Single tone combination teleprinter („Einton - Kombinationsschreiber“) / ETK; developed by Dr. Edgar Gretener AG, Zurich.
After World War II, Dr. E. Gretener developed the paper tape teleprinter „Einton - Kombinationsschreiber“ (ETK) on his own initiative. The transmitting and printing method is based on a unique 14 element code specially developed for this set. In contrast to the Baudot code CCITT No. 2, in which even the failure of one single character (for example, in a disturbed radio transmission) leads to mutilation of the rest of the message, the text in 14 segment code can usually still be deciphered, even in case of transmission-related failure of a segment.
The ETK was initially used on wired teletype connections, only the improved version ETK-R (Model 55) with improved filters can make use of wireless transmission due to better transmission quality possible.
The communications system was enhanced with the Telekrypto 53 cipher machine for real-time encryption. It turned out, that quite often, due to propagation disturbance or interference on the radio transmission frequency, reception was interrupted and even one single bit or character caused the ciphering machines at the transmitter and receiver site went out of synchronisation - the rest of the radio message could not be decoded correctly and the teleprinter printed messages in „Chinese“…
It was only the successor, the KFF58 („Krypto-Funkfernschreiber 58“) which came with automatic synchronisation, which could cope with transmission interruptions - after this set had been introduced, the Swiss Army really got one of the world's best wireless teleprinter systems with real-time encryption.
In research of a replacement for the mechanically complex teletypewriter „Schreibtelegraf 34“, Dr. Ing. E. Gretener constructed a paper tape teleprinter, which could set new standards in Switzerland by its ingeniously simple construction and low total weight, in the late forties.
Unlike the Teletypewriter, the ETK used a specially developed combination alphabet: All letters are composed of 14 character elements, which are printed by character revolver on a paper tape. The fact, that in the event of a segment failure, mutilated characters are still recognizable - which gives the system better fault redundancy than teletypewriters which use the Baudot CCITT No.2 code.
In the Single tone combination teleprinter ETK, the electronics with the line amplifier are housed in the transport case, the actual teleprinter with the keyboard and the paper tape printing mechanism is pulled out of the case for operation.
For each character, a sequency consisting of a start pulse (20 msec.), fourteen pulses (0 or 1, i.e. active current or no current) and a stop pulse (30 msec.), is transmitted each time a character key is pressed on the keyboard. For each character, the control shaft does a 360° turn. The „impulse program“ for one character thus has a duration of 200 msec.; so in one second, five characters can be transmitted.
The collector distributor, on which the coding for the segments of a character is defined, and the printing revolver, on which the fourteen printing segments are present like on a daisy wheel, are mounted on the same axis, the printer control shaft - thus they all rotate synchronously.
In transmit mode, the start pulse is triggered by pressing any key, the start magnet is de-energized, the control shaft begins to turn. The collector brush sweeps over the contacts of the collector distributor, so the tone signal with the fourteen pulses for the transmission of the character is generated.
In receive mode, the incoming start pulse triggers the turning of the control shaft. In the following phase of the fourteen segment steps, the type revolver is rotated around it's axis and the characters are printed.
In the presence of signal current, the printing of the corresponding segment is suppressed; during the de-energized state during the period of the corresponding character step, the segment is printed on the paper tape by the printing unit, i.e. the printing elements during the current gaps of an impulse program are printed. The paper feed is done during the start-stop step.
To operate the teleprinter, the amplifier in the case is connected to the AF output of the receiver and to the microphone input of the transmitter. A multipole cable connects the amplifier / modem to the teleprinter.
To feed the paper, the lid must be opened and the paper tape inserted according to the instructions. It is important that the felt roll in the printing unit is soaked with the recommended amount of ink.
On wired lines, the ETK teleprinter runs in single tone mode, i.e. during one impulse program, the „de-energized“ state stands for state 0 and a 1500 Hz signal for the state 1.
After experiments with wireless operation with the ETK 47/50 it turned that, especially during the „powerless“ pauses between the character steps with 1500 Hz tonemodulation, the noise level rises and atmospheric disturbances - that are virtually always present in AM reception - do impair reception and readability.
So for wireless operation, dual-tone mode has been implemented in the ETK-R, in this set „0“ and „1“ state are no longer „de-energized“ resp. 1500 Hz, but two different tones: the „space“ frequency of 1255 Hz (corresponding to the „0“ state) and the „mark“ frequency of 1145 Hz (corresponding to the „1“ state). So continuously a sound frequency is transmitted, the program content resp. the character steps are frequency modulated, so the system is less sensitive to propagation and atmospheric disturbances.
As a speciality of the set, in the ETK-R, the first harmonics (2400 Hz ± 110 Hz instead of 1200 Hz ± 55 Hz) are filtered, which improves decoding during fading („Funk Fading“ („Wireless Fading“)) mode active).
The ETK-R teleprinter can also be set from dual-tone to the more troublesome single-tone mode operating on 1500 Hz, if the set is communicating with an ETK-47/50 (for example in wired operation).
To generate the punched paper type, the teletypewriter Siemens T. typ. 68 can be used, so that at the same time, the text can also be printed in plain text.
To replace the Schreibtelegraph 34, the olt teletypwriter came in a total of eight crates in addition to a petro generator, during the the last years of World War, Dr. Ing. E. Gretener developed the „single tone combination teleprinter“ at his own risk.
Unlike standard teletypewriters, the ETK based on on a unique combination alphabet, all letters are composed of 14 character elements, which are printed by a daisy wheel on paper tape. The fact, that even mutilated characters, can still be recognized, this gives a better error redundancy compared to the teleprinters working with the Baudot code CCITT No.2.
A first badge of 50 single-tone combination teleprinters ETK Model 47 for wired use was delivered in 1947, after improvements to the keyboard, the more reliable model 50 was created.
Since the lack of synchronisation caused problems, when wireless operation was used, an improved variant of the ETK was ordered by the „War Department“. In a modem, a frequency-shift AF subcarrier was generated to directly control a grid-modulated transmitter in telephony mode, so to avoid necessity to conversion of the transmitter to frequency shift keying (FSK).