Crosstalk occurs when a signal "leaks" from one adjacent channel to another, essentially inducing electronically, signals into places it would be deemed unwelcome.

This can happen with cables, wires, printed circuit boards, tape and magentism etc.

A classic example of crosstalk would be the "bleeding" of tape tracks on an analogue tape machine, or between two adjacent channels of a mixing board.

Crosstalk is considered undesirable and is quoted as a negative dB quantity.
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Recording Studio Suntans