dbx 161 Manuel d'instruction - Page 3
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or limiting the dbx 160 and
161
provide superior perfor-
mance at a reasonab le price.
Stereo
Tracking
Some compressor/ limiters have
jacks
that ostensibly
equip the units for stereo
operation. The
jacks
"strap"
the
gain control sect ions of the
two
sing le-channel units
together so
that
the stereo image remains stable even when
the signa
l
level is radica lly
different
in the two channels.
dbx's
Model
160
and
161,
are not equipped for stereo
operation.
T
he lack of
"strapping"
provisions
is
no over-
sight;
it is
a well thought out engineering dec ision. 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
ab ility of a single -channel unit to
meet its spec ifications
.
Even with better
than
average com-
ponent tolerances, component-to-component
variation
is
typically 10%; most potentiometers
have 20% to lerances.
Such
tolerances are perfectly acceptab le, and wi ll.not
degrade the performance
of an
individual
single-channel
un it.
Internal
trimmers are adjusted to provide any needed
correction,
and any crit ical components
are matched or
have prec ision tolerances.
Wh ile the front pane
l
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 to lerance diffe rences will cause the stereo image
to sh ift
rapidly
from right to left.
These
normal component
tolerances on ly become a prob lem 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, individua
l
component
tolerances in each un it
would have
to
be very tightly controlled
. T
his approach
would not
improve
the performance
of any one single-
channe
l
unit,
but it would make the cost of a ll 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 prob lems
:
(The expensive way out)
insta
ll
highly
accurate, precis ion components
throughout
the compressor for accurate stereo tracking performance,
and charge every purchaser for the stereo capabi
I ity,
whether
or not
it is
needed, or (The hard way out for you)
install
enough external tr im-pots so that, if you had the
right
test
equipment,
you cou ld adjust the tracking accuracy
yourself
...
as often as required by component
aging or
touring abuse.
None
of the above solutions were very pa latable to dbx
(nor wou ld the resu lts have been to you, the user). dbx
decided that omitting the stereo tracking feature would
improve
the overall va lue of the product, and wou ld
help
dbx to
keep
its commitment
to
product excel lence at
reasonab le cost.
Th
ere
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/ lim iter designed and
bui lt
for
the user who needs precise stereo tracking. The
162
utilizes matched components
and close-to lerance 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 thi
n
k 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.
Fi_gure 4, Curve
A
shows the envelope of an input signa
l
w1~h ~o compression.
(The
"envelope"
of a signal
is
a graph
of its
instantaneous
leve l.) Curve B shows the envelope of
the same signa
l
after
2:
1 compression
has
reduced
its level.
Cur~e. C shows the results of extreme 20:
1
compression
(l1m1t1ng). At this extreme compression ratio, the output
level is essentially constant for any input above the
threshold.
misused. In fact, many people refuse to
use
compress ion
even when it wou ld improve the audio qual ity, because they
have heard compression being misused a
ll
to often
.
One
of
the nicest aspects of dbx compressor /
Ii
miters
is the
fact that
th~y are easy to use and they sound better than competitive
units
...
even when using more extreme compression.
Much of the character of music is contained be low the
attacks or peaks. If
the
t
hr
eshold is set
1
OdB
or more above
the average level of the
input
signa
l,
then compression will
take place primari ly on the
uppermost peaks
or attacks of
the signal, minimizing musica
l
a lteration.
However if the
threshold is set to a leve
l
that
is lower with respect to the
average leve l, noticeable musica
l
alteration
may occur.
With a 2: 1 compression ratio, a 2dB increase
in
input
le~el
(above threshold) causes the output to
rise
only 1 dB.
With a 4:1 ratio, a 4dB increase at
the
input results
in
a 1dB
increase at the output. With a
10: 1
ratio, a
1
OdB
increase
at the
input
resu lts in a
1 dB
increase at the output, and so
forth.
Therefore, the amount of musical a lteration a lso
depends on the compression
ratio
used, as wel
l
as the
th reshold setting.
Music listen ing pleasure is greatest with ful
l
una ltered
dynamic
range.
Because dbx noise reduction systems afford
a useab le dynam ic range of over
1 OOdB,
dbx recommends
their use whenever practical. Lim iting or compression
can
be used in conjunction
with dbx noise reduction systems
for effect on ly.
'
A compressor/limiter,
as with any useful tool, can be
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