LINEAR PHASE EQUALISATION EXPLAINED.
You may have seen products such as Fab Filter’s Pro-Q 3, or IK Multimedia’s T-RackS Linear Phase EQ and wondered how linear phase EQs differ in performance and function to minimum phase options.
Today we will be delving into this, and assessing where, when and how best to use linear phase EQs to your advantage.
A logical first step would be to briefly introduce the concept of phase (or polarity, for you fancy types).
A little bit about phase…
Phase (Ø) refers to the relation of coherence between independent audio signals when summed.
All sound waves consist of cycles of positive and negative pressure. This is reflected graphically by the peaks and troughs of waveforms.
A signal is considered to be out of phase when two or more audio signals begin their respective waveform cycles at different points in time. This can lead to a variety of acoustic outcomes, both desired and undesired.
We will delve deeply into phase and phase shift in future articles, but for now, all you need to know is that it relates to the relationship of combined audio signals, and that this can have various sonic implications.
So what is linear phase processing?
All forms of analog and digital equalisation will cause some degree of phase shift, this can cause undesired (or desired) sonic outcomes through affecting the amplitude of varying frequencies.
A linear phase EQ is designed to ensure that its phase shift is spectrally consistent. They use Finite Impulse Response processing to guarantee that frequencies pass through the filtering process with an even delay time.
This processing is often more CPU intensive than minimal phase options, but is potentially a useful tool to help to reduce the tonal effects of phase shift.
If phase shift occurs with all forms of equalisation, why does linear phase processing matter?
Generally speaking, most phase smearing is unnoticeable, or may even add pleasant effects to your audio signal. However, steep filtering can cause phase distortion on transient material, degrading its audio quality.
These issues can become even more noticeable when you mix phase altered content with similar, unaffected audio or when applied in parallel processing such as NYC Compression.
With this in mind, here are some instances where I might use a linear phase EQ:
To negate the effects of phase cancellation in any instance.
Mastering.
Parallel or buss processing.
When mixing audio that was recorded using stereo microphone techniques.
Audio Demo
The audio below demonstrates NYC Compression (with minimal phase equalisation) and NYC Compression (with linear phase equalisation).
As parallel processing involves duplicating a signal, and processing the duplicate, linear phase EQs can help solidify the signal, negating the effects of polarity on the return track.
If you want to read more about parallel processing - you can find our in-depth article here:
Caution!**
As buss processing would impact the dynamics, I have left the audio uncompressed and at pre-master volume (-24 LUFs). Please be wary of this and be careful with any audio you feed to your headphones or monitors afterwards as it might be significantly louder!
Something to keep in mind…
As good as linear phase EQs are at reducing phase smearing, they can also create undesirable artefacts - known as “pre-ringing.” These resonances are generally very difficult to remove from a signal as they are symptomatic of the EQ shifting the phase of the signal.
You can reduce their audibility by:
Experimenting with Q width. Broadening the Q will reduce pre-ringing duration, but increase its amplitude, narrowing it will do the opposite.
Avoiding drastically boosting EQ curves.
Focusing your EQ activity on higher frequencies (the resonant duration decreases as the harmonic frequency increases).
Concluding remarks
Whilst I would not advise you rush to download a plug-in purely for its linear phase features, there are many excellent plug-in bundles that include phase linear functionality. As I mentioned in an earlier article, the Fab-Filter or T-RackS bundles would be great options.
Linear phase equalisation does not necessarily improve or degrade an audio signal. Thousands of records have been made without this technology. Keep it in mind as a useful tool in your belt, and trust your judgment as you work.
Article Sources / Further Reading:de Boer, Maarten. (2022). Linear phase EQ. [Online]. https://www.fabfilter.com. Last Updated: 10 February, 2022. Available at: https://www.fabfilter.com/learn/equalization/linear-phase-eq [Accessed 16 February 2023].Crave DSP. (2017). Linear Phase EQ Explained. [Online]. https://cravedsp.com. Last Updated: 30 May, 2017. Available at: https://cravedsp.com/blog/linear-phase-eq-explained [Accessed 16 February 2023].