Deep Thoughts on Filters: Part 1
Updated: Jan 20, 2020
With that in mind the point I wish to make for consideration is to try and keep the phase distortion of high-pass filters at hand when choosing to use them. All (non-linear) high-pass filters introduce significant phase distortion at and around their cutoff frequency by nature of their very design. This is true for both analog and digital filters and may be a topic of conversation for a later date. The amount of distortion is relative to the slope of the filter, the stronger a filter affects frequencies below its cutoff point the greater its phase distortion. While there are a number of different designs of filters, each with their own filter shapes and respective phase distortion, none can completely eliminate this effect and mainly just seek to change the behavior of this phase distortion in a way that is more pleasing for certain applications.
The most common place I find the issues with high-pass filters come into play is when removing sub frequency material from bass instruments or the master bus of a mix (hence the bad pun in this title). While many of the following comments are true in all applications of HP filters, this is the area where I find their side effects most problematic.
Firstly you have to be very careful when making the decision to do any kind of equalisation or processing on the master bus, while it can certainly be a powerful tool, it is important to remember that it can only ever affect the sum of the total mix and not individual elements before they come together at the master bus. In other words if you are high-passing your mix to get rid of excess sub frequencies you should be aware that those frequencies are still interacting earlier on in the chain and may be triggering compression, saturation, or other processing in an undesirable way.
This may seem obvious but it is important to consider that if you are correcting a problem on the master bus it is almost always better to find that problem earlier in the signal chain and fix it there. This could mean placing a high-pass filter on only the elements of the mix that are providing too much bass, or even simply turning them down. The point is to minimise the application of any processing only to where it is necessary.
With that in mind the point I wish to make for consideration is to try and keep the phase distortion of high-pass filters at hand when choosing to use them. All (non-linear) high-pass filters introduce significant phase distortion at and around their cutoff frequency by nature of their very design. This is true for both analog and digital filters and may be a topic of conversation for a later date. The amount of distortion is relative to the slope of the filter, the stronger a filter affects frequencies below its cutoff point the greater its phase distortion. While there are a number of different designs of filters, each with their own filter shapes and respective phase distortion, none can completely eliminate this effect and mainly just seek to change the behaviour of this phase distortion in a way that is more pleasing for certain applications.
The point is that when you use a high-pass filter, it is important to think beyond just the curve represented in your plugin or DAW and really listen and focus on the way that the filter is interacting with the phase near to the point you have placed the filter. For example sometimes a filter can cause the transient of the kick to move forward or back in a mix or can change the way that a mix sounds in more than just the reduction of certain frequencies would suggest. The way that the phase distortion of a high-pass filter will interact with a full mix can be hard to predict and it would be inappropriate to recommend anything other than paying attention when using them and taking the time to make A/B comparisons between the processed and unprocessed signals.
By no means am I recommending against high-pass filters, again they are a powerful and versatile tool. What I am saying is that they need to be more than an automatic decision. Where and how you chose to use them can have a profound effect on the rhythmic and tonal nature of a mix if you are not careful. Being aware of this fact is an important skill to develop and learn, something that I myself am often practicing. Once you are aware of how filters can interact with the phase of a mix it can even become a viable technique to deliberately chose some filters over others specifically for their phase interactions that you might deep suitable for a certain sound or effect.
Again I may chose to get into further detail in a later post but for now maybe play around with some different filter settings on your master bus and see what happens. If you are interested in learning more I highly recommend starting with this excellent article by Ian Stewart (1). Good luck!
Footnotes:
(1) Ian Stewart. Center That Sub! (A Guide to Monoing Your Low End) - Blog - Flotown Mastering [Internet]. 2018 [cited 2020 Jan 6]. Available from: http://ianstewartmusic.us/blog/center-that-sub