Where protection from electromagnetic interference and freedom from earth references are required, a balanced interface is used. The term ‘balanced’ refers to identical (balanced) impedances to ground from each of two signal carrying conductors which are enclosed within an all-embracing overall electrostatic screen. This screen is grounded (to catch and remove unwanted RFI), but plays no part in passing the audio signal or providing its voltage reference. Instead, the two signal wires provide the reference for each other — the signal is conveyed ‘differentially’ and the receiver detects the voltage difference between the two signal wires. Any interference instils the same voltage on each wire (called a common mode signal) because the impedance to ground is identical for each wire. As there is therefore no voltage difference between the two signal wires, any interference is therefore ignored (rejected) completely by the receiver.
Signals conveyed over the balanced interface may appear as equal half-level voltages with opposite polarities on each of the two signal wires — which is the most commonly described technique. However, modern systems are increasingly using a single-sided approach where one wire carries the entire signal voltage and the other a (grounded) reference for it. This configuration is known as 'Impedance Balanced' and it offers the advantages of needing a less complicated balanced driver output stage, and connection to an unbalanced destination provides the correct signal level (where as a symmertical balaned output would lose 6dB of signal level). The interference rejection properties of an impedance-balanced output are exactly the same as a symmetrical balaned output since effective interference rejection only requires identical impednaces to ground from both signal wires.