Tape Recording 751which the magnetic patterns on the tape are converted back to an audio signal suitable
for subsequent amplifi cation and application to a loudspeaker. Finally, the tape is wound
onto the take-up reel. When in playback mode, the erase head and the record head are not
energized. Correspondingly, in record mode, the playback head may be used to monitor
the signal off-tape to ensure that recording levels and so on are correct. Less expensive
cassette tape recorders combine the record and playback heads in a composite assembly,
in which case off-tape monitoring while recording is not possible.
26.3 The Physics of Magnetic Recording .....................................................................
In a tape recording, sound signals are recorded as a magnetic pattern along the length
of the tape. The tape itself consists of a polyester-type plastic backing layer, on which
is applied a thin coating with magnetic properties. This coating usually contains tiny
particles of ferric iron oxide (so-called ferric tapes), although more expensive tapes may
use chromium dioxide particles or metal alloy particles, which have superior magnetic
properties (so-called chrome or metal tapes, respectively).
The properties of magnetic materials take place as a result of microscopic magnetic
domains—each a tiny bar magnet—within the material. In an unmagnetized state, these
domains are effectively aligned randomly so that any overall, macroscopic magnetic
external fi eld is canceled out. Only when the ferrous material is exposed to an external
magnetic fi eld do these domains start to align their axis along the axis of the applied
fi eld, the fraction of the total number of domains so aligned being dependent on the
strength of the externally applied fi eld. Most signifi cantly, after the external fi eld has been
removed, the microscopic domains do not altogether return to their preordered state and
the bulk material exhibits external magnetic poles.
The relation between the magnetizing fi eld ( H ) and the resultant induction ( B ) in an iron
sample (assumed, initially, to be in a completely demagnetized condition) may be plotted
as shown in Figure 26.3. Tracing the path from the origin, note that the fi rst section of the
looped curve rises slowly at fi rst (between O and B1), then more rapidly (between B1 and
B2), and fi nally more and more gradually as it approaches a point where only a very few
magnetic domains remain left to be aligned. At this point (B3) the ferrous material is said
to be saturated. Signifi cantly, when the magnetizing force ( H ) is reduced, the magnetic
induction ( B ) does not retrace its path along the curve B3–B2–B1–O, instead it falls
along a different path, B3–B4, at which point the magnetizing force is zero again, but the