Apple Soundtrack Pro 3 - Manual (Page 518)

About Balanced Audio Signals
Audio cables can be either balanced or unbalanced, depending on their intended use.
For long cable runs, especially when using relatively low microphone levels, a three-wire
balanced audio circuit reduces noise. Balanced audio cables use the principle of phase
cancelation to eliminate noise while maintaining the original audio signal.
A balanced audio cable sends the same audio signal on two wires, but inverts the phase
of one signal by 180 degrees.
Original signal
Inverted signal
(reverse phase)
When noise is introduced into the cable, it is introduced equally to both the original and
the inverted signal.
Noise on line
(affects both signals)
518 Appendix C Working with Professional Video and Audio Equipment