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 EQ/compression
technique
for isolated sound
sources, such
as a
single channel of vocal or
a
single
instrument in a multi-track
program. Unlike overall
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
preampMfied 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 EQ 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
sibilance).
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, this arrangement greatly attenuates the ''essing"
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.
Frg. 8
—
Equalizer
or
Filter
Used with the
Level Detector Input
Use of a
Filter
in the
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
considerably more compression than those
frequencies outside the passband). Because a passive
filter can have insertion 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
threshold indicator
LEDs.
Use of a Time
Delay Line in the Signal
Path
but not in
the Level Detector Circuit for Zero or Negative Attack
&
Release Times
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 (digital or analog) can be used in
this instance. By
feeding the program directly
to
the
165's
detector input, but delaying 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.
The 165 As A Line Amplifier
To use
the
165 as a
line amplifier, adjust the
COMPRESSION RATIO control to ^lly
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 RATIO and the THRESHOLD controls
to
the desired settings.
9
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