Reference - Are XLR 40 Analogue and XLR 40 Digital the same?
Answer:
Many people misunderstand balanced line thinking that the two signals in the hot and cold wires just have to be equal and opposite for it to work and while this is often true, it is not this that makes the transmission line “balanced” or which cancels the noise. The line is called balanced because each line must have exactly the same impedance with respect to ground and just like a Wheatstone Bridge* when the two arms (i.e. hot and cold) are balanced the noise signal which is between them is nulled and it is this which cancels the noise not the phase difference between the two signals. Now, when you have hot, cold and screen the impedance between the grounded screen and the twisted pair can vary slightly along the cable due to the twisting and non-perfect concentric geometry. So, in the Analog XLR QED adds a third wire to the twisted pair to act as ground because we can control the geometry more easily and we ground the screen at the sending end only. This approach makes for a more uniform and more accurately balanced line while still retaining the overall screen which now becomes an electrostatic one giving enhanced immunity to high voltage noise signals (like relays and switches) rather than just high current ones (like mains cables) which it could only protect from before. The electromagnetic screening and ground reference is thus provided by the third wire added to the central twisted pair.
For the digital XLR the only requirement from AES EBU is that the twisted pair have a 110 ohms characteristic impedance. So, for this cable the third wire is omitted and the screen is connected both ends as is customary.
* A Wheatstone Bridge is an electrical bridge consisting of two branches of a parallel circuit joined by a galvanometer and used for determining the value of an unknown resistance in one of the branches.