Raise Your Shields
Many types of interference can affect an audio signal traveling along a wire. A common type is radio-frequency interference (RFI). If you’ve ever picked up a radio station in your system, you’ve experienced RFI. Typically, the culprit is a poorly shielded audio cable acting like an antenna.

RFI is a kind of electromagnetic interference (EMI) that is caused by radio-frequency transmissions. Other types of EMI include emissions from various sources, such as coils in motors, fluorescent lights, and rheostat lighting dimmers. Electrostatic (ES) noise is another potential source of problems in an audio system. Electrical sparks and static electricity are types of ES noise, either of which can cause artifacts and distortion in an audio signal.

Cable shielding helps protect audio from problems that are caused by RFI, EMI, and ES noise. In general, audio wire employs shielding between the conductor and the jacket to keep interference from reaching the conductor. For some types of EMI, however, cable shielding is not effective, and the only solution is to use balanced cables (see the pop-up “Balancing Act”). Shielding is essential for mic, guitar, and line cables. That’s because the signals from these sources are very low in level and must be amplified—which in turn amplifies the interference to the point of being audible.

Brazen Braids
One type of cable shielding consists of a braid of wire that completely encircles the conductor (see Fig. 1). Called braided shields, they offer excellent structural integrity, flexibility, and flex life, which is why they are commonly used in mic and guitar cables.

Braided shields typically cover 90 to 97 percent of the conductor. The higher the percentage of braid coverage, the more effective the protection. Cheap cables frequently have loosely wrapped braiding that covers a smaller percentage of the conductor. In this case, interference noise can still penetrate through the gaps in the braid and be picked up by the conductor; as the cable loosens with age, the problem only gets worse.

Foiled Again

FIG. 3: A dual-conductor cable with foil shield and drain wire. Note the lack of filler material.

Another type of cable shielding uses metal foil wrapped completely around the conductor (see Fig. 3). Called foil shields, these consist of aluminum foil laminated to a polyester film. Not only are foil shields lighter and less bulky than braided shields, but they are less expensive to make as well.

Foil shields provide 100 percent coverage of conductors, which improves protection from RFI, EMI, and ES noise. In addition, foil shields use a drain wire to aid in grounding electrostatic charges. This wire runs along the length of the foil and attaches to the connector at the output end, which leads these charges away from the destination of the signal. If you’ve ever cut open a cable with foil shielding and found one more wire than you expected, the extra one was the drain wire.

Foils shields have a shorter flex life than braids, so they are best suited for permanent cable installations. This makes foil-shielded cable a poor choice for any application that requires the cable to be moved, flexed, or otherwise handled regularly. Foil shielding is therefore not used for guitar and mic cables, because the foil would break with use.

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Reprinted with permission from Magazine, January, 2001
© 2001, Intertec Publishing, A Primedia Company All Rights Reserved