Cable shielding intercepts electrostatic interference; balanced twisted pairs cancel electromagnetic interference

Two distinct noise sources require two distinct cable defenses. Electrostatic (capacitive) interference — from fluorescent lights, dimmers, RF fields — is intercepted by a conductive shield surrounding the inner conductors; the shield drains the induced charge to ground. Electromagnetic (inductive) interference — from power transformers, motors, hum magnetic fields — is rejected by using a twisted pair of conductors; any field induces equal voltages in both conductors, and a balanced differential circuit cancels these common-mode signals. Braided shields provide better coverage but add capacitance; foil shields are lighter but can have small gaps. Spiral (serve) shields are flexible but less complete. For very noisy environments (near dimmers, power amplifiers), ‘star quad’ cable (four conductors in a double-twisted configuration) provides additional electromagnetic rejection beyond standard twisted-pair balanced cable.

Examples

A mic cable routed alongside a dimmer pack picks up 60 Hz hum even though it is balanced. Replacing it with star-quad cable eliminates the hum by providing double cancellation of the magnetic field induction.

Assessment

Why does lifting the shield at one end of a balanced cable (to break a ground loop) not prevent electrostatic noise pickup? What does it prevent?