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CIA NRP EC ← SRR-91
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This transmitter was not developed by the
NRP
but by an unknown third party, probably in the US.
It uses 40 kHz subcarrier audio masking and requires an SC-module
in the SRR-91 or SRR-90 receiver.
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Developement of the system was started around 1968, with the first prototypes
using an early version of the DP audio masking scheme.
After several improvements, further miniaturisation of the transmitters,
and the move to a lower DC voltage, the first operational parts of the
system were released in 1974, consisting of the
SRT-91 transmitter and
the SRR-91 surveillance receiver.
The system introduced a new
audio-masking scheme for pulse-modulated
transmitters, that became known as Dirty Pulse (DP) or '91' masking.
It works by converting analogue audio into a series of position-modulated
pulses (PPM) of which the front edge is shifted in time under control
of a noise generator, making it impossible to decode the audio in a
non-compatible receiver.
The SRR-91 receiver had a novel modular design, that made it possible
to quickly repair a faulty unit in the field, simply by swapping a couple
of modules. Immediately after the release of the first units however, the
CIA put in a request to study further improvements, such as different
form factors and the possibility to add compatibility with subcarrier
modulated transmitters [F].
The study resulted in the release in 1975, of two new modular receivers
that were initially known as the SRR-91 Mark II,
but were later renamed by the CIA to SRR-90A
and SRR-90B. Being fully upgradable, they would
become the standard CIA receivers for the next two decades
for the majority of CIA bugs.
Also in 1975, the SRT-90
was introduced, which was in fact a low-power
variant of the SRT-91.
It was housed in a nearly identical enclosure, but was about 1 cm shorter.
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Other surveillance systems
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- Ontwikkeling (development) SRS-91
NRP (notes), August 1968 - January 1971. CM302629/A.
- Manual for SRS-91 Engineering Model
NRP, September 1968. CM302629/B.
- Operating Manual for SRS-91 Equipment
NRP, January 1970. CM302629/C.
- Technical Manual for SRS-91 Equipment 1
NRP, January 1970. CM302629/D.
- Technical Manual for SRS-91A (prototype, draft)
NRP. March 1973. CM302629/E.
- Study of Further SRS-91 System Developments
NRP, October 1973. CM302629/F.
- Operating and Test Manual for SRS 90-91 Dual-Channel Equipment (prototype)
NRP, March 1976. CM302629/G.
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To be used with SRS-52 or SRS-56 Technical Manual.
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- NRP/CIA, Collection of documents related to SRS-91
Crypto Museum Archive, CM302629 (see above).
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© Crypto Museum. Created: Saturday 06 May 2017. Last changed: Sunday, 25 December 2022 - 12:08 CET.
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