Answer 7—A magnetic coil is normally used for recording and playing back audio signals. When recording, the magnetic field impressed on the medium is directly proportional to the current in the record head. However, when playing back, the current in the playback head is proportional to the rate of change of the field in the medium. In other words, the playback process acts as a mathematical differentiator. This causes the system’s overall gain to have a value that rises with frequency. Equalization is required to compensate for this.

Contributor: David Tweed