Twisted Pair Wiring; Shielded And Unshielded Cable; Balanced And Unbalanced Lines - Toa A-503A Manual

TOA Electronics Amplifier Guide
The two most popular methods to reduce the pickup of induced noises through sound system wiring
are the use of twisted pair wiring, and the use of shielded cable.

Twisted Pair Wiring

Twisted pair wiring is just what it sounds like: two insulated conductors are twisted around each other
over the length of the cable run. The twisting has the effect of rejecting certain types of induced
noise, since each half-turn of the wire exposes it to the noise source with the opposite polarity of the
preceding half-turn. The effect also works in reverse: twisted pairs generate less noise than pairs run
in "flat," untwisted wire. This fact helps to reduce the effect of "crosstalk" between pairs when multi-
ple lines carrying similar signals are bundled together. Twisted pairs have been used by telephone
companies for the better part of a century to carry voice communications, and are now the standard
type of cable for Ethernet networking and other data transmission protocols (for example, CAT 5
wiring is simply a set of twisted pairs).
In sound systems, twisted pairs are often used for speaker wiring, especially over longer distance runs.
For other sound system applications, twisted pair wiring is seldom used, except in conjunction with
shielding and balancing (see Balanced and Unbalanced Lines, below). So, while CAT 5 may be the cat's
meow in data networking, you don't want to use it for your microphone wiring, or you risk serious
noise problems.

Shielded and Unshielded Cable

Shielded cables are the most common, and a more effective, line of defense against noise pickup in
audio applications. They protect the signal path from noise pickup by surrounding one or more of the
cable's conductors with another conductor (the shield) that is tied to ground at one or both ends of
the line. Shielded cables should always be used for microphone wiring. They should also be used for
all unbalanced line level wiring, such as the outputs of CD players, tape decks, or many other common
music sources. Standard stereo RCA patch cords are a common example of shielded wiring for unbal-
anced sources.

Balanced and Unbalanced Lines

The most effective defense against the pickup of induced noise through the wiring is to use a "bal-
anced" circuit for the connection between equipment. This method involves not only using the right
cable, but also having a certain type of input and output circuit. In sound systems, balanced circuits,
or balanced lines, are typically run using three conductors — a twisted pair of inner conductors sur-
rounded by a shield conductor. Running a balanced line requires the use of balancing output and
input circuits, which work by splitting the signal into two paths, then inverting the polarity of one
path, so that each conductor carries a signal that is the exact electrical opposite of the signal on the
other conductor. While the signal is carried by the two conductors in opposite polarity, the noises that
accumulate on the line will have the same polarity on both conductors. When the polarity of the
reversed "low side" conductor is reversed again at the receiving end, any noise picked up by the line
will be cancelled out. The combination of this balancing action with the use of shielded cable, and the
twisted inner pair makes this arrangement the best for protecting audio signals from noise pickup.
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