We all use equalisers and it's easy to take them for granted without considering just how clever they really are - Being able to selectively raise or lower the level of a specific frequency range within an audio signal. These days we probably take it for granted that it possible to digitally disassemble complex sounds using Fast Fourier Transforms and do things with them that our analogue-only forebears would find completely incomprehensible. But we've been able to do most of the equalisation we rely on today for decades, and that makes me wonder how that happens.
Many will be aware that conventional EQ also affects the phase of the audio being processed. I suspect that these days it's the common availability of phase linear equaliser plugins that first brings the fact that other EQ designs affect phase to the attention of novice engineers. Most of us probably see the phase rotation which occurs with conventional equalisers, whether based in analogue hardware, or in digital plug-ins just as a fact. An inevitable byproduct of EQ, and don’t think about it any further.
In terms of what practical difference this phase rotation makes to us as engineers, the answer, most of the time, is actually very little. An inevitable byproduct of linear phase filters is an increased latency, which makes them less suitable for some applications. But the advantages of linear phase EQ when used on channel EQ duties don't really outweigh the practical disadvantages. Also linear phase filters can display pre-ringing, a very a unnatural phenomenon in which artefacts precede the audio. So there are practical advantages to standard designs even though there is now an alternative. The phase rotation introduced by standard EQ is typically anywhere from inaudible to benign and it's only in complex audio that linear phase EQs tend to see much use. This is why they are seen by many as useful in mastering applications.
But returning to the matter of phase rotation, actually, the phase rotation is more than just a byproduct of equalisers, it is the method by which they are able to do what they do. That, taken for granted but clever when you think about it, trick of being able to selectively raise or lower the level of specific parts of the audio spectrum happens because of phase shift, not alongside it.
Phase Rotation
Phase rotation is much misunderstood. For our purposes it can be thought of as delay which affects different frequencies differently. The way in which this delay affects the different frequencies compared to the original signal can be plotted as a curve on a graph and this curve can show the phase relationship rotating from fully in phase, through fully out of phase and back again over the audio spectrum. This effect is hard to hear in isolation but if the phase-rotated audio is heard in context with the original audio the effect is much more apparent.
Rather than EQ resulting in phase change, a more instructive way to view what is happening is to see the frequency-specific gain change of an equaliser as being caused by manipulation of the phase of the signal rather than the other way round.
We are familiar with different types of audio filters. High pass, low pass, shelf and bell. These everyday audio filter types are well understood, but at some point most of us have come across the mysterious ‘All Pass Filter’. An all pass filter lets all frequencies pass at the same level and I know my first reaction when I heard of these were to wonder how there was any point having such a filter? The answer is that while all pass filters don't affect frequency levels, they do affect the phase of signals, or to put it another way not all frequencies pass through an all pass filter at the same speed. Many will leave it at that, with their curiosity, satisfied but without having any real answers.
In the following video from the inimitable, Dan Worrall, we see an investigation into this area. Dan uses all pass filters to create different common audio filters, and sees how, by changing the arrangement of these components, very different filter types can be created. This achieves two things. The first is to see how some of the filters we take for granted can be created from simple components, but more interestingly, it illustrates the point that EQ filters rely on phase shift to achieve what they do.
Watch the video and you'll see what we mean. This is definitely one for the for the more curious amongst us. You don't need to know this to be able to operate an equaliser but if you're anything like me and you like to know how the things you use every day work its a valuable watch.
Tell us what you think in the comments. Does this help you to understand phase better or are you even more confused?