Magnetic Properties of Tape
1967 Electronics World
you are a collector of vintage high-end audio equipment, chances are
you owned a reel-to-reel tape player. I remember back in the 1970s that
anybody wanting to call himself an audiophile had better own a rack-mounted
reel-to-reel player. Of course the funny part is that many of those
people could not afford to buy original recordings on tape, so they
would dub from an LP on a turntable or from a cassette or, gasp, 8-track
tape. This article from the August 1967 edition of Electronics World
delves into the technical aspects of magnetic tape, daring to introduce
such terms as intrinsic cohesive force, residual induction, and flux
- heavy stuff for the layman. Of course, regurgitating such terms while
wowing their friends with a rolling tape held to keep the subject off
of whether the music being played on a $1000 reel-to-reel player had
been dubbed from a $75 cassette deck.
August 1967 Electronics World
of Contents] People old and young enjoy waxing nostalgic about
and learning some of the history of early electronics. Electronics World
was published from May 1959 through December 1971. All copyrights are hereby acknowledged.
Electronics World articles.
all the available
Electronics World articles.
Magnetic Properties of Tape
The tape user looks at magnetic tape in terms of its electrical
performance on the recorder, expecting a certain frequency response
or a specified signal-to-noise ratio. The tape maker must translate
these requirements into magnetic properties which, when present in the
tape, will assure the specified machine performance. Since magnetism
is the operating principle in tape recording, it follows that the magnetic
properties determine the electrical performance of the tape. The chemical
and physical attributes have a very pronounced effect on the magnetic
behavior of the tape, but their main role is to assure the best possible
magnetic characteristics for a given purpose.
Most tape makers
design and predict the electrical performance of their products by controlling
the magnetic properties throughout the manufacturing process. This control
is exercised predominantly prior to the actual coating operation, because
after this point the tape is largely finished and little can be done
to correct any faults. The knowledge of the valid relationship between
magnetic and electrical properties is, therefore, of vital importance
to the manufacturer, but it should be of value to the user as well to
enable him to utilize this medium more effectively. It appears worthwhile
to describe briefly some of these relationships, to help the reader
in forming a clearer picture as to what the tape manufacturer is doing
and what parameters he is manipulating to make the tape better. It must
be understood, however. that this coverage is necessarily incomplete
and greatly simplified; it is meant only to establish a few rules of
Hysteresis of Magnetic Tape
The figure shows a typical hysteresis loop of a magnetic tape. The
symbols indicated are the ones usually listed in technical data sheets
and other tape literature and are, therefore, quite appropriate to this
discussion. Most data sheets specify the magnetic characteristics at
a fixed magnetizing force. Hm, of 1000 oersteds. For all
practical purposes, a force of 1000 oersteds is sufficient to saturate
the majority of magnetic tapes. By strict definition, however, saturation
is not reached until the tapes are subjected to several thousand oersteds.
For this reason, the symbols shown in the figure lock the sub-index
"s" which would denote saturation. For instance. Br (residual
induction) is used here instead of Brs (retentivity); Bmi
(maximum intrinsic induction) is shown instead of Bs (saturation
induction). Many tape data sheets do not make this distinction and employ
the saturation symbols and terminology with the tacit assumption that
1000 oersteds is indeed Hs (magnetizing force high enough
to produce saturation). These side remarks may prove helpful in clearing
up seeming inconsistencies among various data sheets and specifications.
Intrinsic Coercive Force
on the abscissa of interest here is Hci (intrinsic coercive
force). Hci, by definition, measures the demagnetizing force
that is necessary to bring the induction to zero. It therefore indicates
the tape's ability to resist demagnetization whether intentional or
accidental. A case of intentional demagnetization is the erasure of
a recording with a head or a bulk eraser, the higher Hci
requiring a higher erasing force for the some degree of signal reduction.
Accidental demagnetization does not refer to pushing the record button
by mistake, but to selferasure of short wavelengths by the self-demagnetizing
action of the recorded signal. Higher Hci tape, therefore.
may be expected to have reduced short wavelengths losses. i.e. better
In addition to defining the resistance
to demagnetization or erasure Hci also determines the tape's
resistance to magnetization or recording. Accordingly. a higher Hci
tape, when compared to an otherwise identical tape but having lower
Hci, will require a higher bias and record current for equal
output and distortion.
Nearly all magnetic tapes utilizing gamma
ferric iron oxide as the active ingredient fall within the range from
230 to 330 oersteds, with 250-270 being most common (at Hm
of 1000 oersteds). Given the impetus by modern instrumentation and computer
tapes which put high-frequency response and resolution as the major
requirements, the industry is moving slowly but inexorably toward higher
Hci tapes. High coercive force tapes, 400 to 600 oersteds,
are around the corner for the more exotic tapes, but it will be some
time before they are used in audio work.
The second magnetic characteristic to be considered
is Br (residual induction or flux density) measured in gausses.
Br is a calculated value obtained from the expression, Br
= Φr/A, where Φr is the residual flux, measured in maxwells,
and A is the tape cross-sectional area in cm2. Cross-section
is the product of tape width and coating thickness.
is directly proportional to the tape width and thickness, at a constant
Br. To put it another way, the same Φr may be
achieved with half the thickness, but doubling the Br for
the same width.
Φr determines the amount of magnetization
remaining in the tape after the magnetizing force has been removed.
Φr thus establishes the magnitude of the playback output.
Br on the other hand, defines the coating thickness necessary
to achieve the required Φr.
In very general terms,
the output at long wavelengths - within the limits of the 6 dB per octave
unequalized playback slope - will increase with Φr, providing
the record head is capable of biasing the entire thickness. An increase
of thickness and, consequently, of Φr, beyond this point
will not raise the output any further. A tape with a higher Br
though would allow for an increase of Φr with no change in
thickness and thus result in an increased output.
In short wavelength
recording - starting beyond the peak on the unequalized playback curve
- the surface of the coating nearest to the head produces most of the
output. The contribution to the output of the layers farther away from
the head diminish with decreasing wavelengths. The short wavelength
output therefore depends on the Φr of the top layer of the
coating. It is clear then that increasing the Φr by a thicker
coating is useless and will not improve the high-frequency output. The
solution is to raise the Φr within the active layer, which may be accomplished
only by a higher Br.
These examples illustrate that
high Br is generally advantageous in sound recording, especially
if a full frequency spectrum is to be recorded at slow speeds. Unlike
Φr, however, which may be changed pretty much at will simply
by varying the coating thickness, Br is subject to more limitations.
Br is limited by the available induction of iron oxide, oxide
concentration in the coating, coating density, and magnetic losses.
Present tapes run from about 700-1400 gausses, the most common ranging
from 800-1100 gausses. Φr of the present tape ranges from
about 0.2 to 1.2 maxwells per 1/4-inch width, with 0.6 maxwell being
The coating thickness range is from about 150 to 800
microinches, with about 450 microinches average. (Note, Φr
must be expressed as so many maxwells per given width, predominantly
1/4 inch. Otherwise, Φr is meaningless.)
Maximum Intrinsic Induction and Flux
(maximum intrinsic induction) and Φmi (maximum intrinsic
flux) have the same units and are derived in the same way as Br
or Φr. As the figure shows, they denote the maximum value
of flux or induction while the magnetizing force of 1000 oersteds is
applied to the tape. This property is an important control parameter
for the tape manufacturer, but of little use per se to the sound recordist.
When compared with Br however, it yields squareness is the
Squareness, as it
is commonly but not quite correctly called, is the ratio Br/Bmi
or the numerically equivalent Φr/Φmi. Since Bmi
is determined while the magnetizing force is applied, the demagnetizing
losses are zero. Br is determined at zero force where the
demagnetizing losses are maximum. The ratio of these two properties
is thus a measure of the internal losses in the coating. These may be
caused by a variety of reasons including faulty dispersion, poor quality
or damaged oxide particles, wide distribution of particle shapes, insufficient
orientation, and other factors. Some tape manufacturers have special
tests to determine the exact cause of low squareness, but they cannot
be discussed here. The range of squareness in current tapes is from
0.63 to 0.82 (at 1000 oersteds) the typical being about 0.76. Since
the ideal squareness is 1, the 0.76 indicates a demagnetizing loss of
24% resulting in a corresponding loss in Br. Values ranging
from 0.85 to 0.93 have been achieved in laboratories.
is important not only because of its direct influence on Br
but even more so by its effect on output losses caused by self demagnetization
by the signal itself. This effect is closely related to the accidental
demagnetization mentioned previously in connection with Hci.
These two parameters, squareness and Hci, must be considered
together as the interaction between them can either offset or multiply
the individual effects.
The matter of interrelation among the
different properties is worthy of special emphasis. These interrelations
are often quite complex and could lead to wrong conclusions if considered
without sufficient data or without the necessary experience. Readers
are advised, therefore, to be cautious in making decisions about tape
quality on the sale basis of the magnetic properties as listed in tape
data sheets. The rules of thumb presented here are very useful but tell
only part of the story.