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Covert listening device with RP audio masking

SRT-56-F is a covert listening device (bug), developed in 1968 by the Dutch Radar laboratory (NRP) for the US Central Intelligence Agency (CIA), as part of a long-term research contract under the codename Easy Chair (EC). The device features the Rejected Pulse (RP) audio masking scheme.

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.
SRT-56-F (later version)

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.

Two variants of the SRT-56-F
SRT-56-F with Sleevex antenna
SRT-56-F with Sleevex antenna
SRT-56-F (later version)
SRT-56-F compared to the size of a hand
Wiring of the later version
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Two variants of the SRT-56-F
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SRT-56-F with Sleevex antenna
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SRT-56-F with Sleevex antenna
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SRT-56-F (later version)
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SRT-56-F compared to the size of a hand
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Wiring of the later version
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Audio masking
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).

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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

Along with the SRT-56, the SRR-56 receiver was developed. It was suitable for the reception of DP-masked bugs and was similar to the SRR-52 receiver that was developed for the SRT-52.

Some time later, the existing SSR-52 receivers were modified to make them suitable for the reception of DP-masked bugs as well. The image on the right shows the SRT-56 in front of the upgraded SRR-52-M receiver.

 More about the SRR-56
 More about the SRR-52

SRT-56 in front of an SRR-52-M receiver

Signals from the SRT-56-F can be received and demodulated with the following receivers:

Surveilance receiver SRR-52
Surveilance receiver SRR-56
Surveilance receiver SRR-91
Surveilance receiver SRR-90-A
Surveilance receiver SRR-90-B
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

PR-100 portable monitoring receiver and HE-300 anenna

Power Supply Unit
Dynamic microphone
Sleevex antenna
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.

Power supply unit   UWP-56
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
UWP-56 power supply unit

  1. The UPW-56 is identical to the UWP-52 PSU of the SRT-52 transmitter.

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].

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
Knowles BA-1501

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
Yellow Sleevex antenna

Technical specifications
  • 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
  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.

  3. Wikipedia, Mercury battery
    Retrieved, April 2017.
Further information
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Crypto Museum. Created: Thursday 09 March 2017. Last changed: Saturday, 03 August 2019 - 07:06 CET.
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