Interesting pdf, I didn't know about differences between constant-Q and non constant-Q. I use parametric filters a lot for EQ'ing headphones & speakers using REW software to model the effects of the parametric filters on the measured frequency responses of the headphones & speakers, and that uses constant-Q from my understanding of your pdf. In fact, every piece of EQ software I've ever used stipulates Q in this same way - as in the filters are universal & interchangeable (give the same results) between all the different EQ software I've used. I'll list the different EQ software and "systems" I've used: REW / EqualiserAPO (& Peace) / Neutron Player / AutoEQ (github) / miniDSP - it's also the same definition of filters used by everyone in the headphone EQ arena - Oratory1990 / Crinacle / Resolve / Innerfidelity / jaakkopasanen(@AutoEQ). I've never come across any implementation that uses non-constant Q. (I think you made a mistake or typo in your first or second sentence, because you're referring to constant Q in both so there's a logical mismatch happening there regardless.) And I think you must be wrong in saying miniDSP / Roon / Pierre don't use constant Q filters.
EDIT: unless there's a 3rd type of Q that isn't described as constant-Q or non-constant-Q. Either way all the parametric filters I've ever used behave like the following pic in your pdf you linked (note within each graph below the Q value is the same between each of the different graphed lines (within a graph), and the only difference is the amount of Gain, which is where the misunderstanding could be happening with those who have glossed over the pdf):
View attachment 254748
and they've never behaved like the following which is non-constant Q (note that the pdf is saying the Q value of the following two graphed filters are the same, only the Gain is different):
View attachment 254749
EDIT #2: I noticed @pierre
put a "Like" on your post, I think he didn't read the pdf in-depth as he's an experienced guy using EQ, so I think he just made an assumption that "constant-Q" meant a Q of 1.41 rather than a person being able to stipulate their Q-value, but that's not what the pdf is referring to in terms of what is meant by "constant-Q".
EDIT#3: There might be a third type called "proportional Q" (after I googled the topic), which might be what most EQ software uses, but that is certainly not the same as the "Figure 2" graph above, and instead looks more like the "Figure 3" graph above (when I model it in REW) which is what Storm Audio is saying is Constant-Q. I'm not sure that Storm Audio have covered off all the different types of Q. @oratory1990
, could you clear this up for us, any misunderstandings that are happening in my post & the one I replied to? (Meanwhile & also @GaryH
, I know you like understanding & using EQ, have you come across any of these definitions? I was surprised I didn't know about them.)