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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