Tape bias

Tape bias is the term for two techniques, AC bias and DC bias, that improve the fidelity of analogue tape recorders. DC bias is the addition of direct current to the audio signal that is being recorded. AC bias is the addition of an inaudible high-frequency signal (generally from 40 to 150 kHz) to the audio signal. Most contemporary tape recorders use AC bias.

When recording, magnetic tape has a nonlinear response as determined by its coercivity. Without bias, this response results in poor performance, especially at low signal levels. A recording signal that generates a magnetic field strength less than the tape's coercivity cannot magnetise the tape and produces little playback signal. Bias increases the signal quality of most audio recordings significantly by pushing the signal into more linear zones of the tape's magnetic transfer function.