Recording - November 2013 - 63

allows the signal level from the external
source to determine how much gain
reduction is applied to the signal traveling down the audio path of the processor.
In simple terms, keying allows one source
to compress another.
A couple of sidechaining techniques are
popular. De-essing is perhaps the most
commonplace, as it allows the reduction of
annoyingly loud “SSSSS” sounds in a
vocal track. De-essers are constructed by
placing an equalizer into the control path
of a fast acting compressor/limiter.
As shown in Figure 5, the vocal enters
the processor and is split to both the
audio and control paths. The eq, placed
on the control path only, significantly
boosts the “S” frequency range (usually
between 5 and 10 kHz). The control signal, now hyper-exaggerated with S’s,
triggers the gain reduction circuit on the
non-eq’d audio path. Like magic, “S”
sounds are being limited. When the
singer sings a savory syllable, this setup
squashes the sound!
Interestingly, it is the very act of signal
processing that makes “S” sounds so loud
in the first place. High-frequency boosts,
commonly exaggerated by mics and eq,
make S’s louder. Traditional compression
also contributes to the problem, as louder,

longer vowel sounds generally see
greater squash, inadvertently making
consonants louder. Luckily de-essing can
be very effective, either through dedicated de-essers, sidechaining an eq into the
control path of a compressor, or by simply automating fader moves.
Note that the de-essing principle can
also be applied in other situations. By varying the frequency of boost in the side
chain, loud bass slaps, acoustic guitar
squeaks, and weird loud-note resonances
can be controlled. A variety of dynamic
equalizer and de-esser plug-ins are available to help solve problems using this
approach. Many of these processors use a
topology slightly different than the keyedcompressor concept discussed above.
Key my kick
Another cool sidechaining trick is to
place a fast-acting limiter on a bass guitar and key it with the kick drum. See
Figure 6 for a graphic explanation. To
use this technique, mult (split) the output
of the kick so it is sent to both its normal
mixer channel and to the key (sidechain)
input of the limiter. Mix the kick as normal
through its own channel. On the bass limiter, set the processor to key to the external input and adjust the threshold so each

kick triggers a few dB of gain reduction.
The bass, passing through the audio path
of the compressor, will see limiting whenever the kick is hit.
While this technique may seem subtle
at first, it can have noticeable impact on
the feel of a song. Once keyed, the bass
moves out of the way of the kick, sometimes making it sound as if the two instruments are tighter in performance. This
technique is especially useful when one
or both of the musicians are inconsistent
in their timing. The trick also can help the
balance of a mix. With the bass keyed,
simultaneous bass and kick notes as less
likely to stick out. This secures the foundation, paving the way for a tight, highly
controlled mix.
With a variety of simple and complex
applications, compression is considered
by many engineers to be their most
favorite form of signal processing. To
master the effect, however, knowledge of
compression parameters is paramount,
and tweaking of default settings is almost
always required. Next month this column
will continue its look at compression and
limiting, investigating stereo bus processing and the resultant loudness wars that
have enveloped the audio community in
the last two decades.

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Recording - November 2013

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