Zero Phase XO's - PEQ-only? . . .

Steve_Lee

IS it really a thing as discussed by this guy and have you experienced it/done it?

Subject post -->

" The objective of the DSP crossover configurations that I use is to flatten the phase across the entire audible band. If I can do that with first order crossover filters and/or shelf filters, then I do it. So there isn't a right/wrong approach to doing it--it's the results in terms of flattened phase response that count.

One other thing I should mention: most crossover filter designs for loudspeakers don't pay attention to phase response--especially the "cookbook" methods found in all books on the subject of DIY loudspeakers. This subject--flattening the phase response of loudspeakers--is what I believe what makes loudspeaker designs like the Danley Synergy series, Dunlavy Audio Labs/Duntech loudspeakers, the much earlier B&O "phase link" designs, and even full-range drivers sound so realistic in my experience, as well as most studio monitors that are sold today for higher-end studios that use active studio monitors employing DSP crossovers with FIR filtering.

I couldn't believe what I was hearing when I first applied the "zero phase" crossover filters to the Jubilees using PEQs only. I was stunned, and the experience ranks as probably the most revealing discovery for me about loudspeakers (and not in any way to downplay the full-range directivity and low modulation distortion of horn-loaded loudspeakers like Roy and PWK designed).

It turns out that there are very good reasons why flat phase or even linear phase loudspeakers sound so lifelike and realistic (i.e., subconscious sound quality), and the reason is related to how humans hear the harmonics of musical instruments and voices, and even spoken word--even more strongly. I recommend reading Dave Griesinger's presentation on clarity (look closely at slides 12-19, and particularly slides 17-18). This simple explanation is missed by just about everyone in the audio engineering world, it seems.

So for the purpose of this thread and others where I talk about flattening phase--it's the flat phase response that's important, and not so much how that is achieved. All I did was apply what other smart guys have been saying for some time to DSP crossover configurations. The results are, in my experience, startling.

Chris "

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Link to thread --> https://community.klipsch.com/index.php?/topic/188177-double-stack-ess-amt-1-with-wings-possible-kit-for-heritage/page/33/

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If so, the new MiniDSP Flex-8 is in my future as it offers 10 PEQ's per driver/output rather than the 5 PEQ's provided by the MiniDSP 4x10HD that I now have in my possession . . .

DCIBEL - any thoughts on this, Sir?

Steve.

dcibel

My personal opinion is that applying any phase specific EQ is splitting hairs on actual results above bass frequencies where group delay can be significant. At the end of the day you have a balance of amplitude and phase that each contribute to overall power / directivity of the speaker, I'm not convinced that correcting every little phase bump in a speaker response provides anything more than you get from a well designed speaker using IIR filters, you just have a bit more control over the end result with FIR. Maybe there's some imaging magic to be had by providing some frequency dependent phase shift to convert the min phase shift to 0 phase, I haven't gone down that rabbit hole, the process sounds somewhat "unnatural" to me.

If it were me, I'd be more concerned with achieving a good step response than correcting every little phase error. In VituixCAD its under View Menu -> Impulse response, check the box for step response, and signal to show -> total SPL. The trick here is that better step response results are achieved with low order filters, high order filters can provide real poor step response from inherent group delay. The downside is that getting all the other aspects of a speaker right with low order filters requires some special speaker drivers and a lot of design care. Maybe you can "fix it" with FIR filters, I haven't tried.

Careful with over-correcting low frequency phase, the group delay correction requires many taps and the delay required can be significant if you like to watch video along with your audio and have it be in sync.

Anyway, if you want to go down that rabbit hole...From why I read about the 4x10HD, it allows for 5 PEQ bands on input and output, so you can apply more than just 5 bands per channel. But it’s mostly irrelevant since the 4x10HD is IIR only, you’ll want the FIR filters available with the flex-8 to control amplitude and phase independently. For FIR filters in VituixCAD, it's the same PEW block, just select the shape dropdown and change it from "Parametric EQ" to "Gain EQ" or "Phase EQ" for FIR functions. High pass and low pass functions have "linear phase" options as well.

Alternatively, you don’t need a dedicated DSP to play with this stuff, just a sound card with multiple channels, many PC motherboards have 5.1 or 7.1 channels built in so you can use VituixCAD along with APO EQ as the DSP engine. I posted some basic instruction on how to process any filter you can design in VituixCAD through APO EQ using the impulse response convolver here:
https://diy.midwestaudio.club/discussion/1832/simulating-crossover-filters-with-vituixcad-and-eq-apo

A bit off-topic but DSP related, Something else that may interest you is the “transfer function” block in VituixCAD which can create a filter that is essentially the inverse response of the speaker to correct it to perfection by mirroring the response. Of course, perfection is only the correction of the measurement provided, but it’s a cool feature that can’t be done with standard hardware DSP.
https://diy.midwestaudio.club/discussion/1836/mirror-eq-with-vituixcad-and-apo-eq-replacing-ue

Ed_Perkins

The audibility of linear phase (aka Transient Perfect or TP) filters is certainly a gray area. They used to be quite rare since they were very hard to implement passively, but with the advent of cheap DSP have become more popular. And with today’s tools (e.g. Rephase, Bodzio’s Ultimate Equalizer, etc.) you can take a typical crossover like a Linkwitz Riley and make it linear phase. I’ve read several studies and they seem to reach the same conclusion: with headphones and non-standard sources (e.g. clicks) linear phase can be detected. But with speakers in a room playing music the difference disappears. My favorite story about linear phase filters concerns John Kreskovsky. In the early 2000’s he was a very strong linear phase advocate. He did all the convoluted math and published several linear phase passive crossovers. However, a decade or so later he said this (HERE):

“As you well know I used to be a very strong advocate of TP design. I could swear I could hear differences with could only be attributable to TP design. The problem today is that with codes like the Phase Arbitrator and the Bodzio UE I can correct any system to be linear phase while leaving everything else un altered (axial response, vertical and horizontal polar, power response...). And I can rapidly switch back and fourth between linear and nonlinear phase. The results I have found in doing so are that only on very limited material and under very limited conditions can I detect a difference between TP and nonTP systems. I am forced to conclude that ultimately the differences I used to hear had more to do with differing power or polar response than TP vs non TP.”

Siegfried Linkwitz had a similar opinion. My personal opinion is to first ignore linear phase and concentrate on the more important aspects. Then you can always add it in later and see what you think. YMMV.

dcibel

John Kreskovsky said:
I am forced to conclude that ultimately the differences I used to hear had more to do with differing power or polar response than TP vs non TP.

This is exactly the point I was trying to make above. We used to not have such convenient tools to analyze the real polar/power/directivity of the complete system, which often led to incorrect conclusions based on single axis response results and assumptions on the rest. VituixCAD has made that analysis a feature that anyone can utilize for free. As well, the combination of VituixCAD and APO EQ can ultimately handle anything that Rephase or UE can do, of course you can choose whatever software suits you best for the task.

Steve_Lee

dcibel & Ed_Perkins --> Thanks for your thoughts, clarifications & insights. I completely agree from experimenting that the phase alignment stuff is far more noticeable in the lower frequencies than the higher concerning XO.

I may do some experimenting with the Sub/MW XO using PEQ-only for my own edification here soon . . .