If interested, here is what a Dirac developer had to say…
It’s also said in the forum thread that …
“IIR filter cannot be used for phase correction”
… which is wrong. It can be used.
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Hi Flavio, your first post is an out of the blue response from a thread that is over a year old…
Nope, excess phase correction at low frequencies is something Dirac does not do. IIR filters are being used. And while IIR filters can correct for “minimum phase”, it is a technical impossibility for IIR filters to correct for excess phase.
Preringing has been a mathematically solved problem since 2011: http://www.acourate.com/freedownload/AcouratePRCen.pdf
Anyway, tell your developer that he needs to do better than that 
PS. If you want someone to really put Dirac ART or DLBC or whatever product can do digital crossovers and room correction, through its paces and offer an independent technical review, please feel free to contact me.
All the best to you in 2024!
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I’ve been using REW and the great video that @OCA has produced on YouTube “Digital Room & Speaker Correction Workshop Part II - All New FIR filters” to produce convolution filters for Roon (small, squarish 
dedicated room ~43m3).
I use an M1 MacBook as the Roon Server (small library pls Qobuz) and stream both wirelessly (at native sample rates) to an Auralic Altair G1 (where it is upsampled to max).
The end result is VERY good. SQ is improved significantly. Much kudos @OCA . Thanks.
I’ve been researching Focus Fidelity (which looks like an excellent piece of software) as a next step and I suppose I’m wondering whether Focus Fidelity derived filters might prove to be even better again? Especially in terms of improved step response and phase corrections?
Any thoughts anyone? @Mitch_Barnett?
I don’t mind paying AU$400 for the software but it would be handy to know if I’m likely to get better sounding convolution filters than I already have?
Or is it a “suck it and see” situation?
I hope it is okay to ask this type of question…
Kind regards,
Peter (Melbourne, Australia).
PS. I second @OCA 's suggestion of improving Roon’s Convolution engine to include HLC Convolver.
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Hi Mitch,
I stumbled into this thread with the alluring title of “Understanding the state-of-the-art of digital room correction” and, even if over a year old, I thought that a comment was due.
As you know, I appreciate your work and we do not intend to open a discussion here about which room correction solution is preferred by whom and for what reasons, but since the above post contains factually wrong statements about our products, we believe a couple of comments are in order.
“excess phase correction at low frequencies is something Dirac does not do. IIR filters are being used. And while IIR filters can correct for “minimum phase”, it is a technical impossibility for IIR filters to correct for excess phase.”
This is not correct.
First, robust excess phase correction is a non-trivial subject, since in practice one never measures the exact same positions as one listens in. We have two ears and we are never sitting completely still. Excess phase correction therefore may induce pre-ringings in positions you didn’t measure, since excess phase distortions (especially the room-induced ones) are never 100% static with respect to listener position. A robust correction must work throughout a soundfield , not just a specific measurement point. The challenge is to make it robust enough, and there one should always be cautious not to overly correct for excess phase behavior as the auditory system is much more sensitive to pre-ringings than post-ringings. On a conceptual level, most of the fundamental insights into this problem were known and published by Michael Gerzon in 1991 in an excellent article in Studio Sound, see reference [1] below. Dirac does broadband excess phase correction, bass included, to the extent possible within constraints defined by the maximum allowed pre-ringing duration and level. Dirac’s phase correction approach is thus intentionally cautious in order to avoid pre-ringing artifacts. The workings behind this pre-ringing constrained approach is based on peer-reviewed research done by Dirac, presented in reference [2]. To our knowledge, reference [2] presents the so far most thorough mathematical analysis of the origin of the pre-ringing problem, and the fundamental conditions around how to treat it.
As was stated earlier, the kind of ‘more aggressive’ phase correction offered by some DRC softwares can indeed work well and sound good if done with care by an expert, but it normally requires a great deal of experimental user interaction before the right filter is found (e.g., iterative testing of different smoothing or windowing parameter settings until artifacts are below the acceptable level).
Second, the statement about IIR filters is a very common misconception. An IIR filter is a filter with both poles and zeros, and as such IIR filters do not have to be minimum phase. An obvious example of non-minimum phase IIR filters is an all-pass filter. Dirac Live has been implemented both with high-order IIR and high-order FIR filters and in both cases the filter is non-minimum phase, always with the aim of realizing the desired mixed phase filter transfer function to the higest possible precision. Which one of all possible filter structures is chosen is not a question of minimum- vs. mixed-phase, but depends on the hardware on which it is supposed to run (e.g., memory and CPU resource tradeoffs). Dirac Live has never been implemented as purely IIR in the bass and FIR at higher frequencies however.
There is a large amount of scientifically peer reviewed material on these topics. The interested reader may dive deeper at for example:
[1] https://www.audiosignal.co.uk/Resources/Digital_room_equalisation_A4.pdf
[2] L.-J. Brännmark, and A. Ahlén, “Spatially Robust Audio Compensation Based on SIMO Feedforward Control”, in IEEE Transactions on Signal Processing, vol. 57, no. 5, pp. 1689-1702, May 2009.
[3] L.-J. Brännmark, “Robust Sound Field Control for Audio Reproduction. A Polynomial Approach to Discrete-Time Acoustic Modeling and Filter Design”, PhD Dissertation, Uppsala University, Uppsala, Sweden, 2011.
[4] B. Bank, “Perceptually Motivated Audio Equalization Using Fixed-Pole Parallel Second-Order Filters”, in IEEE Signal Processing Letters, vol. 15, pp. 477-480, 2008.
That’s all, as I said we don’t want to start a debate so I leave you the last word
All the best!
Flavio
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Oh Flavio, you don’t want to start a debate, but here you are starting it. And leave me with the last word. Lol! Nothing like being passive aggressive with the ol “I stumbled across this and here let me dump some FUD and then run away” sales tactic. Folks, Flavio is a familiar sales person at Dirac.
I have reversed engineered Dirac’s filters and can show there is no excess phase correction below a certain low frequency cutoff point. It is as simple as that. I can publish the results and the step by step instructions on how to do that so every one can see and confirm for themselves. But that is in bad form, just like stumbling in on someone’s thread, dumping a load of FUD, and running away.
So here is my challenge to Dirac. Send me your best DRC software and I will do a bakeoff. I will publish the objective results and the step by step procedures at Audiophile Style.
I will drop a mic at and around the listening position and take some basic acoustic measurements with the DRC engaged to show the differences between the DRC programs and have a discussion about it.
You have my contact info.
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Hi Peter,
David’s Focus Fidelity Designer software is excellent and highly recommended. I wrote an article about it a few years ago:
David’s approach has a unique way of dealing with excess phase correction, including a new way to integrate multiple measurement locations without losing the time domain information for each measurement.
I know David has further improved on his algorithms since my review and has recently released a Mac version. I am not sure if David offers a trial period or not, but if you reach out to him at his site, I know he is flexible.
If you go for it, I would like to hear about it.
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Thanks for your insights, Mitch. Much appreciated.
Your article, and your video on SOTA DRC, were some of the reasons I considered Focus Fidelity as a next step option.
I usually do a fair bit of research before trying/buying new products and I think I’ve read every article on the 'net about David’s software 
Anyway, after I put up the above post I thought I should give David the courtesy of contacting him directly and expressing my interest in his software.
To his credit, he got back to me very quickly and we’ve had a very productive series of emails back and forth re using his (free) measuring software Impala (which, assuming you’ve got a microphone, makes recording measurements around the listening position very easy) and also then about what Focus Fidelity might achieve re improving impulse and step response anomalies in the data I had gathered.
The upshot is that I bought and downloaded the Focus Fidelity Filter Development software (Mac version) this morning (AU time) 
And have already developed a set of convolution filters using the default settings and trialled them briefly in Roon.
At this early stage, I can categorically say my money has been well spent.
Even the “straight out of the box” default filter settings offered subtle, but definitely noticeable, improvements in the sense of “ease” that music has (less “digital”…), in terms of clarity of vocals/lyrics and also better sound-staging, especially in terms of depth.
We are minding our grandchild for the rest of today, so I will spend the next few days refining a set of filters and report back in more detail.
Very positive to date, though.
Kind regards,
Peter.
.
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To be fair, @Flavio_Fellah wasn’t the one to criticize your product here. You stated specific things about his company’s product, and he posted a rebuttal to what you said. I fail to see how that’s starting a debate.
And accusing him of having a sales motive seems at best a little tone deaf, if not outright hypocritical given the circumstances.
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Long time user of various DRC products … including Dirac Live, REW/Rephase, @OCA’s recent processes, etc. To make a long story short, I decided to purchase @Focus_Fidelity Designer today and am very happy with the results. The two applications (Mac at least) are very well designed and the workflow of the filter designer app is really well thought out. It looks simple but there are actually a lot of graphical controls - one can really dial in the filter parameters and limits in a very intuitive way. It’s easy to go back and tweak things and re-export convolution filters for HQPlayer. And it has been error free. After a few hours of tweaking things, I now have better sound than I ever had with REW/Rephase and with much less effort. This product is not as well known as some of the other standards so just wanted to give it a shout out Happy with the $249 spend (perpetual license as well apparently).
[Edit: One big concern I had after reading some reviews was that I would have to correct the entire frequency range. Some reviewers were confusing ‘correction bandwidth’ with frequency range limits - not quite the same thing, one is time based and one is frequency based. But it turns out you can easily set the filter high frequency limit with a slider. I have variations of filters with different upper limits under 500Hz …]
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I fully agree regarding @Focus_Fidelity 's Filter Designer being an excellent product both regarding GUI experience and the quality of the resulting filters. I started out with the windows version running in a VM on my Mac and was very delighted when the native Mac Version arrived - at no extra cost to the existing windows license!
Edit: You can just transfer your existing license from Windows to Mac.
I especially like the possibility to load the convolution filters in the (free) measuring application called “Impala” which makes it very easy to actually verify the results and compare those to the calculated results you’re presented with in the Filter Designer application.
Also David (the creator of Impala and Filter Designer) is very responsive and helpful via email if you have any questions and/or suggestions.
All in all the asking price is to be considered a bargain especially in terms of increase of sound quality vs. money spent. And you can always start over with new target curves and settings tweaking to further optimize the result.
Highly recommended!
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The decision appears to have been merely influenced by constraints on filter size and computational power, aiming to achieve sharper cutoffs with lower orders than min-phase filters. However, this choice inevitably introduces stability issues due to poles outside the unit circle and produces ringing artifacts around sharp transitions. You assert that these challenges have been addressed through the incorporation of additional feedback loops and windowing algorithms. But the compromise is still way too significant to be comparable to the SOTA algorithms designed by the likes of Mitch and David with nearly unlimited FIR taps which we have easy access to with Roon. It is also noteworthy that the papers you cite are all authored by Dirac.
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Hi Keith. I’ve PM’ed you.
Happy to join. Thanks.
Happy to report back that, IMHO, Focus Fidelity is very good DRC software.
It produces impressive results.
I agree with ALL of the comments that @nquery and @Roland_von_Unruh have posted above.
“Better sound… with much less effort”. “A bargain… in terms of increase in sound quality” All true.
I’ve tweaked things a bit from the default settings but nothing too drastic and I suppose the best anecdote I can give is that I’ve spent the last few days listening to, and enjoying, music rather than worrying about my system’s deficiencies.
Happy camper here.
Thanks, David @Focus_Fidelity.
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Thank you all for the feedback.
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I switched from Genelecs GLM to Focus Fidelity and the GLM is a really simple software compared to Focus Fidelity which yielded really great results. I decided against Acourate and REW/rephase because I found their learning curves to steep for me. Focus Fidelity really makes a complex process accessible for a lot more people.
When I run the program I was always wandering if there is some affiliation with Acourate or if Focus Fidelity is a complete independent development?
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Hi @Daiyama
Thanks for your feedback, very much appreciated.
Focus Fidelity’s software has been developed entirely in house and from scratch, is completely independent to all other DRC products.
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David, do you have any update for us about the future developments you mention on your website, i.e., digital crossovers, bass management, and multiple subwoofer optimization? Any timeline you can share?
Thanks!
Hi @dathzo
There is something new in development which will lead to those new features.
I can’t share details right now.
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Hi @Focus_Fidelity, can Focus Fidelity generate a set of PEQ filters as well as convolution files? If not, any plans to implement that feature?
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Hi @Anthony_B
No support for PEQ filters at the moment, only FIR filters. Maybe that might change in the future but no definitive plans right now.
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