Spy radio
Burst encoders
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Proton   R-353
Burst encoder

Proton was a sophisticated electromechanical device for recording and transmitting pre-coded messages in morse code at very high speed (typically 250 WPM) in order to avoid interception and radio direction finding (RDF). Such devices are commonly known as burst encoders. Messages are stored onto magnetic tape by means of a recording device with a rotary dial, similar to the dial of an old telephone set, without the need for electricity. It is only used on the R-353 spy radio set, which has a built-in automatic keyer on which the the supplied tape cassettes are played back.
The encoder is housed in a metal hamerite-painted enclosure that measures 10 x 8 x 4 cm. It has a black dial at the top, much like the dial of an old telephone set, with the number 0-9, the letter P and the morse correction sign (···). At the right hand side of the dial is a metal bracket that marks the end-of-dial, known as the stop. The holes in the dial are too small for a finger.

Most encoders had their serial number written or engraved in the top right corner. The image on the right shows an R-353 burst encoder in near-mint condition with written serial number 160.
R-353 burst encoder

At the rear of the encoder is a metal panel that can be removed. The panel gives access to the cassette bay and exposes the interior of the encoder. Inside the metal panel is a short stylus, or pen, with a metal tip at one end. It is held in place by two metal clips and can be removed easily.
With each burst encoder, one or more cassettes were supplied. Each cassette measures approx. 9.5 x 4.5 x 2.5 cm and contains two spools with magnetic tape on them. The tape is supplied by the leftmost reel and picked up by the one on the right. So, tape direction is from left to right.

Inside the cassette is a belt-driven friction coupling that ensures a contant tape-tension and prevents the tape from rolling out of the cassette. The tape is 6.25 mm wide and each cassette contains approx. 10,6 metres, enough for storing 300 words (300 groups of 5 digits).
Magnetic tape cassette

Although the transmitter can be used for an uninterrupted period of 1.5 hours, it is advised to keep the messages as short as possible in order to minimise the risk of interception and radio direction finding. The tapes can be wound forward or reverse by means of the two small fold-out cranks on top of the reels. It is also possible to rewind the tape on the transmitter (R-button).
A collection of encoders and cassettes in different packaging R-353 burst encoder Magnetic tape cassette Lid removed, pen stored inside the lid Cassette attached to the encoder Encoder and cassette, ready to be joined Manually winding the tape Dialling the number '3'

Coding a message
Coding a message is a quite laborious task. First of all, a textual message has to be converted to some kind of numbering scheme of which there were many around. Next the numerical message had to be encrypted with some cipher, so that an eavesdropper would not be able to read it.
Although various manual encryption methods were used by Eastern Block spies and agents, such as matrix transpositions and codebooks, the most common one (also the most feared one) was the One-Time pad (OTP) cipher. When used correctly, this cipher is unbreakable.

The OTP was a small booklet with very thin pages, each of which contained a sequence of random numbers. Only two copies of the OTP existed: one in the hands of the agent and one at the receiving end. Each page was used only once and was destroyed immediately after use.
Close-up of an OTP

The message is encrypted by adding each of the digits on the OTP to one character of the plain­text. If the OTP consists of truely random numbers, the result bears no relationship to human-produced text and will defeat any frequency analysis. At the receiving end, all that has to be done is subtract the numbers again to reveal the original message. One of the disadvantages of the OTP is the problem of key-distribution: both parties need to have sufficient supply of OTP sheets.

 More information about the OTP
Recording a message
Recording a message is pretty straightforward. For now we assume that the textual message has been converted to a numeric one, possibly encrypted with the unbreakable One-Time Pad (OTP) cipher. The message is typically written down in groups of 5 digits (0-9), separated by spaces. Before starting a recording, ensure that the tape has been fully rewinded either manually, using the fold-out cranks on top of the cassette, or by means of the keyer of the R-353 (see below).
First take away the rear panel of the encoder and remove the brown pen that is stored inside that panel. Now attach the cassette by 'hinging' it onto the metal axle inside the encoder and firmly locking it onto the body of the encoder.

The cassette is now attached to the encoder and we can start the recording of a message. Take the first digit of the message, for example: '3'. Place the metal tip of the brown pen in the corresponding hole of the dial (3), press it fully down and then use it to rotate the dial towards the stop lever. You will feel some resistance.
Cassette attached to the encoder

In the last 2 cm before reaching the stop, the tape will be transported. Once you have reached the stop, do not release the pen but use it to rotate the dial a little bit further clockwise. This will move the stop down a little bit and you will probably hear a 'click'. This is the moment that the data is actually written on the tape. Repeat this procedure for every digit in the message. Use P to insert spaces after each 5th digit (Pause) and use (···) if you have made a mistake.
Encoder and cassette, ready to be joined Opening the lid Lid removed, pen stored inside the lid Attaching the cassette to the encoder Locking the cassette in place Cassette attached to the encoder Dialling the number '3' Forcing the 'stop' in order to write the digit to the tape

Sending a message
Once the pre-coded message is recorded onto tape, using the encoder as described above, the message is ready for transmission. The cassette has to be removed from the encoder and must be placed on the R-353 transmitter. Removing the tape from the encoder may need some force.
Note that the cassette has to be wound back to the initial position before the message can be transmitted. This can be done manually, using the small cranks on top of the tape reels, or automatically by using the rewind feature of the high-speed keyer of the R-353 (recommended).

Install the cassette on the keyer by 'hinging' it onto the steel axle at the bottom of the keyer and locking it in at the top. A 'click' will tell you that the cassette is installed properly. You may now rewind the tape by pressing the R-button (i.e. the leftmost button just below the cassette).
Message cassette attached to the high-speed keyed of the R-353

Now follow the normal procedure for pre-heating the transceiver, calibrating the frequency scale, selecting the desired frequency and tuning the transmitter's PA and antenna sections. The button 250 (or 500 on some models) will pre-heat the transmitter. Once this is done and all circuits are stable (after approx. 15 minutes), the burst transmission is started by pressing the S-button.
The transmitter now sends an identification signal to the base station. If all goes well, the base station returns an answer signal in the form of a series of dots (······) to acknowledge receipt.

The acknowledge signal will automatically start the high-speed keyer and the burst transmission will commence. Depending on the length of the coded message this may take several seconds.

The image on the right shows the R-355 that was used as the main controller in the USSR (and other Warsaw pact countries) base stations.
R-355 spy radio base station controller

Note that you have to rewind the cassette to beginning of the tape, before it can be used again. The initial version of the keyer on the R-353 was able to send data at 250 WPM (groups or words of 5 digits per minute), indicated by the number 250 on the rightmost button of the keyer. The speed was increased to 500 WPM on later models, reflected by the number 500 on that button.

 More about the R-355 base station
Manually winding the tape Burst encoder tape cassette Placing the tape cassette on the burst transmitter Locking the tape cassette in place Tape cassette attached to the high-speed keyer Message cassette attached to the high-speed keyed of the R-353 Front panel of the R-353 spy radio set, with the cassette installed in the high-speed keyed. R-353 with cassette and burst encoder

Receiving messages
Although the R-353 has a proper built-in receiver, it was typically not used for the reception of messages (i.e. instructions) from the base station. This is because the transceiver was commonly held by the radio operator and not by the agent. Furthermore, the R-353 was usually hidden in a safe place and would only be retrieved in case of an emergency. As everything in espionage is on a need to know basis, the radio operator does not have to be aware of the agent's instructions.


Instead, the agent was often given money to buy a commercial short wave world receiver from a local electronics store, such as the Sony ICF-2001D and the Zenith 1000-D, on which he listened to spoken messages from the so-called Numbers Stations. These messages contained personal instructions for the agent and were commonly encrypted with the unbreakable OTP cipher.

 More about Numbers Stations

Dead drop
The R-353, and hence its burst encoder, were used for a variety of purposes, including Special Forces (SF), diplomatic traffic and espionage. When used for espionage, for example by a Soviet agent operating undercover in Western Europe, intelligence and radio were usually separated.
This means that the agent did not necessarily have to transmit the coded messages himself. Instead he would encrypt it, using an OTP, and record it onto magnetic tape by means of his burst encoder. The cassette was then hidden in a predetermined secret place, known as a dead letterbox, where he picked up a fresh new tape cassette. This process is known as a dead drop.

After signalling the radio operator, by issuing a previously agreed signal or marking in a public place, the latter would visit the dead letterbox, pick up the cassette and leave a blank new one.
Example of a dead drop. An agent leaves a tape cassette with an encrypted message and picks up a blank one.

He would then transmit the message to the base station at a predetermined time, after which he issued a different signal to let the agent know that this his message had been sent. The main advantage of this way of working is that the agent is not exposed when the radio operator is captured by means of surveillance, betrayal or radio direction finding (and vice versa of course).
R-353 transceivers and their burst encoders and OTP booklets have been captured during the Cold War by Western intelligence agencies on a number of occassions. One documented example is the capture of a Dutch man, who acted as an East-German agent in The Netherlands, in 1969. When he was captured, the Dutch intelligence agency BVD (now: AIVD) found a fully operational R-353 in his home, along with a burst encoder, cassettes and a partly used OTP booklet [1][2].

Complete R-353 transceiver with burst encoder and OTP

Tape cassettes
When the burst encoder and/or the cassette was not used it was normally stowed in some kind of packaging to protect it agains dust, moist and other potentially hazardous influences. Two types are known to exist: (1) a wallet-type bag with a zipper and (2) a 'raincoat' with elastic edges.
For use in an urban environment, the zipper bags were probably the most practical way of storing the devices. However, when the system was used in the field, e.g. by Special Forces (SF), the raincoat-type packaging was recommended as it offered better protection against moist.

When used by SF, 1 encoder and 2 cassettes, all in raincoats, were stowed in a special pocket at the side of the canvas R-353 carrying bag. When used for real espionage, cassettes would also be carried around in their raincoats, especially when left behind in a dead letterbox (or: dead drop).

Note that all cassettes came with a rectangular plastic cap that should be placed over the open end, so that the sensitive magnetic tape is properly protected. Once this plastic cap is in place, like on the cassette in the image above, the cassette can safely be transported or stowed away.
Encoder in zipper bag Cassette in zipper bag Removing the protective plastic cap from the cassette Two cassettes: one in a zipper bag and one in a raincoat A collection of encoders and cassettes in different packaging Taking the encoder from its 'raincoat' Taking the cassette from the zipper bag

Like most Russian devices of the Cold War era, the R-353 burst encoder and its tape cassettes are relatively simple yet ingenious systems, that do not require any external energy source, apart from human labour. They are very robust, service friendly and require only basic maintenance.
The cassette consists of two hinged metal shells inside which the two tape spools and some tape guides are housed. The cassette can be opened by releasing a single screw at the bottom. This reveals the mechanism as shown on the right. A drawing in the lid shows how the tape is guided.

The supply reel is at the left and the tape is led via seven tape guides onto the pickup reel at the right. At the centre is a piece of felt (here visible in red) that pushes the tape against the coded heads of the encoder. During playback this felt pushes the tape against the head of the keyer.
Inside the tape cassette

A the right hand side is the driving mechanism. The tape passes two metal guides at the front right of the cassette with a spring-loaded rubber pinch roller behind them. The cassette body has a circular cut-out at this position. When the cassette is placed in the encoder, a rubber wheel engages the tape here. It moves the tape to the right and in doing so it pulls the tape from the supply reel. On the transmitter, the rubber wheel of the keyer drives the tape in the same way.
The two tape reels are coupled by means of two rubber belts, each of which drives the friction coupling of the other one. This allows the tape to be transported in both directions simply by driving the tape either way with a rubber wheel.

The mechanism of the encoder is much more complex, but is nevertheless equally ingenious. The image on the right shows the interior of the encoder after removing the two case shells. At the heart of the device is a large brown bakelite disc that is directly driven by the black number dial, which itself is driven by the brown stylus.
Burst encoder interior

When entering numbers via the dial, the last two centimetres of the circular movement before the stop, sets a rubber drive wheel in motion. This rubber drive wheel is just visible at the rear right corner and advances the magnetic tape inside the cassette by by the actual width of the morse character (max. 9 mm). This means, for example, that 5 (····и) takes less space than 1 (·----). A small magnet inside the encoder, clears any previous recording by fully magnetising the tape.
When the styles reaches the stop, the dial should not be released, but the stylus should instead be pressed further towards the stop, so that the shiny metal stub (i.e. the stop) rotates its axle.

In doing so, the stop lever moves a permanent magnet against the inside of the large brown bakelite disc. This magnetizes one of the 12 rectangular metal coded heads that are present at the circumference of the disc. This action effectively writes the chosen digit onto the tape as a series of pulses divided over two tracks: one for the ON signal and the other one for OFF.
Close-up of one of the coded heads

The image above shows a close-up of one of the coded heads. Click the image to take an even closer look. It shows that each track of the coded head is made up of several vertical layers of magnetically conductive metal. At the junction of two such layers, a tiny gap causes magnetic resistance and, hence, a small disruption of the magnetic flux, which is copied onto the tape.

Writing a morse digit onto the tape

Magnetic tape cassette Opening the tape cassette Top view of the cassette interior Inside the tape cassette Inside the tape cassette Inside the tape cassette Moving the tape manually Close-up of the pinch roller section
Upper case shell removed from the encoder Burst encoder interior Burst encoder interior seen from the rear Gear mechanism at the bottom of the encoder Close-up of the gears Close-up of the drive wheel Close-up of one of the coded heads Side view of the bakelite disc, showing a coded head and the permanent magnet.

The two tape tracks are separated by a horizontal metal layer at the centre. When playing back the tape on the high-speed keyer of the R-353, the small magnetic disruptions cause small electrical pulses which, after amplification, are used to drive a flip/flop. The pulses from TRACK 1 are used to set the flip/flop, whilst the pulses from TRACK 2 will reset it. This way, the morse code representation of the original character is reconstructed. This is further illustrated below.

Enlargement of coded head '4'

The drawing above shows an enlargement of the coded head that represents the morse character '4'. All metal parts are magnetized when the permanent magnet inside the encoder is moved forward, but the junctions between the layers will cause small disruptions in the magnetic field that magnetizes the tape. In the above diagram, the upper two lines shows the pulses that are read by the keyer from track 1 and 2 respectively. The lower line shows the reconstructed digit.
Morse code
Although the burst encoder for the R-353 stores its infomation as real morse code characters, there are some limitations. Due to the width of the coded heads, the maximum length of a morse character is limited to 9 mm. The table below shows how the characters on the dial are translated into morse code. It also shows how much space of the magnetic tape each character occupies.
  Morse Width Remark
1 ·---- 9 mm  
2 ·и--- 8 mm  
3 ·ии-- 7 mm  
4 ·иии- 6 mm  
5 ·ииии 5 mm  
6 -·иии 6 mm  
7 --·ии 7 mm  
8 ---·и 8 mm  
9 ----· 9 mm  
·ии ··  ии 5 mm Repetition sign (ii) is used for Correction
0 - 4 mm Actual code (-----) doesn't fit in 9 mm, so 'T' is used instead
P   3 mm No sign, just a pause

 More about morse code

Repairing a cassette
In order to repair a broken cassette, spare magnetic tape was supplied in the so-called ZIP box that came with the R-353 radio. The ZIP box contains spare parts, tools and other materials for repairing the radio and its accessories in the field. It contains approx. 100 metres of spare tape.
Broken tapes were never repaired or glued back together; they were simply replaced by a fresh new contiguous piece of recording tape. Two types of tape have been found so far. The most common one is the shiny chromium tape shown here, but over time some cassettes were loaded with the more common brown ferro-based tape.

In addition to the spare tape, the ZIP box also contained a piece of suede, used for the friction couplings, and a couple of spare rubber belts.

 More about the ZIP box
Spare recording tape

Like the R-353 radio itself, both the encoder and the cassettes came in two different colours: grey hamerite and green/blue hamerite. The purpose of these colours is currently unknown, but might have been related to the actual organisation or agency that used the R-353 spy radio sets.

In practice however, these colours were often mixed, possibly as a result from earlier repairs and replacements. For this reason, green burst encoders can be found with grey radios and vice versa. The colour might also be related to the manufacturer, the production plant or the era in which they were produced.
Two burst encoders with cassettes, in two different colours.

  1. AIVD, Short description and image of a captured R-353
    Website. Retrieved November 2009.

  2. Chris Vos, et al. De geheime dienst: verhalen over de BVD
    ISBN: 90-8506-181-4 (Book with DVD, Dutch).

Further infomation

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Е Crypto Museum. Created: Saturday 07 November 2009. Last changed: Sunday, 31 July 2016 - 07:57 CET.
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