When a new consumer electronic device such as a computer, DVD player, blender, electric razor or other modern electronic marvel is offered for sale to the public the manufacture has to gain a special certification or authorization from the FCC. This process ensures that when the consumer uses the device that they will not interfere with other devices in the area. For example we don't want a DVD player or blender to accidentally jam all the TV, and cellular telephones in a five-block area due to a poor product design.

The FCC (Federal Communications Commission) and its foreign equivalent has created a series of formal standards which new equipment is evaluated against before it is offered to the public.

These new products are taken into a specialized laboratory, and an engineer completes a complicated battery of tests. These test results are then sent to the FCC who then approves or denies the authorization.

When modern electrical devices operate they generate electromagnetic fields. Digital computers, radio equipment, typewriters, and so on generate massive amounts of electromagnetic signals which if properly intercepted and processed will allow certain amounts of information to be reconstructed based on these "compromising emanations". Basically anything with a microchip, diode, or transistor, gives off these fields.

Compromising emanations are these unintentional intelligence-bearing signals, which, if intercepted and analyzed, potentially disclose the national security information, transmitted, received, handled, or otherwise processed by any information-processing equipment.

These compromising emanation signals can then escape out of a controlled area by power line conduction, other fortuitous conduction paths such as the air conditioning duct work, or by simply radiating a signal into the air (like a radio station).

An excellent example of these compromising emanations may be found in modems and fax machines which utilize the Rockwell DataPump modem chip sets and several modems made by U.S. Robotics. When these modems operate they generate a very strong electromagnetic field which may be intercepted, demodulated, and monitored with most VHF radios. This is also a very serious problem with many speaker phone systems used in executive conference rooms.

This is also a very serious problem with many fax machines, computer monitors, external disc drives, CD-R drives, scanners, printers, and other high bandwidth or high speed peripherals.

If an eavesdropper is using high quality intercept equipment the signal may be easily acquired several hundred feet or more away from the target.

In the consumer markets a slight amount of signal leakage really does not present a problem, however; if a computer processing classified information has a leak the results could be devastating.

To deal with this "signal leakage" issue the government developed a series of standards which lay out how equipment should be designed to avoid such leakage. The TEMPEST standard are really nothing more then several industry measurements standards which were adjusted by the NSA (they gave it steroids).

Really the only difference between a TEMPEST approved computer, and a consumer computer is that the NSA TEMPEST approved one will be in a special heavy metal case, will have special shielding, a modified power supply and a few other modifications which increase its price. The TEMPEST approved unit will also require the use of a special torque wrench anytime you have to work on it.