Handbook for Sound Engineers

Magnetic Recording and Playback 1067

28.3.10 Magnetic Performance Curves

The input-output relationship for typical magnetic mate-
rials is very nonlinear. As shown in Fig. 28-30A, the
magnetization characteristic curve can be broken into
three zones. For low excitation levels, the initial output
is very small and nonlinear. As the excitation increases,
a fairly linear region is encountered, which produces
low distortions. As the level continues to increase, the
magnetic particles finally become fully magnetized or
saturated. Further increase at the input yields no more
magnetization in the material.

The nonlinear initial region must be avoided in audio
recording if low distortion is to be achieved. The
high-frequency bias signal provides enough excitation
to jolt the magnetic particles into an active state. Opti-
mizing the bias level yields the much more linear
transfer characteristic of Fig. 28-30B.
Another representation of the magnetic characteris-
tics is given by the %+ curves of the tape, as shown in
Fig. 28-31. The curves show the amount of magnetic
flux density created within the magnetic material by a
cyclically varying intensity of applied magnetic excita-
tion. Since the particles store part of the magnetic field,
the path for increasing excitation differs from the decay
path for decreasing excitation.

Magnetic recording tapes are typically characterized

by the coercivity and retentivity described previously.

These points on the %+ curve for full saturation are

indicated by +c and %r , respectively.

A figure of merit called squareness is commonly

used to indicate the uniformity of the magnetic

switching characteristics of magnetic coatings. As

shown in Fig. 28-32, the squareness is the ratio of the

remanent output value where the curve crosses the

vertical axis to the saturated output. A perfect square-

ness of 1.0 would mean that every particle switched at

exactly the same excitation level, yielding maximum

output level and low distortion at high output levels.

The squareness ratio improves as more and more

particles are aligned in parallel with the flux lines

produced by the record head. The ideal case would be if

all particles were exactly the same size with a perfect

needle shape and all of the particles were stacked tightly

like cordwood.

The early oxides had many branches or dendrites

stick out of the sides of the needles. The dendrites inter-

fered with the uniform packing of the particles,

reducing the overall ratio of magnetic particles to

binder. Later highly orientable particles (HOP) with

reduced dendrites improved the packing factor. Addi-

tional work in coating techniques took advantage of the

liquid flow of the coating during application to the base

Figure 28-30. Tape transfer characteristics.

Output B Saturated

Linear

Initial

Input (

A. Without bias.

B. With bias.

Figure 28-31. B-H curves.

"r

Flux density B

Drive intensity (

(C

A. Varying levels of magnetization.

Magnetization M

Saturation (arbitrary units)

magnetization

Remanent

magnetization

Coercive force

0 1.0 2.0 3.0

Magnetizing force. ((K0e)

B. Critical parameters.