Cipher Machines


CHM logo

Burst Encoders

For clandestine operations behind enemy lines, sending and receiving radio messages can be dangerous. Not only is there the possibility of having a message intercepted and deciphered, but it can also alert the enemy to the location of the transmitter. There are numerous examples of this occurring during WW2, with deadly consequences for the spies. Burst encoders were developed to rapidly send short bursts of morse code signals so that the message would be hard to pick up and there would not be enough time to determine the location of the transmitter.

Generally, burst encoders use some type of storage medium such as holes punched in paper tape or photographic film, magnetic tape, mechanical storage, etc. to store a message and then have some way to send that stored information rapidly through a radio or telephone. The information is typically coded information so that a few characters could represent whole phrases or words of the message. Instead of taking the chance of sending the information by radio, another option was to leave the burst encoder or the storage medium from the burst encoder in a "drop" site.

West German RT3 Burst Encoder

Top drum stores 25 digits, brass drum "bursts" morse code

The RT3 (for Rapid Transmitter 3) Burst Encoder was developed by West Germany in 1958 and manufactured by Wandel & Goltermann for the Bundes Nachrichten Dienst (BND or West Germany's CIA). It is a small but heavy device with a case made of milled aluminum. There is a drum inside which stores a 25 character message and a second drum that is made up of 10 disks containing the morse code electrical contacts for 10 unique characters. The first drum is made of aluminum and the second is milled out of a single piece of brass. The RT3 is manufactured with typical, that is, precise German engineering.

There is a small crank built into the case which is used to turn the drums in order to store and then send a message. There is also a plug to attach an SP-15 or later an SP-20 spy radio. The RT3 and the radios are pocket-size devices, the SP-15 being one of the first all-transistorized radios. On top is a spring-loaded aluminum door which reveals one of the 25 slots of the storage drum. There is a plastic tab which slides along the slot to one of 10 positions; 1, 2, 3, 4, 5, 6, 7, 8, 9, 0 which represents the 10 characters W, F, V, R, 5, K, B, L, G, M. Note position 5 represents the number 5. These characters were carefully selected because they have about the same length morse code signal but are also different enough from each other to be distinguished in less than ideal transmissions.

1W. _ _
2F. . _ .
3V. . . _
4R. _ .
55. . . . .
6K_ . _
7B_ . . .
8L. _ . .
9G_ _ .
0M _ _

So the cipher is rather simple and hard-coded into the hardware. If the enemy captures one of these devices, he can quickly see the translation of the numbers to letters. The secrecy of the message is dependent on the integrity of the code books used to translate these numbers into the phrases and words they represent.

After setting the tabs to one of these 10 characters on all 25 slots, the spy would use the crank to turn the drum to slot 7, which is the only one red in color and represents the starting position for a transmission. After connecting the radio, he would turn the crank 13 times at a constant speed, which would complete one revolution of the storage drum. This also turns a brass drum which has the morse code signal for the 10 characters embedded twice around its circumference, to improve the chances for a successful transmission. The brass drum rotates 25 times for each revolution of the storage drum, transmitting 25 characters. In this method, 250 wpm can be sent.

There is a second location to insert the crank which is geared 3 times faster than the normal speed, allowing the spy to send messages at 750 wpm. This second location is covered with a black button which must be removed to access this faster geared location.

SP-15 Spy Briefcase
SP-15 Spy Radio Briefcase, including the burst encoder

This table shows the translation of the 10 tab positions with the letters they represent and the morse code for that letter. All the codes sent out using the RT3 would have used only these 10 specific characters, probably coded in 5 groups of 5 characters each, filling up the drum with the 25 character storage. Since the slot is numbered 1 through 0, the spy is most likely sending his message as a code in digital form. The fact these digits (except the 5) are translated into letters for transmission is not something he needs to consider, the translation of these characters back to numbers occurs on the receiving end. After sending this transmission, the spy may have to repeat the process several times to send a complete message. Since each letter is automatically sent twice, there are 50 morse code characters (or some 200 dots and dashes) being sent in just a couple of seconds, less than a second in the faster mode, so burst is a good description of the device.

The SP-15 radio set was developed in 1958 as one of the first fully transistorized radios, allowing it to have a small size and low power consumption. It was made for the BND (the West German CIA) and the full radio set included a receiver, transmitter, RT3 burst encoder, AC power supply, battery power, morse key and accessories all contained in a leather or canvas briefcase. The receiver was made by Wandel & Goltermann, the manufacturer of the RT3. The transmitter was made by Pfitzner.

The transmitter has a maximum power output switchable from 10W to 20W. The RT3 burst encoder or the morse key could be attached to the transmitter for sending morse code. In fact, the transmitter was only suitable for sending CW (morse code).

Russian Burst Encoder

This Russian burst encoder was first produced at the height of the cold war in the early 1960s. It was part of the spy radio set R-353 and allowed the spy to send a pre-recorded message very quickly. It was a very advanced device for its time and was one of the first uses of magnetic tape, in this case using stainless steel tape.

The burst encoder is a two-part device, one part has a telephone style dialer and the other part is a metal tape cassette used as the recording medium. The dialer is engaged by using a stylus which is stored inside the lid of the dialer. As the stylus turns the dial, magnets are spun past the metal tape, encoding it without the need for electricity. At the end of the dial radius, the stylus then advances the tape so it is ready to encode the next digit.

Russian Burst Encoder
Magnetic storage cartridge on top of keyer
R-353 Spy Radio
R-353 Russian spy radio with magnetic storage cartridge attached to front

Note that only digits are recorded, which means that the message has been written in code. This way, the small cassette could be left in a "drop site" without sending the message over the radio. Even if the cassette is compromised, the coded message would still be secure. To send the burst message by radio, the cassette is attached directly to the front panel of the R-353 spy radio. The cassette has a built-in crank to spin the tape, sending the pre-recorded digital message very quickly. This tape is capable of holding a much larger message than the RT-3 burst encoder above.

US GRA-71 Burst Encoder

The AN/GRA-71 burst encoder is a US made device to encrypt and burst morse code messages using magnetic tape as the medium to store the message. It was originally made for the RS-1 spy radio or equivalent Army AN/GRC-109 radio but was also used with the SP-20, PRC-64, PRC-74 or other clandestine or manpack radios. This was first deployed in 1961 and used extensively in the Vietnam war and up to the early 1980s.

Messages are recorded on magnetic tapes using one of the two supplied coders, much like using a domestic tape recorder. One coder has a disc with the 26 letters of the alphabet. It allows letters to be recorded directly in morse code. Alternatively, the dash-dot coder could be used, to allow any morse character to be recorded (e.g. numbers and punctuation marks). Once the recording was complete, the keyer was connected to the transmitter and was used to play back the message at high speed (approx. 300 WPM). The complete kit consists of the following items, which are further described below: MA-9, Recording tape magazine [2x] MX-4495, Tape coder (dot-dash) MX-4496, Tape coder (wheel) Alternative index wheel for MX-4496 MX-4498, Keyer Adapter (fixed) KY-468, Keyer Soft cleaning brush

UK Racal MA-4450 Burst Encoder

The MA-4450 is called a Tactical Data-Entry Device (TDED) and is made by Racal in Salisbury, UK. It was used by NATO Special Forces to encrypt messages and send them in burst mode when connected to a manpack radio. This more recent example of a burst encoder was used in the 1980s through the 1990s, using solid state memory to store up to 2000 characters for transmit messages.

The MA-4450 has a full QWERTY keyboard with 10 digits, a spacebar and 6 special function keys, marked Enter, Send, Read, Light, Print, Store and Off. There is a 32 character LCD display and 4 connectors on the rear of the device for attaching the headset, radio, printer and remote. The MA-4450 uses 24V power, which is supplied via the printer or remote port.

Up to six transmit messages can be stored, each message up to 475 characters for a maximum of 2000 characters total. Up to 16 received messages can also be stored. These messages can then be read out on the 32 character LCD display or printed. Four station addresses can also be stored.

In addition to being a burst encoder, the MA-4450 encrypts the messages and is able to store four 16 bit alphanumeric cipher keys for that purpose.

See the Entire Collection of Cipher Machines

Burst Encoders
See detailed pictures of the German, Russian, US and UK burst encoders.

The German RT3, or Rapid Transmitter 3, was used by the West German spies with the SP15 and later the SP20 transmitters. These were used during the cold war by spies behind the Iron Curtain. The detailed pictures will show an RT-3 with serial number 1000 and the precision engineering inside this device.

The Russian Burst Encoder was first introduced in the 1960s during the height of the Cold War. It was one of the first uses of magnetic tape, in this case using permanent magnets to rotate past the tape to encode the signal. There was no electrical or battery connection needed. Note this device has no markings to identify it or its country of origin.

The GRA-71 is a clandestine cipher machine and burst encoder set designed to be used with the CIA RS-1 radio set or the equivalent US Army GRC-109. It was also later used with the PRC-77 radios. The GRA-71 was first manufactured by Stenographic Machines Inc. in 1961, which was the company that developed this device, later GRA-71s were manufactured by Arvin Industries. The pictured machine was made in 1963 and has serial number 645.

The Racal MA-4450 Burst Encoder was used in the 1970s by the clandestine operators from the UK and other NATO countries.