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".
From the Rane library at http://www.rane.com/library.html
http://www.rane.com/note151.html Grounding and Shielding Audio Devices
http://www.rane.com/note110.html Sound System Interconnection
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")
Part one: http://www.prosoundtraining.com/site/articles/shield-current-induced-noise-causes-and-solutions/
Two good papers, that are often referenced:
J. Brown and B. Whitlock, Common-Mode to Differential-Mode Conversion in Shielded Twisted-Pair Cables (Shield-Current-Induced Noise), Audio Engineering Society 114th Convention, 2003.
http://audiosystemsgroup.com/AES-SCIN-ASGWeb.pdf or http://www.gotham.ch/images/content/cable/aespaperneilmuncy.pdf
N. Muncy, Noise Susceptibility in Analog and Digital Signal Processing Systems, Journal of the Audio Engineering Society, June 1995, pp. 435-453.
http://www.gotham.ch/images/content/cable/Muncy8588.PDF
Demonstration of interference into cables, with one having high rejection and one having low rejection: https://vimeo.com/155057552
Procedure and tools for pinpointing a problem
http://web.mit.edu/~jhawk/tmp/p/EST016_Ground_Loops_handout.pdf (by Bill Whitlock)
Same tutorial, in slide format: http://www.bennettprescott.com/downloads/grounding_tutorial.pdf
Tools inspired by the tutorial above, in Danish: https://plus.google.com/u/0/+NikolaiBeier/posts/CR9t3G8QBB4