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← Easy Chair CIA NRP
The device is based on the design of the
SRT-56 and is in fact a combination
of the SWE-56 video encoder
and the SRK-35 RF-unit,
redesigned to fit in a single rectangular metal enclosure,
and cast in a strong epoxy.
The device operated in the 315 - 385 MHz band,
was powered by a DC source between 5 and 8V, and consumed
4.5mA, wilst delivering a peak-output-power of 75mW.
The device is compatible with the SRT-56
and can be decoded with a
SRR-52-M,
SRR-56,
SRR-90
or SRR-91 receiver.
It was powered by the UWP-56 module,
or by a 4-cell mercury battery.
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The device was commonly used in combination with a
Sleevex Band 2 antenna,
whilst an SRN-9H
or SRN-9 antenna was used at the listening post.
The first prototypes of the SRT-56-F were ready for evaluation in
April 1968. The device was first used in the field in February 1970 and
was in production until at least 1973.
The SRT-56-F is part of the CIA's
SRS-56 surveillance system.
For further information please refer to our page about
the SRT-56.
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To hide the RF carrier and its modulation from regular
surveillance receivers,
professional bugs often use a special technique
that is known as
audio masking.
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
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Signals from the SRT-56-F can be received and demodulated with the following receivers:
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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
Scanlock
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
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The transmitter is housed in a rectangular metal enclosure and is basically a
combination of the SWE-56 video encoder
and the SRK-35 RF unit,
albeit in a different (non-cylindrical) enclosure.
The video encoder converts analogue audio into
a masked Pulse Position Modulation (PPM) signal, using the
Rejected Pulse (RP) masking scheme,
also known as the 56 scheme.
The RF-unit is the actual pulse transmitter, which operates in the
315 - 385 MHz band.
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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
networks. More...
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
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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 [3]. 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 [3].
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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
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The SRT-56 was commonly used in combination with a so-called
Sleevex antenna,
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
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Frequency 315 - 385 MHz (pre-determined spot frequency) Audio masking Rejected Pulse (RP), also known as Type 56 modulation Power +45 to +8 V Current 4.5 mA Output 75 mW (peak output power) Supply UWP-52, UWP-56 or 4-cell battery Antenna Sleevex Band 2
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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.
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- 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
Retrieved, April 2017.
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© Crypto Museum. Created: Thursday 09 March 2017. Last changed: Saturday, 03 August 2019 - 07:06 CET.
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