Humbucking

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For the purposes of this article, "hum" is defined as "an unwanted signal, generally at the frequency of the local A.C. electrical supply" (Mains hum).

The process by which electrical voltage and/or current (signal) levels are reduced by combining them with otherwise identical signals that are shifted or reversed in phase, is called "bucking".

Humbucking is a process in which "hum" that is causing objectionable artifacts, generally in audio or video systems, is reduced or eliminated (bucked) in electrical and/or electronic systems and/or devices. This is a term used when the reduction is accomplished through the use of devices that introduce a π degree phase shift only in the undesirable portion of the signal in such a way that, when the modified signal is combined with the original, the unwanted part of the signal is reduced to a point where it is no longer noticeable and the desired part of the signal is not noticeably affected.

This is also described as common-mode rejection, and is the phenomenon which dictates the use of balanced lines (twisted pairs) in communication cables.

Some common applications of this process are:

Humbuckers (Humbucking guitar pickups).

Humbucking microphones (the Sennheiser MD421, Shure SM63 and others).

Humbucking transformers or coils used in video systems.

Telephone (and other audio) system and computer communications wiring.