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Gretag ETK-47
14-bit teleprinter system

ETK-47 was a teleprinter system developed by Dr. Edgar Gretener (later: Gretag AG) in Zürich (Switzerland) in 1947. Unlike other teleprinter systems, that are based on the common Baudot standard, the ETK uses a novel technique to build the image of the characters from a series of individual segments or elements (combination writer). It was developed by Kurt Ehrat, who had joined the company a year earlier, and was based on an original idea of Edgar Gretener himself.
 
The machine uses a 14-bit digital signal that can be carried over telephone lines or radio links by means of a single 1500 Hz tone, and is known as Einton-Kombinationsschreiber (ETK): single-tone combination writer. 1 The image on the right shows a typical ETK-47 Fernschreiber as it was built by Gretener during the 1950s.

It consists of a keyboard with 50 keys, three of which are not used. The top row of keys is used for the numbers whilst the rest is for letters and puctuation marks. A special key at the left, marked KZ (Kennzeichen), could be programmed with a unique station identification character.
  
ETK-47 14-bit teleprinter

Gretener and Ehrat thought that there was a market for a small light-weight teleprinter that could compete with the existing 5-bit teleprinters. As the ETK used a single 1500 Hz tone for transmission of the data, the ETK could be used over a standard telephone line, whilst 5-bit machines required a special telex network. Gretag even developed a special modem for the ETK.

The Swiss Army soon became interested in the lightweight and robust ETK and started ordering it in quantities. The fact that is did not comply with the existing 5-bit standard was no problem for the Army. Gretag later developed additional devices to convert the 14-bit code to 5-bit Baudot and vice versa. A few years later, in 1953, Gretag introduced a high-end automatic online cipher machine, the TC-53, that allowed the ETK-47 to be used for secure communication as well.
 
  1. Although ETK is commonly used as the abbreviation of Einton-Kombinationsschreiber (single-tone combination writer), some literature (including Gretag documentation) sometimes describes it as Eintonkleinschreiber (single-tone small writer) and even as Einton Telegrafie Kombinationsschreiber (single-tone telegraphy combination writer). In either case, the abbreviation is identical: ETK.
ETK-47 14-bit teleprinter ETK-47 half-way out of the storage case of the ETK-R ETK-R amplifer in case without the ETK-47 Control panel of the ETK-R Close-up of the keyboard of the ETK-47 Opening the top cover Paper compartment Rear view of the ETK-47

 
Complete setup
Although it was possible to use the ETK-47 as a stand-alone teleprinter, it was often used as part of an encryption/decryption system with the Gretag TC-53 at its heart. Furthermore, additional interfaces and or peripheral devices might be needed to connect the ETK-47 to a telephone line or to transmit over radio links. A complete installation consists of the following items:
 
  • ETK-47 Fernschreiber (teleprinter)
  • ETK Amplifier
  • TC-53 Telekryptogerät (cipher machine)
  • Army Field Telephone
Versions
The ETK-47 was introduced in 1947 but remained in production until the late 1960s. The initial version had a die-cast keyboard with 47 keys (plus room for a 48th key at the bottom left). The top cover fitted only the rear part of the machine, whilst the keyboard was a separate unit.

The keyboard was later made separately, so that it could be adapted for different languages and different customer requirements. The top cover of that version fitted over the entire machine and had a large rectangular cut-out at the front, through which the keyboard protruded. The keyboard of the later version had 50 keys, with room for another three. This version was approx. 1 cm longer, but fitted the same case as the early ones.
 
Development
The ETK was developed by Gretag for several reasons. The existing teleprinters of the era were relatively large and heavy, which was mainly caused by their complex printing mechanism that had to move over the paper horizontally. Furthermore, the 5-bit code used by the CCITT-2 standard (ITA-2 or Baudot) was prone to errors. As the characters are assigned randomly to the available codes, a single bit change causes a completely different character to be printed.

On the ETK, the characters are created from 14 basic elements, each of which is assigned to a single bit. This allows the image of the character to be recreated at the receiving end. In most cases, simple bit errors only have a limited effect on the readabily of the text. More importantly however, it simplifies the construction of the printer. In addition, the ETK prints its text on a paper strip rather than on a sheet, which further reduces the size and weight of the printer.
 
14-element code
All characters printed by the ETK are built from the 14 basic elements that are present on a rotating print head. The design of the characters can be compared to the creating of numbers on a 7-segment display. The initial design of the characters was made by Edgar Gretener and was modified several times before the machine was taken into production. Furthermore, the typeface was slanted somewhat (italic) in order to improve the readability, resulting in these segments:


Nearly all of these segments can be combined in a variety of manners in order to create virtually any printable letter, number and even some punctuation marks. The dot (element 13) was added as a full stop and for the construction of the question mark. The last segment (element 14) breaks with this rule, as it prints the rounded number 8 and can not be used for anything else.


The presence of the separate '8' was for historical reasons [2]. For the development of the ETK existing 5-bit components are used. As a result, the characters have to be built with 5 segments or less. Creating the '8' with the initial character set (not the one shown above) involved the use of 6 elements which was not possible. A dedicated '8' was then added. Other designs for the '8' were considered but were not estethically approved, although the rightmost design above was later used on a civil ETK variant, whilst the one in the middle was used on the later KFF-58.


The table above shows the complete character set of the ETK-47. Note that the numbers are clearly different from the letters (even the 0 and the 1) so that they are easily distinguised. The zero has a vertical line through it: Ø. A space is represented by setting all 14 bits to '0'. Please note that the character set, and also the assignment of the individual elements, is different from the later KFF-58, making the two machines incompatible. The timing is as follows [2]:


Each character takes 200 ms, including the start and stop bits. This is equivalent to 5 characters per second (CPS), only a little bit slower than an 50 baud 5-bit teleprinter which does 6.66 CPS. The advantage of the ETK however, is that it never has to switch between letters and figures, which makes up for the loss in speed. If a text contains many numbers and punctuation marks, the ETK might even be faster than a standard 5-bit teleprinter which inserts shift-characters.

Example of a Cyrillic character set (Russian) generated on an ETK teleprinter

Boris Hagelin, who would later become one of the main competitors of Gretener, was also interested in the ETK teleprinter. As it doesn't use a fixed character set, it was ideally suited for his foreign customers and could easily be adapted for different languages, such as Russian. In March 1947, Gretag's development engineer Oskar Stürzinger, who would later become Hagelin's first Swiss employee, informs him that he has successfully tried to use the ETK printing elements for Cyrillic and Greek characters [3]. Above and example of his Cyrillic (Russian) alphabet.
 
Cipher machine
In 1953, Gretag introduced the TC-53, a high-end cipher machine that could be inserted between the ETK-47 and the amplifier. It used an electro-mechanical scrambler to encrypt and decrypt messages in 14-bit ETK format.

The image on the right shows a typical Gretag TC-53 cipher machine in its black transport case. When not in use, the front was covered by a lid that also carried the cables, accessories and spare parts. The TC-53 has 12 wheels or rotors, four of which are used for the actual scrambler, whilst the remaining eight control the stepping of the scrambler mechanism.

The Swiss Army used many ETK-47 and TC-53 machines in the field, often combined with the SE-222 transceiver from Zellweger.

 More information
  
Gretag TC-53 cipher machine with Key Generator

 
Interior
The ETK-47 measures only 34 x 38 x 14 cm and was a truely compact device for its time. The interior can easily be accessed by removing two bolts from the sides and two from the rear of the machine. After lifting the black wrinkle-paint cover, the beautiful mechanism is exposed.
 
All functions of the electro-mechanical device are controlled by a main axle that is located at the left side of the machine. A horizontally placed motor is located just behind the keyboard and drives the main axle from the front. Towards the rear end of the axle is the print head.

The print head rotates with a continuous speed and is inked by a ink-roller at the back. The paper strip moves past the lower half of the print head, but is halted when the character is being printed. After all 14 segments have passed by the paper strip is advanced by one position.
  
Main axle of the ETK-47

 
Interior of the ETK-47 Main axle of the ETK-47 Electrical contacts at the keyboard Print head Notched rings controlling the timing of the ETK-47 Print head seen from the front

 
References
  1. Gretag AG, ETK-R-Fernschreibanlage Mod. 55
    Wartungsvorschrift für den Gtm. Nr. 501-11. Maintenance instructions (German).
    Date unknown.

  2. Walter Schmid, Der Krypto-Funk-Fernschreiber KFF-58
    October 2008.

  3. Dr. Edgar Gretener AG, Internal memo to Mr. Frey (German)
    Walter Schmid, Der Krypto-Funk-Fernschreiber KFF-58 [2].
    October 2008. p. 55.

Further information

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