If you like to watch television while you use your computer, you may have noticed something funny happening when the channel is turned to certain stations. With the computer on, channel two on my television is complete static, while channels 3 and 4 get decreasingly snowy. This happens when electromagnetic fields radiating
from my computer and cables are picked up by the televi-sion antenna. If I'm watching channel 2, 1 can even make out a very fuzzy representation of what I see on the computer screen.
There is a simple reason for this happening. The various components of a computer - amplifiers, cables, the coupling between cables, the power supply to power line coupling, switching transis-tors, the ground loop, internal wires, and even printed circuit boards - all act as antennae to con-duct electromagnetic radiation. The
components, cables and whatnot will not only pick up the radia-tion, but transmit it as well, sometimes re-emitting it at some distance from the source equipment. Nearby electrical wiring and metal pipes can further act as antennae. Computers operate at radio frequencies and so they are also radio transmitters.
That's why the Federal Communications Commission must ap-prove all computers (and many other electronic appliances) before they can be sold in the United States. The FCC wants to make sure those radio emissions aren't strong enough to interfere with other licensed radio receivers (such as television sets). In fact, there
have been cases of unregistered computer monitors whose screens have been picked up on the next-door-neighbor's television set. This sort of thing is more likely to occur when the neighbor has a black and white television and the computer has a composite monitor, because a black and white set can more easily adapt the syn-chronization signals that it picks up from a com-posite monitor (especially if the TV has an antenna amplifier attached).
When my television receives computer fre-quencies, it is doing so accidentally. Imagine the consequences of someone setting out to purposely receive radiated information. Indeed, such a thing is possible, and has been going on for quite some time. For years the Department of Defense has stashed away its most hush-hush
computers and communications devices in copper-lined rooms to prevent radiation leakage. They have also pro-duced guidelines for a security standard called TEMPEST ( Transient Electromagnetic Pulse Emanation Standard. ) which defines how military computers are to be constructed so that the radiation leaking from
them is minimal.
Special military computers might be well pro-tected, but your run-of-the-mill PC or terminal is not. The FCC ensures that equipment won't inter-fere with other equipment; it makes no promises that equipment is safe from prying eyes. In fact, those eyes don't even have to be at the scene of the crime. There is an electronic
marvel called the Van Eck device which picks up your favorite leaked radiation and projects it onto a television screen. Hook up a VCR to the television and you've got a living document of everything that goes on in your target's computer account.