Ampex 351 Series - THEORY OF MAGNETIC TAPE RECORDING

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THEORY OF MAGNETIC TAPE RECORDING
If a material ca pabl e of being magnetized
is placed in th e proximity of a ma gn etic fi eld
the mol ec ul es of th at m ater ia1 will be oriented
according lo the direcUon oJ the field. Any of
severa l methods may be used to produ ce a mag-
netic fi eld. but of mo st interes t in ma gnetic
recmding is the field produced by a current
flowing through a coil of wire. Th e c urr e nt its e lf
may be derived from a transducer suc h as a
microphone wh ich converts the m ec hanical
energy
of sou nd to elec tr ic c urr e nt.
Magnetic recording tape consists of finely
divided iron-oxide pa
rt i cl es deposited upon a
plastic backing. D uring the reco rdin g pro cess
this tape is moved thro ugh a magnetic field
in whjch th e magnetizing force is alterna ting,
and the iron oxide par ticl es arc align ed ac-
cord
ing to th e instantan eous dir ection and mag-
nitude of the field.
TAPE
s N
s
FLUX
s
N
:=tt
:=t
N
s
/'\ SIGNAL
-~----1-~~.-----.---e
Magnetization of Tape
The magnetic field is produced in the gap
of a recording head ( which is essentia lly an
electro-magnet) over whi ch the reco rding tap e
passes.
Th e head consists of an incomplet e
ring of highly per meable materia l inserted in
a coil of wire. Th e discontinuity in the ring
forms the gap. and the ring its e lf is the core of
th e electromagnet. The recording head and its
g ap thus constitute a series magnetic circuit .
4-2
GAP
TAPE
.__ _____ e _ _ _ _ _ .....,
Recorcl H ead
The magnetization c urve of the iron oxide
used
as the r ecord ing medi um is similar to that
shown as the heavy line in the illustr ati on
above.
B
(INDUCTION)
H
\MAGNETIZING FORCE)
MAGNETIZATION
CURVE
COMPOSITE SIGNAL
AT RECORD HEAD
I
I
SATURATION
LEVEL
1 I
1-1- --
1 I
I l
I I
SIGNAL
RPco rding medium magnetization curoe