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or limiting the
dbx 160 and
161 provide
superior perfor-
mance
at
a
reasonable
price.
Stereo Tracking
Some
compressor/limiters have
jacks that ostensibly
equip
the
units for stereo
operation.
The
jacks "strap" the
gain
control sections
of the
two
single-channel units
together so that the stereo
image remains
stable
even
when
the
signal
level
is
radically different
in
the
two
channels,
dbx's
Model 160 and
161,
are
not
equipped
for stereo
operation.
The
lack
of "strapping" provisions
is
no
over-
sight;
it
is
a
well
thought out
engineering
decision.
The
reason
for this
decision
is
simple,
if
not immediately
obvious.
Stereo tracking
between two
separate compressor/limiters
requires great
accuracy. This tracking
accuracy has
little
to
do
with
the quality or
the
ability
of a single-channel unit
to
meet
its
specifications.
Even
with
better
than average
com-
ponent
tolerances,
component-to-component
variation
is
typically
10%;
most
potentiometers have
20%
tolerances.
Such
tolerances are perfectly acceptable,
and
will
not
degrade
the
performance
of
an
individual single-channel
unit.
Internal
trimmers
are
adjusted
to
provide
any needed
correction,
and any
critical
components
are
matched
or
have
precision tolerances.
While
the front panel
settings of
two
"strapped"
units
may
be
identical,
component
tolerances
can cause
the actual
performance
to
vary
from
unit to unit
by
as
much
as
20%.
Even
small tolerance differences
will
cause the
stereo
image
to
shift
rapidly
from
right
to
left.
These normal
component
tolerances
only
become
a
problem
if
two
units are
strapped
together
for stereo
operation.
If
any
given
pair
of single-channel units ever
manufac-
tured
could be strapped
together
for
accurate stereo
tracking, individual
component
tolerances
in
each
unit
would
have to be very
tightly
controlled.
This
approach
would
not improve
the
performance
of
any one
single-
channel
unit,
but
it
would
make
the cost of
all
units
prohibitive.
There
are
three other
ways
to build
compressors with
stereo strapping
jacks:
(The easy
way
out)
Put
in
the
jacks
on
production
units that are
not
critically
matched, and
ignore the resulting
problems: (The expensive
way
out)
install
highly accurate, precision
components
throughout
the
compressor
for
accurate
stereo tracking
performance,
and
charge every purchaser
for
the stereo
capability,
whether
or
not
it is
needed,
or
(The hard
way
out
for
you)
install
enough
external trim-pots so
that,
if
you
had
the
right test
equipment, you could
adjust
the tracking accuracy
yourself ...
as
often
as
required
by
component
aging or
touring abuse.
None
of
the
above
solutions
were
very palatable
to
dbx
(nor
would
the
results
have been
to
you, the
user),
dbx
decided
that
omitting the stereo tracking
feature
would
improve
the
overall
value of the product,
and
would
help
dbx
to
keep
its
commitment
to
product
excellence
at
reasonable
cost.
There
is
no
reason
for
people
who
need
a single-channel
compressor/limiter to
pay
for precision
parts
or
matching
that
they
do
not need. Neither
is
there
any
reason
for
someone
who
needs
a
stereo
compressor/
limiter to
accept
inferior
performance from
units
with
strapping
jacks
but
no
matched
or precision
components
that
would
provide
precise stereo tracking
accuracy.
Instead,
dbx
decided
to offer
another model,
the 162,
which
is
a
two-channel compressor/limiter designed and
built for
the user
who
needs
precise stereo tracking.
The
162
utilizes
matched components and
close-tolerance parts
to
achieve
precise tracking,
with
easy,
single-knob
adjust-
ments.
The
162
does
the job
at
an
attractive price.
It
makes
a lot
of
sense
when
you
think
about
it:
a
precise
dbx
compressor/limiter
for single-channel
jobs,
and another
for
precise
stereo operation.
The
Compressor/Limiter
as
a
Tool
Set
up
properly,
a
compressor
can be
a
useful device.
Figure
4,
Curve
A
shows
the
envelope
of
an
input
signal
with
no
compression. (The "envelope"
of a
signal
is
a
graph
of
its
instantaneous
level.)
Curve B
shows
the
envelope
of
the
same
signal after
2:1
compression
has
reduced
its
level.
Curve
C
shows
the
results
of
extreme
20:1
compression
(limiting).
At
this
extreme compression
ratio,
the
output
level
is
essentially
constant
for
any
input
above
the
threshold.
Much
of the character
of
music
is
contained
below
the
attacks or peaks.
If
the threshold
is
set
10dB
or
more
above
the
average
level
of the input
signal,
then
compression
will
take place primarily
on
the
uppermost
peaks
or attacks
of
the
signal,
minimizing
musical
alteration.
However,
if
the
threshold
is
set
to a
level
that
is
lower with
respect to the
average
level,
noticeable musical
alteration
may
occur.
With
a
2:1
compression
ratio, a
2dB
increase
in
input
level
(above threshold) causes the
output
to
rise
only IdB.
With
a
4:1
ratio, a
4dB
increase
at
the input
results
in
a
IdB
increase at
the output.
With
a
10:1
ratio, a
10dB
increase
at
the input
results
in
a
IdB
increase
at
the output,
and
so
forth.
Therefore, the
amount
of musical
alteration also
depends
on
the
compression
ratio
used,
as
well as
the
threshold
setting.
Music
listening
pleasure
is
greatest
with
full,
unaltered
dynamic
range.
Because
dbx
noise
reduction
systems
afford
a
useable
dynamic
range
of
over
lOOdB, dbx
recommends
their
use
whenever
practical.
Limiting or
compression can
be used
in
conjunction with
dbx
noise reduction systems,
for effect only.
A
compressor/limiter,
as
with
any
useful tool,
can be
misused.
In
fact,
many
people
refuse to
use
compression,
even
when
it
would
improve
the
audio
quality,
because they
have heard compression being misused
all
to often.
One
of
the nicest aspects
of
dbx
compressor/limiters
is
the
fact that
they
are
easy
to use
and
they
sound
better
than competitive
units
.
.
.
even
when
using
more
extreme
compression.