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Burst encoders
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High-speed electronic burst encoder

MMP was a fully electronic high-speed burst encoder that was developed by a hitherto unknown manufacturer, probably in Germany, around 1980. It was designed as a replacement for the aging mechanical burst encoders, such as the RT-3 and the GRA-71, and the earlier Speicher. It allowed letters, number and some other characters to be transmitted at very high speed in morse code.
Compared to the earlier burst encoders, the MMP was far more advanced. It allowed the use of both numbers and letters and even some punctuation marks, and was able to send the information at a much higher speed (1200 bps).

The image on the right shows a typical MMP-B. It is a fully self-contained unit that measures only 14 x 10 x 4 cm. It is powered by internal 6V NiCd batteries that can be recharged via the headphones socket (MON). The unit can hold up to 175 groups of 5 letters each. It can send data at various speeds ranging from 15 to 1200 bps.
High-speed morse burst encoder MMP-B

The MMP was initially designed for use with the 1970s SP-20 spy radio set in Germany, but was also used in combination with the 1960s valve-based SP-15, although the latter had to be modified for use at the highest data rates. Evidence of the use of the MMP with the SP-15 was found in The Netherlands [3] where it was used by the Stay-Behind Organisation alongside or in place of the Speicher burst encoder. An MMP-A was later donated to the signals museum [3].

Apart from a photograph in Louis Meulstee's Wireless for the Warrior, Volume 4 [2] and another photograph taken by ourselves at the Dutch Signals Museum in 2009 [3], we could not find any further publication. We therefore believe that this is the first detailed public description of it.
High-speed morse burst encoder MMP-B Power switch MODE selector Connecting the transmitter Cable for connection to SP-15 (FS-7) Cable for connecting the MMP to the SP-20 spy set MMP connected to the FS-7 transmitter (SP-15) MMP-B in operation

All controls and connections are on the control panel, on top of the unit. At the bottom right is a 4-position rotary dial that is used as the power switch. In the leftmost position the MMP is turned off. The rightmost position can be used to check the voltage of the internal battery. The other two positions (L and F) are for entering Letters and Figures (numbers) respectively. On an older version of the MMP (probably the MMP-A) a slide switch was used for turning power ON/OFF.

MMP control panel layout

At the top right is the MODE selector. Is has one position for checking the keyboard (CHECK), one for recording the text (REC) and eight for playing back the text at various speeds (PB). The three buttons at the top (STOP, START and RESET) are used to control the internal text counters. To the left of the STOP button is a 2-pin LEMO socket for connection to the transmitter, and at the far left is a 2.5 mm jack socket for connection of a high-impedant earphone (MON). The MON-socket is also used for recharging the internal NiCd batteries from an external 9V DC source.
So far, two versions of the MMP have been found:
  • MMP-A
    This is probably the oldest version and was developed in the late 1970s. It is pictured in Louis Meulstee's excellent book Wireless for the Warrior, Volume 4 [2]. An MMP-A with serial number 357 is held in the collection of the Dutch Signals Museum.

  • MMP-B
    This version has a few improvements over the earlier MMP-A, but is otherwise functionally identical. The power switch and the L/F switch are both replaced by a single rotary dial at the bottom right. The MMP-B is featured on this page.

There are different ways of expressing the speed of a burst encoder. Although the speed is often specified in characters per second (CPS), groups per second (GPS) or groups per minute (GPM), the best way of doing this is by specifying it in bits per second (BPS or Baud), as each character consists of a varying number of dots and dashes. In this case, a dot counts for 1 bit, whilst a dash counts for 3 bits. The space between dots and dashes is 1 bit long, whilst the space between letters is 2 bits and a word spacing is 4 bits. The same method is used for specifying the speed of the American AN/GRA-71 burst encoder. The MMP can sent text at the following speeds:
  • 15 baud
  • 75 baud
  • 100 baud
  • 300 baud
  • 400 baud
  • 600 baud
  • 1200 baud
Operating the MMP is rather straightforward but requires some understanding of the operation of the circuit. This is described in more detail in the section 'Interior' below. When recording a message, place the MODE dial (top right) in the REC position and turn the device ON by placing the power switch in the L-position (Letters). Next, press RESET in order to set the internal counter to the first memory position. Then press START to start recording.

Now enter the text in groups of 5 characters. Letters can be entered directly by pressing the corresponding keys (A-Z). Numbers can be entered by setting the power switch to 'F' (Figures) and then pressing one of the first ten keys on the keyboard. Return to 'L' to enter letters again. Note that the device stops accepting input once five characters have been entered. You then need to enter a Group Space (gs) before you can enter the next group of five characters.

At the bottom row of the keypad are some special characters. From left to right are the last two letters of the alphabet (Y and Z), followed by the equals sign (bt), the question mark (ini), the plus sign (ar) and the correction sign (the blank key). At the far right is a pilot tone key marked (mp).

The MMP has room for approximately 1000 characters. When the memory buffer is full, the Overload LED (OVL) will light up and no further input is accepted. Once the text has been entered, press the (mp) button. You should now hear the pilot tone. Set the MODE dial to the required transmission speed (e.g. 1200 baud) and press RESET in order to set the internal counter to the first position again. Now turn on the transmitter and press START to start the burst transmission. Once all characters have been sent, you'll hear the pilot tone again. Then turn off the transmitter.
The MMP is housed in a grey metal case in which a separate front panel is mounted. The PCBs that contains the electronic components and the batteries are mounted to the back of the front panel. The entire interior can be extracted by removing 4 M3 screws from the side of the case.
The image on the right shows the interior of the MMP after it has been removed from the case.

The front consists of a 5 mm thick aluminium panel that holds two keypads with 16 buttons each, two connectors and all controls. A piece of blank PCB, with a cut-out at the bottom left, is used as the actual front panel with the copper side facing outwards. The text on the front panel is etched out of the copper layer (i.e. negative).

Both the front panel and the PCBs are protected against corrosion by a conformal coating.
MP-B interior removed from the case

Observing the build quality of the MMP, gives a bit of a mixed feeling. One the one hand it is a very professional device built with first-class components, and yet, at the same time, its PCB and its front panel look totally amateuristic. This clearly demonstrates the highly secretive nature of this kind of devices. They are probably built by an internal technical department of one of the agencies, such as the Bundesnachrichtendienst (BND) or the Stay-Behind Organisation itself.
The main PCB is single sided, which means that all components are at the top and all tracks are at the bottom. This results in a rather complex maze of tracks. Furthermore, the PCB has no solder mask and no silk screen (white print). It seems to be made in a home-based laboratory.

Nevertheless, the design of the circuit is very professional, especially considering that is was built in the early 1980s. Only first class parts are used on the PCB. The device uses an EPROM and (battery-backed) CMOS RAM, which was pretty new and was not commonly available in 1980.
Main PCB (right) and battery board

For input of the text and numbers, two separate keyboards with 16 buttons each are used. Using a multiplexer, the two keyboards are connected in parallel and supply 4 bits each. Once the data is latched, it is stored in the current location of a battery-backed CMOS RAM. After this, the address counter of the RAM is incremented by one position, ready to store the next character.

At the same time, the data on the DATA bus of the RAM is fed to the ADDRESS bus of an EPROM that holds the bit pattern of each morse code character. The bit pattern is then serialized into a series of '0' and '1' signals, representing the dots and dashes of the morse code character. This signal is fed to a MOSFET switch (VQ1000CJ) that drives the KEY-input of the transmitter, and to an audio circuit that produces the required tones for the monitoring output.

The entire control logic is built from the MM74C-series of National Semiconductors (now: Texas Instruments); the first generation of CMOS ICs, known for their low power consumption. It allowed the MMP to have many hours of trouble free operation without charging the batteries.

The simple circuit at the bottom right of the diagram above shows how the battery charge circuit is combined with the audio monitoring output (MON). The capacitor (C) blocks the DC voltage from the charger. Power is supplied via a red LED, directly to the batteries. This way, the voltage from the external PSU (approx. 9V) drops to the 7.5V needed to charge the 6V battery. A 9V1 zener diode is used to protect the batteries, and hence the rest of the circuit, against excessive voltages. In our MMP this zener diode was shorted as a result of this, and had to be replaced [1].
MP-B interior removed from the case Bottom view of the main PCB with the battery board in place Main PCB (right) and battery board View from the top of the device View from the bottom of the device Close-up of the main PCB (solder side) The name MMP-B in a corner of the PCB MMP-B interior: main PCB and front panel

Connecting the MMP to the SP-20
Although it was also used with other spy radio sets, the MMP was originally developed for use in combination with the German SP-20 radio set, in particular as a replacement for the Speicher burst encoder. The digram below shows the wiring between the MMP and the KS-30 synthesizer.

Connecting the MMP to the SP-15
The MMP was initially designed for use with the solid state SP-20 spy set of the 1970s. As this radio set was fully transistorized, it could be keyed at the highest possible speed of 1200 baud. The device was also suitable for the earlier SP-15 that was developed in the early 1960s.
The transmitter of the SP-15 (i.e. the FS-7) is valve-based however, and is therefore far more difficult to key at high speed. In the original design, there were two ways of creating a morse (CW) signal. When used with the manual morse key, the cathode of the oscillator valve (EL95) is switched, allowing speeds up to 100 baud.

When used with a burst encoder, such as the RT-3, a different method was used. By switching the grid voltage of the PA (EL81) speeds up to 800 baud are possible. In the image on the right, the MMP is connected to the key socket of the FS-7.
MMP connected to the FS-7 transmitter (SP-15)

Neither of these methods is suitable for running the MMP at the highest possible speed of 1200 baud however. This means that a third method for modulating the CW (morse) signal had to be developed. This was done by adding Frequency Shift Keying (FSK) to the FS-7 transmitter.
Cable for connection to SP-15 (FS-7) Crystal earphone MMP connected to the FS-7 transmitter (SP-15) MMP-B in operation FSK modulator and crystal MMP-A in the collection of the Dutch Signals Museum MMP-A in the collection of the Dutch Signals Museum Close-up of the zener diode at the side of the PCB

FSK modulator   wanted item
A few years ago, whilst visiting a collector in Austria, we discovered a small hitherto unknown plug-in unit that was found with an SP-15 radio set. The device was clearly intended to fit the crystal socket of the FS-7 transmitter, had its own crystal socket on top and a knob at the front.
A short piece of cable came out of the device at one of its corners, but seemed to go nowhere. At the time it didn't have a connector at the end.

We had the impression that the device was used to slightly alter the frequency of the crystal by supplying a signal to the cable, but could only be certain if we were allowed to look inside and reconstruct the circuit diagram.

In the summer of 2013 we had the chance to borrow the device for a few weeks in order to determine its functionaly and study the interior.
FSK modulator and crystal

The circuit diagram below shows that the device was indeed what we had initially thought: an FSK modulator in its most primitive form. Two capacitors, C1 and C2 are connected in series with the crystal and will change its frequency somewhat. Whilst C2 can be adjusted with the knob at the front, C1 is shorted by the burst encoder in the rythm of the morse signal, causing small shifts in frequency. One of the pins of the crystal socket of the FS-7 is already connected to ground, which is why this circuit works, even if no burst encoder is connected.

The way the device is constructed prevents it from being inserted the wrong way around. It should be inserted into the existing crystal socket in such a way that the crystal socket of the FSK modulator is at the top. The crystal can then be inserted into the socket of the FSK modulator.

The final part of our investigation involved testing the device in combination with an FS-7 transmitter and the MMP burst encoder. After mounting a 2-pin LEMO plug to the loose end of the modulator's cable, we connected it to the OUT socket of the MMP and tried to send a pre-recorded message and... it worked! For this to work, a special 5-pin DIN connector with pins 1, 2 and 3 shorted, had to be inserted into the DIN socket on the transmitter. Alternatively, the morse key had to be kept depressed during the burst transmission. At the receiving end, a suitable modem would have been necessary to decode and record the hight speed burst transmission.

PLEASE HELP - Unfortunately we had to return the device shown here to the rightful owner once our investigation was complete. We are currently looking for this type of FSK modulator for our collection. If you have one available, please contact us.
The device as it was discovered in Austria in 2010. FSK modulator with LEMO connector FSK modulator and crystal Crystal inserted in the socket of the FS-7 Inserting the FSK modulator with crystal into the FS-7 FSK modulator fitted to the FS-7 Adjusting the frequency shift FSK modulator in use, connected to the FS-7 and the MMP
Opened FSK modulator FSK modulator interior FSK modulator interior FSK modulator interior FSK modulator interior Complete setup with MMP and morse key Switching on the MMP Keeping the morse key depressed during the burst transmission

MMP burst encoders are very rare. Only a relatively small number of them were built and most of them were destroyed when they were taken out of service or after the Stay-Behind operations were terminated in the early 1990s. If you find one, you may have trouble getting it to work.
The device is powered by internal NiCd batteries that were manufactured in the early 1980s. These batteries will most likely be dead by now.

Although it is possible to power the device externally by connecting a 9V DC mains charger to the MON socket, it can not be operated this way, as the connection is also needed for the headphones. If you want the MMP to work, remove the batteries and replace them with good (modern) alternatives. Removing them might be a good idea anyway, in order to prevent damage caused by leaking batteries.
Repaired battery board

In our case we replaced the five 1.2V NiCd cells by two Panasonic 3V Li-ION cells, as shown in the image avbove. The nominal voltage stays the same (6V) but the charge current should be lower. These cells are typically charged for 25 hours with just 4 mA. A 7V DC charger will be suitable.
When using this type of replacement cells, ensure that the charge voltage never exeeds 7.5V DC as the higher current might damage the batteries. Use a current limiter if possible.

The next thing to check is the 9V1 zenerdiode that is connected in parallel to the batteries. It is there to protect the batteries against excessive voltages. If someone has tried to power the device with, say, 12V DC, there is a good chance that this diode is broken and causes a full short circuit. Check wether the zener diode has become black or check it for a short circuit.
Close-up of the zener diode at the side of the PCB

If it is broken (it was in both MMPs we've seen so far), remove it from the PCB, clean the PCB thoroughly, and replace it with a new 9V1 zener diode. The diode is located at the edge of the board, close to the CHARGE led. The image above shows the MMP board with a new 9V1 zener.

Set the power switch to 'OFF' and charge the batteries sufficiently long to ensure normal operation. Remove the battery charge cable and connect a crystal earpiece to the same socket. Set the speed dial to 15 and turn on the device by setting it to 'L' mode. The device should now start sending morse code. If it doesn't, press RESET followed by START.
  1. Paul Reuvers & Marc Simons, Investigation of a working MMP
    Crypto Museum, March 2013.

  2. Louis Meulstee, Wireless for the Warrior, volume 4
    ISBN: 0952063-36-0, September 2004.

  3. Museum Verbindingsdienst, MMP-A burst encoder
    Crypto Museum, photographed at Dutch Signals Museum, 25 February 2009.

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Crypto Museum. Created: Saturday 30 March 2013. Last changed: Tuesday, 04 October 2016 - 13:09 CET.
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