immunity of a single-ended system. To see why, consider a single-ended digital system with supply voltage . The high logic level is and the low logic level is 0 V. The difference between the two levels is therefore . Now consider a differential system with the same supply voltage. The voltage difference in the high state, where one wire is at and the other at 0 V, is . The voltage difference in the low state, where the voltages on the wires are exchanged, is . The difference between high and low logic levels is therefore . This is twice the difference of the single-ended system. If the voltage noise on one wire is uncorrelated to the noise on the other one, it takes twice as much noise to cause an error with the differential system as with the single-ended system. In other words, differential signalling doubles the noise immunity. Comparison with single-ended signalling In single-ended signalling, the transmitter generates a single voltage that the receiver compares with a fixed reference voltage, both relative to a common ground connection shared by both ends. In many instances, single-ended designs are not feasible. Another difficulty is the electromagnetic interference that can be generated by a single-ended signalling system that attempts to operate at high speed. Relation to balanced interfaces When transmitting signals differentially between two pieces of equipment it is common to do so through a balanced interface. An interface is a subsystem containing three parts: a driver, a line, and a receiver. These three components complete a full circuit for a signal to travel through and the impedances of this circuit is what determines whether the interface as a whole is balanced or not: "A balanced circuit is a two-conductor circuit in which both conductors and all circuits to connected to them have the same impedance to ground and to all other conductors." Balanced interfaces were developed as a protection scheme against noise. In theory, it can reject any interference so long as it is common-mode (voltages that appear with equal magnitude and the same polarity in both conductors). There exists great confusion as to what constitutes a balanced interface and how it relates to differential signalling. In reality, they are two completely independent concepts: balanced interfacing concerns noise and interference rejection, while differential signalling only concerns headroom. The impedance balance of a circuit does not determine the signals it can