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* broadband harmonics of the various 'random' digital signals such as the video signal.<ref name="Eck1">[[#Eck1|Eck, 1985, p.2]]</ref>
Known as compromising emanations or [[Tempest (codename)|TEMPEST]] radiation, a code word for a U.S. government programme aimed at attacking the problem, the electromagnetic broadcast of data has been a significant concern in sensitive computer applications. Eavesdroppers can reconstruct video screen content from radio frequency emanations.<ref name="Kuhn1">[[#Kuhn1|Kuhn,1998, p.1]]</ref> Each (radiated) harmonic of the video signal shows a remarkable resemblance to a broadcast TV signal. It is therefore possible to reconstruct the picture displayed on the video display unit from the radiated emission by means of a normal television receiver.<ref name="Eck1"/> If no preventive measures are taken, eavesdropping on a video display unit is possible at distances up to several hundreds of meters, using only a normal black-and-white TV receiver, a directional antenna and an antenna amplifier. It is even possible to pick up information from some types of video display units at a distance of over 1 kilometer. If more sophisticated receiving and decoding equipment is used, the maximum distance can be much greater.<ref name="Eck2">[[#Eck1|Eck, 1985, p.3]]</ref>
==== Compromising reflections ====
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