dbx 3BX - Page 2

RMS
Detection
The 3UX
uses
a detection
method
that
acts
on
the
RMS
(Root
-Mean-Square)
value
of the
input
signal.
RMS
de-
tection
is
different
from
either
peak
or
average
detection.
An
RMS detection
circuit
will
not
over-react
on
musical
transients
or
noise
spikes,
yet it
responds
precisely
and
accurately
to
all
changes
in
input
level,
and
produces
natural
sounding
expansion. In fact,
the
human
ear
judges
sound
levels by
their RMS
values,
which
means
that
the
RMS
detection circuit
in
the
3BX
electronically
parallels
the way
the
human
ear
hears music.
Until
recently,
how-
ever,
RMS
detection
was
very complex
and
costly,
dbx
has
pioneeied
die
development ol
moderately
pi iced
RMS
detection
circuitry,
and has led
the
iiulustiy
in
applying
RMS
detection
to
expanders,
compressor/limi
teis
and
tape
noise
i
eduction systems.
Linear
dB Expansion
Once the signal
has been "detected,"
the
expander
knows
when
to increase
or
decrease
its
level.
The
circuit
that
actually
performs this level
change is
known
as
a
"voltage
controlled
amplifier"
or
"VCA."
The
"AVC"
(automatic
volume
control),
and
"ALC"
(automatic
level
eontroilon
many
cassette
recorders
are
examples
of
voltage
controlled
amplifiers,
as
are
the
level
changing
circuits
in
any
modern expander,
compressor
or
limiter.
The
voltage
horn the detection circuit
increases
or
decreases
the
gain of the VC
A
which
increases
or
decreases
the
level
of the
program.
While some expanders
may
increase
or
decrease the
program level by
a
lixed
amount,
the
3BX
increases
or decreases the
level
of
the
program
on a
"linear
decibel" basis.
This
means that the
output
dynamic
range
and the input
dynamic range
are
linearly
related
by
the
"expansion
ratio"
over
the entire
dynamic
range
(as
desciibed earlier)
for
a
smooth, natural
sound.
Attack
and Release
Times
The
expander
must
decide
how last
to
react
to
changes
in
program
level.
The
length
of
time
between
the
detection
of
an
input
signal
level,
and
its
expansion
is
known
as
the
ATTACK
TIME.
After
the
expansion
of
an
input
signal,
the
expander
allows
the input
signal to
return
to its
normal
level.
The
amount
of
time
to
return to
normal
is
known as
RELEASE
TIME
.
These
terms
also
apply
(or
compressors
and
limiters.
In
general,
an
expander's
detection
method,
and its
attack
and
release
times
are
not
directly
related.
For
example,
an
expander
that
uses
peak
detection
may
detect
an
increase
in
program
level
very
rapidly,
yet not
act
on
that
increase
(by
expanding tt)
until
much
later.
Thus,
peak
detection
does
not
necessarily
imply
fast
attack
lime;
the
expantler
just
cited
wuuld have a
slow
attack
time.
Con-
versely,
an
expander
using
average
detection
might
have a
very
fast
attack
time.
While
such
a
unit
might
not
detect an
increase
in
input
level
until the
actual
level
had
begun to
decrease
again,
the
expander
could
react
quickly,
expanding
the
program
upward
at a
point
where it
should
be
expanded
downward.
These
are
extreme
examples,
but
they
illustrate
the
distinction
between
attack
and
release
times
and
detec-
tion
method.
Even
art
expander
using
an
RMS
detection
scheme
may
not
have
optimum
attack
and
release
times
for
every type
of
musical
program.
The
reason
is
that
the
attack
and
release times
ol
most
expanders
(and
compressor/
limiters)
are
fixed
(unchanging).
In
those
units
where
the
attack
and
release
times
are
adjustable
via
front
panel
controls,
the
times
are
still
"fixed"
at
the
control
settings”
fixed
since
they do
not
change as
the
program
requires.
Diffeiem
types
of music
require
different
attack
and
release
times.
For
example, a
smooth
classical
string
quartet
number
might
sound
best
when
expanded
with a
slow
attack
and
slow
release
time,
where a
punchy
rock
and
roll
number
might sound best
when
expanded
with a
fast
attack
and
release
time.
Other
programs
might
sound
best
with
a
fast
attack
and
slow
release time.
The
point
is
that
attack
and
release
times
should
be
allowed
to
vary
according
to
the
program
content
for the
most
natural
sound.
The
3BX
does
just
that.
The
3BX's
attack
and
release
times
auto-
matically
and
continuously
follow
the
rate
of
change
of
the
"envelope"*
of the
program.
In
fact,
because
they
are
not
fixed,
the 3BX'$
attack
and
release
times
are
specified
as
rates
which
change
in
response
to
different
program
envelopes.
The
result is
a
smooth
action
that
does
not
alter
the
character
of the music as
dynamics
are
expanded
and
noise
is
lowered.
Single
vs
Multi-Band
Expansion
dbx
produces
three other
models
of
expanders,
the 1
17,
1
1 8
and
1
1
9,
and a
combination
expander/dbx
1
1 tape
noise
reduction
system, the
model
128.
These
expanders
are
single
band units
that
expand
the
entire
program
at
all
frequencies
at
the
same
time.
A
single
band
expander
is
less
expensive
than
a
multi-band
unit,
and
this
method
is satis-
factory
for
most
types
of
program
material.
In
fact,
the
117,
1
1
8,
1 1 9
and
1 28
outperform
every
expander
marketed
except
the
new
3BX.
However,
in some
cases,
when
the
program
has
a very
heavy bass
line,
single
band
expansion
can
cause
audible "breathing"
as
it
also
raises
high
fre-
quency
noise
when
a
loud
level,
low
frequency
note is
detected.
Fortunately, the
high
frequency
content
of
most
program
material masks this
high
frequency
noise so
that
the
breathing
effect is
usually
unnoticable.
A
multi-band
expander
has
the
advantage
of
being
able to
control
the
expansion
of
different frequency
bands
separately.
eliminating
the
possibility
that a
strong
note
in one
band
will
cause
audible
effects
in
another
frequency
band.
After
careful
consideration,
dbx
decided
that
three
bands
represented
the
optimum
number
of
frequency
divisions
for
an
expander.
The
3BX
Devices
such as
"noise
gates,"
"correlators"
and
"horizontal
filters"
may
be
able
to
subtract
noise
from
certain
types
of
programs,
but
they
do
not
significantly
improve
the
dynamic
range
of
the
program
in any
other
way.
In
addition,
such
devices
may
be
misled
by
certain
types
of music,
causing
undesirable
changes
in
the sound,
or they
may
change
the
frequency
response
of
the
pro-
gram, or
even
remove
parts of
the
music.
The
3BX
represents a
new
state
of
the
art
in
expansion.
With its
three
band
linear
dB
expansion,
RMS
detection,
and
program-controlled
attack
and
release
rates,
it
avoids
the
midrange
coloration
of
pre-emphasized
units (so
called
"weighted"
expanders),
the
"breathing"
of
single
band
units,
and
the
"noise
modulation"
of
units
that
divide the
spectrum
into
many
bands.
The
3BX
does
not
subtract
any
frequency
components
from
the
music.
Instead,
it
reduces
its
gain
during
quiet
passages
and
increases
its gain
during
loud
passages,
achieving
35dB
or
more
of
natural
sounding
expansion.
*
The
envelope is
a
graph of
the
program
level
versus
time;
see
the
Glossary
for
a
further
definition.