I had a session with
@UliBru last night, and he made me a new filter. As usual, I learnt a lot. TLDR: his sounds better, but mine measures better. Read on to find out why.
First, let's take a look at the verification measurements of the two filters:
These are verification measurements of
@UliBru's filters (red/green, upper), and mine (blue/brown, lower). These used almost the same target curve. His curve has a notch at 4.5kHz to avoid overcorrecting the tweeter, whereas mine does not. Other than that, you can see that they both follow very similar shape. Apart from the target curve, the only differences between these filters is the driver linearization procedure (his vs. mine), and workflow settings for room correction. The XO points and slopes are exactly the same.
I had a friend come over to listen to the two filters. The difference is more obvious with his choice of music vs. mine (he listens to pop, I listen to classical). Dr. Uli's correction has a certain openness and spaciousness, and even the height of the soundstage is different (his is lower, mine is higher). The tone is also different, his correction seems to be more neutral, whereas mine sounds a bit darker. My filters sound really tight, whereas his filters sound relaxed. As my friend said, mine sounds boxy, his sounds open. Instruments with my correction seems to be tightly located in a box in front of you, whereas the placement of his instruments seems more believable. I am sorry, I am not so good at describing the sound, but be assured that the differences are real!
If we overlap the verification measurements and only show the left speaker, you can see the difference between his correction (red) and mine (brown). OK, so we know that a difference in the target curve will produce different tonality. So that part is explained. But why does it sound more spacious? Or, as my friend put it, my correction sounds more "boxy". So let's look at some other curves.
Comparison of his correction (red) vs. mine (brown), with phase extraction to show the min phase version. Look at the phase angle. He did not linearize the phase of the tweeter with a reverse AP filter, whereas I did. This is why mine shows almost flat phase response from 200Hz up to 24kHz, whereas the red curve shows some phase rotation at the XO point to the tweeter.
On a graph, my correction certainly looks prettier. But his sounds better. I can't wrap my head around WHY but Uli said that the behaviour of my tweeter is odd, and correction might make it worse, so it is better to leave it alone.
This is the unwrapped non-minphase version, i.e. without phase extraction. You can see the difference between his correction and mine.
Could the difference be due to group delay? Not really, my version (in brown) has better GD than his.
One notable difference is that his step response looks much better. He spent a lot more effort on time alignment than I did. Above in red/green is his step response.
And this is mine. Both graphs are displayed in the same scale. Mine has more pre-ringing and the alignment of all the pulses isn't as good as his.
As mentioned, there were a few differences in his workflow vs. mine:
1. He did not linearize the tweeter, because he thought it behaved oddly. Mine was linearized until it was absolutely flat.
2. He did not linearize the subwoofer, because he thought that an in-room measurement of the sub would be contaminated by the room, making correction impossible. He feels the same about lower woofer frequencies, but he linearized the woofers anyway.
3. He does his driver linearization with the mic 1m from the drivers. When I do mine, the mic is almost touching the driver. I forgot to ask him why he does this. It may be to capture the baffle step response, I don't know.
4. When he does the excess phase correction, he repeats iterations whilst watching for group delay. This messes up the step response and introduces pre-ringing, which he then compensates for with Acourate's pre-ringing compensator. When I do mine, I ONLY watch the step response. Despite this difference in workflow, the results are opposite - my GD looks better than his, but my step response looks worse than his.
5. He uses three different time alignment procedures to align the subwoofer and he cross-checks them with each other. I only use one, and my only check is my verification measurement where I look at the impulse and step response.
6. I use 65536 taps. He prefers 131k or 262k taps, but for me the latency is excessive. He offered, I declined. For those who don't know, latency can be calculated by:
(N-1) / (2Fs), where N = number of taps, and Fs = sampling frequency. So if we assume we have a perfect convolver, the latency for 65536 taps at 48kHz is 0.682 seconds. For 131k taps, it is 1.37 seconds, and for 262k taps it is 2.73 seconds. The advantage of going with higher taps is finer correction; the resolution of 64k taps at 48kHz is (48000/65536) = 0.73Hz, 128k taps is half of that (0.37Hz), and 256k is half that again (0.18Hz). I personally don't think I could ever hear the difference between a 0.73Hz correction and 0.18Hz correction, but I would certainly notice the increased latency which doubles every time.
7. He seems to prioritize volume over correction, whereas I tend to sacrifice volume for correction. As seen above, he even notched out the tweeter XO point to gain more volume. As a result, his filter is louder than mine and noticeably lumpier. Don't worry, I volume levelled the two filters before listening (Acourate Convolver has a volume levelling feature).
Because I also use Acourate Convolver, he made some modifications to my convolver settings (specifically AcourateFLOW and AIR). Separate discussion to follow.
So in summary: my corrections generate prettier looking graphs. But his sounds better. And it's not even close. I suspect it comes down to knowledge of what to correct and what to leave alone that has created this discrepancy. I look at online graphs, I know what perfect looks like, and I try to generate a perfect looking graph from imperfect measurements. I succeed, but I end up correcting for things I shouldn't and it sounds worse.
To be honest, right now I am feeling a little bit lost, as if something has been taken away from me. There is obviously a large gap in knowledge and learning between him and me, only this time I don't know where to look to improve my learning. I will have to hit the books again to look for answers, there must be something I skimmed over.