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Audiotel MCX
Power line bug

The MCX is an eavesdropping device (bug) that uses the mains power line as its transmission medium, also known as a Mains Carrier Transmitter (MCX). The MCX was developed by Audiotel in Corby (UK) and was intended for training sweep teams on their Scanlock receivers, but was also suitable for covert operations when used in combination with the matching MCR receiver [1].
 
The MCX is housed in a small plastic case and measures 10 x 6 x 3 cm. It can be connected directly to the mains power (220V AC) and has a small sensitive electret microphone at the front, that picks up voice conversations many metres away. The MCX then modulates them onto a 120 kHz carrier that is send back into the mains.

The special MCR receiver is used to demodulate the mains carrier signal, after which a clear wide-band voice signal will be heard. Rather than using a dedicated receiver, it was also possible to use a modified power line intercom.
  
Interior of the SL-MCX power line bug

Power line communication was used extensively during the 1970s and 80s for intercoms and baby monitoring systems. It was a simple and effective means for eavesdropping as well, as it was possible to build relatively small transmitters that could be hidden inside a wall socket. In order to eavesdrop on a conversation in another room, the receiving end had to be connected to the same power phase as the transmitter. As three mains phases are commonly used to wire a house (R, S and T), some experimentation and luck were often necessary. As an example:

Example of a power line bug hidden in a room

Power line bugs could be detected with special bug finding equipment, such as the Scanlock 2000 and the Scanlock ECM. The MCX was created by Audiotel especially for training sweep teams, but could also be used as an effective bug. Today, power line bugs are hardly usable as their signal is disturbed by switched mode power supplies, Power Line Communication (PLC) Modems, and mains filters in many domestic and industrial appliences.
 
SL-MCX power line bug SL-MCX power line bug Microphone at the front panel Audiotel serial number tag Interior of the SL-MCX power line bug Interior Interior Top view

 
References
  1. Audiotel International Ltd., MCX/MCR Mains Carrier System
    Sales leaflet. © Copyright 1989, Audiotel.

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Crypto Museum. Created: Friday 15 May 2015. Last changed: Thursday, 06 April 2017 - 17:05 CET.
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