Click for homepage
← Easy Chair
  
CIA
NRP
  
SRT-56-F
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
SRT-56-F compared to the size of a hand
Wiring of the later version
Connector
A
×
A
1 / 8
Two variants of the SRT-56-F
A
2 / 8
SRT-56-F with Sleevex antenna
A
3 / 8
SRT-56-F with Sleevex antenna
A
4 / 8
SRT-56-F (later version)
A
5 / 8
SRT-56-F
A
6 / 8
SRT-56-F compared to the size of a hand
A
7 / 8
Wiring of the later version
A
8 / 8
Connector

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).

Click to see more

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


Receivers
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
Countermeasures
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

Parts
Transmitter
TX
Power Supply Unit
PSU
Dynamic microphone
Mic
Sleevex antenna
Transmitter
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.
  
SRT-56-F

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.

Batteries
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].

Microphone
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

Antenna
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
Documentation
  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.
References
  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
Any links shown in red are currently unavailable. If you like the information on this website, why not make a donation?
Crypto Museum. Created: Thursday 09 March 2017. Last changed: Saturday, 03 August 2019 - 07:06 CET.
Click for homepage