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Radio Frequency Interference (RFI) is a growing problem with PC-class machines. Today's processor speeds (above 33MHz) are such that the electromagnetic noise generated by a PC's circuitry in normal operation can degrade or jam radio and TV reception in the neighborhood. Such noise is called Radio Frequency Interference (RFI). Computers, as transmitting devices, are regulated by the Federal Communications Commission (FCC).
FCC regulations recognize two classes of computer:
If a PC is to be used in a home or apartment, it must be certified to be FCC class B. If it is not, neighbors have a legal right to prevent its use. FCC class A equipment is allowed in industrial environments.
Many systems are not FCC class B. Some manufacturers build boxes that are class B and then ship them with class A monitors or external disk drives. Even the cables can be a source of RFI.
It pays to be cautious. For example, the Mag MX17F is FCC class B. There are less expensive versions of the MX17 that are not. The Mag MX17 is a great monitor (I wish I had one). It would be painful to own one and not be allowed to use it.
An upgradeable system poses special problems. A system that is FCC class B with a 33 MHz CPU might not be when the CPU is upgraded to a 50 or 66 MHz CPU. Some upgrades require knockouts in the case to be removed. If a knockout is larger than whatever replaces it, RFI can leak out through the gap. Grounded metal shims can eliminate the leaks.
Even Class B systems don't mix will with wireless phonesets (not cellular phones, but the kind with a base station and antennaed headset). You'll often find a wireless phone hard to use withing 20 feet of a Class B machine.
To cut down on RFI, get a good metal case with tight joints, or at least make sure any plastic one you buy has a conductive lining. You can also strip the painted metal-to-metal contacting parts of paint so that there's good conductive metal contact. Paint's a poor conductor in most cases, so you can get some benefit from this.