1064 Chapter 28
flux field. For a tape speed of 30 in/s (76 cm/s) and a
typical decay length of the erase head field of 0.005 in
(125μm), the drop from saturation to zero occurs in
0.17 ms. If the tape is to experience at least 20 complete
cycles during the decay, the erase frequency must be
(28-10)
A conventional coil-and-core head that has a very
long gap can produce the long flux field required for
erasure. Although such heads will produce approxi-
mately 50 dB of erasure, some of the original signal will
still remain. A second pass over the erase head will
provide the additional erasure that is required to erase
the unwanted signal completely.
The reason for the incomplete erasure is a phenom-
enon known as gap jumping. As the tape leaves the
saturation zone of the erase gap, the flux level experi-
enced by the tape particles will pass through a level that
creates a recording zone similar to the trapping plane of
the record head. Any audio variations in the erase field
would be recorded at this point. Such audio variations
are created by the unerased program that is starting to
enter the erase field at the other side of the gap. This
incoming flux adds to the erase head flux, creating an
unwanted recording at the trailing edge of the erase field
just as if the audio signal had jumped across the gap.
Complete erasure can be achieved without multiple
passes if the erase head contains two magnetically
isolated gaps. The tape is erased by the first gap, and
then immediately reerased by the second gap. A wide
center spacer isolates the two gaps so that flux cannot
jump both gaps.
Although both bulk erasers and erase heads are
capable of completely erasing all recorded material
from a tape, the residual noise level left by the erase
head will be slightly higher than the virgin-tape level
achieved by the bulk eraser. Possible sources of this
excess noise include small changes in the erase field
caused by the tape-to-head contact variations, the tape
particle-to-particle magnetic variations, and the
recording of Barkhausen noise from the erase core. The
record head biasing field also produces similar increases
in the noise level. The excess noise perceived by a
listener due to the erase and record heads may rise as
high as 6 dB above the virgin-tape noise floor.
28.3.8 Head Degaussing (Demagnetizing)Early tape recorders used permanent magnets rather
than an ac high-frequency signal to bias the tape so that
small signals could be recorded without high distortion.
These fixed magnetic fields produced a very high back-
ground noise level that severely limited the SNR of the
taped recording. The introduction of ac bias upgraded
the tape recorder from a voice-grade recording instru-
ment to a true high fidelity recorder for music.
Modern recorders all use ac bias, but occasionally
the background noise on a tape will be well above the
normal level. The culprit is usually a permanently
magnetized head, guide, or capstan that is acting like
one of the old biasing magnets. The problem is most
commonly created by touching a magnetized tool such
as a screwdriver or razor blade to a component in the
tape path. On rare occasions a faulty electronic circuit
will create a dc current in one of the heads, leaving a
residual magnetic field. (Loud clicks or thumps may be
symptoms of dc currents.)
Since there are no commonly available instruments
which can detect the very small magnetic fields which
will result in noise, the best strategy is to frequently
demagnetize all magnetic components in the tape path
with a head degausser, Fig. 28-29.
The head degausser in Fig. 28-29 is an electromagnet
with an extended core. The extension probe conducts an
alternating magnetic flux generated in the coil to the tip
of the probe. The probe is passed close to the magnetic
components on the tape deck so that the alternating flux
can flood the components. The actual demagnetizing
occurs as the probe is slowly withdrawn from the
component, creating the gradually decreasing alter-
nating magnetic field mentioned previously in the
discussion of bulk degaussers and erase heads.
Caution! Before using a head degausser, always
verify that the tip of the probe is covered by a soft mate-
rial that will not scratch the face of the magnetic heads.
If necessary, wrap the tip with vinyl electrical tape or a
similar tape.
Degauss the heads and other steel tape-guiding parts
with a commercial-grade head degausser as follows:- Although a typical head degausser will not disturb a
recorded tape that is more than a few inches from
the degausser, always remove all tapes from the
vicinity of the transport prior to energizing the
degausser. - Hold the degausser at least 1 ft from the tape trans-
port when applying power to the degausser. The
degausser will produce a large voltage in the play-
back and record heads, which will probably not
ferase number of cycles
decay of time= ----------------------------------------20
0.0017----------------== 120 kHz.