Most professional microphones today use XLR 3-pin connectors. Originally called "cannon" plugs after the original manufacturer, "XLR" became the standard name. Microphone cables with male and female XLR ends are "balanced" cables. With two or more wires inside a common shield, balanced cables have noise-cancelling properties that allow cables to extend to long lengths without signal loss. XLR cables do have a standard for pin numbering at connectors, but wire colours vary between manufacturers.
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Balanced vs. unbalanced
Audio needs only two conductors to move a signal from place to place. Electromagnetic interference from equipment or power cables can induce noise in a two-wire, unbalanced system over a long cable length. Three-wire, balanced systems send identical audio signals along the hot and cold wires, except the polarity is reversed on the cold wire. When signals are combined, any noise introduced into both wires over the length of the cable will be cancelled out. For this reason, proper wiring is essential in an XLR balanced system to ensure proper decoding of the audio signal.
Standard microphone cable
The most basic balanced mic cable is made up of three conductors; a pair of wires surrounded by a shield of wrapped or braided wire. The shield connects to ground, and is always connected to Pin 1 of an XLR plug or jack. Wires inside the shield are usually light and dark; white and red, clear and blue, white and black, or similar. Pins 2 and 3 are hot (positive) and cold (negative) respectively. There is no standard colour assignment to match wires with pins, but Pin 2 connects using the same wire at both male and female ends, as does Pin 3.
For superior noise performance, the use of four-conductor (quad) cabling with XLR 3-pin connectors is preferred. Quad cabling consists of two pairs of wires twisted within the shield. The dual pairs substantially increase the noise-cancelling ability of the design. Quad cabling with two white and two blue wires is preferred for this, so that each white wire is connected together on either Pin 2 or 3, with each blue wire connected to the other pin. Quad cable with four different colours is available, but potentially confusing for mic cable use.
Two-conductor microphones do still exist, often inexpensive mics with cables permanently connected with a 90 cm (3 foot) or 1.8 m (6 foot) cable ending in a 6 mm (1/4 inch) mono phone-plug. While these microphones usually use one-conductor cable with a braid or foil shield, it is possible to find them with white and black, red and black, or white and red wires. In most cases, the lighter coloured wire of the pair is used as the hot (positive) wire.
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