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Thursday, January 10, 2013

Q. What is side‑chaining, and what do you use it for?

Sound Advice : Mixing
This might be a very big topic, but I’m hoping that you can help to clear up some confusion. Side‑chaining seems to be something that is used a lot, but I don’t really understand what it is. Can you explain?
Kim Nguyen via email

Normally, compressors and gates use the signal that’s being processed to control the amount of gain reduction taking place, as in the top arrangement in the diagram to the right. Some devices, however, allow you to use a secondary input to control the gain of the first input (below). This allows you to, for example, compress a bass guitar using a kick drum as the trigger, or ‘side‑chain’ input.
SOS Reviews Editor Matt Houghton replies: 
This is a huge topic, and it would be well worth reading some of the past SOS features about it (see the archive on our web site). Essentially, though, any dynamics processor (for example, a gate, expander, compressor or limiter) uses two input signals: the incoming audio itself and a side‑chain, which feeds the detection circuitry that determines whether or not the processor acts on the material. Simple processors take their side‑chain signal directly from the audio input. A more sophisticated approach is to split that signal, and allow you to process the side‑chain with high‑ or low‑pass filters.
Many professional devices also have a second physical input called the external side chain, so that you can feed the processor’s detection circuit with any audio signal, which can be totally unrelated to the main audio input. A common example is ducking, where you might feed the kick‑drum signal into a compressor on the electric bass and set it up with a fast attack and release time so that the bass is attenuated by 1‑2 dB every time the kick exceeds the threshold. Another example would be to use a signal to ‘key’ a gate: you could place a gate on a synth pad, for example, and use a percussive loop to make the gate open and close rhythmically with the groove of the loop, without ever needing to hear the loop itself!

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