The alternating current, usually of frequency several times higher than the highest signal frequency, that is fed to a record head in addition to the signal current. AC bias serves to linearize the recoding process and is universally used in analogue recording. Generally, a large AC bias is necessary to achieve maximum long-wavelength output and linearity, but a lower value of bias is required to obtain maximum short-wavelength output.

The mechanism of AC bias can best be explained in terms of anhysteresis.