Q. Why do my mixes clip when I apply a high-pass filter?

By Sam Inglis

 
Here we see the characteristic 'hump' just above the turnover frequency of a shelving filter.

There's something happening when I master my mixes that I can't find an explanation for. This has been bothering me for nearly two years, so I thought it was time to call the experts! After maximising my mix, so that the level of the audio is just below the point of clipping, if I insert a high-pass filter at, say, 40Hz, suddenly the audio starts to clip. I thought that after inserting a high-pass filter the level should drop, but this is not the case. I've tried it with several EQs but always with the same result. What is happening? I've been using digital EQ, but will this still happen with analogue EQ?

Johan De Visser

Features Editor Sam Inglis replies: It's surprising, but true, that using EQ can cause clipping whether you are cutting or boosting. There are two reasons for this.

One is that EQ changes the phase relationships between the different frequencies that make up a complex signal, such as a full mix. The result of this is that even though you're cutting the low frequencies, you could be shifting other frequencies around in such a way that they reinforce one another at a point where they had not previously done so. It's also possible that whatever signal element you removed was actually serving to 'cancel out' another element at some points, so removing it has created larger peaks at these points.

The other reason is that some EQ designs actually have a resonant peak at the corner frequency. If this is the case then applying a low-pass filter at, say, 40Hz might actually introduce more energy than it removes, assuming there was nothing below 40Hz in the signal to start with.

It's possible to experience gain increases when cutting with both analogue and digital EQ — it's in the nature of equalisation that this will happen. The answer is to always do loudness maximising as the last process in the mastering chain (apart from dithering) — always use EQ before the maximiser, not after.

Published December 2005