Homepage
Crypto
Spy radio
Burst encoders
Intercept
Covert
Index
Glossary
Cameras
Recorders
Radio
Bugs
Microphones
Dead Drop
Lock picking
Stories
Radio
PC
Telex
People
Agencies
Manufacturers
• • • Donate • • •
Kits
Shop
News
Events
Wanted
Contact
About
Links
   Logo (click for homepage)
Audiotel SCX
FM radio sub-carrier bug

The FM/SC is an eavesdropping device (bug) that can pickup an conversation in a room and transmit it via radio, using Frequency Modulation (FM) or modulated onto a Sub-Carrier (SC). The transmitter was developed by Audiotel in Corby (UK) and was intended for training sweep teams on their Scanlock receivers, but was also suitable for actual covert bugging operations.
 
The image on the right shows a typical FM/SC bugthat 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 SC.

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

In this mode (SC) audio is modulated in AM onto a non-audible carrier. To the average interceptor, the transmitters appears to be sending a silent carrier, whereas in reality is carries the full sound 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

 
Further information

Any links shown in red are currently unavailable. If you like the information on this website, why not make a donation?
Crypto Museum. Last changed: Thursday, 16 May 2013 - 06:28 CET.
Click for homepage