Sidechains are effective mixing and musical tools. Tips include how to use a kick drum signal to compress the bass line, how to use a second vocal track through a compressor sidechain as a de-esser and using a sidechain as a rhythmic noise gate.

Next time you watch TV (you do still watch the old-school tube, don't you?), pay attention to the commercials. Notice how there is usually a musical theme, and when the mind-numbing advertiser's voice speaks, the music gets softer. Then, when the voice finishes its pitch, the music once again gets louder. This same thing occurs in many radio broadcasts. For example, many news shows begin with a familiar soundtrack as an introduction, and on cue, as the host begins to speak, the music fades into the background. A further example is one for you DJs: Have you ever actually made use of that ubiquitous “talkover” switch? Perhaps more commonly applied by wedding jocks, the idea is that the DJ's voice “talks over” what is currently playing, and the music magically gets softer, “stepping aside” for the voice to be clearly heard. All of these are basic examples of the concept of sidechaining.


Most often used in the studio in conjunction with dynamics processors such as compressors and gates, sidechaining is a method by which a control signal (often referred to as a “key input” or a “sidechain input”) is used to apply some type of processing to a separate, primary source signal. The processing applied can be as simple as a volume drop of the source signal, it can be as common and utilitarian as applying compression, or it can be more of a special effect such as gating a reverb track. The classic TV and radio examples cited earlier are usually called “ducking.”

While the act of dropping Muzak down to background level may be useful, there are unlimited musical applications for ducking as well. For example, as anyone who has mixed even a dozen rock, house or hip-hop tracks knows, there can often be a conflict in the low end, with bass lines drowning out kick drums or vice versa. By placing a compressor on a bass track and feeding a copy of the kick track into the compressor's sidechain input, you can use the kick as a trigger to compress the bass wherever they overlap. As usual, setting your attack, threshold, ratio and other compression settings will naturally affect the overall balance between the two sounds, so experiment until you find the right brew for the particular task at hand — presets don't apply here. This type of ducking can be the glue that holds clean mixes together, yet you need to use caution: In any such compression sidechaining application, you'll want to listen carefully to make sure that “pumping” doesn't occur. The term “pumping” means that signal-level dips can be clearly heard in the mix. The ducking routine is not limited to bass and kick drums, by the way. Let's say you are mixing a pop track with guitar and vocals. You typically want your vocals to stand front and center in your mix, so you can use a copy of the vocal track to compress the guitar; wherever the two intersect, you can ensure that the vocals always come to the fore.


Another common use for sidechaining is to turn a normal compressor into a dedicated de-esser. In this instance, you can run a vocal track through the compressor while running a duplicate version of the vocal into the compressor's sidechain input. But the magic here is equalization. First, route the duplicate track through a multiband EQ, strip out the bass and lower-mid frequencies, and target the mid or higher frequencies where the “s” and “t” sounds reside. Accentuate those frequencies. Now, by driving your compressor with that frequency-emphasized source, you can effectively knock down those harsh sounds in your vocal tracks while teaching your old dog (the compressor) new tricks. Once again, you will need to work your compressor's controls to achieve optimum results. But in this instance, it can be very useful to have attack and release controls. Start out with short times for both, and then slowly lengthen them to smooth over the resulting sound and reduce any pumping that might result.


A still common yet more creative use for sidechaining involves a noise gate in conjunction with any form of rhythmic music. Dancefloor tracks such as techno or synth-pop can be enhanced by placing a gate on a synth line or pad and using a sharp repetitive sound such as a hi-hat, clap or conga loop for the key input. Now, each time the relevant percussive element triggers the gate, the synth line will be chopped up, resulting in a rhythmically locked arpeggio of sorts. I can imagine you trance addicts licking your chops now in anticipation of your next studio session. By adding this simple ingredient into the stew, your tracks can churn and bubble along just a little bit funkier. If you synth-pop aficionados want to get all anachronistic with this, try busing a vocal track's reverb from your DAW into a gate audio input. Then use an amplified or normalized copy of the same track for the gate's key input but chop out all but a few beats or a bar right after each vocal phrase has ended through the end of each reverb tail. That controls the gate to truncate the reverb at the end of each vocal pass, and — just like Marty McFly — you are suddenly back in time straight to the '80s, the golden era of hair mousse, hot pink and Yamaha DX-7s.

Sidechaining isn't a complex feat, yet its use can open new, creative doors. Limited mostly to hardware for years, DAWs and plug-ins with sidechain capabilities now abound. The major multitrack platforms such as Pro Tools, Logic and Reason each have their own form of sidechaining, and a slew of old-school and new hardware compressors, limiters, gates and de-essers also feature sidechains. Explore these sidechaining basics, and then go nuts.