"Balanced" cables provide protection against noise by having a copy of the audio signal in the opposite polarity. The receiving device will invert one of the signals and combine them, getting rid of any noise present in both signals (known as "common-mode interference"), since the noise will be "out of phase" with itself.

The same thing happens if you mix two copies of the same audio file and invert the phase on one; the audio will disappear.

Of course, a balanced cable (which could otherwise be used for a stereo signal) can only transmit a mono signal.

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