← Easy Chair
The SRT-56 consists of two or three cylindrical modules that contain
the SWE-56 audio masking unit,
also known as the video encoder,
an RF-module (the transmitter),
and a power source. Depending on the selected RF module, the set
operated in the 350 MHz or the 1500 MHz UHF band.
A complete 350 MHz set is shown here.
The basic version of the SRT-56 is adjusted at the factory to a spot
frequency in the 315-400 MHz range, and can generally be recognised by
the green colour 1 of its cases. The high-band version works between
1300 and 1600 MHz.
The SRT-56 is very similar to the SRT-52,
which was developed around the same time, but uses a different
frequency range and a
different audio masking scheme.
Development of the SRT-56 was started in 1966, with the first prototypes
being ready for evaluation in March 1968 [A]. It was first used in the field
in September 1969 and was in production until at least 1974, but probably
much longer. The high-band version of the SRT-56 was succeeded
in 1974 by the integrated SRT-107.
The SRT-56 was developed as part of the CIA's
SRS-56 surveillance system,
that existed alongside the SRS-52.
It is compatible with, and can be decoded by, the
receivers, although the SRR-145 down-converter
is needed if the SRT-56 contains the
high-band SRK-145 RF-module.
A compact version of the SRT-56 is known as the
Initially the modules of the SRT-56 were coated in green hamerite paint.
Later versions were cast in a strong two-component dark green epoxy.
Both types of coating are visible in the image above.
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The individual modules are each housed in a cylindrical brass enclosure
with a standard diameter of 26 mm and a length of approx. 65 mm.
The modules could be fitted inside a pre-drilled 1 1/8" hole in, say,
a piece of furniture, a window pane or a concrete wall. All parts are
connected with high-quality 6-pin Socapex plugs, to a
that is part of the wiring of the video encoder.
and the microphone were connected via BNC plugs, or
The diagram above shows a typical SRT-56 target element, complete with
a Knowles BA-1502 microphone
and a suitable Sleevex antenna. In this
example, the target element is powered directly from the mains, by means
of the UWP-56 that is part of the kit. In situations were no mains
power was available, long-life military-grade mercury batteries
were used instead.
- Standard version (350 MHz)
Standard version of the SRT-56 that operates in the 315-385 MHz band.
In a minimum configuration it consists of an SRK-35 transmitter
and an SWE-56 video coder. When necessary, the SRT-56 could be
made to work outside its regular frequency range.
- Compact version (SRT-56-F)
Same version as above, but in a compact rectangular enclosure, smaller than
a pack of cigarettes,
known as the SRT-56-F.
It contains flat versions of the SRK-35 RF-module
and the SWE-56 video encoder in a single enclosure.
- Alternative version (290 MHz)
Special version that operates in the (lower) 256-315 MHz band.
This was done by swapping the SRK-35 RF-module
for the SRK-29 that was normally used with the
- Non-air version (SRT-56-C)
Standard version of the SRT-56, of which the
video encoder (SWE-56)
is modified for use with a non-air microphone, such as
the SWM-25 contact microphone.
- High-band version (1500 MHz)
In 1971, the CIA decided to move the operating frequency of
their bugs from 300 MHz to the 1500 MHz band. In the SRT-56
this was done by swapping the SRK-35 RF unit for an
and replacing the Sleevex antenna
by an SRN-58 plexiglass antenna.
It was this combination that was shown by the Soviets
on a press conference on 10 April 1987.
To hide the RF carrier and its modulation from regular
professional bugs often use a special technique
that is known as
The SRT-56 uses a sophisticated masking scheme, based on Pulse Position
Modulation (PPM), known as Rejected Pulse (RP).
This masking scheme is characterised by an AM carrier with a rather large
bandwidth (~ 7 MHz) and a multitude of sidebands at either side,
caused by the short square-wave pulses, as shown in the diagram
above. There are currently no known commercially available surveillance
receivers that can readily demodulate an RP-masked signal.
Most receivers won't even lock onto the signal.
➤ More about RP audio masking
Signals from the SRT-56 can be received and demodulated with the following receivers:
Detection and discovery of the bug is possible, but is not evident.
As far as we know, there are no commercially available
surveillance receivers that can readily
demodulate an RP-masked signal. Furthermore, existing bug tracers like the
do not lock onto its signal at all.
Finding and locating the bug is possible with a portable spectrum analyzer,
such as the
Rohde & Schwarz FSH-3,
and with a modern monitoring receiver like
the R&S PR-100 shown on the right.
➤ Read the full story
In 1987, the American government accused the Soviets of bugging their
embassy in Moscow. The Russians replied with a press conference
on 10 April 1987, saying that the American claims were unfounded, but that
instead the Soviet buildings in the US had been bugged by the Americans.
The image on the right shows the press meeting that day, on which
the Soviets had made a real showcase from a variety of bugging devices that
had so far never before been seen by the public. On the wall behind the table
were several large panels with the actual devices and photographs of the
buildings in which they had been found.
Soviet spokesmen explained to the assembled press that these were bugging
devices that had recently be found in the walls of their buildings in the US,
and that they had most likely been planted there by the
Central Intelligence Agency.
Unfortunately, the quality of the above picture is not very good, and the
available footage is not very clear either. But after carfully studying the
available material, Crypto Museum has been able to identify one bugging device
that was developed for the CIA, under the Easy Chair contract.
At the bottom of the second panel from the left, a group of four devices
with a cluster of cables is visible. They are marked here with a red circle.
The image on the right shows the same items as they appeared on national
television that night.
The set appears to be a high-band version of the SRT-56, with its 1500 MHz
plexiglass SRN-58 antenna
clearly visible at the right. The leftmost unit is
the SWE-56 video encoder that provides the audio masking facilities.
The small unit next to it, is the SRK-145 high-band RF-unit,
that is connected to the antenna and the video coder.
The 3rd item is a bit of a mystery and can not be easily identified.
It is unpainted, which suggests that it might have been a prototype
or a special.
Judging from the photograph, it appears to be a long shiny cylindrical
enclosure with a
military-grade mercury battery
clearly visible at the top.
It is likely that this item is a stack of four 1.35V mercury batteries
(for a total of 5.4V)
mounted inside a metal cylindrical enclosure. The top of the enclosure appears to
be removed for the picture. Only two wires seem to come out at the bottom
of the cylinder, which confirms that this is a battery. It might have provided
energy to the SRT-56 for a period of three to six months. 1
The image on the right shows nearly the same set, assembled from objects in our
collection. The set that is shown here, is fully operational, but the mercury
battery is replaced by a PSU.
The differences in colour between the discovered set and our reconstruction,
are easily explained, and can help to determine the age of the equipment.
The combination that was found by the Soviets, was manufactured
between 1971 and 1973 and was probably installed within that time frame.
The leftmost unit was initially painted hamerite green, but from 1973 onwards
it was cast in dark green expoxy.
From the photograph, it seems likely that the hamerite version is shown.
This narrows the time frame down to 1971-1972. Since the battery was 15 years old by
the time of the press conference, we can assume that the set was no
longer operational. It seems therefore unlikely that it had been discovered
recently, as claimed by the Soviets. It is far more likely that,
in order to emphasize their claim, the Russians showed everything they
had found over the years.
This period could be extended significantly, by adding a switch
receiver, such as the QRR-25,
and turning the transmitter off between sessions. However, such a
switch receiver was not shown by the Soviets.
The video clip below shows part of the original footage as it was aired
in the US on the evening of 10 April 1987. It shows only part of the
press conference and unfortunately it comes without any sound.
If anyone has access to a more complete or better quality newsreel,
please contact us.
A complete SRT-56 transmitter consists of one or more of the following items:
This is the SRK-35 transmitter that operates on a fixed frequency
between 315 and 385 MHz, driven by a pulse shaper, that in
turn is driven by extremely short pulses from the video encoder.
The pulse shaper ensures that all pulses are of the same amplitude
and length (approx. 0.5 µs).
The RF unit is powered by a DC source between +5V and +12V, and consumes
between 1.5 and 5.5 mA. It produces a peak output power of 75 to 300 mW.
It can be driven by an
or SWE-56-C video coder.
In 1971, it was replaced by the SRK-145 for use at 1500 MHz.
➤ Look inside the SRK-35
In SRK-35, the suffix 35 indicates the center of its frequency
range, i.e. 350 MHz.
In situations were the SRT-56 could be powered from the mains, the UWP-56 1
power supply unit (PSU) could be used. Like the RF unit, it is housed in
a cylindrical brass enclosure, and is cast in epoxy. Inside the cylinder is
a miniature toroid transformer that is suitable for 110V and 220V AC mains
Powering a bug from the mains, virtually gives it an endless life, but
increases the chance of discovery. For this reason a
QRR-25 switch receiver
was sometimes added to the setup.
➤ More about the PSU
In situations where it was not possible to power the SRT-56 from
the mains, a series of stacked long-life Mercury cells was sometimes
used. Although this reduces the operational life of the bug, it make
it's installation a lot easier.
Mercury cells use a reaction between mercuric oxide and zinc electrodes
in alkaline electrolite, and deliver 1.35V per cell . When using four
stacked cells, the battery provides 5.4V, which remains practically constant
during discharge. Due to the presence of toxic elements,
mercury batteries are now banned in most countries .
Although the SRT-56 can be used with virtually any type of
sensitive dynamic microphone, it was commonly used in combination
with a Knowles BA-1501
or BA-1502 element.
Measuring just 10 x 10 x 5 mm, it was one of the smallest
dynamic microphones available. It has an excellent
dynamic behaviour and a good frequency response
curve, and was commonly used in military equipment for many years.
➤ More information
Instead of the normal dynamic (air) microphone, it was als possible to
use a so-called non-air or contact microphone, such as the
SWM-25 shown in the image on the right. The microphone is housed in a
brass cylinder that has the same diameter as the SRT-56 (26 mm), and is
filled with oil, to provide internal damping.
The SWM-25 uses a piezo-ceramic element in shear-mode and can be used
to listen through the thick concrete walls of a large building. 1
➤ More information
When using the SWM-25 contact microphone with the SRT-56, a special
version of the video encoder with a modified frequency response curve,
the SWE-56-C, should be used.
The SRT-56 was commonly used in combination with a so-called
which was also developed by the NRP.
Made from a piece of rigid coax cable, Sleevex antennas
were available for a variety of frequency ranges.
Furthermore, different types of Sleevex antennas were available
for embedding in a variety of environments, such as wood and concrete.
➤ More information
In 1971, following a series of studies into the use of the higher frequency
bands for covert listening devices, the SRK-145 RF unit was introduced
as an alternative for the existing SRK-35 units. It is somewhat shorter
than the existing ones, and is normally painted in the same colour as
the SRT-56. The one shown here is a laboratory model, which is why it is beige.
The SRK-145 works on a spot frequency in the 1300 - 1600 MHz band
and requires the use of the SRN-58 antenna instead of the
➤ Look inside the SRK-145
When the SRK-145 high-band RF unit was used, the
had to be swapped for the small end-fed vertical dipole shown
in the image on the right. The antenna is fore-shorted and is embedded
in a plexiglass (perspex) stick that has the same diameter as the
The plexiglass SRN-58 antenna was also used
with the high-band version of the SRT-52
and with the later integrated SRT-107
➤ More information
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Showing the interior of the SRT-56 is difficult, as all of its modules
were soldered hermetically. Furthermore, the video encoder and the
PSU are fully cast in black epoxy, making it impossible to show
the interior. Nevertheless we are able to show the interior of some
laboratory models below.
The standard SRK-35 RF-unit is the only module of the set, that is not cast in
expoxy, because of the dielectric effects of the material on the RF components.
Furthermore, it has to be possible to adjust the transmission frequency
from the outside of the case. Instead, the components are protected against moist
by a conformal coating.
The image on the right shows the interior of the RF unit of an SRT-52.
At the centre is the main oscillator, built around a
TR1062 transistor. The transistor is directly
coupled to the tuned circuit that determines the bug's operating frequency.
The oscillator is keyed by a pulse-shaper that is located at the top of the
unit (assuming that the wiring is at the bottom), and is built around a
2N3866 transistor. It is driven by the modulated pulses from the video encoder,
and shapes them so that they are of equal length and amplitude.
The most complex part of the SRT-56 is without doubt the video coder.
It converts the sound that is picked up by the
microphone, into a series
of seemingly random pulses that are used to key the RF unit.
This is known as audio masking.
The video encoder is housed in a brass cylinder, roughly the same size
as the transmitter, and contains five electronic circuit blocks,
known as cordwood structures.
The entire unit is cast in an elastic substance and is fixated inside
the brass cylinder at both ends with a strong black epoxy,
due to which the SWE-56 cannnot be serviced.
For this photograph, we've removed the enclosure from a broken SWE-56
video encoder, so that we can see how it is constructed internally.
The five cordwood circuit blocks are clearly visible and are separated
by isolated discs, that have cut-outs for the wirings. A long blank wire,
running over the full length of the assembly, provides ground (0V)
to each of the cordwood modules.
The power supply unit (PSU) was initially much longer than the
RF unit and the video encoder, but was later redesigned to fit
inside the same size enclosure.
As the entire PSU is cast in black epoxy, we are
unable to show its contents, but from the documentation,
its circuits are known.
At the heart of the PSU is a miniature toroid transformer that is
connected directly to the mains network. It has separate windings
for 110V and 220V AC.
It was probably made by the Radio Corporation of America, especially
for the UWP-56. It came with an individual test sheet.
The transformer had to be this small in order to be fitted inside
a standard brass cylinder that has an inner diameter of 25 mm.
The rest of the PSU's circuit is placed on
a single cordwood module.
Like the PSU, the battery provides two voltages, in order to allow adjustment
of the transmitter's output power between 75 mW and 300 mW.
In 1971, new frequencies in the 1500 MHz band were allocated for covert
listening devices, and the SRK-145 module was developed to replace the standard
The image on the right shows a lab model of the new SRK-145.
Due to the higher frequency, the tuned circuit
is smaller, as a result of which the entire unit is shorter.
The cordwood structure at the right
provides the -20V oscillator keying voltage.
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- Manual for SRS-56 Protype Equipment
CM302491/A, March 1968.
- Operating Manual for SRS-56 Equipment
CM302491/B, September 1969.
- Technical Manual for SRS-56 Equipment
CM302491/C, September 1969.
- Manual for SRR-56 Receiver
CM302491/D, January 1974.
- Manual for SRR-56L Receiver
CM302491/E, March 1978.
- Manual for SRR-56H Receiver
CM302491/F, September 1979.
- NRP/CIA, Collection of documents related to SRS-56
Crypto Museum Archive, CM302491 (see above).
- NRP/CIA, Collection of documents related to AGC ignition interference
Crypto Museum Archive, CM302626.
- Wikipedia, Mercury battery
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© Crypto Museum. Created: Monday 17 April 2017. Last changed: Tuesday, 13 June 2017 - 06:23 CET.