Handbook for Sound Engineers

Magnetic Recording and Playback 1061

28.3.5.4 Magnetostrictive Noise

The magnetic core material also exhibits magnetostric-
tion—a change in magnetic field due to stress. The
microscopically rough surface of the magnetic tape will
therefore produce a small magnetic field change in the
core as the tape slides across the head. This field change
generates a magnetostrictive noise component in the
winding.
Both the Barkhausen and magnetostrictive noises are
absent when no tape is moving over the surface of the
head. The residual standby noise, which is measured
under these conditions, is the absolute noise floor for the
reproduce head and amplifier. The comparison of this
standby noise level with the bulk-erased and biased noise
levels is covered in the test and maintenance section.

28.3.6 Record Heads

The magnetic core and gap of a reproduce head obey
the principle of reciprocity, which states that the roles of
an excitation source and sensor can be interchanged.
For a head used in the reproduce mode, external flux at
the gap produces a voltage across the head winding. If,
instead, a voltage is applied to the head winding, a
concentrated external flux field will be generated at the
gap and can be used to record a signal on a piece of
moving tape.
The shape and strength of the magnetic field at the
gap is the basis for the operation of a recording head. The
flux generated in the core by the current in the winding
must jump across the gap to complete a closed magnetic
path. The gap, which is a very poor magnetic path
compared to the core, produces an obstruction that forces
the flux to spread sideways, as shown in Fig. 28-23.

An analogous situation occurs with a crowd of
people moving down a hallway. If the hallway widens
for a crossing corridor or small lobby, the crowd will
broaden out into the open area and then narrow down
again to reenter the continuation of the hallway. The

broadening will increase if the pressure within the

hallway should increase due to an emergency such as a

fire. The stress is greatest at the transitions between the

wide and narrow spaces since this is where people are

squeezing to try to change the shape of the flow.

A magnetic tape passing over a record head gap

experiences a similar buildup and decline in the

magnetic recording field as it moves across the gap. To

produce a permanent recording on the tape, the flux

must first rise to a level sufficient to overcome the

magnetic memory force of the tape, which normally

keeps the magnetic particles on the tape from changing

state spontaneously. In the central zone of complete

excitation, the tape particles will follow any change in

the input signal driving the head. As the tape particles

exit the strong central zone, a well-defined point will be

reached at which the driving flux drops below the

memory force, leaving a fixed magnetic image

impressed on the tape. This transition region in which

the image freezes at the trailing edge of the gap is called

the trapping plane.

The shape of the trapping plane depends primarily

on the gap size and the thickness and magnetic charac-

teristics of the tape. Since trapping planes that are

narrow and vertical will produce short-wavelength

recordings that are more easily reproduced, several

techniques have been developed to sharpen the transi-

tion zone, as shown in Fig. 28-24.

The focused-gap technique in Fig. 28-24B uses a

highly conductive gap shim made of silver to serve as a

barrier to flux jumping straight across the gap. Eddy

currents in the shim force the flux away from the shim,

squirting the flux deeper into the tape. The reduction in

Figure 28-23. Record head flux field.

Flux field

Oxide layer

Record head gap

Figure 28-24. Focused-gap and cross-field (X-field) head.

Cross-field bias head

A. Cross-field (X-field) recording.

Tape travel

Recording region

Record

head

B. Focused-gap record head.

Eddy currents in silver shim reduce shunting effect