August 1967 Electronics World
Wax nostalgic about and learn from the history of early electronics. See articles
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
If 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
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 thumb.
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
The symbol 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 high-frequency response
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.
Residual Induction and Flux
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.
Φr 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 typical.
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
Bmi (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 result.
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.
Squareness 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
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.