Apple Soundtrack Pro 2 - Manual (Page 495)

Appendix C Working with Professional Video and Audio Equipment 495
A balanced audio cable sends the same audio signal on two wires, but inverts the
phase of one signal by 180 degrees.
When noise is introduced into the cable, it is introduced equally to both the original
and the inverted signal.
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.
Original signal
Inverted signal (reverse phase)
Noise on line
(affects both signals)
Inverted signal
(inverted again)