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 three-pin 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.
Applications.
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.
Interference reduction.
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 signalone 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.
The term "balanced" comes from the method of connecting each wire to identical impedances at source and load. This means that much of the electromagnetic interference will induce an equal noise voltage in each wire. Since the amplifier at the receiving end measures the difference in voltage between the two signal lines, noise that is identical on both wires is rejected. The noise received in the second, inverted line is applied against the first, upright signal, and cancels it out when the two signals are subtracted. 
This differential signal recombination can be implemented with a differential amplifier. A balun may also be used instead of an active differential amplifier device.
The wires are also twisted together, to reduce interference from electromagnetic induction. A twisted pair makes the loop area between the conductors as small as possible, and ensures that a magnetic field that passes equally through adjacent loops will induce equal levels of noise on both lines, which is canceled out by the differential amplifier. If the noise source is extremely close to the cable, then it is possible it will be induced on one of the lines more than the other, and it will not be canceled as well, but canceling will still occur to the extent of the amount of noise that is equal on both lines.
The separate shield of a balanced audio connection also yields a noise rejection advantage over an unbalanced two-conductor arrangement (such as used in typical home stereos) where the shield must also act as the signal return wire. Therefore, any noise currents induced into a balanced audio shield will not be directly modulated onto the signal, whereas in a two-conductor system they will be. This also prevents ground loop problems, by separating the shield/chassis from signal ground.
Differential signaling.
Signals are often transmitted over balanced connections using the differential mode, meaning the wires carry signals of opposite polarity to each other (for instance, in an XLR connector, pin 2 carries the signal with normal polarity, and pin 3 carries an inverted version of the same signal). Despite popular belief, this arrangement is not necessary for noise rejection. 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. A simple method of driving a balanced line is to inject the signal into the "hot" wire through a known source impedance, and connect the "cold" wire to the signal's local ground reference through an identical impedance. 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:
Internally balanced audio design.
Most audio products (recording, public address, etc.) provide differential balanced inputs and outputs, typically via XLR or TRS phone connectors. However, in most cases, a differential balanced input signal is internally converted to a single-ended signal via transformer or electronic amplifier. After internal processing, the single-ended signal is converted back to a differential balanced signal and fed to an output.
A small number of audio products have been designed with an entirely differential balanced signal path from input to output; the audio signal never unbalances. This design is achieved by providing identical (mirrored) internal signal paths for both the "non-inverting" and "inverting" audio signals. In critical applications, a 100% differential 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. Fully balanced internal circuitry has been promoted as yielding 3 dB better dynamic range, though at increased cost over single-ended designs.
Connectors.
Three-pin XLR connectors and quarter-inch (¼" or 6.35 mm) TRS phone connectors are commonly used for balanced audio signals. Many jacks are now designed to take either XLR or TRS phone plugs. Equipment intended for long-term installation sometimes uses terminal strips or Euroblock connectors.
With XLR connectors, pins 1, 2, and 3 are usually used for the shield (earthed or chassis), the non-inverting signal, and the inverting signal, respectively. (The phrase "ground, live, return", corresponding to "X, L, R", is often offered as a memory aid, although the inverting signal is not simply a "return.") On TRS phone plugs, the tip is non-inverting, the ring is inverting, and the sleeve is ground. 
If a stereophonic or other binaural signal is plugged into such a jack, one channel (usually the right) will be subtracted from the other (usually the left), leaving an unlistenable L − R (left minus right) signal instead of normal monophonic L + R (left plus right). Reversing the polarity at any other point in a balanced audio system will also result in this effect at some point when it is later mixed-down with its other channel.
Telephone lines also carry balanced audio, though this is generally now limited to the local loop. It is called this because the two wires form a balanced loop through which both sides of the telephone call travel. Note that the telephone line is balanced for AC (audio) signals but is actually unbalanced at DC, as one 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 while transmitting DC power to the telephone and allowing simple on/off hook detection. 
Digital audio connections in professional environments are also frequently balanced, normally following the AES3 (AES/EBU) standard. This uses XLR connectors and twisted-pair cable with 110-ohm impedance. By contrast, the coaxial S/PDIF interface commonly seen on consumer equipment is an unbalanced signal.
Converters.
Unbalanced signals can be converted to balanced signals by the use of a balun, often through a DI unit (also called a "DI box" or "direct box"). 
If balanced audio must be fed into an unbalanced connection, the electronic design used for the balanced output stage must be known. In most cases the negative output can be tied to ground, but in certain cases the negative output should be left disconnected.