A minimum phase EQ is just another name for your normal everyday equalizer. Your Neve 1073, API 550, your Pultec EQP-1A. All of these equalizers experience phase shifts due to the latency created from changing the amplitude of specific frequency bands. This latency or delay of the frequencies causes what’s known as 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 other 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 plug-in coding and processing power started to become more advanced, developers realized they could finally do what engineers have wanted to do for this whole time. Linear phase equalizers are impossible in the analog world, but in plug-in land anything is possible. Programmers eventually developed an EQ that does not alter the phase at all— the phase is completely linear.
The irony of Linear Phase EQs is they were originally conceived because there was a 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 here were new problems and artifacts to overcome.
Pre-ringing is a negative artifact commonly associate 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 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 is so important a Linear Phase EQ will make sure this relationship 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 signal will change when combined with unprocessed channel. This may in fact make it sound better and you may like the change that was made or it may make it sound worse and 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.