dbx 128 - USING THE DYNAMIC RANGE ENHANCER SYSTEM; Independent Expander Use

Subsonics and Interference
The
dbx
128
incorporates
very
effective
bandpass
filters
that
maintain
full
frequency
response
throughout
the
audi-
ble
spectrum
(-3
dB
at
27
Hz
and
27kHz).
Their
purpose
is
to
reduce
sub
and
supersonic
(bias)
frequencies
to
prevent
them
from
introducing
errors
into
the
encode
or
decode
process.
However,
in
some
extreme
instances,
rumbles
from
passing
trains,
trucks,
aircraft,
or
even
air
conditioning
may
be
picked
up
by
your
microphones
and
fed
to
the
128’s
compression
circuitry.
These subsonics can cause the RMS level sense detector
to
modulate
the
program,
even
though
the
low
frequencies
themselves
are
not
audible
and
are
not
passed
onto
the
tape
(a
tape
machine
usually
cannot
record
frequencies
below
30
Hz). As a result, the low frequency information is missing
upon playback, and the decoder is “fooled”, causing a mys-
terious and unwanted deviation in program level.
Should
you
encounter
this
rare
problem,
it
can
be
elimi-
nated
by
using
a
high
pass
filter
between
the
microphone
preamplifier
and
the
128
record
input;
the
“rumble”
filter
on your amplifier or receiver is suitable. If unexplained play-
back level deviations occur after copying a conventional
phonograph disc onto a tape (using dbx II tape noise reduc-
tion encoding), use of the “rumble” filter during recording
will solve the problem by cutting subsonic frequencies.
USING THE DYNAMIC RANGE ENHANCER
SYSTEM
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How
to
Use
the
Expander
Alone,
Without
dbx
11
Tape
21
Noise
Reduction*
(Linear
Expansion)
1. Set the 128’s front panel switches and controls as
shown.
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2.
Set
the
EXPANSION/COMPRESSION
control
to
the
desired
expansion
ratio
(on
the
right
hand
side
of
the
con-
trol
scale).
An
expansion
ratio
of
1.1
creates
a
10%
increase
in
dynamic
range,
1.2
creates
a
20%
increase,
1.3
a
30%
increase,
and
so
forth...
up
to
2.0,
which
creates
a
100%
increase
in
dynamic
range.
The amount of expansion is continuously adjustable
from
1.0
(a
1:1
ratio
for
no
effect)
to
2.0
(a
1:2
ratio
for
maximum effect), for two reasons. First, different
listeners desire varied degrees of expansion to suit their
musical
tastes.
Second,
various
program
sources
have
differ-
ing amounts of compression, and will therefore require
different amounts of dbx expansion to achieve similar
results. Most users prefer to set the EXPANSION/
COMPRESSION
control
between
1.2
and
1.5
(1:1.2
to
1:1.5)
depending
on
the
recorded
material.
It
is
recom-
mended
that
settings
of
1.2-1.4
be
used
for
classical
music
anid
1.3-1.5
for
popular
music.
Too
much
expansion
often
*Combined
expansion
and
noise
reduction
are
covered
in
the
next
section.