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D-C NOISE results when the recording medium has been magnetized by a d-c field, which may be produced by a record head, a permanent magnet, or a solenoid. Modulation noise is the variation in reproduced amplitude when an a-c signal of constant amplitude is recorded. Bias noise occurs when a-c bias is applied to the record head with no signal current. Bulk-erased noise appears when the medium has been thoroughly demagnetized by a cyclic field. It occurs because the magnetic medium is not continuous but is composed of individual magnetic domains or particles; each of these always remains fully magnetized, and only the direction of magnetization can change. When the medium is demagnetized on a macroscopic scale, equal numbers of particles are magnetized in opposite directions so that the net magnetization in any one direction is zero. The magnetic field from each particle will still couple to the reproduce head, thus giving rise to the basic or bulk-erased noise which occurs when the medium is demagnetized on a macroscopic scale.
D. Eldridge (Tue,) studied this question.