Common Mode Rejection is a measure of a balanced circuit's ability to cancel noise and interference.

It's what makes balanced connections so effective against noise.

A common-mode signal is any unwanted signal (such as electrical hum or radio interference) that is picked up equally by both wires of a balanced cable. The balanced input circuit (a differential amplifier) ​​works by "looking" only at the difference between the two wires. The audio signal is sent with inverted polarity on each wire, so the difference between them is large. The noise is equal on both wires, so the difference between them is zero. By subtracting one signal from the other, the circuit completely cancels the noise (the common-mode signal) and, at the same time, restores and amplifies it. the desired audio signal.

The effectiveness of this cancellation is measured in decibels (dB) and called the Common Mode Rejection Rate (CMRR).

The higher the CMRR value of a device, the better it is at rejecting noise.