Mixing – Sidechain Processing Tips

The sidechain is perhaps a dynamics processor’s most powerful feature.
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The sidechain is perhaps a dynamics processor’s most powerful feature.
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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.

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.

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 compression.

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.

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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.

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Fig. 3. Kick-drum hits key a Waves C1 Gate placed on a bass guitar track, locking the two tracks’ timing.

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.

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).