The purpose of a balanced line is to transfer a “signal” from one place to another while rejecting “ground noise”, which is not white noise or hiss, but power line related hum and buzz. To accomplish this noise rejection, two signal lines are used and the IMPEDANCE of the two lines to ground must be equal or “balanced”. Since anything connected to the line affects its impedance, always consider the “system” consisting of the driver, the line itself, and the receiver. It is a popular belief that the signals must have opposing polarity and equal amplitudes, or symmetry. Signal symmetry has NOTHING to do with noise rejection. The system must reject noise even when there is no signal and, in fact, this is usually how system noise testing is done. Illustrated below is a generalized balanced system schematic, rearranged to show that it takes the familiar form of a Wheatstone bridge.

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