Magnetic Recording and Playback 1053required to slide the tape up or down is determined by
the tape tension and the coefficient of static friction.
The tension component is identical for the stationary
guide, but in this case the coefficient of sliding friction,
which is typically half the static value, is used.
Although both stationary and rotating guides are
commonly used in tape transports, rotating guides are
slightly more prone to damage the edge of the tape.
Guides with large rotating flanges can produce ruffles
on the edge of the tape if the tape edge contacts the
outer radius of the moving flange. Most guide designs
taper the flange to minimize this hazard, but a small flat
area at the bottom of the taper is still required if the
guide is used for precise tape positioning.
The edge-only guide is very limited in effectiveness
since any appreciable force on the edge of the tape may
cause the tape to twist rather than move up or down.28.3 Magnetic HeadsAlthough magnetic tape is covered in a later section, the
following discussion of magnetic heads requires a few
very simple assumptions regarding the composition and
dimensions of the magnetic tape. First, assume that the
magnetic coating consists of microscopic particles of
magnetic materials that have been bonded to one surface
of a thin plastic backing or substrate. Second, each
magnetic particle is assumed to function as a small inde-
pendent magnet, allowing patterns of varying magnetic
polarity and intensity to be stored along the tape. Last,
the thickness of the magnetic coating for the audio tape
example will be assumed to be 0.6 mil (15μm).28.3.1 Geometric Characteristics
Most of the characteristics of magnetic heads are
controlled by the geometry of the head and the magnetic
tape. Since wavelength on tape is determined by the
recorded frequency in hertz and the relative
tape-to-head speed, there can be many combinations of
frequency and speed that will result in the same effects
in a head. For example, the wavelength of a 15 kHz tone
on a mastering recorder at 15 in/s will have the same
wavelength as a 240 kHz signal on a high-speed tape
duplicator running at 240 in/s. The geometric consider-
ations for both applications are identical, despite the
16:1 difference in tape speed.
Not all of the characteristics are geometric,
however. Eddy current losses, for example, depend on
the frequency in Hz rather than the wavelength.
28.3.1.1 Gap Length LossEach of the tiny magnetic particles on the surface of the
tape produces a magnetic force or flux in the space
surrounding the particle. This invisible magnetic effect,
called a magnetic field, will interact with other nearby
magnetic particles. To measure the strength of this field,
a flux concentrator in the form of a reproduce head is
scanned along the tape. The resulting electrical output
from the head is dependent on the flux pattern recorded
on the tape.
The reproduce head must be able to collect flux
selectively from a very small span of tape. For example,
flux patterns on a compact cassette may be as small as
100 millionths of an inch (100 × 10–6in or 2.5μm) in
wavelength. To achieve this fine resolution, a small gap
must be created in a ring of magnetic material, as shown
in Fig. 28-14A.
The length of the gap ranges from two ten-thou-
sandths of an inch (2 × 10^4 inch or 5μm) for studio
mastering recorders down to less than 30 millionths of
an inch (30 × 10^6 inch or 0.75μm)—the wavelength of
red light for cassette and high-density digital recorders.
Since no slicing technique is available to cut accurate
gaps that short, the core is usually fabricated as two pole
pieces that are fastened together with a shim spacer of
the desired dimension inserted in the gap. Fig. 28-14B
shows a typical studio head core drawn full size, with
the critical gap area at the pole tips and adjacent tape
magnified in Fig. 28-14C.
The operation of the gap, which serves as a sensing
aperture, can be analyzed in terms of a flux pickup
focused at the surface of the tape. The amount of flux
picked up by the core, and thus made available to
generate an output voltage in the winding, is determined
by the net magnetic flux from pole tip to pole tip across
the gap area. If the tape segment at the gap consists of a
strong magnetization of only one polarity, the flux in the
core will be maximized. If, on the other hand, the
segment contains two strong portions of opposite
polarity that cancel each other, the net flux in the core
will be zero.
The efficiency of the gap due to this averaging
effect is illustrated in Fig. 28-15. The output of the head
declines, slowly at first, and then quite rapidly to zero as
the wavelength decreases to the length of the gap. As
the gap length becomes longer than the wavelength, an
output of opposite polarity appears. When the wave-
length drops to half the gap length, another null will
occur. This pattern of diminishing peaks of alternating
polarity is repeated over and over, with nulls occurring