A minimum phase EQ is just another name for your standard, everyday equalizer. Your Neve 1073, API 550, your Pultec EQP-1A. All of these equalizers experience phase shifts due to the latency created by changing the amplitude of specific frequency bands. This latency or delay of the frequencies causes what’s known as a phase smear. Smearing leaves audible artifacts in the signal, which can be undesirable. Many times you can’t hear smearing at all, other times, you may like what it’s doing, but in different scenarios, you may want an equalizer that keeps the phase consistent (more on those later).
In the analog world, phase smear was just something that product designers tried to minimize or mold into something that sounded pleasing. In the digital world, all bets are off. When plugin coding and processing power started to become more advanced, developers realized they could finally do what engineers have wanted to do this whole time. Linear phase equalizers are impossible in the analog world, but in plugin land, anything is possible. Linear Phase EQ is equalization that does not alter the phase relationship of the source— the phase is entirely linear.
The irony of Linear Phase EQs is that they were initially conceived because of an engineer’s desire to eliminate phase smearing, which was thought to be a negative byproduct of using analog hardware equalizers. Once software programmers were able to develop a Linear Phase EQ, they soon realized that there were new problems and artifacts to overcome.
Pre-ringing is a negative artifact commonly associated with using Linear Phase EQs, which affects the initial transient. Instead of starting with a sharp transient, there is a short but often audible crescendo in the waveform before the transient hits. Since it happens before the transient, it sounds very unmusical and displeasing. This affects the overall tonality of transients which people do not find desirable.
The next obvious question… When are you supposed to use one? What are they good for?
Well, that answer depends on the engineer you ask. There are a lot of engineers that might tell you there is never a good reason to use a Linear Phase EQ. There were thousands and thousands of records made before plugins and Linear Phase EQs existed, and a lot of them sound pretty damn good. I can’t fault an engineer who doesn’t even bother with ever using one.
Other than never, there are a few scenarios where you might want to try a Linear Phase EQ. One of those is when boosting or cutting on sources that were multi mic’d. Since the phase relationships between each mic are so important, a Linear Phase EQ will ensure the phase coherence stays intact even with processing.
Another time you may want to pull out the ol’ Linear Phase EQ is when equalizing parallel tracks. When you have two of the same tracks, and you insert an equalizer, the phase of when the signal will change when combined with the unprocessed channel. This may, in fact, make it sound better, and you may like it, or it may make it sound worse; in this case, you could try reaching for a Linear Phase EQ to retain the phase relationship while still being able to boost and cut frequencies on the parallel channel.