Example EQ Phase Plots                           Back to fader8's audio blog Index

Parametric equalizer design is an interesting subject. While many plug-ins can exhibit the same performance,

when set up to provide identical filter shapes, the difference in sound often comes from the design of the user 

interface and how the controls interact with each other. Unless the plug-in is extremely well documented, it can 

be difficult to tell just what these differences are, but they typically include various degrees of gain-Q coupling, 

Cut-boost Q symmetry, filter symmetry, and even Q limits with selected "Types". These control differences account 

for much of the attributable "preferences" users have from one EQ to another.

The first series of measurments below were done by matching actual filter shapes as closely as possible and 

capturing phase response for each setting. The second series is a set of "analog emulations" that are usually 

dubbed as "colourful".

Minimum Phase EQ

I was going to write something myself here about non-linear phase (aka "minimum phase") EQ, but I came across 

this little section of the SSL Duende manual for the X-EQ, (designed by Algorithmix) that 

outlines this subject quite well:

4.4 Non- Linear Phase EQ

Phase shift in equipment is probably one of the most misunderstood topics in audio. Phase shifts often get blamed

for anything that is not classed as typical distortion. The reality is that all analogue EQs produce a deviation from linear phase

which is specifically related to the shape of the EQ curve, and it is precisely these phase changes which produce the required

gain reductions. This is a very natural process which can be found in the audio characteristics of rooms, materials and

elsewhere. 

Though it is not possible for us to detect phase in isolation, the phase shift of an EQ can often be the cause of the ‘magical’

sound of that device once the EQ’d signal is added back into the mix. For example, the Duende channel EQ derived from

the SSL analogue consoles – plenty of phase shift and plenty of magic!

In contrast, a linear-phase EQ delays all frequencies by the same amount. This can only be achieved in the digital domain

by using special techniques. While this process is very effective in certain situations (eg. ‘surgically’ removing

troublesome frequencies), it should not be considered as a superior tool for the majority of EQ tasks.

Linear phase equalisers have become popular over the past few years due to the proliferation of powerful digital

processors. We decided not to go down that route with X-EQ and we think you’ll agree with that decision. Maybe linear

phase EQ is something we will explore in the future but… who knows? An equaliser not only changes the relative

frequency balance of a signal but also its phase. If you EQ a sound until it sounds right or natural, then the chances are that

you are EQing to make it sound in phase with the rest of the mix. We need say no more on this subject.

(Thanks SSL. I couldn't have said it better!)

This PSP MasterQ is very typical for a "minimum phase" EQ.

After matching their Q's at -3dB, lowering the Epure's gain to -6dB shows that the gain to Q coupling is different from the above PSP EQ.

The Epure is not constant Q, which I've found to be more musical. 

 SSL stuff, very typical, good minimal phase EQ's.

Same for Sonalksis . . .

The Duende Channel Strip can't achieve a tight Q, so not included.

So much for high quality minimum phase EQ's. Understandably very close in phase performance.

"Colourful" EQ's

This Brainworx EQ is interesting. I've included a trace of this with a null setting. As  you can see, this thing will change 

things quite a bit, even with all the bands disabled. Apparently it has an all-pass filter always enabled. Good to know! 

Here's a few analog hardware emulations, all with a -6dB cut at 700Hz (or 800Hz). For the most part, they cannot 

achieve the filter shapes of the EQ's above, so I'm just showing comparable filter settings. Note that some of 

these exhibit varying degrees of harmonic distortion with boost, but those measurements are for another day!

Impulse responses are in another section of this blog though.

Nomad Blue Tubes PEQ-2A (Not too radical, this one.)

URS A-Series (API 550)

UAD Helios EQ:

UAD Pultec Pro:

UAD Neve 1081:

UAD Neve 1073:

UAD Neve 88RS Channelstrip:

Back to the audio blog Index