Common mode

 Referring to equal voltages induced in the two wires of a signal-carrying pair. In a balanced line circuit, the signal voltages are of opposite polarity in the two signal wires. Any voltage which appears with the same polarity on each wire is called a common-mode voltage. Usually noise, such as a 60Hz hum, is induced in audio cables equally and in the same direction, and so is a common-mode voltage. If the signal is connected to a differential amplifier input, the common-mode voltages will cancel, while the signal voltages, being of opposite polarity on each input terminal, will add together. This is the reason why balanced lines are less prone to induced noise from external influences. See CMRR.