Balanced Audio is basically a 3-wire audio path. The wire paths
are conveniently labeled as: (+) Positive, and (-) Negative. This is to reflect the fact that an audio signal is traveling
down both paths at the same time, with each path 180 degrees out of phase from it's counterpart. This "dual" path provides
for induced noise immunity. Since both paths arrive at the receiving end and are 180 degrees out of phase from one another,
any noise that is induced due to powers line inductance or miscellaneous signals will be equal in both paths. but in-phase.
Therefore when the balanced receiver (be it a transformer, or an active combiner) recombines the audio the two out of phase
signals result in a mirror image of the original signal. The noise (which is not out of phase on the audio pairs) self
cancels. This is because the signals are 180 degrees out of phase in the receiver after the balanced line receiver combines
them into one signal (since the noise is not balanced it becomes negative and positive, and therefore cancells itself out.
While a Balanced Line is a 3-wire link. Only a single pair is usually connected for any great distance.
This is because the center or common lead is usually an earth ground. The signal is distributed equally between the wires in the pair. The transformers at
each end ensure they are symetrically ballanced. An example is given in the figure below.
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