Handbook for Sound Engineers

Magnetic Recording and Playback 1063

Heads with very low inductance typically require a
step-up transformer to achieve adequate playback
SNRs, but the transformer will also contribute some
additional small amounts of distortion, noise, and
frequency response anomalies.
Lengthening the record head gap will reduce
shunting and give better bias penetration into the tape,
but the short-wavelength sync/overdub response will
suffer greatly.
A more straightforward approach to optimize the
record head for both recording and playback is to use
separate flux paths or windings for each of the func-
tions. One simple method of switching windings and
flux paths is to use parallel paths that can be selectively
blocked. As shown in Fig. 28-27, when the high-induc-
tance playback winding is shorted, flux will be blocked
from the playback shunt magnetic leg of the core, effec-
tively eliminating this path and thereby forcing all of
the flux from the low-impedance bias winding to the
front of the head. During reproduce, when the bias
winding is shorted, the flux picked up from the tape will
pass only through the reproduce winding. Although the
cost of this dual-winding head is significantly higher
than for a conventional single-path design, each coil can
be optimized for its intended function without the need
for compromise, yielding playback-to-record induc-
tance ratios of up to 1000:1.

28.3.7 Erase Heads

A major advantage of magnetic tape recording is the
ability to erase easily and reuse the magnetic tape.
Although physical wear may eventually degrade the
performance of the tape, the magnetic properties of the
tape never wear out.
Erasure of the tape can be accomplished by remag-
netizing the tape with either a very strong static field or

a very strong alternating field. For audio applications the

alternating field, which produces a completely random

flux pattern that is very quiet, is used exclusively.

A very large electromagnet, known as a bulk eraser

or degausser, is used for rapid erasure of an entire reel

of tape, Fig. 28-28. The coil and core of the degausser

are similar to a large recording head. The very strong

flux field created across the eraser gap penetrates the

magnetic tape, driving the magnetic particles to

complete saturation. Any magnetic patterns on the tape

are completely erased when all the tape particles are

alternately saturated in one direction and then the other

by the changing field.

To leave the tape in a neutral stage, the strength of

the erasing field must gradually decrease from hard satu-

ration to zero. A common technique is to move the tape

slowly away from the eraser so that the 60 Hz excitation

field will drop gradually from one cycle to the next.

A few degausser models include control devices that

gradually reduce the current in the eraser coil to zero,

thereby eliminating the need for the operator to move

the reel. Other models contain motor-driven actuators

that slowly remove the tape from the field automatically.

A dc current or permanent magnet can also be used

to erase unwanted signals from the tape, but the tape

particles will not be left in a neutral state. A dc-erased

tape will usually produce a very noisy recording that

contains high levels of even-order harmonic distortion

components.

Selective erasure of small portions of a reel of tape

requires the use of an erase head on the tape recorder.

The function of the head is similar to the bulk eraser in

that the tape is slowly withdrawn from a saturating ac

Figure 28-27. Dual winding record head.

Figure 28-28. Tape eraser. Courtesy Taber Manufacturing

& Engineering Co.