dbx 165A Manuale di istruzioni - Pagina 10
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loud sounds are held back.
Of
course, when compression
does
occur,
the
level
of the entire program
is
affected.
For this
reason,
it
may be more useful
to
reserve this
combined
EO/compression
technique for isolated sound
sources, such
as
a single channel of vocal or a single
instrument
in a multi-track
program.
Uni
i
ke overal
I
program
equalization, EQ of the
level
detector will take
effect
only
when signals are above
threshold
(or when
those frequencies affected fall above the set threshold).
Depending
on the
threshold setting,
lower-level
funda-
mentals
or harmonics will not cause compression, and
the program is not subject to
the
phase shift normally
caused by program equalization.
For example, consider a single channel carrying the
preamplified signal from a microphone placed near a
cymbal and a tom-tom. Set up the 165 with an equalizer
in the level detector path, as depicted
in
Figure 8.
The
equalizer can be adjusted
for
boost with a peak at about
5 kHz,
causing the cymbal to be compressed on a very
loud
crash, preventing tape saturation at high frequencies
where
there
is less headroom.
However,
gentle tapping
of a drumstick
or
brushing of
the
cymbal will not be
held back.
Assuming the
tom-tom is a lower frequency
instrument, and can be
better
tolerated by the tape,
there is less need for compression on it. The equalization
in
the
detector circuit means that the compressor will
not
be triggered as readily by a
loud
tom beat as by an
equally loud cymbal crash.
Another application for this type of
EQ
boost in the
level
detector is for
increasing
the sustain of a guitar, bass,
etc; this requires EO boost in the dominant frequency
range of
the
instrument, along with a fairly
low
threshold
and a moderate compression ratio.
Still
another related application involves de-essing of
vocals ( reduction of si bi
I
ance). Use a parametric equalizer
in the level detector circuit and set
it
for
high
frequency
boost
in
the specific frequency range where the vocal
"hiss" or lisp occurs. This pre-emphasizes the already
"hissy" vocal input to the detector.
Used in
conjunction
with a moderate
to
high
threshold
and compression
ratio, th is arrangement greatly attenuates the "essi ng"
without affecting the basic sound quality or balance of
the voice. While it is true that all frequencies are
lowered
in
level
when the compressor is triggered,
generally the "sss" sound occurs alone, before or after
the dominant tone in the voice. ( If this seems unlikely,
just
try to hum and hiss at the same time.)
The
converse of the above EQ techniques may be
used;
dipping
the
equalizer will cause any sound in the
affected
register
to pull the
level
up because
it
will seem
to require
less
compression than the other
frequencies.
-
- ·
i
INPUT
OUTPUT
F
I
LTER OR
EQUA
L
IZER
PROGRAM
SOURCE
,-~.-=::
.....
+
t -
..
,.,
. ,
_ ,.
•
-·-
0
Fig. 8
-
Equalizer or Filter Used with the
Level
Detector Input
Use
of
a Filter
in th
e Level Detector Circuit
The results of inserting a filter
in
the level detector
circuit are basically the same as obtained with an equali-
zer, as previously described. Those frequencies passed by
the filter are subject to compression (or at least they
are subject to
c.onsiderably
more compression
than those
frequencies
outside
the
passband).
Because
a passive
filter can have
in$ertion
loss, it may be necessary to
lower
the
165's
Threshold setting to maintain a given
amount of gain
reduction
within the
filter
passband; this
can be
determined,
as usual, by monitoring the 165's
thresho
ld
indicator
L
EDs.
Use
of
a
Tim
e Delay Line in the
Signal
Path but not in
the
Level
Detector
Circuit
fo
r
Zero or Negative Attack
&
Release Ti mes
While
the
Model
165
can be set for incredibly fast
attack times,
there
will always be some small transient
that
"gets past" the level detector. In some cases - such
as maximum modulation broadcasting
-
it may be
desirable
to
preserve the sonic quality obtained with a
slower attack time, yet it may not be permissible for
even the slightest overshoot to get past the compressor
/
limiter. A
delay
line
(digita
l
or analog) can be
used in
this instance. By
feeding
the program directly to
the
165's
detector
input,
but delay ing the feed to the 165's
signal input, the unit can "anticipate"
the need
for
a
gain change. (Refer to Figure 9.) With some experimen
-
tation,
the effect can be that of "zero" attack time.
Additional
signal delays beyond the "zero" time
established above would then cause
the
compressor to
finish changing gain before
the leading
edge of the loud
passage enters the signal input, suppressing program
which
is
not above
threshold. Also,
the
165
would begin
to recover from compression (release) before the input
signal has dropped back to the set threshold, causing the
output to surge
higher in level
as the note or passage is
decaying. This special effect obtained with
the
time
delay might sound akin to reverse playback of a tape
recording.
Access
to the 165's level detector makes possible
a whole range of effects not normally available. The
more you think about it and experiment, the more use-
ful this capability can become.
PROGRAM
SOURCE
.-I-
----
~
DELAY
INPUT
OUTPUT
LINE
,-
=
.....
-
.,
...
. .
-·
•·••· ---··
•
... ...
.......
_
-·-
Fig. 9
-
Delay Line Used With The Signal Input
The 165 As A Line Ampl ifier
0
To use the
165
as a
I
ine amp I ifier, adjust the
COMPRESS ION
RATIO
control to fully counterclock-
wise (1:
1
position), THRESHOLD to full clockwise
position (+10) and OUTPUT GAIN to whatever setting
is required for
the
application.
Remember
that, as with
any amplifier, excessive gain may
lead to
output clipping
of high
level
signals.
To
add compression, adjust the
COMPRESSION RAT IO
and the
THRESHOLD
controls
to
the desired settings.
9