Tape Head Action

An electric current in a coil of wire produces a magnetic field similar to that of a bar magnet, and that field is much stronger if the coil has a ferromagnetic (iron-like) core.

Tape heads are made from rings of ferromagnetic material with a gap where the tape contacts it so the magnetic field can fringe out to magnetize the emulsion on the tape. A coil of wire around the ring carries the current to produce a magnetic field proportional to the signal to be recorded. If an already magnetized tape is passed beneath the head, it can induce a voltage in the coil. Thus the same head can be used for recording and playback.
Tape heads for stereo cassette tapes must record on two tracks simultaneously.
Index

Sound reproduction concepts

Tape recording concepts

Reference
Backus
p318
 
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Cassette Tape Head Arrangement

The basic tape head action involves an oscillating current in a coil. The magnetic field produced in a ring of ferromagnetic material fringes out to the tape material at the gap. For stereo cassette tape heads, there are two such mechanisms to record and playback from parallel tracks on the tape.

This is the head structure in a Kenwood tape deck. The structure to the left of the head may be for erasing. The head is about 7 mm from left to right in the image.

Index

Sound reproduction concepts

Tape recording concepts

Reference
Rossing
Science of Sound
Ch 21
 
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Magnetic Emulsions

The recording medium for the tape recording process is typically made by embedding tiny magnetic oxide particles in a plastic binder on a polyester film tape. Iron oxide has been the most widely used oxide, but chromium oxide and metal particles provide a better signal-to-noise ratio and a wider dynamic range. The oxide particles are on the order of 0.5 micrometers in size and the polyester tape backing may be as thin as 0.5 mil (.01 mm). The oxide particles themselves do not move during recording. Rather their magnetic domains are reoriented by the magnetic field from the tape head.

Tape heads for stereo cassette tapes must record on two tracks simultaneously.

Discussion of biasing
Index

Sound reproduction concepts

Tape recording concepts

Reference
Rossing
Science of Sound
Ch 21
 
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