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
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.
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.
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.
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
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.
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
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.
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 . 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 :
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
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
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 .
Above an example of his Cyrillic (Russian) alphabet.
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.
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
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.
- Gretag AG, ETK-R-Fernschreibanlage Mod. 55
Wartungsvorschrift für den Gtm. Nr. 501-11.
Maintenance instructions (German).
- Walter Schmid, Der Krypto-Funk-Fernschreiber KFF-58
- Dr. Edgar Gretener AG, Internal memo to Mr. Frey (German)
Walter Schmid, Der Krypto-Funk-Fernschreiber KFF-58 .
October 2008. p. 55.
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