Editing Balanced audio (section)

== Differential signaling ==

Signals are often transmitted over balanced connections using the [[differential signaling|differential mode]], meaning the wires carry signals that are equal in magnitude but of opposite [[Electrical polarity|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.<ref name="Blyth">{{cite web
|url=http://www.soundcraft.com/support/white_papers.aspx#
|title=Audio Balancing Issues
|access-date=2010-12-30
|author=Graham Blyth
|author-link=Graham Blyth
|work=White Papers
|publisher=Soundcraft
| archive-url= https://web.archive.org/web/20101204065754/http://www.soundcraft.com/support/white_papers.aspx| archive-date= 4 December 2010 | url-status= live}}</ref><ref>{{cite book
 | title = Sound system equipment
 | edition = Third
 | year = 2000
 | publisher = [[International Electrotechnical Commission]]
 | location = Geneva
 | id = IEC 602689-3:2001
 | pages = 111
 | chapter = Part 3: Amplifiers
 | quote = Only the common-mode impedance balance of the driver, line, and receiver play a role in noise or interference rejection. This noise or interference rejection property is independent of the presence of a desired differential signal.
}}</ref>  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 <em>quasi-balanced</em> or <em>impedance-balanced</em> 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:
* Though the signal level would not be changed due to [[nominal level]] standardization, the maximum output from the differential drivers is twice as much, giving 6&nbsp;dB extra [[headroom (audio signal processing)|headroom]].<ref name="Blyth"/>
*Increasing cable capacitance over long cable runs decreases the signal level at which high frequencies are attenuated. If each wire carries half the signal voltage swing as in fully differential outputs then longer cable runs can be used without the loss of high frequencies.
* Noise that is correlated between the two amps (from imperfect [[power supply rejection ratio|power supply rejection]], for instance), would be cancelled out.
* At higher frequencies, the output impedance of the output amplifier can change, resulting in a small imbalance.  When driven in differential mode by two identical amplifiers, this impedance change will be the same for both lines, and thus cancelled out.<ref name="Blyth"/>