What’s That? Equalisation Pt.3

Welcome to part 3 ! You’re probably sick of EQ by now, but hang on a little longer because this post could save you from making some bad mixing whoopiedasies. Today I am going to talk about “linear phase” EQ and when (or when not) to use them.

What is Linear Phase EQ?

Linear phase EQ is a method of applying equalisation where the phase shift of the original audio source is preserved, which is done by adding a delay to the entire signal (and consequently adding latency to the whole project). This is different from the standard “minimum phase” EQ method, which only imparts phase on the EQ curve of frequencies changed.

(Note: It’s actually the phase difference that causes EQ, not the other way round!)

Linear phase can be a very powerful tool for multiple tracks where phase differences can make a huge impact on the sonic quality - for example a multi-mic’d drum kit.

For both the typical “Snare Top” and “Snare Bottom” microphones, it is paramount to maintain the correct phase relationship to avoid the body of the snare being cancelled out. With a “high pass filter”, whilst using a minimum phase EQ the phase can change 180 degrees above the cut-off, which may cancel out the impact of the snare body. However, with linear-phase EQ the snare body would be maintained since phase would stay aligned.

This is also particularly useful for bass production, especially if the engineer is using the double-DI method (typically used in metal) of processing one DI for low end and the other (copied) DI for upper harmonics/distortion. Adding low or high pass filters to either DI track with minimum phase filters can drastically alter the sound of the final result, so be sure to listen carefully to your mixing decisions!

Additionally, linear phase is considered a much more transparent method of EQ than their minimum phase counterpart. This is because phase shift is what usually impacts the “character” of the EQ, and this is why different audio engineers desire the sound of different EQs for different audio sources.

Sounds Great! … So What’s the Catch?

Unfortunately nothing in life comes without its drawbacks ! Aside from the extra latency and processing power that is used for linear phase, this EQ method can cause a terrible artefact called “pre-ringing”.

This artefact is defined by a weird “swell” before a transient, which causes them to lose their impact and clarity. This is usually caused when the EQ moves are too steep, especially in the low end. To avoid this, the engineer must make sure to closely monitor the transients, in addition to making sure that they don’t go too crazy with their EQ decisions!

Here’s another video from the genius Dan Worrall, explaining linear and minimum phase EQs:

I hope that this article helped you learn something new. I’m sure you’re glad to know that I’m done with EQ for now … onto compression next !

Thanks for reading :)