Before passing over the record head, a tape in a recorder passes over the erase head which applies a high amplitude, high frequency AC magnetic field to the tape to erase any previously recorded signal and to thoroughly randomize the magnetization of the magnetic emulsion. Typically, the tape passes over the erase head immediately before passing over the record head.

The gap in the erase head is wider than those in the record head; the tape stays in the field of the head longer to thoroughly erase any previously recorded signal.

Biasing

High fidelity tape recording requires a high frequency biasing signal to be applied to the tape head along with the signal to "stir" the magnetization of the tape and make sure each part of the signal has the same magnetic starting conditions for recording. This is because magnetic tapes are very sensitive to their previous magnetic history, a property called hysteresis.

When a magnetized tape passes under the playback head of a tape recorder, the ferromagnetic material in the tape head is magnetized and that magnetic field penetrates a coil of wire which is wrapped around it. Any change in magnetic field induces a voltage in the coil according to Faraday's law. This induced voltage forms an electrical image of the signal which is recorded on the tape.

Problem: The magnetization of the magnetic emulsion is proportional to the recorded signal while the induced voltage in the coil is proportional to the rate at which the magnetization in the coil changes. This means that for a signal with twice the frequency, the output signal is twice as great for the same degree of magnetization of the tape. It is therefore necessary to compensate for this increase in signal to keep high frequencies from being boosted by a factor of two for each octave increase in pitch. This compensation process is called equalization.