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Covert listening device with SC audio masking - this page is a stub

SRT-93 was a covert listening device (bug), developed in the early 1970s by an unknown party, for the US Central Intelligence Agency (CIA). The device featured a subcarrier (SC) audio masking scheme with a frequency modulated subcarrier and noise injected into the baseband signal.

At present we have no further information about the SRT-93. If you can provide more details or a photograph, please contact us. New information will be added to this page as and when it becomes available. In the meantime, check out the other members of the SRS-91 family here.   

Suitable receiver
The SRT-93 was developed around the same time as the SRT-90 and SRT-91, which is why the CIA ordered the manufacturer of the SRR-90 receiver – the NRP – to develop a set of plug-in modules to make the receiver compatible with it.

 More information
SRR-90B receiver

Audio masking
In order to reduce the chance of discovery, professional bugs often feature a special technique known as audio masking. The SRT-93 features a popular FM-based subcarrier audio masking scheme, in which a frequency modulated (FM) subcarrier is frequency modulated onto an RF carrier, whilst at the same time injecting random noise into the unit's baseband signal.

 More about subcarrier (SC) audio masking

Despite the fact that the SRT-93 uses a sophisticated audio masking scheme, it is relatively easy to discover. First of all it transmits a continuous wave (CW) signal, which will exhaust the battery after several hours of use.

Secondly, a professional bug-tracer of the era, such as the ScanLock Mark VB, can detect and demodulate its signal within seconds.

 More about the ScanLock
Scanlock Mark VB in operation

  1. SRT-93R, technial manual (incomplete)
    Date and author unknown. Retrieved March 2017.
Further information
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Crypto Museum. Created: Thursday 09 March 2017. Last changed: Tuesday, 12 July 2022 - 09:11 CET.
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