XLR cables use differential designs and, as their name implies, they only let differences pass. They use two signal paths, inverted (negative) and non-inverted (positive), plus a ground. The inverted and non-inverted signals are exact reflections of each other, so where one has a peak, the other has a trough. This differential design helps cancel any noise injected in the line and may provide better sound than RCA connections.

Single-ended signals use a single conductor in the cable which typically also uses a shield wire to make a ground connection. The shield cannot be 100% effective, so noise picked up by the signal conductor becomes forever part of the audio signal is amplified through the signal path.