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DEGAUSSING (DEMAGNETIZING)
A little stray magnetism can become quite a big
nuisance in tape recording. It only takes a small
amount (.2 Gauss) to cause trouble on the record
head. Playing 10 cassettes will put about that
much charge on the heads. A little more than
that (.7 Gauss) will start to erase high frequency
signals on previously recorded tapes. You can
see that it's worth taking the trouble to degauss
regularly.
DEGAUSSING IS ALWAYS DONE WITH THE
RECORDER TURNED OFF. If you try it with the
electronics on, the current pulses produced by
the degausser will look just like audio signals to
the heads. These pulses are around 10,000
Gauss, and will seriously damage the electronics
and/ or meters. Turn off your 488, then turn on
the degausser at least 1 m (3 feet) away from the
recorder.
Be certain that your degausser has either a
plastic cover or plastic tape covering the tip.
Make sure that no metal ever touches the tape
heads as it will scar them and ruin them.
Slowly move in to the tape path. Move the
degausser slowly back and forth, touching lightly
all metal parts in the tape path. Slowly move it
away again to at least 1 m (3 feet) from the
recorder before turning if off.
Be sure to concentrate while you are degaussing.
Don't try to hold a conversation or think of
anything else but the job you are doing. If the
degausser is turned on or off by accident while it
is near the heads, you may put a permanent
magnetic charge on them that no amount of
careful degaussing will remove. You will have to
get the heads replaced. Make sure you are wide
awake for this job.
A clean and properly demagnetized tape
recorder will maintain its performance without
any other attention for quite a while. It won't
ruin previously recorded material, nor will
getting it back to original specifications be
difficult.
CAUTION: If the surface of the unit gets dirty,
wipe the surface with a soft cloth or use a diluted
neutral cleaning fluid. Clean off thoroughly. Do
not use thinner, benzine, or alcohol, as they may
damage the surface of the unit.
How the dbx Works
The DBX is a wide-band compression-expansion
system which provides a net noise reduction
(broadband, not just hiss) of a little more than 30
dB. In addition, the compression during
recording permits a net gain in tape headroom of
about 10 dB.
A compression factor of 2:1 is used before
recording; then, 1:2 expansion on reproduce.
These compression and expansion factors are
linear in decibels and allow the system to
produce tape recordings with over a 90 dB
dynamic range - an important feature, especially
when you're making live recordings. The DBX
employs RMS level sensors to eliminate
compressor-expander tracking errors due to
phase shifts in the tape recorder, and provides
excellent transient tracking capabilities.
To achieve a large reduction in audible tape hiss,
without danger of overload or high-frequency
self-erasure on the tape, frequency pre-emphasis
and de-emphasis are added to the signal and
RMS level sensors.
SUBSONICS AND INTERFERENCE
The DBX incorporates an effective bandpass
filter. This filter suppresses undesirable subsonic
frequencies to keep them from introducing errors
into the encode or decode process. However, if
rumble from trains or trucks is picked up by your
microphone and fed to the DBX, modulation of
the program material during low level passages
may occur. This low-frequency component will
not itself be passed through the recorder and so,
will not be present at reproduce for proper
decoding. It this low-level decoding error is
encountered, and subsonics are suspected, we
suggest the addition of a suitable high-pass filter
in the Microphone Line.
45
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