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Covert listening device with RP audio masking

SRT-56 is a covert listening device (bug), developed around 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. The device features the Rejected Pulse (RP) audio masking scheme [A].

The SRT-56 consists of two or three cylindrical modules that contain the SWE-56 audio masking unit, also known as the video encoder, an RF-module (the transmitter), and a power source. Depending on the selected RF module, the set operated in the 350 MHz or the 1500 MHz UHF band. A complete 350 MHz set is shown here.

The basic version of the SRT-56 is adjusted at the factory to a spot frequency in the 315-400 MHz range, and can generally be recognised by the green colour 1 of its cases. The high-band version works between 1300 and 1600 MHz.
SRT-56 transmitter consisting of RF unit, video coder and PSU

The SRT-56 is very similar to the SRT-52, which was developed around the same time, but uses a different frequency range and a different audio masking scheme. Development of the SRT-56 was started in 1966, with the first prototypes being ready for evaluation in March 1968 [A]. It was first used in the field in September 1969 and was in production until at least 1974, but probably much longer. The high-band version of the SRT-56 was succeeded in 1974 by the integrated SRT-107.

The SRT-56 was developed as part of the CIA's SRS-56 surveillance system, that existed along­side the SRS-52. It is compatible with, and can be decoded by, the SRR-52-M, SRR-56, SRR-90 and SRR-91 receivers, although the SRR-145 down-converter is needed if the SRT-56 contains the high-band SRK-145 RF-module. A compact version of the SRT-56 is known as the SRT-56-F.

  1. Initially the modules of the SRT-56 were coated in green hamerite paint. Later versions were cast in a strong two-component dark green epoxy. Both types of coating are visible in the image above.

SRT-56 transmitter consisting of RF unit, video coder and PSU Complete SRT-56 kit Complete SRT-56 kit SRK-35 RF-module SWE-56 video encoder UWP-56 power supply unit SWM-25 contact microphone SRK-145
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SRT-56 transmitter consisting of RF unit, video coder and PSU
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Complete SRT-56 kit
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Complete SRT-56 kit
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SRK-35 RF-module
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SWE-56 video encoder
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UWP-56 power supply unit
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SWM-25 contact microphone
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The individual modules are each housed in a cylindrical brass enclosure with a standard diameter of 26 mm and a length of approx. 65 mm. The modules could be fitted inside a pre-drilled 1 1/8" hole in, say, a piece of furniture, a window pane or a concrete wall. All parts are connected with high-quality 6-pin Socapex plugs, to a common connector that is part of the wiring of the video encoder. The antenna and the microphone were connected via BNC plugs, or soldered directly.

The diagram above shows a typical SRT-56 target element, complete with a Knowles BA-1502 microphone and a suitable Sleevex antenna. In this example, the target element is powered directly from the mains, by means of the UWP-56 that is part of the kit. In situations were no mains power was available, long-life military-grade mercury batteries were used instead.

  • Standard version (350 MHz)
    Standard version of the SRT-56 that operates in the 315-385 MHz band. In a minimum configuration it consists of an SRK-35 transmitter and an SWE-56 video coder. When necessary, the SRT-56 could be made to work outside its regular frequency range.

  • Compact version (SRT-56-F)
    Same version as above, but in a compact rectangular enclosure, smaller than a pack of cigarettes, known as the SRT-56-F. It contains flat versions of the SRK-35 RF-module and the SWE-56 video encoder in a single enclosure.  More

  • Alternative version (290 MHz)
    Special version that operates in the (lower) 256-315 MHz band. This was done by swapping the SRK-35 RF-module for the SRK-29 that was normally used with the SRT-52.

  • Non-air version (SRT-56-C)
    Standard version of the SRT-56, of which the video encoder (SWE-56) is modified for use with a non-air microphone, such as the SWM-25 contact microphone.

  • High-band version (1500 MHz)
    In 1971, the CIA decided to move the operating frequency of their bugs from 300 MHz to the 1500 MHz band. In the SRT-56 this was done by swapping the SRK-35 RF unit for an SRK-145, and replacing the Sleevex antenna by an SRN-58 plexiglass antenna. It was this combination that was shown by the Soviets on a press conference on 10 April 1987.
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).

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 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

Discovery by the Soviets
In 1987, the American government accused the Soviets of bugging their embassy in Moscow. The Russians replied with a press conference on 10 April 1987, saying that the American claims were unfounded, but that instead the Soviet buildings in the US had been bugged by the Americans.

The image on the right shows the press meeting that day, on which the Soviets had made a real showcase from a variety of bugging devices that had so far never before been seen by the public. On the wall behind the table were several large panels with the actual devices and photographs of the buildings in which they had been found.

Soviet spokesmen explained to the assembled press that these were bugging devices that had recently be found in the walls of their buildings in the US, and that they had most likely been planted there by the Central Intelligence Agency.

Unfortunately, the quality of the above picture is not very good, and the available footage is not very clear either. But after carfully studying the available material, Crypto Museum has been able to identify one bugging device that was developed for the CIA, under the Easy Chair contract.

At the bottom of the second panel from the left, a group of four devices with a cluster of cables is visible. They are marked here with a red circle. The image on the right shows the same items as they appeared on national television that night.

The set appears to be a high-band version of the SRT-56, with its 1500 MHz plexiglass SRN-58 antenna clearly visible at the right. The leftmost unit is the SWE-56 video encoder that provides the audio masking facilities. The small unit next to it, is the SRK-145 high-band RF-unit, that is connected to the antenna and the video coder.

The 3rd item is a bit of a mystery and can not be easily identified. It is unpainted, which suggests that it might have been a prototype or a special. Judging from the photograph, it appears to be a long shiny cylindrical enclosure with a military-grade mercury battery clearly visible at the top.

It is likely that this item is a stack of four 1.35V mercury batteries (for a total of 5.4V) mounted inside a metal cylindrical enclosure. The top of the enclosure appears to be removed for the picture. Only two wires seem to come out at the bottom of the cylinder, which confirms that this is a battery. It might have provided energy to the SRT-56 for a period of three to six months. 1

The image on the right shows nearly the same set, assembled from objects in our collection. The set that is shown here, is fully operational, but the mercury battery is replaced by a PSU.
High-band version of the SRT-56, as shown by the Soviets at a press conference on 17 April 1987.

The differences in colour between the discovered set and our reconstruction, are easily explained, and can help to determine the age of the equipment. The combination that was found by the Soviets, was manufactured between 1971 and 1973 and was probably installed within that time frame. The leftmost unit was initially painted hamerite green, but from 1973 onwards it was cast in dark green expoxy. From the photograph, it seems likely that the hamerite version is shown.

This narrows the time frame down to 1971-1972. Since the battery was 15 years old by the time of the press conference, we can assume that the set was no longer operational. It seems therefore unlikely that it had been discovered recently, as claimed by the Soviets. It is far more likely that, in order to emphasize their claim, the Russians showed everything they had found over the years.

  1. This period could be extended significantly, by adding a switch receiver, such as the QRR-25, and turning the transmitter off between sessions. However, such a switch receiver was not shown by the Soviets.

News footage
The video clip below shows part of the original footage as it was aired in the US on the evening of 10 April 1987. It shows only part of the press conference and unfortunately it comes without any sound. If anyone has access to a more complete or better quality newsreel, please contact us.

A complete SRT-56 transmitter consists of one or more of the following items:

RF-unit (transmitter)
Video coder (audio masking unit) Power Supply Unit
Dynamic microphone
Contact microphone (non-air) Sleevex antenna Alternative RF-unit (transmitter) for the 1500 MHz band Plexiglass antenna for the 1500 MHz band
RF unit   SRK-35
This is the SRK-35 transmitter that operates on a fixed frequency between 315 and 385 MHz, driven by a pulse shaper, that in turn is driven by extremely short pulses from the video encoder. The pulse shaper ensures that all pulses are of the same amplitude and length (approx. 0.5 µs).

The RF unit is powered by a DC source between +5V and +12V, and consumes between 1.5 and 5.5 mA. It produces a peak output power of 75 to 300 mW. It can be driven by an SWE-52, SWE-56, or SWE-56-C video coder. In 1971, it was replaced by the SRK-145 for use at 1500 MHz.

 Look inside the SRK-35
SRK-35 RF-module

  1. In SRK-35, the suffix 35 indicates the center of its frequency range, i.e. 350 MHz.

Video encoder   SWE-56
This is the part that converts the analogue audio signal into a masked Pulse Position Modulation (PPM) signal, using the Rejected Pulse (RP) audio masking scheme, also known as the 56 scheme.

In this scheme, the audio signal from a dynamic microphone is sampled and converted into a noise-controlled random sequence of pulses that drive the transmitter. Without the video encoder, the RF unit does not transmit a signal.

 More about Rejected Pulse audio masking
 Look inside the video encoder \

SWE-56 video encoder

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 [2]. 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 [2].

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

Contact microphone
Instead of the normal dynamic (air) microphone, it was als possible to use a so-called non-air or contact microphone, such as the SWM-25 shown in the image on the right. The microphone is housed in a brass cylinder that has the same diameter as the SRT-56 (26 mm), and is filled with oil, to provide internal damping.

The SWM-25 uses a piezo-ceramic element in shear-mode and can be used to listen through the thick concrete walls of a large building. 1

 More information
SWM-25 contact microphone

  1. When using the SWM-25 contact microphone with the SRT-56, a special version of the video encoder with a modified frequency response curve, the SWE-56-C, should be used.

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

High-band version   SRK-145
In 1971, following a series of studies into the use of the higher frequency bands for covert listening devices, the SRK-145 RF unit was introduced as an alternative for the existing SRK-35 units. It is somewhat shorter than the existing ones, and is normally painted in the same colour as the SRT-56. The one shown here is a laboratory model, which is why it is beige.

The SRK-145 works on a spot frequency in the 1300 - 1600 MHz band and requires the use of the SRN-58 antenna instead of the Sleevex.

 Look inside the SRK-145


Plexiglass antenna
When the SRK-145 high-band RF unit was used, the Sleevex antenna had to be swapped for the small end-fed vertical dipole shown in the image on the right. The antenna is fore-shorted and is embedded in a plexiglass (perspex) stick that has the same diameter as the SRT-56 modules.

The plexiglass SRN-58 antenna was also used with the high-band version of the SRT-52 and with the later integrated SRT-107 transmitter.

 More information

SRN-58 sleeve antenna for 1500 MHz

SRK-35 RF-module SRK-35 RF-module SWE-56 video encoder SWE-56 video encoder Connector on the video encoder cable Connector on the video encoder cable Reverse side SRK-35 RF-module
1500 MHz RF unit (SRK-145) SRK-145 Collection of Sleevex antennas
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SRK-35 RF-module
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SRK-35 RF-module
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SWE-56 video encoder
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SWE-56 video encoder
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Connector on the video encoder cable
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Connector on the video encoder cable
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Reverse side
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SRK-35 RF-module
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1500 MHz RF unit (SRK-145)
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Collection of Sleevex antennas

Showing the interior of the SRT-56 is difficult, as all of its modules were soldered hermetically. Further­more, the video encoder and the PSU are fully cast in black epoxy, making it impossible to show the interior. Nevertheless we are able to show the interior of some laboratory models below.

The standard SRK-35 RF-unit is the only module of the set, that is not cast in expoxy, because of the dielectric effects of the material on the RF components. Furthermore, it has to be possible to adjust the transmission frequency from the outside of the case. Instead, the components are protected against moist by a conformal coating.

The image on the right shows the interior of the RF unit of an SRT-52. At the centre is the main oscillator, built around a TR1062 transistor. The transistor is directly coupled to the tuned circuit that determines the bug's operating frequency.
SRK-35 interior

The oscillator is keyed by a pulse-shaper that is located at the top of the unit (assuming that the wiring is at the bottom), and is built around a 2N3866 transistor. It is driven by the modulated pulses from the video encoder, and shapes them so that they are of equal length and amplitude.

The most complex part of the SRT-56 is without doubt the video coder. It converts the sound that is picked up by the microphone, into a series of seemingly random pulses that are used to key the RF unit. This is known as audio masking.

The video encoder is housed in a brass cylinder, roughly the same size as the transmitter, and contains five electronic circuit blocks, known as cordwood structures. The entire unit is cast in an elastic substance and is fixated inside the brass cylinder at both ends with a strong black epoxy, due to which the SWE-56 cannnot be serviced.
SWE-56 video coder (RP audio masking)

For this photograph, we've removed the enclosure from a broken SWE-56 video encoder, so that we can see how it is constructed internally. The five cordwood circuit blocks are clearly visible and are separated by isolated discs, that have cut-outs for the wirings. A long blank wire, running over the full length of the assembly, provides ground (0V) to each of the cordwood modules.

The power supply unit (PSU) was initially much longer than the RF unit and the video encoder, but was later redesigned to fit inside the same size enclosure. As the entire PSU is cast in black epoxy, we are unable to show its contents, but from the documentation, its circuits are known.

At the heart of the PSU is a miniature toroid transformer that is connected directly to the mains network. It has separate windings for 110V and 220V AC. It was probably made by the Radio Corporation of America, especially for the UWP-56. It came with an individual test sheet.
Miniature toroid transformer

The transformer had to be this small in order to be fitted inside a standard brass cylinder that has an inner diameter of 25 mm. The rest of the PSU's circuit is placed on a single cordwood module.

Like the PSU, the battery provides two voltages, in order to allow adjustment of the transmitter's output power between 75 mW and 300 mW.

In 1971, new frequencies in the 1500 MHz band were allocated for covert listening devices, and the SRK-145 module was developed to replace the standard SRK-35 transmitter. The image on the right shows a lab model of the new SRK-145. Due to the higher frequency, the tuned circuit is smaller, as a result of which the entire unit is shorter. The cordwood structure at the right provides the -20V oscillator keying voltage.
Inside the SRK-145

The 1500 MHz RF-unit was also used as a replacement for the 290 MHz RF-unit of the SRT-52, where it was combined with the SWE-52 video coder. It was also used as the basis for the design of the SRT-107 transmitter, in which the RF unit was integrated with the SWE-56 video coder.

SRK-35 interior RF-module (interior) SWE-56 video coder (RP audio masking) Side view SWE-56 SWE-56 video encoder (TP) Inside the SRK-145 Tuned circuit of the SRK-145
-20V keyer for the oscillator Two cordwood structures of an SWE-52 PSU
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SRK-35 interior
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RF-module (interior)
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SWE-56 video coder (RP audio masking)
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Side view
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SWE-56 video encoder (TP)
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Inside the SRK-145
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Tuned circuit of the SRK-145
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-20V keyer for the oscillator
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Two cordwood structures of an SWE-52 PSU

  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.
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Crypto Museum. Created: Monday 17 April 2017. Last changed: Thursday, 07 March 2019 - 09:46 CET.
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