Balanced audio
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.
The balanced interface guarantees that induced noise appears as common-mode voltages at the receiver which can be rejected by a differential device.
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.
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.
Since the differential device at the receiving end only responds to the difference in voltage between the two signal lines, noise that is identical on both wires is rejected.
As long as the impedances are balanced, noise will couple equally into the two wires (and be rejected by a differential amplifier), regardless of the signal that is present on them.
Due to common misconceptions about differential signalling, this is often referred to as a quasi-balanced or impedance-balanced output, though it is, in fact, fully balanced and will reject common-mode interference.
However, there are some minor benefits to driving the line with a fully differential output: Professional audio products (recording, public address, etc.)
In critical applications, a 100% balanced circuit design can offer better signal integrity by avoiding the extra amplifier stages or transformers required for front-end unbalancing and back-end rebalancing.
As telephones require DC power to operate and to allow simple on/off hook detection, extra circuitry was developed where one signal wire is fed from the exchange power bus, typically −50 volts, and the other grounded, both via equal value inductors which have about 400 ohms DC resistance, to avoid short-circuiting the wanted AC signal and to maintain impedance balance.
As a last resort a balanced audio line can be fed into an unbalanced input and vice versa as long as the electronic design used for the output stage is known.
