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Audiotel SCX
FM radio sub-carrier bug

The SCX is a professional covert listening device (bug), developed around 1990 by Audiotel in Corby (UK). The device features professional subcarrier (SC) audio masking. Although it was intended for training purposes, it was also suitable for actual covert listening operations.
 
The image on the right shows a typical SCX bug that operates at approx. 170 MHz. The device measures 7 x 6.5 x 2 cm and is powered by an standard internal 9V battery. At the front are two switches: one to turn the power ON and OFF, and one to switch between FM and subcarrier.

When set to FM, the transmitter behaves like a common radio bug with Frequency Modulation (FM). In this mode, the bug is detectable with all common intercept receivers. When set to SC however, the sound that is picked up by the microphone is modulated onto a subcarrier (SC).
  
FM/SC bug

In SC mode, audio is Amplitude Modulated (AM) onto a carrier well outside the audible range. To the average eavesdropper, the transmitter appears to be sending a silent carrier, whereas in reality is carries the full audio picked up in the room. The sound can only be demodulated with a suitable SC receiver or with a bug tracer, such as the Audiotel Scanlock Mark VB or later receivers.

Checking the signal of the transmitter on a spectrum analyzer reveals what is happening. If we monitor the signal from the bug in FM mode, we see a carrier of which the frequency is altered by the audio signal. Watching the same signal in SC mode, shows a fixed carrier with two sidebands.
 
FM/SC bug FM/SC training transmitter with wire antenna Opened FM/SC bug with battery Interior with battery FM signal as seen on a spectrum analyzer SC signal as seen on a spectrum analyzer Testing the transmitter in SC mode

 
Training
The SCX was often supplied as a training device with Audiotel bug tracers such as the Scanlock ECM shown in the image on the right.

 More information
  
Scanlock ECM

 
Further information

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Crypto Museum. Created: Friday 15 May 2015. Last changed: Wednesday, 12 April 2017 - 09:47 CET.
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