Noise suppression for wired signal transmission, or The art of getting a signal through a cable without adding noise from electromagnetic interference to the desired signal.
A twisted pair of wires, subjected to radiated electro-magnetic interference (EMI), will pick up some of that interference. The more equal the impedance between earth ground and each twisted wire is, the less noise will appear as a voltage difference between the twisted wires. This is utilised to transmit a desired signal as a differential voltage on a balanced line, which enables a receiver to suppress/disregard/ignore any common mode signal, and detect the desired differential transmission with a good SNR and EMI suppression. A mirrored/symmetric voltage does not improve common mode rejection in itself. The concept of balanced lines relies on impedance balance, and the transmitter, cable and receiver can all cause an imbalance that degrades the common mode rejection.
Balanced lines, as seen from an electrical engineering point of view:
http://www.douglas-self.com/ampins/balanced/balanced.htm
SOS describes how things are typically named when products are sold: http://www.soundonsound.com/sos/oct13/articles/qanda-1013-4.htm
Not everyone thinks "balanced" is short for "impedance balanced".
Elliot Sound Products (ESP) has three pages/projects for line drivers and DI’s
And two pages for describing balanced lines
Practical Line-Driving Current Requirements (regarding cable capacitance loading): http://www.rane.com/note126.html
Protection of inputs and outputs: http://www.thatcorp.com/datashts/AES5335_48V_Phantom_Menace.pdf Discharging the large coupling capacitors for DC-blocking may cause harmful current peaks.
From the Rane library at http://www.rane.com/library.html
Earthing: http://sound.westhost.com/earthing.htm
Electromagnetic fields from outside the cable may cause interference, and the shield current inside the cable may as well.
Described at http://www.rane.com/note166.html . Republished in 2010 under "Shield Current Induced Noise – Causes and Solutions" (from "The Syn-Aud-Con")
Two good papers, that are often referenced:
http://audiosystemsgroup.com/AES-SCIN-ASGWeb.pdf or http://www.gotham.ch/images/content/cable/aespaperneilmuncy.pdf