Bias is a high-frequency ultrasonic signal that is mixed with the audio signal during analog tape recording to ensure a clean, high-fidelity recording.

The magnetic particles on a blank tape are "lazy" and resist aligning with a weak magnetic signal, a problem known as magnetic inertia. Trying to record audio directly onto tape without bias would result in very low volume and extremely distorted sound, especially in the quieter parts.

The bias signal acts as a magnetic "lubricant." It is a very high-frequency alternating current (usually between 80 kHz and 150 kHz), far above the range of human hearing. It "agitates" and "energizes" the magnetic particles, taking them out of their state of inertia and allowing them to respond linearly and precisely to the audio signal being recorded.

The same bias signal is also sent to the erase head to completely erase any previous recording, preparing the tape for a new recording.

The amount of bias is one of the most critical calibration adjustments in a tape recorder, as it directly affects the frequency response, distortion, and recording level.