Ampex 351 - Page 31

BASIC COMPONENTS OF A MAGNETIC TAPE RECORDER
OB Odb
B
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2
A console mounted Ampex Model 354, two channel
recorder/reproducer.
2CCOO
0021
FREQUENCY IN CYCLES PER SECOND
The difference in response between polished (curve
A) and unpolished (curve B) tape is indicated on this
graph. Readings were taken using new tape from the
manufacturer (B) and again after mechanical polish 
ing by running the oxided surfaces against each other
(A).
Ki
WHY MAGNETIC TAPE?
There are many advantages to recording on to 
day ’ s high quality magnetic tape, using professional
grade equipment. No other device can offer com 
parable fidelity of reproduction. Tape provides the
convenience of immediate playback without proces 
sing, and the economy of being able to erase and re 
record. It furnishes a large storage capacity in a
minimum space. Technically one of its greatest attri 
butes is a gradual overload characteristic which ex 
acts a minimum penalty for slightly incorrect record
level adjustments. Audio recordings can be stored
indefinitely or replayed thousands of times with no
deterioration of signal. And tape still is the only prac 
tical means of recording professional quality stereo 
phonic sound; though two track discs are used ex 
tensively in home music systems, the original master
recordings for those discs are made on tape.
Magnetic recording has made possible the presen 
tation of three, four, and six channel sound in the
motion picture theater. In this instance, of course,
the magnetic material is striped on film rather than
on the usual plastic backing.
Any lack of uniformity in the coating implies a
lack of perfect flatness at the tape .surface, so separa 
tion of the tape from the heads will vary. This will
affect the output capabilities (sec Frequency Re 
sponse). Suitable polishing of the tape after manu 
facture will reduce this variation, and some manu 
facturers are now pre-polishing their professional
grade tape. This polishing also minimizes head wear
for equipment that will continually run new tape,
such as duplicating systems for the commercial re 
cording industry.
Tape width variations can also cause trouble when
Magnetic Tape
Modern magnetic tape consists of a plastic back 
ing. on which is deposited a layer of magnetic ma 
terial consisting of iron oxide particles suspended in
a synthetic resin binder. The iron oxide material is
the actual magnetization medium, and since it is in
the form of minute particles the recording process
must depend on the size, shape, orientation, and uni 
form distribution of these particles on the tape.
Manufacturers have greatly increased the quality
of magnetic tape over the past few years, but it re 
mains true that variations in magnetization within
individual wavelengths will occur. The magnitude of
these variations will depend on the factors noted in
the preceding paragraph.
A random packing density of the oxide particles
will impose a random variation of amplitude of a re 
corded signal which will appear as noise in the repro 
duced output. In high frequency applications, where
only a surface layer of the tape is involved, the signal-
to-noise ratio will be particularly affected.
If the backing which supports the medium is not
uniform in thickness, it will create variations in the
deposit of oxide coaling at the base. In low frequency
work the under layers assume importance and such
variation in coating will, again, be reproduced as
noise.