Balanced vs. Unbalanced
There are two basic ways to transport an audio signal in a system, that is through a microphone-level (balanced) or a line-level (unbalanced):
Unbalanced line — Utilizing a two-conductor cable, it transports the signal as the voltage between them. Electromagnetic interference (EMI) can sum to the signal as undesired noise. Connectors that carry unbalanced signals have two pins, such as RCA (phono) and ¼ inch TS (6.35mm, often referred to as mono jack). 3-pin connector such as XLR (cannon) may also carry unbalanced signals if one of the pins is unused.
Balanced line — Utilizing a three-conductor cable, one of them acts as a shield against electro-magnetic noise and is the ground conductor. The other two have the same voltage with respect to the ground conductor but with opposite signs. The noise that cannot be rejected by the shield affects both signal conductors in the same way. At the device’s input the two signals get summed with opposite sign, so that noise is cancelled out while the program signal doubles in level. Most professional audio devices use balanced inputs and outputs. Connectors that can carry balanced signal have three pins, such as XLR (cannon) and ¼ inch (6.35mm) TRS (stereo).