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DEGAUSSING (DEMAGNETIZING)
A little stray magnetism c an bec om e quite a big
nuisanc e in tape recording. It only t ak es a small
amount (.2 Gauss) to c ause 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 w orth taking the trouble to degauss
regularly.
DEGAUSSING IS ALWAYS DONE WITH THE
RECORDER TURNED OFF. If you try it w ith 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 electronic s
and/ or meters. Turn off your 488, then turn on
the degausser at least 1 m (3 feet ) aw ay from the
rec order.
Be certain that your degausser has either a
plastic cover or plastic tape covering the tip.
Mak e sure that no metal ever touches the tape
heads as it will sc ar them and ruin them.
Slowly move in to the tape path. Move the
degausser slow ly bac k and forth, touching lightly
all metal parts in the tape path. Slow ly move it
aw ay again to at least 1 m (3 feet) from the
recorder before turning if off.
Be sure to concentrat e w hile 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 w hile 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. Mak e sure you are w ide
aw ak e for this job.
A clean and properly demagnetized tape
recorder will maintain its performance w ithout
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,
w ipe the surfac e with a soft cloth or use a diluted
neutral cleaning fluid. Clean off thoroughly. Do
not use thinner, benzine, or alcohol, as they m ay
dam age the surfac e of the unit.
How the dbx Works
The DB X is a w ide-band compression-expansion
system w hich 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, espec ially
w hen 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 ac hieve a large reduction in audible t ape hiss,
w ithout danger of overload or high-frequency
self-erasure on the tape, frequenc y pre-em phasis
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 k eep them from introducing errors
into the enc ode or dec ode process. How ever, 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
m ay oc c ur. 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, w e
suggest the addition of a suitable high-pass filter
in the Microphone Line.
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