In telecommunications and professional audio, a balanced line or balanced signal pair is a transmission line consisting of two conductors of the same type, each of which have equal impedances along their lengths and equal impedances to ground and to other circuits.  The chief advantage of the balanced line format is good rejection of external noise. Common forms of balanced line are twin-lead, used for radio frequency signals and twisted pair, used for lower frequencies. They are to be contrasted to unbalanced lines, such as coaxial cable, which is designed to have its return conductor connected to ground, or circuits whose return conductor actually is ground. Balanced and unbalanced circuits can be interconnected using a transformer called a balun.
Circuits driving balanced lines must themselves be balanced to maintain the benefits of balance. This may be achieved by differential signaling, transformer coupling or by merely balancing the impedance in each conductor.
Lines carrying symmetrical signals (those with equal but opposite voltages to ground on each leg) are often referred to as balanced, but this is an entirely different meaning. The two conditions are not related.
Transmission of a signal over a balanced line reduces the influence of noise or interference due to external stray electric fields. Any external signal sources tend to induce only a common mode signal on the line and the balanced impedances to ground minimizes differential pickup due to stray electric fields. The conductors are sometimes twisted together to ensure that each conductor is equally exposed to any external magnetic fields that could induce unwanted noise.
Some balanced lines also have electromagnetic shielding to reduce the amount of noise introduced.
A balanced line allows a differential receiver to reduce the noise on a connection by rejecting common-mode interference. The lines have the same impedance to ground, so the interfering fields or currents induce the same voltage in both wires. Since the receiver responds only to the difference between the wires, it is not influenced by the induced noise voltage. If twisted pair becomes unbalanced, for example due to insulation failure, noise will be induced. Examples of twisted pairs include Cat-3 Ethernet cables or telephone wires.
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