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Ultrasonic Erase and Bias

Author: N.H. Crowhurst

Erasing and biasing action

The best form of erase an ultrasonic oscillator. As the tape passes the magnetic gap, the magnetization due to the oscillator signal alternates several times because of its very high frequency. As a result, the magnetization cancels out as the tape leaves the gap, ultimately demagnetizing it. This type of erase is necessary because of the hysteresis effect in any magnetic material. Too little reverse magnetism does not demagnetize; too much re-magnetizes in the opposite direction. Repeated magnetization at ever decreasing intensity reduces the magnetism until the tape is completely demagnetized.

This hysteresis action in magnetic material is responsible for very drastic distortion, unless something is done about it. The simplest way to eliminate this distortion is to use ultrasonic bias. The audio is combined with some of the same high-frequency signal used for erase. The supersonic bias is quite strong, compared to the smaller audio fluctuations at least (full-amplitude audio will be about equal to the bias). As the tape leaves the gap and the demagnetizing action of the ultrasonic frequency fades out, the magnetism left on the tape is the audio component, which does not fade out, because its fluctuation is much slower (only a fraction of a cycle, instead of many times, while the tape is passing the recording head).

Last Update: 2010-11-03