Apple Final Cut Pro 5 - Manual (Page 816)

Chapter 2 Setting Up Audio Equipment 29
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When the signal arrives at its destination, the inverted signal is put back in phase and
both signals are combined. This puts the original and inverted signals back in phase,
but it causes the noise signals on each line to be out of phase.
Now, both audio signals are in phase, but the noise is inverted, causing the noise to be
canceled. At the same time, the original signal gets a little stronger because it is sent
on two wires and combined. This helps compensate for the reduction in signal strength
that occurs naturally on a long cable run.
Any noise introduced into the cable across its long run is almost completely eliminated
by this process.
Note: Unbalanced cables have no way of eliminating noise, and are therefore not as robust
for long-distance cable runs, microphone signals, and other professional applications.
Inverted signal
(inverted again)
Combined signals
(noise eliminated)