I tried again this evening to get rid of that notch. I failed miserably. My "idea" turned out to make the situation worse. For now, this is probably as good as I can make it. I will have to go watch some OCA on Youtube now to see if I can learn anything.
OTOH, that all pass filter has a dramatic effect on the sound, and I am NOT talking about the frequency response. In fact, over the past week I have been experimenting with all pass filters. There is a real disconnect between what I hear and what the measurements show, and I can't wrap my head around why.
I will compare and contrast my attempt at the "ultimate" filter with the filter made for me by Dr. Uli (which is a really great sounding filter). With my attempt at the "ultimate" filter, every driver was linearized nearfield with both FR and phase flattened using a reverse AP filter. Every correction that I know how to apply was applied to this set of filters. I spent a LOT of time crafting it to perfection, and indeed ... a look at the measurements would suggest that it
is perfect.
Comparison of my correction vs. Uli's. I staggered them for clarity. I would even say that my correction looks a little bit better. Both curves use the same XO points, same target curve. The only difference is that I corrected my drivers more aggressively than he did.
In fact, VERY aggressively. Red is my correction, black is Dr. Uli's. This is the nearfield phase linearization for the woofer. Dr. Uli used a reverse AP filter. I came up with this idea to invert the phase guideslope and use that to flatten the phase. When I ran my idea past Uli, he said that it is not a good idea. Regardless, I had already created the filters so I went ahead and completed the procedure. You can see that my correction is ruler flat, whereas Dr. Uli's is looser. Mine is a measured verification response, Uli's is a sim that I created (he did not do a verification measurement).
Above is Uli's step response ...
... and this was mine. I think I can say that mine objectively looks better. Left and right are almost carbon copies of each other, and there is no pre-ringing.
By this stage I was really pleased.
This is the minphase and windowed version of the verification measurement of Dr. Uli's filter. You can see a step at the tweeter at the crossover point. This was because Dr. Uli elected not to correct the tweeter phase with an AP filter. Also the phase slope continues to roll gently downwards.
... and this is mine. You can see how aggressive the correction is. There are filters everywhere, correcting everything I can see. I was quite chuffed when I saw this result as well.
Group delay, left channel only, Uli's (red) vs. mine (brown). Almost the same, except that mine shifts the spike from 60Hz to 50Hz, probably due to that AP filter in my subwoofer. Dr. Uli has no AP filter in his sub correction.
So, in summary ... the differences between Dr. Uli's filters and mine are:
- he has reverse AP filters in the woofers and mids only. Mine has phase correction in every driver.
- his reverse AP filters are designed more gently. Mine use an inversion of the guideslope to correct the driver phase to flat.
- in nearly every case, I think that my measurements look more pleasing than his.
Now - frequency response is the same, time alignment is nearly the same (mine might be a little bit better), the only major difference is how aggressively the phase was corrected. Since phase is supposed to be inaudible, I was expecting my new, carefully crafted filters to sound awesome.
Listening Impressions
Except that they don't. The main difference is how "lively" the sound is. On Uli's filters, percussion instruments have a natural sounding decay, and you can hear them shimmer. Mine simply go "clang" with no shimmery decay. Just as bad - instruments seem localized to each speaker on mine, in fact it is almost as if there are two columns of sound in front of me with emptiness in between. With Uli's, the ensemble is realistically spread out and you can almost reach out and touch them.
This was a real WTF moment for me. I had done something terrible, despite the nice looking measurements. I went and made another set of filters, removing ALL phase corrections, then sat down and had a listen.
The difference was quite unbelievable. The sound suddenly came to life, veils were lifted, etc. The speakers were really unleashed, they were so aggressive. Where all the phase corrections seemed as if the soft pedal on the piano was being pressed, this one sounds as if the dampers were removed. Really it was all a bit too much, in fact it felt as if I was being assaulted by sound. It was exciting for a while, and then it became tiresome. I am pretty sure that some of you rock fans would love it. There is so much impact. With my typical classical music, it was excessive.
Uli's correction is a nice mixture of polite and aggression, whereas my two filters were on either extreme. The version with no phase correction was less objectionable than the highly corrected version though. Even though it was ultimately more fatiguing, it sounded more believable than the corrected version where the sound was stuck in a straightjacket.
But above all, I am confused (again). I thought that phase was not audible? How do I explain this? Clearly I don't know as much about measurements as I thought, and I don't understand phase as well as I thought. Lesson learnt - be gentler with the corrections, don't go around chasing perfect looking measurements. I think this is an easy trap to fall into with DSP, you read a textbook, see what a perfect measurement looks like, and then try to replicate it.
I am fortunate that I have Dr. Uli's filters to compare my efforts to. They really do sound amazing. If I didn't have them, I would have settled on the perfect looking measurements and then come to ASR to tell you that I have cracked the nut. It looks perfect, so it must sound perfect. But I have to be honest with myself here. In my zeal to produce the best looking correction, I have severely damaged the sound - and the only reason I know this is because I have a better sounding filter to compare mine with.