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Covert listening device with DP audio masking

SRT-91 was a covert listening device (bug), developed in the early 1970s by the Dutch Radar laboratory (NRP) for the US Central Intelligence Agency (CIA), as part of a long-term research contract code­named Easy Chair. The device featured the Dirty Pulse (RP) audio masking scheme and is part of the SRS-91 family of devices, which also includes the SRR-91 and SRR-90 receivers.
The device is housed in a rectangular metal enclosure that measures 13.5 x 22.5 x 110 mm, and is in fact the high-power variant of the SRT-90. It consists of an RF module that operates between 270 and 400 MHz, and a video coder for the Dirty Pulse (DP) 1 audio masking scheme.

When the SRT-91 was released for field use in June 1974, it marked the transition from conventional components and manufacturing techniques, to surface mount devices (SMDs), resulting in a much reduced size and a more efficient integration of the individual circuits.
SRT-91 transmitter

The device is powered by a DC source between +2.25V and 3.25V and consumes typically 800µA, whilst delivering a peak-output-power of 20mW. A low-power variant of the SRT-91 was made available as the SRT-90, which is slightly smaller and consumes 160µA whilst delivering 1mW. The first concepts of the SRT-91 were delivered to the CIA for evaluation in March 1970. After several improvements and further miniaturisation, it finally went into production in June 1974.

 More about its history
  1. Dirty Pulse (DP) audio masking is also known as Type 91 modulation.

SRT-91 transmitter SRT-91 transmitter Prototype SRT-91 aside a production unit Colour coded dots

According to an intermediate development study of October 1973 [3], the following models of the SRT-91 existed. Note that these designators are conflicting with the later frequency scheme shown below. We therefore assume that the module numbers below were for internal use only.
  • SRT-91 A
    270-400 MHz, DP masking, low-power → later: SRT-90
  • SRT-91 B
    270-400 MHz, DP masking, medium power
  • SRT-91 C
    L-band version (235-360 MHz)
  • SRT-91 D
    Binaural version (stereo)
  • SRT-91 E
    Wideband version (30 kHz audio)
The SRT-91 operates on a pre-determined spot frequency between 270 and 400 MHz, that is factory adjusted. The selected frequency is presented as a suffix to the transmitter's model number (e.g.: SRT-90-A). The following frequencies were used (± 5 MHz):
  1. 275 MHz
  2. 295 MHz
  3. 315 MHz
  4. 335 MHz
  5. 355 MHz
  6. 375 MHz
  7. 395 MHz
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-91 uses a sophisticated masking scheme, based on Pulse Position Modulation (PPM), known as Dirty Pulse (DP) masking.

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. In addition, the front porch of each pulse is shifted in time, under control of an internal random noise source. There are currently no known commercially available surveillance receivers or bug tracers that can readily demodulate a DP-masked audio signal. Most receivers won't even lock onto the carrier.

 More about DP audio masking

Along with the SRT-91, a new modular receiver was introduced that was capable of decoding the new Dirty Pulse (DP) masked audio signals. It was known as SRR-91, and was just 6 cm high, so that it could easily be fitted inside a standard executive style Samsonite briefcase or the era.

By installing the decoder module the other way around, the receiver could also be used for decoding RP-masked bugs, such as the SRT-56.

 More information
Lifting the hinged cover to get access to the modules

Signals from the SRT-91 can be received and demodulated with the following receivers:
Surveilance receiver SRR-91 Surveilance receiver SRR-90-A Surveilance receiver SRR-90-B

A complete SRT-56 transmitter consists of one or more of the following items:
SRT-91 transmitter
Dynamic microphone
Sleevex antenna Mercury battery cells

Transmitter   SRT-91
The transmitter is the core part of each bugging system. The SRT-91 measures 110 x 22.5 x 13.5 mm and houses the RF unit (transmitter) as well as the video encoder (the audio masking unit) in a single enclosure. As a result, wiring the unit for operation use has become extremely simple.

The red and black wires are used to provide a DC power source between 2.25 and 3.25V, whilst the two white coaxial cables are used for the connection of an antenna and a microphone.
SRT-91 transmitter

Although the SRT-91 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-91 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

Bugs like the SRT-91 were generally used for a limited period of time, which makes them ideal for battery-powered operation. Due to the fact that an SRT-91 consumes less than 800µA of current, the transmitter can work reliably for several months on just two mucury battery cells of 1.35 V each. If a longer life was needed, several mercury cells could be used in parallel.

If an even longer life was needed, the unit could also be operated from a dedicated mains power converter.

Top view of the interior of the SRT-91

The SRT-91 is housed in a strong brass enclosure that is sealed (soldered) hermetically. The cable end of the enclosure is then cast in a strong two-component epoxy, after which the entire unit is sprayed in grey or beige. The serial number is present as a series of three coloured dots.
Getting access to the interior of an SRT-91 is very difficult, and is not possible without badly damaging the unit. Luckily, we were able to obtain the laboratory model of the transmitter, which was used as an example during the manufacturing process in the mid-1970s.

As the model is not soldered and cast in expoxy, the interior can be extracted from the enclosure quite easily. The first half of the unit is taken by the pulse transmitter, also known as the RF unit, whilst the other half houses the video encoder. This part is responsible for the audio masking.
Video encoder (audio masking unit)

The video encoder consists of three stacked PCBs that are populated with the first generation of surface mount components, also known as surface mount devices (SMDs). The stack consists of a microphone amplifier and conditioner, a noise generator and the actual DP audio masking unit. The latter converts the audio signal into a series of short position-modulated pulses (PPM) that drive the RF unit, using noise to randomly change the position of the leading edge of each pulse.
Prototype SRT-91 aside a production unit SRT-91 laboratory model Removing the transmitter from the enclosure Transmitter partly removed from its enclosure, revealing the RF unit Interior Side view Top view SRT-91 interior compared to the size of a hand
Connections at one of the short sides Close-up of SRT-91 wiring RF unit (detail) RF unit Video encoder (audio masking unit) Side view of the video encoder RF-unit of the SRT-91 (top) and SRT-90 (bottom)

Block diagram
The block diagram below illustrates the operation of the SRT-91. At the left are the three stacked PCBs, of which the bottom one contains the microphone amplifier and the Automatic Gain Control (AGC). The PCB in the middle contains the random noise generator and the power regulator.

The upper PCB contains the actual video encoder, which is based on a 20 kHz master oscillator and a flip-flop (FF), that is set by the phase of the audio + noise signal, and reset by the phase of just the audio signal. This results in a series of short pulses with an average duration of 1µs, spaced 50 µs apart, that are used to drive the keyer of the 340 MHz pulse transmitter at the right.

The development of the SRT-91 dates back to 1966, when the first concepts of its audio masking scheme were tested, based on the Dirty Pulse (DP) and Super Pulse (SP) developments. The scheme is based on Pulse Position Modulation (PPM) with a repetition rate of 20 kHz. Masking is obtained by varying the pulse width randomly, based on the output of a (true) noise generator.
In the earliest implementation, the noise was used to alter the position of the trailing edge of each pulse, but this was later changed to the leading edge, so that only the trailing edge of each pulse could be used to recover the audio.

The first concept prototypes were delivered to the CIA in March 1970, and consisted of two separate enclosures: an SRK-43 RF unit, and an SWE-91 video encoder. At that time, the bug was still powered at 5.2V, just as previous bugs like the SRT-56. This was later lowered to 2.7V, so that it could be powered by two mercury cells.
Early prototype of the SRT-91, consisting of an SRK-43 RF-unit and an SWE-91 video encoder

After several tests, improvements, new concepts, a lower DC power supply, simplified wiring and a much smaller enclosure, the first production units were delivered to the CIA in June 1974, soon followed by the low-power variant SRT-90 in 1975. The SRT-91 was an important product for the NRP and the CIA, as it marked the transition from conventional components to SMD technology.

The use of SMD parts significantly reduced the size of the transmitter, especially in comparison with earlier transmitters like the SRT-56. In order to reduce the size even further, the CIA also commissioned other contractors to build pulse-type transmitters based on the NRP design of the SRT-91. One example of a compatible pulse transmitter that was made elsewhere is the SRT-99.
Early prototype of the SRT-91, consisting of an SRK-43 RF-unit and an SWE-91 video encoder Early prototype of the SRT-91, consisting of an SRK-43 RF-unit and an SWE-91 video encoder SRK-43 RF-unit (part of SRT-91 concept) SWE-91 video encoder (part of SRT-91 concept) SRK-43 interior SRK-43 RF-unit Transmitter detail

Path loss survey system   URS-1
A bugging system generally consists of a small transmitter with a low RF power ouput, in order to reduce power consumption and minimise the change of discovery.

In order to establish the requirements, the CIA commonly used a path loss survey system, such as the one shown here, in an environment which resembled the actual situation at the target area.

 More information
URT-1 transmitter and URR-1 receiver

  1. Provisional Technical Manual for SRT-91A
    NRP, March 1973. CM302504/A.

  2. Protype Evaluation of the XSRT-91A Pulse Position Modulated Transmitter
    CIA, project 74-1032. Date unknown, but probably June 1973.
    CM302504/B. 9 pages.

  3. Provisional Technical Manual for SRT-91B
    NRP, September 1973. CM302504/C.

  4. Provisional Technical Manual for SRT-91C
    NRP, October 1973. CM302504/D.

  5. Technical Manual for SRT-91A
    NRP, July 1974. CM302504/E.

  6. Technical Manual for SRT-91 (concept)
    NRP, June 1975. CM302504/F.

  7. Technical Manual for SRT-91
    NRP, march 1976. CM302504/G.

  1. NRP/CIA, Collection of documents related to SRT-91
    Crypto Museum Archive, CM302504 (see above).

  2. NRP/CIA, Collection of documents related to SRS-91
    Crypto Museum Archive, CM302629.

  3. Study of Further SRS-91 System Developments
    NRP, October 1973. CM302629/F.

Further information

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Crypto Museum. Created: Thursday 09 March 2017. Last changed: Saturday, 13 May 2017 - 07:22 CET.
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