During a quick search on a popular online music retailer, I found no less than 342 different models of equalizers. Some plugin brands produce as many as twenty different EQ models. With all those options, how is one supposed to choose an appropriate EQ for a given situation? My usual advice to beginning engineers is to limit your choices to a few devices and use them until you feel like you really understand their strengths and limitations. After that add another device to your arsenal. This way you build a vocabulary of processors the way a guitarist builds a collection of guitars. Each instrument or processor has a unique personality and you will begin to instinctively reach for a particular tool for a specific application.
I will admit that many of my EQ choices are based simply on the user interface and the features of the plugin. If I want to make my drum overheads brighter, I’ll reach for an EQ plugin that has an easy-to-read and uncomplicated high-frequency section. I don’t want to flip through pages or adjust more than two or three parameters to simply “make this brighter.” I may also limit my choices to EQs that also provide a polarity switch. Many times I’ll flip the polarity while I EQ a sound to make sure that I’m aware of its interactions with other instruments. This is especially true for acoustic drums and low-end instruments (drums, basses, synths) that tend to mask each other. I have also become quite attached to EQs that let me solo individual frequency bands.
Common EQ types include graphic, parametric, semi-parametric, shelving and dynamic equalizers. Circuitry options would include solid-state, tube-based, inductor-based or passive circuits—and all these modeled in plugins. These may be styled after constant-Q or proportional-Q circuits and some EQs even automatically hunt down and correct tonal variations and resonances.
As the market saturates with reproductions and variations of so many classic equalizers, companies have begun to bombard us with something referred to as a Linear Phase Equalizer. Marketers would have us believe that audio Nirvana can finally be reached via this elusive and magical tool. Should we jump in and replace our beloved equalizers with linear phase equalizers? As we all know, there is no free lunch, so let’s dive into the world of linear phase and see what’s what.
Typical equalizers (and all analog EQs) are referred to as minimum phase equalizers since one of the design goals for equalizers is to minimize phase shift artifacts. Simply put, analog equalizers rely, in part, on capacitors and inductors, which induce tiny time delays and cause phase shifts that vary with frequency, filter shape and the amount of gain applied. Phase shift seems like it may be bad, but I think of phase shift like salt. Too much salt certainly tastes bad, but the right amount of salt brings forth dimension and complexity to flavors. Phase shift, like salt, imparts interesting depth and exciting dimension to musical sounds. Too much phase shift, unsurprisingly, smears harmonics and muddies up our musical intentions. Each brand and model of minimum phase equalizer owes a lot of its personality to the particular phase shift artifacts they impart.
There are some situations where wish to avoid phase shift from our EQ. Thanks to digital signal processing, we can have equalizers that do not produce any phase shift artifacts. These EQs are called Linear Phase Equalizers. These equalizers avoid phase shift by analyzing the frequency content and applying gain with FIR filters, a process that takes a lot of time (latency). The processed audio is subsequently shifted earlier to try to keep everything in time. This time shift produces an audible echo, called a pre-ring, that immediately precedes sounds with strong transients, like drums. This pre-ring can smear or weaken the attack of drums and picked or plucked instruments. Getting back to the no free lunch idiom, linear phase equalizers trade phase shift for pre-ringing artifacts. We have now established that both minimum-phase and linear phase equalizers have good and bad attributes. Now let’s explore the best uses of linear phase EQ.
For most applications of EQ, like sweetening instruments or mastering mixes, the actual phase shift is minimal because we use broad bandwidths and gentle boosts and cuts. For extremely narrow cuts or boosts, like when removing a resonant frequency from a snare drum, phase shift will drastically affect the frequencies surrounding the EQ’d frequency. In this case, linear phase EQ would be a wise choice and would remove the offending resonance without affecting the overall sound of the drum. Likewise, a steep high-pass filter that removes rumble from a vocal mic or guitar cabinet could wreak havoc on phase at frequencies well above the cutoff frequency. Phase shift artifacts may cause instruments to sound strange, so linear phase EQ would be a great choice. We can say that narrow cuts and boosts and steep bandpass filters are two situations where you might consider linear phase over minimum phase EQs. Keep reading—there are more uses for these magical devices.
Parallel processing instruments or subgroups in a mix would be another situation for linear phase EQ. Let’s say we have a vocal track and wish to duplicate the track and add some high-frequency shimmer by applying a high pass filter and compression to the duplicate (parallel) track. Minimum phase EQs, especially high pass filters, can induce phase shift that adversely affects the combination of the two tracks, causing unintended tonal changes. You can’t simply “flip the phase” and fix this problem. So, for parallel EQ duties, the linear phase EQ, with its lack of phase shift, may be our new best friend. You should audition both linear and minimum phase for parallel processing since in some cases you might find that phase artifacts sound interesting.
How about blending a top and bottom snare mic? For similar reasons to the above example, a linear phase high pass filter on the bottom snare mic (or hi-hat mic) may help retain the natural body of the snare while removing some low-frequency mud. The same goes for EQ’ing multi-miked guitar cabinets. For stereo miking, however, minimum phase EQ may be just fine since we are typically applying the same eq curve to both mics and both mics contain very similar source audio. Again, try both linear phase and minimum phase to hear what each does in your mix.
Another consideration when deciding whether to use linear-phase EQ is latency and CPU power. Many linear phase equalizers load your CPU slightly more than traditional equalizers and all linear phase equalizers can impart extreme amounts of latency. Linear-phase EQ latency can range from around 3,000 samples to over 20,000 samples. At a 44.1kHz sample rate, that translates to between 100 milliseconds and more than half a second. While most DAWs provide plugin delay compensation, you may near the compensation limit once you exceed 10,000 samples. Pro Tools, for example, provides about 16,000 samples of delay compensation at a 44.1kHz sample rate.
Many modern plugin equalizers, like those from Izotope and Fabfilter, provide linear-phase, minimum-phase, and even mixed-phase modes. Mixed modes provide some benefits from each type of EQ. You should audition these modes on various sources to find the best uses of each type. Further, many equalizers provide resolution or quality settings to further optimize your preferences of sonic character vs latency.
To answer the question of “Should I use linear phase EQ?” the answer is, as usual, it depends. Minimum phase equalizers, with their familiar personalities, would be my first choice most of the time. In certain situations, like narrow bells and steep filters, linear phase probably has the advantage. For parallel processing and multi-miked sources linear phase also probably comes out on top.
The best recommendation I can make is for you to add a linear phase EQ to your toolbox and learn where it best works for you.
Popular Linear Phase Options:
Logic Linear Phase EQ (Stock Logic Plugin)