Answer 6—DC bias is added to an AC (e.g., audio) signal in order to shift its zero point to the center of one of the two linear regions of the recording medium’s transfer curve. DC bias, which is simple to implement, was commonly used on early tape recorders, but it has several significant drawbacks, mostly related to the fact that components in the tape path tend to acquire permanent magnetization, producing noise in the recording.

High-frequency AC bias was developed to solve that problem. It requires an oscillator running at a few hundred kilohertz with an amplitude that’s comparable to the maximum audio signal that the recorder can handle. This bias signal is added to the audio signal. During recording, the tape is magnetized to alternate polarities by the bias signal. When there is no audio signal, these points lie near the centers of both linear regions on the tape. A positive peak on the audio signal increases the strength of the positive magnetization and decreases the strength of the negative magnetization (and vice versa). The playback signal, which is effectively the net result of adding the effects of the positive domains and the negative domains, is extremely linear.

Contributor: David Tweed