A page about cabling considerations; mostly about noise.
Three pin connector. pins are wired (1)eXternal; (2)Live; (3)Return (cold), however, this is NOT the definition of 'XLR' - A company called Cannon had an 'X series' range of circular 3-pin connectors. They made a new version with a locking latch on it called the "XL series". This is the connector design now commonly used in Professional Audio. Much later, cannon decided to mount the contacts on the female ends out of a tough rubber material. They named this new connector "The XLR series". No-one else makes a rubber mounted XL female, yet the whole world (it seems) uses the term XLR when they actually are referring to XL...
As cables are a core in close-proximity to the screen, they act like a
capacitor, with the capacitance increasing as the cable gets longer. The
degree of capacitance is specified in pico-farads per metre.
Especially when used with high-impedance microphones, cable capacitance can lead to handling noise
48v DC, where present is delivered down the signal pins of balanced-line systems. The signal is then resolved using +48v as a "ground" level. Both of the signal lines have +48v present on them, with ground used as 0v. This allows any balanced, non-powered equipment to "see" the same voltage across the voice coil and thus not be damaged. HOWEVER, there is a significant chance that, during connection, one pin of the connector will close slightly before the second, allowing a voltage imbalance and damaging the microphone. SO, it's advisable, where possible to turn-off phantom power, or to only "power up" when all connections have been made.
Most people know that the output signal, rather than being a 2-pole
"signal and shield" (unbalanced) is 3-pole "signal, inverse
signal and shield".
But why does it work?
Inside the source equipment, the signal is split into two opposite, equal-amplitude phases.
These are fed into pins 2 and 3 of the XLR; the two central cores of the cable.
At the receiving end of the cable, the signal is 'made' from the difference between the two cores.
Because the cores of the wire are physically close together, the chances are, any induced interference (noise) will occur equally in both cores. When the receiving equipment examines the difference and thus produces one signal, any 'noise' should be equal in both cores and so there is no 'noise' difference thus no noise on the signal.
HOWEVER, it's not perfect, especially at higher frequencies, so all the usual considerations of minimising cable length and careful routing avoiding power supplies etc. etc.
On a more technical note, the great benefit is that the signal is derived relative to the inverse signal voltage, rather than relative to ground (ie: a conventional 'unbalanced' signal is the voltage difference relative to ground). Any factors to help ensure that noise is present equally will help (good transformers presenting identical impedance, good cable etc.)
COMMON MODE REJECTION
The capacity of a balancing transformer to cancel noise common to two lines of a cable; expressed in dB usually at a given frequency (usually 1kHz).