Magnetic recording
techniques are one of the most common way of recording signals. The system
relies on the imposition of a magnetic field, derived from an electrical
signal, on a magnetically susceptible medium, which becomes magnetized. The magnetic
medium employed is magnetic tape: a thin plastic ribbon with randomly oriented
microscopic magnetic particles glued to the surface. The record head magnetic
field alters the polarization (not the physical orientation) of the tiny
particles so that they align their magnetic domains with the imposed field: the
stronger the imposed field, the more particles align their orientations with
the field, until all of the particles are magnetized. The retained pattern of
magnetization stores the representation of the signal. When the magnetized
medium is moved past a read head, an electrical signal is produced.
Unfortunately, the process is very non-linear, so the resulting playback signal
is different from the original signal. Much of the circuitry employed in an
analog tape recorder is necessary to undo the non-linear distortion introduced by
the physics of the system.
Recording
The record head converts an
electrical signal into a magnetic field which can be used to create a
pattern of magnetization in the
tiny magnetic particles of the tape. The head consists of a torroidal core with
a small air gap. A coil of wire is wound around the core, which is made of a
magnetically permeable metal. Much like a transformer, the record head converts
an electrical signal into a changing magnetic field. As the tape moves away from
the gap, the magnetic flux (the magnetic equivalent of current) decreases as
the inverse square of distance. At some distance, it is no longer strong enough
to change the magnetic particles on the tape and the magnetization pattern then
present is retained. This means that the actual recording takes place at the so-called
“trailing edge” of the gap, rather over the entire gap length. The process of
recording information to magnetic polarizations involves the interaction of imposed magnetic field with a magnetizable
layer on the tape.
As the field strength increases, it begins to
magnetize some particles. For some amount of signal level increase, the
magnetization left on the tape increases linearly. At high levels, there are
fewer and fewer magnetic particles left to magnetize and the tape becomes
saturated until no unmagnetized particles are left. This results in what is called
hysteresis: tape magnetic domains are not linearly changed by the imposed
signal. This results in distortion of the recorded signal.
Reproduction
The reproduce process is
conceptually the reverse of the recording process: as the magnetized
tape is moved past the reproduce
head gap, it’s magnetic field induces a flux in the head. This flux then causes
a current to flow in the coil of wire which is wrapped around the head core.
Unlike the recording head, the length of the reproduce head gap is critically
related to the ability of the head to reproduce the frequencies recorded on the
tape. This is because the flux generated in the gap is created over the entire gap
length rather than at the trailing edge as in the recording process.