Equalization for Tape Playback

When a magnetic image of a sound signal is recorded onto tape with optimum bias, the magnetization of the tape is linearly proportional to the signal voltage applied to the head.

The playback process, depends not only upon the magnetization of the tape which passes under the playback head but also upon its frequency. By Faraday's law:

The output signal voltage then increases by a factor of two for each octave rise in frequency f, so an equalization circuit must be employed to reduce the signal back to proper balance. A factor of two in voltage is 6 decibels in power, so the equalization circuit must increase its attenuation 6 dB per octave.

Iron and Chromium Comparison

Phonograph Cartridge

The movement of a coil of wire in a magnetic field generates a voltage according to Faraday's law. The tracking of a groove on a vinyl record by the needle on a phonograph cartridge may cause a tiny coil to move in a magnetic field, generating an electrical image of the signal recorded in the groove.

Other types of cartridges may use moving magnets in a coil. A stereo record has different signals on the two sides of the groove, moving the needle in perpendicular directions so that they can be separated by perpendicular coils.