Balanced audio

Balanced audio is a method of interconnecting audio equipment using balanced lines. This type of connection is very important in sound recording and production because it allows the use of long cables while reducing susceptibility to external noise caused by electromagnetic interference.

Balanced connections typically use shielded twisted-pair cable and three-conductor connectors. The connectors are usually XLR or TRS phone connectors. When used in this manner, each cable carries one channel, therefore stereo audio (for example) would require two of them.

Many microphones operate at low voltage levels and some with high output impedance (hi-Z), which makes long microphone cables especially susceptible to electromagnetic interference. Microphone interconnections are therefore a common application for a balanced interconnection, which cancels out most of this induced noise. If the power amplifiers of a public address system are located at any distance from the mixing console, it is also normal to use balanced lines for the signal paths from the mixer to these amplifiers. Many other components, such as graphic equalizers 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 cost introduced by the extra circuitry they require.

Balanced audio connections use a number of techniques to reduce noise.

A typical balanced cable contains two identical wires, which are twisted together and then wrapped with a third conductor (foil or braid) that acts as a shield. The two wires form a circuit carrying the audio signal; one wire is in phase with respect to the source signal, the other wire is reversed in polarity. The in-phase wire is called non-inverting, positive or "hot" while the out-of-phase wire is called inverting, phase-inverted, anti-phase, negative or "cold". The hot and cold connections are often shown as In+ and In− ("in plus" and "in minus") on circuit diagrams.

...
Wikipedia