High-speed electronic burst encoder
MMP was a fully electronic high-speed burst encoder,
developed and built around 1980 by the secret
Dutch stay-behind organisation O&I
at Villa Maarheze in Wassenaar (Netherlands).
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 high speed in
The device was also supplied to Belgium and Germany.
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.
The MMP was initially designed for use with the
FSS-7 spy radio set,
which is the Dutch stay-behind version of the
It was later also used with the German SP-15 and its successor
Note that the SP-15 had to be modified for use at the highest
Apart from a photograph in Louis Meulstee's Wireless for the Warrior,
Volume 4  and another
photograph taken by ourselves
at the Dutch Signals Museum in
2009 , we could not find any further publication.
We therefore believe that this is the first detailed
public description of it.
Two versions of the MMP are known to exist:
This is the initial version of the MMP. It was developed in the late 1970s and
is featured in Louis Meulstee's excellent book Wireless for the Warrior,
Volume 4 . An
MMP-A with serial number 357
is held in the collection of the Dutch Signals Museum. 1
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 was used for the descriptions on this page.
Currently known as the Signals Service Historical Collection.
The diagrams below show the control panels of the MMP-A and MMP-B.
The MMP-A has its power switch at the bottom right. Shift upwards to
turn the unit on. Above the ON/OFF switch is another slide switch
marked 'L/F'. It is used to select between Letters and Figures.
At the top right is a rotary switch that is used to check the unit
and to select the required transmission speed (baudrate). The highest
possible baudrate that is supported by the MMP-A is 1000 baud.
The MMP-B is very similar, but has the ON/OFF switch and the L/F switch
combined in a single rotary selector,
that is located at the bottom right.
The highest possible speed is 1200 baud and the unit has been extended with
an OVERLOAD indicator, a CHARGE indicator and a battery voltage meter.
Furthermore, the KEY output is available on a 2-pin LEMO socket
rather than SMC.
The MODE selector at the top right
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. At the left
is a socket for connection to the transmitter (OUT) plus a jack socket for
connection of a high-impedance earphone (MON).
The MON-socket is also used for recharging the batteries. 1
The internal NiCd batteries should be charged with 9V DC.
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 (baud):
1200(1000 on the MMP-A)
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
(top right) in the REC position and turn the device ON by placing the
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.
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
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.
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
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
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 broken as a result
of this, and had to be replaced
Connecting the MMP to the SP-20
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.
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.
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
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
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
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.
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
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
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
At the receiving end, a suitable
modem would have been necessary to decode and record the hight speed
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.
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
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.
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.
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.
- Paul Reuvers & Marc Simons, Investigation of a working MMP
Crypto Museum, March 2013.
- Louis Meulstee, Wireless for the Warrior, volume 4
ISBN: 0952063-36-0, September 2004.
- 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, 10 October 2017 - 05:39 CET.