dbx 122 - Page 27

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never have been audible in conventional recordings can
become an annoyance when captured on a quiet dbx
recording. For this reason, the ambient room noise should
be heid to an absolute minimum.
Subsonics and Interference
The dbx 122 and 124 incorporate very effective band-
pass filters that maintain full frequency response through-
out the audible spectrum (—3dB at 27Hz and at 27kHz).
Their
purpose
is
to
reduce
subsonic
and
supersonic
(bias)
frequencies which might otherwise introduce errors in the
encode or decode process. Howaver, in some extreme cases,
rumbles from passing trains, trucks, aircraft, or even air
conditioning may be picked up by your microphones and
fed to the dbx's encoding circuitry.
These subsonics can cause the rms level detector to
modulate
the
program,
even
though
the
low
frequencies
themselves are not audible and are not recorded on the _
tape (a tape machine usually cannot record accurately fre-
quencies below 30Hz). As a result, the low frequency
information that created encoding changes is missing upon
playback
so
the
decoder
is
‘‘fooled.’”
The
result
may
be
a
mysterious and unwanted deviation in program level.
Should you encounter this rare problem, it can be
eliminated
by
using
a
high
pass
filter
between
the
micro-
phone preamplifier and the dbx RECORD INPUT; the
LOW
CUT,
SUBSONIC
or
RUMBLE
filter
on
your
pre-
amplifier may be suitable, although on some preamplifiers
the
cut-off
frequency
may
be
higher
than
necessary.
If
unexplained
playback
level
deviations
occur
after
copying
a conventional phonograph disc onto a tape (using dbx II
tape noise reduction encoding), use of the RUMBLE filter
during
recording
will
solve
the
problem
by
cutting
sub-
sonic frequencies.
When to Use dbx I Tape Noise Reduction Plus Expansion
By
recording
and
playing
back
a
program
with
dbx
II
tape
noise
reduction,
then
expanding
it
with
a
dbx
dynamic
range expander such as our Model 3BX or 118, you can
reduce
the
noise
and
increase
the
dynamic
range
of
the
original
program
source.
This
process
is
especially
effective
on
FM
broadcasts
or
noisy
phonograph
discs.
By
itself,
the
dynamic
range
enhancer
can
lower
the
noise
and
increase
the
dynamic
range
of
conventional
(non-dbx-encoded}
phono-
graph discs, FM programs, pre-recorded tapes or other
sources.
Together,
the
dbx
II
tape
noise
reduction
system
and
the
dbx
dynamic
range
enhancer
can
significantly
increase
your
listening
pleasure.
The
dbx
Model
128
is
a
combination
unit
containing
basically
the
same
circuitry
as
the
Model
122
(dbx
II
tape
noise
reduction
unit)
and
the Model 118 (dbx dynamic range enhancer).
How to Playback with Expansion
1. Connect your system as shown in Figure 18.
2. Set the 122 or 124 as shown below:
dbz De wcccs i dbx BD nscacc mn csrew 124
@ @00 008 80
i@e
oO: oO:
@-.0 O-ourd
3. Set your preamplifier’s TAPE MONITOR Switch to
the TAPE position and select AUX.
4. Set the dbx dynamic range enhancer’s controls for
the desired degree of expansion (as suggested in that unit's
owner’s manual).