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Surveillance receiver

SRR-91 is a surveillance receiver for the reception of a several types of covert listening devices (bug), developed around 1974 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 has a modular design and can easily be adapted to new and emerging audio masking schemes. The receiver was initially known as SRR-90 Mark I, but was renamed SRR-90-A after modification. 1
The SRR-91 had a flat rectangular design and was no higher than 60 mm, so that it would fit nicely inside a common briefcase of the era.

All controls are at the front half of the top panel, whilst all connections are located at the left side. The rear half of the receiver contains the various modules, and can be accessed by lifting the hinged lid that is held in place by two screws. The device is suitable for the reception of bugs that use the sophisticated Dirty Pulse (DP) audio masking scheme, also known as the 91 scheme, that was used by the SRT-90 and SRT-91 bugs.
SRR-91 surveillance receiver

It is also suitable for the reception of transmitters that use the Rejected Pulse (RP) audio masking scheme, such as the SRT-56, SRT-56-F and SRT-107 2 , although for these bugs, the DEMOD B plug-in had to be installed the other way around. For this reason the DEMOD B plug-in has a connector at either end. The SRR-91 superceeded the SRR-56 receiver, but not the SRR-52, as it is not suitable for transmitters with Triple Pulse (TP) audio masking scheme, such as the SRT-52.

The SRR-91 was developed during the course of 1973 and 1974, with the first production devices being delivered in November 1974. It was in production until at least 1976. About a year after its introduction, in 1975, the SRR-91 was succeeded 3 by the more advanced SRR-90 A/B, that was available in two variants: a straight-up tabletop model, and a flat one for use inside a briefcase.

 More about its history
  1. As part of the modification, the modules were immunized against pulse-type interference caused by the ignition of cars and motor cycles.
  2. For the reception of the SRT-107, the SRR-145 down-converter is required as well. This is also the case for the high-band version of the SRT-56.
  3. Although its model number suggests otherwise, the SRR-91 was developed and produced about one year earlier than the SRR-90. The reason for this is the fact the SRR-91 was initially known as the SRR-91 Mk I and the enhanced version, which would be produced later, as the SRR-91 Mk II. However, when the SRR-91 Mk II was ready for release, the CIA decided to rename it SRR-90.

SRR-91 with cables SRR-91 with closed cover Protective lid installed over the control panel SRR-91 SRR-91 surveillance receiver Lifting the hinged cover to get access to the modules SRR-91 with open cover Connections at the left side

The SRR-91 is designed for installation in a common slimline Samsonite executive style briefcase. As it consumes little power and does not produce heat, it can be operated safely from inside the briefcase. Under normal circumstances, the hinged lid over the modules, that are installed in the rear half of the unit, remains closed. The front half contains the controls as shown below.

Prominently visible is the large square indicator (meter) which is used for several measurements, controlled by the function selector at the bottom centre. To the right of the meter is the tuning knob, which has two scales: one for the LO and one for the HI band, selectable with the toggle switch at the bottom right. The unit is switched ON with the Power selector at the bottom left.

When tuning into the signal of a bug, the pulse locking knob should be tuned for the best quality of the reproduced sound, using the indicator to find the optimum. By default, the receiver was suitable for the reception of DP-masked bugs, like the SRT-91, but by installing the DEMOD B plug-in the other way around, it was made compatible with RP-masked bugs like the SRT-56.
SRR-91 control panel Mainframe rear view Connections at the left side Demodulator B plug-in with connectors at either end Dual demodulator module

Compatible bugs
The SRR-91 is suitable for receiving and decoding the following transmitters:
350 MHz bug with RP audio masking 350 MHz bug with RP audio masking SRT-90 transmitter SRT-91 transmitter
SRT-107 transmitter

  1. This transmitter requires the use of the SRR-145 down-converter.

SRR-91 modular receiver Listening Post (LP) antenna for 300 MHz Headphones Optional down-converter for the 1500 MHz band Listening Post (LP) antenna for 1500 MHz

Receiver   SRR-91
At the heart of the listening post is an SRR-91 receiver. It is just 6 cm high, allowing it to be fitted inside the thinnest Samsonite briefcase.

The receiver is fully modular, so that it can be repaired in the field within minutes, simply by swapping some of the modules. This also allows the receiver to be adapted to different masking schemes, although it is doubted whether this was actually done, as it was soon superceeded by the more flexible SRR-90 receiver.
Lifting the hinged cover to get access to the modules

LP antenna   SRN-9
A suitable directional antenna for the SRR-91 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 suitcase, along with the SRR-91 receiver and its accessories.

 More information
Sen from the rear

The SRR-91 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.

Down-converter   SRR-145
The frequency range of the SRR-91 (260 - 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-56 units that were equipped with a high-band SRK-145 RF module. It was also needed for the reception of the SRT-107 transmitter.

 More information
SRR-145 down-converter

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

Plug-in modules
Each module should be installed in a dedicated slot of the mainframe, as indicated by the name that is printed above each socket. The modules have a rectangular shape and have a connector at one of the short sides that mates with the socket on the mainframe. Eight modules are present:
  1. RF Converter (tuner)
  2. Filter
  3. IF Amplifier
  4. Demodulator A
  5. Demodulator B ← Interchangable
  6. Audio
  7. Regulator
  8. Battery
    Dual demodulator module
Demodulator B
The fifth module (DEMOD B), is the only one that has a connector at either end, allowing it to be installed the other way around. This was done to make the device compatible with both RP audio-masking (56) and DP audio-masking (91).

Only one 1 audio-masking scheme is supported, with the one readable on the label being the one that is in use. Installing the module the other way around, selects the alternative scheme. Other masking schemes could be supported by issuing replacements for the DEMOD B module, although in practice this never happened. 2
Demodulator B plug-in with connectors at either end

  1. The later SRR-90 receiver selects automatically between RP and DP masking.
  2. Modules for alternative masking schemes were developed for the later SRR-90 receiver however.

Plug-in slots Empty slots Plug-in modules Plug-in units seen from the right Plug-in units seen from the rear Dual demodulator module Demodulator B plug-in with connectors at either end Inserting the RF converter (tuner)

The SRR-91 measures 24.5 x 24.5 x 6 cm and weights 3.1 kg. It consists of a square aluminium frame of which the front half contains the so-called mainframe, that carries the power supply unit (PSU), plus all controls and connections. It can be covered by a protective beige aluminium panel.
The rear half contains the actual receiver which consists of eight plug-in units, or modules, each of which is installed in a dedicated slot at the rear of the mainframe. This section is covered by a hinged aluminium lid that is held in place by two screws that are inserted from the top.

The wiring for each module is unique, which means that a module can only be used properly when it is installed in its own slot, as identified by the name that is printed above each socket. Note that the modules are not compatible with those of the early prototypes, as shown below.
Selector switches

The SRR-91 is extremely well built, using only first-class components and wiring. The circuits inside the plug-in modules are covered in a conformal coating, in order to protect them against the environment. As a result, the unit featured on this page still works after 25 years of storage.
Plug-in slots Mainframe bottom view Plug-in slot wiring Spare fuse Power supply unit Selector switches Aux connector wiring Wiring detail

The first concepts for the SRS-91 family of devices, date back to 1968, with the conception of an early variant of the Dirty Pulse audio masking scheme. After several tests and improvements, the first SRR-91 prototype, shown in the image below, was delivered to the CIA in March 1973.
At that time, the modules were still housed in plain uncoated brass enclosures, which were identified with coloured dots rather than with descriptive names. The prototypes were used by the CIA to evaluate the modular concept and test the SRS-91 system (i.e. the SRR-91 receiver and the SRT-91 transmitter) under field conditions.

Although the receiver passed all tests, there were several recommendations and request for further features. Some of these suggestions made it into the final product, resulting in a number of changes in the wiring of the slots.
First prototype of the SRR-91

As a result of this, the modules of the prototype version can not be used in a production device and vice versa. A second evaluation candidate was released and approved in November 1974, immediately followed by full-scale production of the SRR-91-Mark I a month later in December 1974. In May 1975, following a modification, the SRR-91 Mark I was renamed SRR-91-A.

The recommendations and requests that did not make it into the final design, were implemented in its successor, the SRR-91 Mark II (later renamed to SRR-90), that was already in the early stages of its design. The first prototypes of the SRR-90 became available in November 1974 and full-scale production started in late 1975. It would become one of the most successful products.

  1. Manual for SRR-91-MK I receiver
    CM302500/A. November 1974 (draft).

  2. Manual for SRR-91-A Receiver
    CM302500/B. May 1975.

  1. NRP/CIA, Collection of documents related to SRR-91
    Crypto Museum Archive, CM302500 (see above).

  2. NRP/CIA, Collection of documents related to AGC ignition interference
    Crypto Museum Archive, CM302626.

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

Further information

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Crypto Museum. Created: Saturday 06 May 2017. Last changed: Saturday, 13 May 2017 - 07:24 CET.
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