)

The Blattschreiber is designed for 5 characters per second, i.e., a telegraphy speed of 50 WPM. This implies almost 14 sec for a full line of text (69 characters). Assuming a Hell font with 7 columns per character, there are 69 x 7 = 483 pixels per printed matrix line. According to ref. 12, the telegraphy speed of the Blattschreiber is 245 Bd, i.e., shortest pulse duration is 4.1 msec.

In his Swiss patent 378940 of 1964, Hell proposes to increase the printing speed to that of fax machines: about 3000 pixels per sec. A Blattschreiber row-line comprises nearly 500 pixels (69 characters per line, seven row-pixels per character). Each character font uses five printable pixel rows (not counting the blank two line-spacing rows). I.e., 5/6 sec per line instead of 13.8 sec: almost 12 times as fast (i.e., 60 cps). He proposed to do this with a fully electronic control unit (with magnetic-core buffer memory), for row-scanning entire lines of text.

Remote on/off control of the Blattschfeiber is very similar to that of the T empf 14 Presse-Hell printer: a constant tone of at least 1 sec turns the machine on, and a 6 sec tone-pulse turns it off.

VARIATIONS OF THE CONCEPT

Dr. Hell conceived a number of Blattschreiber printer implementations, with various technologies.

His patent 1157258 describes a two-step method, in which text is first printed column-by-column on an endless carrier tape with a conventional inked printer helix. The only difference is that text is printed in mirror image. The printed tape passes from left to right in front of the paper sheet. When an entire line of text is in front of the paper, a hammer bar (item 12 in the figure below) taps against the tape. This causes the entire line of text to be transferred onto the paper. The tape is wiped clean after printing.

Fig. 25: A line-printing Blattschreiber

(source: Fig. 1 in Hell's patent 1157258)

In an similar two-step method, text is first electrostatically "printed" column-by-column on an endless carrier tape, and then transferred to paper, where the carbon particles are heat-fused. The tape is wiped clean after printing. This technology is commonly used in mimeographs ("xerographic" copying machines).

Fig. 26: Line-printer, using xerographic technology

(source: Hell's patent 1178459)

Hell's 1953 US patent 2819941 proposes a single-thread multi-turn printer helix that is installed across the paper. The helix is inked with a long ink roller, or inked ribbon is used. An endless belt or chain is installed behind the paper. One or more hammer blades are attached to it. A hammer bar extends across the paper, and can actuate the blade that sweeps behind the paper. The hammer blade and the thread of the helix sweep across the paper at the same speed. As with the "real" Blattschreiber, this design prints a single line of row-pixels across the paper.

Fig. 27: Many-turn spindle across the paper

(source: Fig. 1 in Hell's US patent 2819941)

Hell's 1950 US patent 2656240 proposes a Blattschreiber with a column-printing mechanism. In this concept, there is a single printer hammer that scans across the paper and then makes a spring-assisted carriage-return. This is not essential, and the endless belt with three hammers would of course also work. The equivalent of a spinning helix is placed behind the paper. It consists of a stack of five thin bars (items 5-9 in the inset Fig. 3 below). This corresponds to five printable pixels per character column. The bars are installed across (and behind)the paper.  The bars are actuated in sequence, by two turning shaft that are placed at opposite ends of the bars (items 28 and 29). The shafts are are equipped with five cams (items 25). As the shafts turn in unison, first the lowest bar is pushed forward against the paper, then retracted while the bar immediately above it is extended, etc. Clearly this creates a continuous column-scanning action. The blade of the printer hammer has the width of the pixels that are to be printed. The height of the blade is that of a full character. When the hammer is actuated, it taps an inked ribbon against the paper, and against the extended bar.

Fig. 28: Blattschreiber with column-printing printer head

(source: Fig. 1-3 in Hell's US patent 2656240)

PATENTS

Below is a listing of patents from Rudolf Hell (and/or his employees) related to the Hell-Blattschreiber.

Patents related to Hell-system page-printers, but not from Rudolf Hell and/or his employees:

Patent office abbreviations:

• BD = Bundesrepublik Deutschland, Deutsches Patentamt [German Federal Patent Office]
• US = United States Patent Office
• CH = Swiss Patent Office (Schweizerische Eidgenossenschaft - Eidgenossisches Amt für geistiges Eigentum)
• DDR = Deutsche democratische Republik, Amt für Erfindungs- und Patentwesen

Patent source: DEPATISnet, the on-line public database of the Deutsches Patent- und Markenamt (DPMA, German Patent and Trademark Office).

The above Siemens-Hell Blattschreiber model is not to be confused with Blattschreiber fax machines that Hell developed some years later. That is: "BS"models such as BS109, BS110, BS112, BS116, BS133, BS134, BS1035, WF103, and WF104 (ref. 5B, 5C, 6, 7), and regular (non-Hell) Siemens-Halske page-teleprinters.

Hellfax-Blattschreiber model BS109

Here are some of Dr. Hell's patents related to Hell-fax printers/plotters:

REFERENCES

• Ref. 1: p. 10 in "Die Firmenneugründung der Firma Dr.-Ing. Rudolf Hell GmbH in Kiel 1947 - Interview zwischen Christian Sütel und Christian Onnasch, gehalten am 1. September 2001" [post-war re-start of the Hell company in Kiel], 12 pp.; source: Hell Verein | Kiel
• Ref. 2: "Telegrafentechnik", Band 6, Teil 6 of "Der Dienst bei der Deutschen Bundespost - Leitfaden für die Ausbildung", Fritz Schiweck (ed.), R. v. Decker's Verlag, G. Schenck, 1960, 970 pp.
• Ref. 2A: Table II, "Bauarten und Betriebswerte von Telegraphenapparaten für den Funkdienst"
• Ref. 2B: "Siemens-Hell-Blattschreiber", p. 67-68 in "Siemens-Hell-Geräte"
• Ref. 3: "Der Blattschreiber für das Siemens-Hell Verfahren", R. Zimmermann, Entwicklungsberichte der Siemens & Halske Aktiengesellschaft, Jg. 15, 25 May 1952, pp. 9-12 (also appeared in VDE-Fachberichte, Vol. 15, 1951)
• Ref. 4: pp. 40, 42, 48, 49, 51, 54, 56, 58, 64, 65, 76, 82, 84, 85-90, 94, 95, 98, 99, 102, 103, 110, 113-116, 118, 19, 122, 123, 128-130 in "News agencies. Their structure and operation.", UNESCO, 1953, 208 pp.; reprinted by Greenwood Press, 1969, 207 pp., ISBN: 0837125014
• Ref. 5: "Die Schreibenden Drucktelegraphen" [47 MB], Fritz Schiweck, section 9.5 (pp. 358-378) of "Fernschreibtechnik", Band 9 of “Lehrbücher der Feinwerktechnik“, 4^th ed., 1962, C. F. Winter, 894 pp.
• Ref. 5A: Section 9.5.2.2 (pp. 360-363)
• Ref. 5B: "Siemens-Hell-Blattschreiber", Section 9.5.2.6 (pp. 368-372) in "Fernschreibtechnik", Fritz Schiweck, Band 9 of "Lehrbücher der Feinwerktechnik", 4^th ed., 1962, C. F. Winter'sche Verlagsbuchhandlung, 894 pp. Note that the Blattschreiber does not appear in earlier editions of this book.
• Ref. 5C: pp. 388, 389 in "Faksimiletechnik", §10 (pp. 381-392) of "Fernschreibtechnik", Fritz Schiweck, Band 9 of "Lehrbücher der Feinwerktechnik", 4^th ed., 1962, C. F. Winter, 894 pp.
• Ref. 6: "Fernkopierer", Adalbert Kukan, in "Kultur & Technik", Nr. 2, 1988, pp. 112-115
• Ref. 7: "Anwendung von Faksimile-Hellschreibern für den Boden-Bord-Verkehr" [Hell fax, Zetfax], Rudolf Hell, pp. 67-75 in "Flugsicherungs-Verfahren und -Technik: Grundsätzliches über die Technik für Flugsicherung und Betrachtung der wichtigsten technischen Hilfsmittel außer Radar", Teil IIIB in "Flugnavigation und Flugsicherung", Band 7 of "Bücherei der Funkortung", proceedings of the "Internationale Jahrestagung des Ausschuß für Funkortung", Berlin; Verkehrs- und Wirtschafts-Verlag, 1958, 107 pp.
• Ref. 8: "Siemens-Hell-Technik", company brochure, Siemens & Halske AG, SH Nr. 725, 6 pp.
• Ref. 9: "BLATTSCHREIBER für das Siemens-Hell-Verfahren", company brochure, Siemens & Halske AG, Wernerwerk für Fernmeldetechnik, SH 1016a, 6513. F Fs, 2 pp.
• Ref. 10: "Press Communication Development" in "World Press News - The National Newspaper of the Press" of 16-Dec-1948; copy of item in file CO 875/40/1 "London Press Service: Hellschreiber machines; CO PR Dept., CFC (1948)" in the collection of The National Archives; material with UK Crown Copyright, used in accordance with the Open Government License [pdf].
• Ref. 11: "Letter of Instruction", memorandum from Chief of Foreign Broadcast Information Service (FBIS), 12 January 1956, 8 pp. Document nr. CIA-RDP83-00586R000300130017-9 (sanitized / declassified / released 7 August 2013). Accessed 25 March 2019.

External links last checked: January 2016