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CIA NRP EC SRS-56 SRR-91 → ← SRR-52
The receiver covers a frequency range of 310 MHz to 400 MHz,
and is powered from the 50 Hz or 60 Hz 90-270V AC mains.
It is suitable for the reception of covert transmitters (bugs)
that use the CIA's super secret
Rejected Pulse (RP)
and Dirty Pulse (DP)
audio masking schemes.
The SRR-52 is fully compatible with the low-band version of the
SRT-56 transmitter,
and is partly (frequency) compatible with the
SRT-90
and SRT-91.
By using the SRR-145 converter,
it is also compatible with the SRT-107,
and the high-band version of the
SRT-52 and
SRT-56.
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The SRR-56 was commonly used as part of a covert listening post (LP),
together with the SRN-9 directional antenna.
As we are not aware of any surviving SRR-56 receivers, we are not able
to provide a full colour photograph of it, but as it is very similar to
the SRR-52 (that was developed around the same time),
we are showing a picture of the latter above. The operation is identical.
Note that the SRR-56 is not compatible with
SRT-52 bugs
(for which an SRR-52 is required).
Development of the SRR-56 was started in 1967, with the first prototypes
ready for evaluation in March 1968. The receiver was first used in the field
in September 1969, and was in production until at least 1979,
after which it was superceeded by the
SRR-91
and by the improved SRR-90.
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- SRR-56
Original version of the SRR-56, suitable for the 310 to 400 MHz frequency range,
and first used in the field in 1969. The Automatic Gain Control (AGC) was later
modified by Motorola to make it immune to peak-pulse interference caused by
the ignition of cars and motorcycles [2].
- SRR-56L
Improved version of the SRR-56, introduced in 1978 and modified for the
lower 240-330 MHz frequency range. Suitable for transmitters with the
SRK-29 RF-module.
- SRR-56H
Improved version of the SRR-56, introduced in 1979, for the original
310 to 400 MHz frequency range. Suitable for transmitters with the
SRK-35 RF-module.
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SRS = Surveillance Radio Set.
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In 1971, the CIA started using the newly allocated 1500 MHz band for
the use of covert listening devices. The diagram below shows how the
existing parts were used in the new setup. At the left is the
SRT-56 transmitter, in which the
SRK-35 RF-module has been replaced by an
SRK-145.
A small SRN-58 plexiglass antenna
transmits the 1500 MHz signal to the listening post (LP) where it
is picked up by an SRN-55 antenna, and passed on
to the SRR-145 down-converter. The latter converts
the 1500 MHz signal into 300 MHz, so that it can be decoded with the
SRR-56.
In the high-band setup, the SRR-56 is also suitable for the reception of
the SRT-107,
which is in fact an integrated version of the SWE-56 video
encoder, the SRK-145 RF-module and the SRN-58 antenna. The diagram above
shows how the SRT-107 is used in this setup.
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A complete SRR-56 listening post consists of one or more of the
following items:
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At the heart of the listening post (LP) of a 56-system,
was the SRR-56 receiver featured on this page.
It covers 310 to 400 MHz, and is capable of receiving
RP-masked pulse transmitters,
like the SRT-56,
and DP-masked
pulse transmitters,
like the SRT-91.
With the use of the SRR-145 down-converter,
the SRR-56 is also suitable for the reception of
the SRT-107.
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A suitable directional antenna for the SRR-52 listening post (LP)
is the SRN-9-L, or the later SRN-9. It offers a gain of 7 dB and is
in fact an adjustable dipole on a horizontal boom (which acts as a
balun), mounted in front of a reflector.
The antenna can be disassembled completely, and the reflector plane
can be folded at the centre, so that the entire unit can be stored inside
a regular briefcase, along with the SRR-56 receiver and its accessories.
➤ More information
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The SRR-56 has two audio outputs: a fixed one for connection of a recording
device, and an adjustable one for connection of a pair of headphones.
Virtually any type of headphones with an impedance of 600Ω can be
used.
It was typically used with American military headphones of the era, such as
the one shown in the image on the right.
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The SRR-56 has a direct video input on its front panel, which can be used
for testing the video encoder of an SRT transmitter. By connecting the video
output of the encoder directly to the video socket of the SRR-56, all RF
components (in the transmitter and the receiver) are bypassed.
By connecting the video detector, shown in the image on the right, to the
video socket of the SRR-56, the RF output of an SRT transmitter can be
converted directly into pulses (video), thereby bypassing the receiver only.
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The frequency range of the SRR-56 (310 - 400 MHz) 1 could optionally be
enhanced with the 1300 - 1600 MHz band, simply by inserting the SRR-145
down-converter shown on the right,
between the antenna and the receiver's input.
This was necessary for receiving SRT-52
and SRT-56 units that were
equipped with a high-band SRK-145 RF module. It was also needed for the
reception of the later SRT-107 transmitters.
➤ More information
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Note that the SRR-56L supported the lower 240 - 330 MHz frequency band.
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When using the SRR-145 down-coverter shown above,
the existing SRN-9 listening post antenna has to be replaced by
one that is suitable for the 1300 to 1600 MHz frequency
range.
The SRN-55 is a flat stacked-dipole antenna that covers the entire
range and offers a gain of approx. 17.5 dB.
➤ More information
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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.
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© Crypto Museum. Created: Monday 17 April 2017. Last changed: Sunday, 25 December 2022 - 11:57 CET.
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