Balanced audio
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Balanced audio connections are extremely important in sound recording and production because they allow for the use of very long cables with reduced introduction of outside noise. The most common balanced connector is the 3-pin XLR, which is used with microphones because of its durable construction. Many microphones have low impedance (Z), which makes long microphone cables susceptible to some forms of outside noise, and a perfect application for a balanced line, which cancels out this outside noise.
A balanced audio connection has three wires. Two of these are used for the signal, and the third is a ground and is used to shield the other two. The signal is the difference between the two signal lines. Much of the noise induced in the cable is induced equally in both signal lines, so this noise can be easily rejected by using a differential amplifier or a balun at the input.
The separate shield of a balanced audio connection also yields a noise rejection advantage over a typical two conductor arrangement such as used on domestic HiFi where the shield is actually one of the two signal wires and is not really a shield at all, but relies on its low, but in practice not zero, impedance to signal ground. Any noise currents induced into a balanced audio shield will not therefore be directly modulated onto the signal whereas in a two conductor system they will be.
If the power amplifiers of a public address system are located at any distance from the mixing desk, it is also normal to use balanced lines for the signal paths from the mixer to these amplifiers. Many other components, such as graphic equalisers and effects units, have balanced inputs and outputs to allow this. In recording and for short cable runs in general, a compromise is necessary between the noise reduction given by balanced lines and the noise and distortion introduced by the extra circuitry they require.
While XLR connections are the most common balanced connector, 1/4" TRS (Tip Ring Sleeve) connectors are also commonly used.