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Balanced Signal

Electronic audio signals consist of an alternating current across a hot wire and an earth wire, this is a typical unbalanced system. Unless shielded, the cable may be subjected to audible interference from sources including nearby electrical cables (causing the introduction of a mains hum) or radio signals (typically AM radio stations and business radio communications). In a live or studio environment using heavily shielded unbalanced cables and complicated cable runs to avoid interference is both expensive and time consuming. Therefore it is common to use a system of 'balancing' the audio before sending it to its destination.

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Balanced Audio

A balanced signal actually consists of two individual copies of the same source. One signal is the original (signal+) while the other is the same signal with its wave phase exactly 180 degress oposite to that of the original (signal-).

      ___
\    /   \     /
 \__/     \___/
  __       ___
 /  \     /   \
/    \___/     \

The top ASCII diagram shows the original waveform while the lower shows the phase inverted or polarity reversed signal. When added together, these two signals would result in zero signal (as they would both exactly cancel). It is important for a balanced cable to use twisted pair wires, this allows any interference to be picked up evenly by each wire. Noise picked up by both wires will actually be inphase with each other, e.g.

      ___
\    /   \      /
 \__/     \_/\_/
  __       _/\_
 /  \     /    \
/    \___/      \

At the receiving equipment, the signal- is flipped back into phase with the signal+ and the two signals can be added, as aforementioned, the now out of phase noise would cancel out leaving a doubled original signal. E.g.

If the received signal is

      ___
\    /   \      /
 \__/     \_/\_/
  __       _/\_
 /  \     /    \
/    \___/      \

Then the signal- (bottom siginal) is flipped to look like

      ___
\    /   \      /
 \__/     \_  _/
            \/

When added to the signal+, you get

          ___
\        /   \         /
 \      /     \       /
  \    /       \     /
   \__/         \___/

This is because

      
_  _ + _/\_ = ____
 \/

It is important to note that balancing is not carried out by digital signal processing but usually by a transformer based preamp or an electrical circuit (creating a pseudo balanced system). This method of noise removal is not perfect and cables for critical tasks (such as microphones) still need to be "screened" or "shielded" but not as heavily as would an unbalanced counterpart over the same distance.

Once inside of the receiving equipment, audio buses are typically not balanced as the runs are very short and are not susceptable to as much interference as, for example, a long cable run through a multicore.

Typically for a cable to be balanced it needs 2 cores and a "shield" or "screen" (ground). For example an XLR cable will have the 3 pins connected 1(ground) 2(signal+) and 3(signal-), however this is not always the case as American and European standards vary. Jack plugs can also be used for balanced audio, a TRS jack is needed. If your equipment only has jack inputs and outputs, check whether its inputs and outputs support balanced audio and use a balanced connection where possible.

Unbalancing

If an input only accepts unbalanced inputs you can "unbalance" a cable by connecting the signal- connection to the ground connection. On a European XLR3 this involves bridging pins 3 to 1. On a Jack connector this is bridging the ring and sleeve (the electrodes in a piece of equipment may bridge between the two). You should only do this on the destination end (ie at the unbalanced input).

It may be useful to have an unbalancing so that standard balanced cables can be used where appropriate.

See Also

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