THE SIDECHAIN is perhaps a dynamics
processor’s most powerful feature. In this
article, I’ll show you three ways to use
sidechains to better your productions. But
first, let’s do a quick review of how these
hidden powerhouses work.
Fig. 1. The iZotope Ozone 5 Dynamics plug-in uses a Tilt internal sidechain filter to make kick drum and bass guitar thunder in a mix. The sloped frequency response of the filter is illustrated by the red line at the top of
the GUI. The steepness of the slope at both ends of the spectrum can be
increased to heighten the effect.
Altered Perception A sidechain is an
ancillary signal path containing a detector
circuit, whether based in hardware or software,
which alters a dynamics processor’s normal
response to signal in its audio path. Signal
routed to the sidechain is not heard in the audio
path (unless you activate a key listen switch to
audition it). The sidechain signal’s sole purpose
is to make its associated gate or compressor
react to it, but the processing is imparted to the
signal you feed into the audio path instead.
Here’s an analogy: A dog approaches a
motion sensor. The sensor reacts to the dog’s
presence by prompting a distant door to open,
letting a cat through. The motion sensor
(sidechain detector) saw the dog (sidechain
signal), but processed the cat (signal input to
the audio path) in response.
Most contemporary dynamics plugins
provide access to their sidechain via a
dropdown menu in the DAW host. When
a signal unrelated to that which is in the
audio path is routed via this DAW facility to
the plug-in’s sidechain, it’s referred to as an
external sidechain signal. Dynamics processors
sometimes also provide user access to an
internal sidechain that’s fed by a mult (copy)
of the audio input. The internal sidechain
is furnished with filters you can adjust to
equalize the mult so that the plug-in “hears”
its audio input having a different timbre.
Most single-band de-essers work this way;
their internal sidechain’s filter boosts high
frequencies so that an attendant compressor
reacts with heightened sensitivity to sibilant
highs in the audio path. Which brings us to
our first sidechain application: frequency-conscious
Tilt for More Bass Because low
frequencies contain more energy than
highs for the same perceived volume,
they generally will most readily trigger a
compressor placed on your stereo mix bus.
As a result, it can be difficult to compress
your overall mix as much as you’d like
without crushing the life out of kick-drum
hits and bass guitar.
The solution is to adjust the mix-bus
compressor’s internal sidechain filter so
that high frequencies are boosted and lows
attenuated in the sidechain (see Figure 1).
With the sidechain’s sensitivity increased for
high frequencies, bright portions of your mix
are more likely to trigger compression. If you
set the compressor’s threshold to be higher
than the level of the kick and bass’s attenuated
signal levels in the sidechain, the compressor
won’t initiate gain reduction during fleeting
moments of bottom-end bravado. The sonic
result will be a thunderous and punchy mix.
Fig. 2. A lead-vocal track is bused to the sidechain of a Waves C1 Compressor plug-in instantiated on a pedal-steel guitar track. Whenever the vocal level (shown in blue) exceeds the compressor’s threshold, gain reduction (level is shown in red) is forced on the pedalsteel track.
Fig. 3. Kick-drum hits key a Waves C1 Gate placed on a bass guitar track, locking the two tracks’ timing.
Duck out of the Way At the end of a solo or
when playing fills, a featured instrument can
readily step on the lead vocal in your mix. You
can dip the instrument’s fader level every time
the singer rejoins the mix, but doing that more
than a few times quickly becomes tedious.
Wouldn’t it be nice to have the instrument dip
automatically whenever the vocalist sings?
You can do just that by ducking the
instrument, using a compressor with a sidechain
configured to be keyed by the lead vocal (see
Figure 2). To set this up, instantiate a compressor
plug-in on the instrument’s track. Bus the lead
vocal—via a send—to the compressor’s sidechain.
Set the compressor’s threshold below the level
of the signal in the sidechain so that the plug-in
initiates gain reduction on the instrument’s track
whenever the vocalist sings. The compressor’s
ratio control will in large part determine how
far the instrument will dip in level during vocal
phrases. Use the compressor’s attack and release
controls to manage how quickly the instrument’s
level ducks and recovers, respectively.
Key the Gate Bass guitar generally sounds
the punchiest when its notes voice on kick-drum
hits. To that end, many bass players
intentionally strip down their phrasing to
mimic the kick’s. The result, if the two players
are tight, is a throbbing groove. But if the bass
player pushes the beat relative to an in-the-pocket
drummer, the feel will be wobbly and
the two players’ combined pulse will sound
weak and diluted. In this case, you could
microscopically shift the bass player’s track
later in time and then laboriously fine-tune each
note’s attack to line up with those for the kick-drum
hits. Call me next year when you’re done!
A better solution is to key the bass track
with the kick, using a gate that offers external
access to its sidechain. The Waves C1 Gate
works better than most other plug-ins for this
application (see Figure 3). Instantiate the C1
Gate on the bass guitar’s track. Bus the kick
drum—using a send—to the gate’s sidechain. Set
the plug-in’s Gate Open value below the level of
the kick in the plug-in’s sidechain. Use super-fast
attack and release times to open and close
the gate in lickety-split fashion. Adjust the gate’s
hold time to sculpt each bass note’s duration to
taste. The Gate Close value should be set low
enough that the gate doesn’t chatter (that is,
open and close indiscriminately). Plunge the
gate’s Floor parameter to “-Inf” to completely
silence the bass guitar in between kick hits. Cue
the disco lights—you’re about to groove!
Michael Cooper is a recording, mix,
mastering, and post-production engineer
and the owner of Michael Cooper Recording
in Sisters, Oregon (myspace.com/michaelcooperrecording).