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Surveillance receiver - this page is a stub

SRR-56 was a surveillance receiver for the reception of covert listening devices (bugs) with RP audio masking, developed around 1968 by the Dutch Radar laboratory (NRP) for the US Central Intelligence Agency (CIA), as part of a long-term research contract codenamed Easy Chair (EC). The SRR-56 was developed as an essensial component of the CIA's SRS-56 surveillance system.

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.

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.

  • 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.
Compatible bugs
350 MHz bug with RP audio masking
350 MHz bug with RP audio masking
Low-power version of the SRT-91
Miniature 350 MHz transmitter (bug) with Dirty Pulse (DP) audio masking
SRT-107 transmitter
Subminiature transmitter developed as part of the Super Pulse project

  1. The SRT-107 can only be received when the SRR-145 down-converter is used as well. The SRR-145 is also needed when the SRT-52 or the SRT-56 is fitted with an SRK-145 RF-module (1300-1600 MHz).

Low-band setup
The diagram below shows a complete SRS-56 1 setup. At the left is the SRT-56 transmitter which consists of a microphone, an SWE-56 video encoder (the RP audio masking unit), and an SRK-35 RF-module with a center frequency of 350 MHz. A sleevex antenna transmits the signal that is picked up by the SRN-9 antenna at the listening post (LP), where it is fed to the SRR-56 receiver.

  1. SRS = Surveillance Radio Set.

High-band setup
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.

A complete SRR-56 listening post consists of one or more of the following items:

SRR-56 receiver
Listening Post (LP) antenna for 300 MHz
External detector (test head)
Optional down-converter for the 1500 MHz band
Listening Post (LP) antenna for 1500 MHz
Receiver   SRR-52
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.
Photograph of SRR-56 not available. The exterior of the receiver was similar to the SRR-52.

LP antenna   SRN-9
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
Sen from the rear

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.

Detector   test head
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.
Video detector

Down-converter   SRR-145
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
SRR-145 down-converter

  1. Note that the SRR-56L supported the lower 240 - 330 MHz frequency band.

1500 MHz antenna   SRN-55
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
SRN-55 directional antenna

  1. Manual for SRS-56 Protype Equipment
    CM302491/A, March 1968.

  2. Operating Manual for SRS-56 Equipment
    CM302491/B, September 1969.

  3. Technical Manual for SRS-56 Equipment
    CM302491/C, September 1969.

  4. Manual for SRR-56 Receiver
    CM302491/D, January 1974.

  5. Manual for SRR-56L Receiver
    CM302491/E, March 1978.

  6. Manual for SRR-56H Receiver
    CM302491/F, September 1979.
  1. NRP/CIA, Collection of documents related to SRS-56
    Crypto Museum Archive, CM302491 (see above).

  2. NRP/CIA, Collection of documents related to AGC ignition interference
    Crypto Museum Archive, CM302626.
Further information
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Crypto Museum. Created: Monday 17 April 2017. Last changed: Sunday, 25 December 2022 - 11:57 CET.
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