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An Introduction to Equalization - A Free Guide from Audio Masterclass

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Why doesn't your equalizer have a side chain input? If it did, what would you do with it?

Although compressors, limiters, expanders and gates all have side chain inputs, equalizers never do. But what if they did? How could that work?


If you look at this article on the side chain of compressors, limiters, expanders and gates, you will see that I comment that equalizers never have side chain inputs, and I explain why.

But the thought occurs that maybe they could, and should have side chain inputs. If they did, what could be achieved?

But let's back track a little... Whereas compressors etc. measure the level of the signal, and act in response to that, as well as the settings of the controls, equalizers do not pay any attention whatsoever to the signal passing through. They act totally in response to the settings of the controls.

But it could easily be possible that a signal varies in frequency balance, and on a moment-to-moment basis sometimes requires high frequency boost, sometimes not. So sometimes, it has all the high frequency energy it seems to need, but at other times is lacking.

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If the equalizer could sense the high frequency content of the signal, then it could automatically apply more boost when necessary.

To explain this in a slightly different way, the effect of boosting the high frequency content of a signal could be 'reverse engineered' to see whether a signal was lacking in this quality. And if so, high frequency boost could be applied.

In fact, this concept could be applied to every control of the equalizer in an individual way, even the Q control. Why not?

This alone would massively increase the range and scope of equalization, and EQ is a pretty powerful tool to start with.

But then the next obvious step is to add an input to the side chain, so it is not the signal passing through the equalizer that controls its own frequency balance, it is another signal entirely.

This opens up so many possibilities that one could speculate all day. But for instance, suppose you are mixing voiceover with a music bed for a radio commercial. It is always tricky to get the right balance between the level of the voiceover and the level of the bed.

The standard technique is to 'scoop out' frequencies around 2 - 4 kHz from the bed. The human voice is strong in this range and will come over more clearly. But the problem is that the music bed now doesn't sound right during the gaps in the voiceover.

But with a equalizer that could sense the frequency content of a signal, and had a side chain input, the voice could be used to inversely control the EQ of the music bed. So when the voice comes along with certain strong bands of frequencies, those frequencies are automatically lowered in the bed.

OK, going to stop now. This isn't just food for thought - it is an entire feast!

By David Mellor Monday December 19, 2005