Magnetic Recording and Playback 1055Use of an excessively short gap will cause an addi-
tional loss in overall head sensitivity due to shunted flux
that jumps the gap rather than traveling through the
core, as shown in Fig. 28-16. For this reason, the repro-
duce head gap length is usually chosen to give the
largest acceptable loss at the shortest expected wave-
length.
28.3.2 Spacing Losses and Thickness
The recording process magnetically aligns groups of the
tiny randomly oriented magnetic particles so that they
act as if they were a single larger particle. We could
visualize these groups as little bar magnets that have
dimensions determined by the tape and signal. The track
width defines the vertical direction and the tape coating
thickness sets the depth. The length is determined by the
wavelength of the recorded signal. To simplify the
example, assume that a 1.5 kHz square wave is recorded
at a tape speed of 15 in/s, yielding a wavelength of
10 mils or 0.010 in. The recorded image is similar to a
series of bar magnets each 5 mils long with alternating
polarity.
Actually, gap length loss and shunting loss are only a
part of what determines the performance of an audio
recorder. The most critical parameter is the relative
thickness of the magnetic coating on the tape. The ratio
of tape thickness to the shortest wavelength to be
recorded has a profound effect on the frequency
response, maximum output, noise, and signal-level
fluctuations.
The magnetic particles at the surface of the tape are
very tightly coupled to the core of the head, producing a
maximum amount of playback flux in the core. Particles
that are buried below the surface of the tape, however,
produce a weaker flux in the core. The amount of flux
that is lost depends on the spacing distance and the
wavelength—just as a small font size is more difficult to
read at a distance than a larger font. An approximate
expression for this spacing loss is
.(28-4)One example of the use of this spacing loss formula
is to determine the playback signal loss due to a piece of
dirt on the surface of a reproduce head. Assuming a
typical recording studio tape speed of 15 in/s (38 cm/s),
a dirt speck only 0.0001 in (2.5μm) high will produce
losses at the following frequencies ofNote that this seemingly insignificant dirt particle
has produced a serious loss in high frequencies.
Spacing loss due to dirt is not the major problem
created by the “nearsightedness” of the gap since proper
head cleaning will keep spacing distances to less than
10 ^5 inch, which is (0.25μm), producing virtually no
error at studio tape speeds. The problem is eight times
more severe for cassette speeds of 1 in/s (4.8 mm/s).
The major spacing problem arises within the tape
itself since the magnetic coating thickness spaces most
of the particles away from the head with other particles.
Consider the tape to be composed of several indepen-
dent layers of oxide, as shown in Fig. 28-17. The
average spacing loss for each layer, calculated using the
midpoint of each layer to determine the spacing
distance, is tabulated for the example with a typical
0.6 mil (15μm) coating thickness.The contributions of layers 2 through 6 fall off so
rapidly due to spacing loss that their combined contribu-
tion is only equal to layer 1 by itself at this wavelength.
Indeed, shaving off layer 6, which constitutes 17% of
the coating thickness, would produce a loss of only 2%
or 0.18 dB in output at this wavelength.
This coating thickness loss can be expressed asFigure 28-16. Gap shunting loss.Useful fluxHead HeadGap Flux wasted by shuntingUseful fluxFigure 28-17. Tape thickness loss.Spacing lossdB 55 distance
wavelength= u------------------------------150 Hz spacing loss 55 0.0001
15
150---------= u----------------=0.055 dB
1500 Hz spacing loss=0.55 dB
15 kHz spacing loss=5.5 dB(^7) » 8
Layer Spacing 15 kHz loss
1 0.05 2.75 dB
2 0.15 8.25 dB
3 0.25 13.75 dB
4 0.35 19.25 dB
5 0.45 24.75 dB
6 0.55 30.25 dB
Total output + 3.63 dB
0.6 mil
Head
0.1 mil
6 5 4 3 2 1