Ampex 351 - Page 33

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In magnetic recording such differentiation is im 
portant. Certain losses — such as amplifier response,
self-resonance of head windings, eddy current losses
in head cores, etc. — are frequency-dependent losses.
Others — reproduce gap losses, head-to-tape spacing
losses, tape thickness losses, etc. — are wavelength 
dependent losses.
Head gap terminology used in this discussion (A) gap
length (B) gap with fC) gap depth.
In reproducing information from a recorded tape,
one important factor is the dimension of the repro 
duce head gap. We have seen that the magnetic flux
on the moving tape induces a voltage in the head
coil, but what actually occurs here is a little more
complex than that simple statement implies.
Actually, the flux must travel to the coil through
each branch of the head core (forced into that path
by the high reluctance of the gap) and must result in
a voltage differential across the coil if a current is to
be created. Therefore, an instantaneous difference
in the magnitude of the moving flux must exist across
Magnetic reproducing equipment in the motion picture
theater. Ampex installation along the far wall (at War 
ner Theater. New York City) provides six channel
stereophonic sound.
Reproducing
Although the reproduce head is constructed al 
most the same as the record head, it functions more
like an electric generator. When we move a conductor
through a magnetic field, as we do in a generator, we
induce in that conductor a voltage whose amplitude
and polarity are functions of the magnitude and direc 
tion of the magnetic field. We can, of course, achieve
the same results by passing the magnetic field across
a stationary conductor, as the only requisite is that
the conductor must cut the lines of force. (Note here
that, assuming a constant field, the amplitude of the
induced voltage is dependent upon the speed with
which the conductor cuts the lines of force.)
Similarly, when we move the recorded tape past
the gap in a reproduce head, the magnetic flux on
the moving tape will induce a voltage in the head coil.
This induced voltage will be proportional to the num 
ber of turns of wire on the head coil, the permeability
of the core material, and the time rale of change of
the magnetic flux.
Assuming a constant tape speed across the head,
the last factor means that the output of a given repro 
duce head will increase directly with frequency (as
frequency rises there is a greater rate of change of
flux across the head gap for a given tape speed).
Erasing
Our major purposes in erasing are to obliterate
any prior recording and to leave the tape quiet, so
that it may be used again and again for different
programs. Permanent magnets will do the erasing
job. but it is difficult to prevent these devices from
magnetizing the tape in one direction — a single pole
on the magnet would magnetize the tape to satura 
tion. and a high noise level would result in the subse 
quent recording. The common practice, therefore, is
to subject the tape to an a-c field which gradually in 
creases to a maximum magnitude then gradually de 
creases to zero.
The erase head functions exactly the same as the
record head, but it is constructed with a relatively
large gap — which allows the flux to leak out over a
relatively large longitudinal area in the tape path
We send a high frequency a-c signal to the head. As
a point on the tape approaches the gap. the alternat 
ing magnetic field gets stronger and stronger until
a maximum magnitude is reached directly al the gap.
Then as the point recedes from the gap. toward the
record head, the field grows weaker and weaker until
it disappears. Remember here that we are talking of
relative distances, and the erasing field will disappear
before our point on the tape approaches the record
head