1066 Chapter 28
gradual (hopefully) drop in playback level at the shortest
wavelength of interest. When these losses exceed the
application’s requirements, the tape is worn out.
Some specialized audio transports designed for
repetitive playback are capable of making over a quarter
of a million passes on a tape. On the other hand, a
poorly maintained studio recorder can destroy a master
tape in ten passes or less! In general, if the abrasive
forces exerted by the transport on the tape are well
below the inherent strength of the binder, the tape will
last virtually indefinitely. Any increase in the abrasive
force due to dirty contact surfaces, excessive tape
tension, or poorly designed tape guiding will accelerate
the wear.
A very rapid catastrophic failure will occur once the
abrasion force becomes sufficient to build up a small
clump of debris on a contact surface. The friction
between the debris and the tape surface is very high due
to both the similarity of materials and the high pressure
exerted by the tip of the clump as it pushes on the tape.
The binder is overwhelmed, causing the clump to grow
rapidly to the point at which the tape will show an
obvious scratch or crease. If this situation should arise,
the source of the problem should be corrected, and a
copy of the damaged tape should be used for subsequent
work.
From the magnetic performance standpoint, the
combination of smoother magnetic particles and newer
binders has enabled the tape manufacturers to use a
smaller quantity of binder material to affix the magnetic
particles. The ratio of useful particles to the magneti-
cally inert binder rose from approximately 40% by
volume for typical mastering tapes in 1970 to approxi-
mately 60% in 1980 with virtually no improvement
since then. This improved magnetic density yields a
higher maximum output for a given particle type and
coating thickness.
28.3.9.3 Magnetic Particles
The ultimate performance of a tape recorder is deter-
mined not by the tape drive, heads, or electronics, but
rather by the physical and magnetic characteristics of
the magnetic particles of the tape. If basic performance
parameters such as maximum output levels, noise, and
distortion are truly determined only by the tape, the
recorder is said to be tape limited. As a practical rule of
thumb, if the noise and distortion products of the
recorder are at least 10 dB lower than the products
produced by the tape, the overall performance of the
machine and tape will be within 0.5 dB of the theoret-
ical levels of the tape alone.
Of primary importance in magnetic recording is the
ability of each magnetic tape particle to assume and
retain a magnetic pattern. These particles are chosen for
their ability to maintain a magnetic field along one
preferred direction or axis, permitting alignment of the
particles for maximum performance. The amount of
preferred orientation or anisotropy in the material
depends on the nature and crystalline structure of the
particles.
The shape of the particles determines the degree of
physical alignment that can be achieved during the
coating process. Smooth cylindrical or spherical parti-
cles that have no jagged edges or branches can be
densely packed, yielding maximum output level.
The size of the particles is determined by the crystal-
line structure of each material. The residual noise of the
tape decreases as the particles become smaller. Small
particles with high anisotropy are therefore most desir-
able. Typical iron oxide magnetic particles for recording
tape are cigar-shaped particles with a length-to-width
ratio in the range of 4:1 to 8:1.
The newest recording products are abandoning
particulate coatings in favor of thin layers that are plated
or evaporated onto the surface of the plastic. These very
thin layers of high coercivity materials are ideal for very
short video wavelengths or very high digital bit densi-
ties. The new technology brings with it a whole new set
of problems such as coating durability and how to
include adequate lubrication in the metallic coating.Coercivity. The coercivity is a measure of the magnetic
force required to cause the tape particles to change
magnetic polarity. High coercivity particles are more
difficult to bias, record, and erase. On the beneficial
side, they are also better able to resist external influ-
ences due to neighboring particles after recording,
reducing the smearing of short-wavelength signals
during storage.Retentivity and Remanence. If the coercivity is
considered to be the input drive, then the retentivity and
remanence are the output of magnetism left in the tape.
Retentivity measures the maximum output per unit
volume of coating cross section; remanence (remanent
flux), which is the output per ¼ inch of tape width,
varies not only with retentivity, but also with coating
thickness. Remanence specifications should be used to
compare the maximum long-wavelength outputs of
different tape types.