The concept of side-chaining is nothing new; studio engineers and mastering specialists discovered long ago how to use one track to control others in the mix with this method. But what is side-chaining and how can it be utilized to get a better mix? Sometimes it is desirable to create a powerful mix without forcing the vocals or more subtle instruments to compete for sonic space. By using the solo tracks to control a compressor on other or the rest of the tracks, the engineer seeks to maintain a relatively uncompressed mix that automatically "steps out of the way of the solos." Subtle side-chain applications are intended to enhance the separation of instruments in the mix without drawing the listener's attention. More dramatic applications can significantly alter the feel of the song and these often become part of the sounds themselves.
What is Side-Chaining?
The term side-chaining here refers to the use of the control input on a compressor. This can also be called ducking. Here we are not trying to use the compressor to control peaks as much as just enhance certain items in the mix. Normally with a compressor the input signal is internally sent to the control circuit of the compressor to compress the peaks of the input signal. Here we are using another channel’s output to control the compression of the channel going through the compressor. When we say “side-chaining the bass guitar to the kick drum” the bass guitar is going through the compressor and a kick drum channel output is going to the control input of the compressor.
Making Space for the Kick Drum
One very popular application is side-chaining the bass guitar to the kick drum track. You might want to employ this method when the mix sounds great with a lot of bass, but you're struggling to get the kick to cut through. Some EQ can help by enhancing the higher frequencies where the tap of the beater on the head is more percussive, but if you overdo it your kick starts to sound like cardboard.
Start by adjusting the EQ on your kick to sound punchy and balanced. Try backing the bass down to where it blends with the mix and the kick is easier to distinguish while you're doing this. Then adjust the kick to get a nice fat, punchy sound. Now you'll side-chain your bass guitar compressor to the kick track. Adjust the threshold to unity gain so the kick triggers the compressor consistently. Set a fast attack setting with a moderate compression ratio and set the compressor to peak so it catches the fast kick drum hits. Adjust the release so it doesn't interfere with the sustain of the bass part. Now you can boost your bass level once again, but the side-chained compressor will duck the bass slightly on the kick drum hits so they aren't buried in the mix. Done correctly you probably won't even notice the bass ducking because the kick should fill the space you've created. Fine tune your settings while carefully listening to the interaction of the bass, kick and the rest of the mix until you achieve the result you're looking for.
Separating the Guitar (or Other Rhythm Instruments) From the Vocals
Because guitars, keyboards, and many other rhythm instruments all "live" in the same frequency bands, it can be difficult to fit them all into the mix and still maintain enough separation to tell them apart. Even worse, you might have a great rhythm mix but you can't get the lead vocals to lay right in the mix. Here is a great opportunity to employ side-chaining. Start by adjusting the EQ and processing of the source track to taste. Then insert another compressor that follows, which is side-chained to the lead vocal track. Use a low compression ratio around 2:1 with a soft-knee and the threshold set to produce about 4-6dB of compression. You'll want fast attack and release settings here. Select the Peak setting on the compressor. This will ensure the compressor will trigger on the high points rather than the average signal over time. With the compressor side-chained to the vocal track, the guitar will automatically duck out of the way when the lead vocals are present. As with the bass and kick you ought to be able to achieve moderate ducking without violating the Prime Directive (does it sound weird?). You can also use this technique by compressing the entire instrumental mix and get some interesting effects.
Getting Separation Between Instrument Tracks
You can use this approach to get any two tracks (or a group of tracks by inserting the compressor on the group channel) to set well together in the mix. A lush string or synth pad can be side-chained to an embellished piano part or fingerpicked guitar to unmask them without taking away the power of the pad in the mix. By side-chaining the part you want to unmask, you can get the background to step out of the way of the foreground instruments. Use the same approach as you take to separate the vocals, but adjust the attack and release to favor the instrument you're compressing. If you want to duck several tracks, a mixer such as the Carvin Audio C1648 has assignable groups with individual outputs to make things easy.
In Part 2 of this series we'll look at more special applications for side-chaining, as well as cover the basics needed to route your system connections. Side-chaining gives you the ability to control one channel automatically with another. Which situations have you encountered where side-chaining might have been a good approach? Let us know in the comments.