(1) Bias is the voltage or current that establishes the intrinsic noise floor of an active device.
(2) In an analog tape recorder, bias is an ultrasonic signal, usually between 100k Hz-200kHz, which is mixed with the audio signal and applied to the recording head, reducing distortion by reducing the hysteresis inherent in the tape recording process. This process is known as AC bias because the bias current is alternating. The ideal setting of analog bias involves a compromise between the MOL of the tape, noise, and third-harmonic distortion. In general, classical recordings use a bias setting with lower distortion and lower MOL; rock or other recordings prefer a higher distortion in order to get the highest S/N ratio. Digital recorders do not require bias as the signal consists only of a bitstream of 0s and 1s, regardless of the audio frequency being recorded.
(3) See electric microphone.