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 (low-Z), which makes long microphone cables susceptible to some forms of outside noise, and a perfect application for a balanced line, which cancels out most of this outside noise.
A balanced audio connection has two wires, one of inverted polarity to the other. (For instance, in an XLR connector, pin 2 carries the signal with normal polarity, and pin 3 carries an upside-down version of the same signal.) However, an XLR plug also carries a third connection - pin 1 is used as an earth to shield the other two. The received signal is the difference between the two signal lines. This signal recombination can be implemented with a differential amplifier where the negated signal is tied to the negative terminal of the operational amplifier. A balun may also be used instead of an active differential amplifier device. Much of the noise induced in the cable is induced equally in both signal lines, so this noise can be easily rejected - the noise received in the second, negated line is applied against the first, upright signal, and cancelled out when the two signals are added together.
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 hi-fi 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. This also prevents ground loops.
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 noise and distortion introduced by the extra circuitry they require.
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[edit] Internally balanced audio design
Most professional audio products (recording, public address, etc.) provide differential balanced inputs and outputs, typically via XLR 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 professional audio products have been designed as 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 pin 2 and pin 3 signals (AKA "hot" and "cold" audio signals). In critical applications, such as classical music recording, a 100% differential balanced circuit design can offer better signal integrity by avoiding the extra amplifiers and/or transformers required for front-end unbalancing and back-end rebalancing.
Examples of fully balanced professional audio products are manufactured by companies such as Millennia Media, 7th Circle Audio, and others.
[edit] Connectors
While XLR connectors are the most common balanced connector, quarter-inch (¼" or 6.5mm) TRS connectors (tip-ring-sleeve) are also commonly used. Many hybrid jacks are now designed to take either XLR or TRS.
On TRS plugs, the tip is "hot" (positive), the ring is "cold" (negative), and the sleeve is ground (earthed or chassis). 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. 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 conversation travel.
Data lines, including digital audio, are also frequently balanced, normally using AES/EBU (AES3) with XLR connectors for pro audio. Eight-channel analog balanced audio connectors like ADAT use DB25 connectors, which can also carry up to 16 digital channels.
If balanced audio must be fed into an unbalanced connection, the negative wire should be tied to ground.
[edit] Converters
Unbalanced signals can be converted to balanced signals by the use of a DI unit
