After copying over the new boosts from the chairside DEQ into the front DEQ, I measured (Unweighted) the sub and panel outputs separately for both channels.
Left/Right balancing by SPL seemed to occur when I attenuated the left channel by about 1.2dB, instead of the 2.3dB I have been using.
However, when I switched to my ultimate "Balance" test, I was right back to the 2.3dB. That test is the Speaker Phase test from the Stereophile Test CD 2. When balance is exactly correct, the out-of-phase (both in John Atkinson's voice and bass guitar) sounds equally loud in both channels, just not centered. Somehow this test is extremely critical, even the smallest imbalance points it one way or the other. You can't tell much from the actual in-phase center image which says roughly centered over a 2dB range.
I'm not sure what explains this difference, or why there should even be a 1.2dB difference in measured levels.
I tried setting the right sub level 1-2dB higher because the right channel actually seemed less bassy. But after some testing that way, I moved it back to the earlier -10dB default. It's not even clear if the right channel is less bassy, it could be more bassy at some points.
I re-measured both channels (subject to the 1dB bass reduction I did yesterday by changing the sub DAC to -6.5dB from -5.5dB...a few days ago it had been at -7.5dB).
 |
| Left Channel (end of Sep 3) cursor at 1kHz |
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| Left Channel, cursor at 35 Hz |
The different cursor positions make it easier to see how high the bass plateau is. Overall this is the smoothest response I've ever achieved, and fitting within the desired room curve very well (only a little bit of up and down all the way). Though 4khz is a bit high perhaps, it doesn't need as much attenuation as 3kHz to sound good (Gundry-Linkwitz dip).
The right channel has a similar bass rise, but the plateau keeps rising all the way down to 20 Hz. The bass looks very smooth, but the midrange looks very bumpy (which I don't understand because I just flattened it in July and August). The upper midrange is curiously smoother.
 |
| Right Channel, end of September 3 |
At this point, I believe the right channel midrange 250Hz-2.5kHz needs to be fixed before going any further. 10dB peaks and valleys are not acceptable (though I've lived with them many many times before if not most of the time).
The EQ is fairly simple in the greater midrange from 200-2000 Hz.
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| Right Channel front panel DEQ |
I'm going to start by turning off all the PEQ's in the 200-2000 Hz range and sweep for new ones.
The current PEQ's in this range are notches at 251, 647, and 853 Hz, and a boost at 1013 Hz.
It's probably going to need a boost below 500 Hz. The left channel now has a boost at 272.
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| Left Channel front panel DEQ |
Both channels have similar cuts at 251 Hz and 654 (or 647) Hz. To get the flattest response in the left channel I lowered the boost to 903 Hz and made it very narrow so as not to exaggerate nearby peaks, whereas I used a wide boost in the right channel at 1013 Hz which exaggerates the peak at 1.2kHz (?), but otherwise it's actually pretty smooth right around 1013 Hz, but has awful peaks in the 600-800 Hz region as well as an awful depression around 400 Hz (curiously very close to where I have a deep cut in the left channel).
I hooked up the chairside EQ to the panel DAC and loaded the pb2 preset (the latest panel work).
Checking the values in the right PEQ quickly, they looked identical.
But when I measured the response with the chairside DAC, it looked much flatter than the right channel measurements I made previously.
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| Right with chairside DEQ |
Now that's more what I remember. There is still stuff around 500-1000 that needs fixing, but the rest of the curve is fine. I measured this with my new "2 dukes" standard where I position the phone two fists (plus a small but comfortable gap) between the back of the chair and the phone. I think this is slightly more accurate (considering the main microphone is at the other end of the phone still) than my old standard which was 2.5-3 dukes (measured with a remote control).
Also I am measuring consistently at center now, whatever demerits this has I think it's basically ok, in my own testing it made very little difference.
I measured it with a completely new adjustment (move the stand arm out of the way and bending the flexible mount, then moving them back) and with 2.5 dukes spaces to get another measurement to be sure this wasn't a microphone positioning thing:
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| Right remeasured at 2.5 dukes |
Basically the same thing, actually the 2 dukes looked better. I then reverted to the front panel DEQ, and that shows the error, a suckout around the crossover point.
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| Right with front panel DEQ |
I know what this is. It's caused by the front panel delay being in Main, which it needs to be in order to fix the left channel. So for the front panel DEQ, the left channel wants Main delay, and the right channel wants Aux delay. Somehow this doesn't apply to the chairside DEQ, perhaps because it's a different vintage DEQ, or perhaps because I'm using optical I/O.
This calls for one thing. RTFM for the Behringer DEQ 2496.
I've found no way to adjust left and right channel delays separately (despite having obviously separately windows) nor any reasonable explanation for what the Main and Aux selections do.
*****
Well, according the the manual the Main selection applies to the "Main" (ie main analog audio) outputs. The Aux applies to the Aux outputs (there are also aux analog audio outputs).
What applies to the digital outputs? It doesn't actually say in this section. But it appears from another I/O menu that the Aux and digital outputs are tied together and you select what Aux/Digital outputs are connected to in menu item 3. So, one would think that whatever applies to Aux applies to digital.
When I started this exercise, in fact right up until around mid September, I had the front panel DEQ set to Aux. Mistakenly (?) I had the chairside DAC set to Main, and I did the tuning there. When I copied the tuning to the front DEQ, it no longer worked, but I found that selecting the Main delay worked.
Now, with the delay line selected for Main, the delay processing is not available for Aux. The manual says you get one or the other but not both. You select the channel delays, and which output they are used for.
So it would seem the chairside DEQ is giving the digital outputs no delay at all. Which basically can't work, because I've designed my system so the subs have less delay (around 8ms) and the panels always have 10ms delay to allow me to set the sub delay to anything. What matters is that the subs have around 2ms less delay because they around 2 feet further back. (Actually, this was fine tuned with impulse analysis using ARTA.)
So it can't work with anything other than 10msec delay, and yet the both channels work fine that way on the chairside DEQ with the delay set to Main (which should mean the delay is then "not available" for the Aux/Digital outputs).
Well it seems I must have understood this operation of the delay selector, since prior to mid september and going back who knows I had both the panel and sub DEQ's set to Aux. But I neglected to do that for the chairside DEQ (and still) all the tuning I do there assumes no delay.
So now the rub seems to be that when I was fixing the right channel on the chairside DEQ, I was not fixing it correctly, I was throwing in a no-delay condition as I had been doing on the chairside to get the same response.
What it appears I need to do is reset everything to Aux, and re-tune the Left channel to work with the 10ms delay instead of working around the lack of it.
(Even though the right channel is problematical, the nature of the problem requires me to fix left channel. Life is like that sometimes.)
What I did for the right channel to get the smoothest bass response ever up to around 300 Hz was to boost the 133 Hz region. That worked with the correct delay. It makes sense because the Acoustat is at a lower level than the sub (because room curve). What I did for the left channel was NOT to boost 133 Hz (which was elevated already) but instead to boost 180 Hz. Well, that worked, but it appears now it was really a work-around.
With everything on Aux, it should be the left channel that looks crappy on everything and the right channel looks fine on everything (well, except for that weird stuff 500-1000 Hz on the right channel).
(A weird angle here is that the Aux signal can also be selected to follow the main output, which is in fact the default on the setting on my units. So in that case, does the delay apply to both? It appears not, because in all cases changing the delay from Main to Aux makes some difference or another, and under that interpretation it could not make any difference.)
I set both DEQ's to AUX. The right channel is smooth to 300 Hz in both chairside and front panel DEQ's. No depression around 125 Hz as would happen with an improper delay. But the left channel is not. It shows a serious depression around 125 Hz, only with the front panel DAC.
I'm back to the "error" I thought I fixed a few days ago when I put the front panel DAC into Main Delay. That wasn't fixing it at all, it should be in Aux to work correctly. There is still an utterly inexplicable difference between the two DEQ's in the Left Channel. All the PEQ's are exactly the same and there are not GEQ's and I just tried another reboot of the front panel DEQ and it didn't help.
I *could* try to fix it by varying the left channel delay in the front panel DAC. Apparently I can chose which channel to change the delay setting for (or both) by holding the A button (more intuitive than it sounds because the A button is the one immediately in front of the Left Right selector button, which defaults to being just one button for both).
Since setting the front panel DEQ to Main delay fixed the problem in the left channel before, and since that should be setting the digital delay to zero, I tried to just change the delay to zero in the left channel, which should have the same effect there as setting the delay to main in just that one channel.
And it worked. It now shows the incredibly smooth bass and conforming curve to what I believe is the best room curve using the front panel DEQ still in the Aux Delay setting.
I have no idea why this hack is necessary. The use of optical connectors has no effect on the right channel, which measures the same with the correct 10ms delay dialed in as with the chairside DAC. A delay time bug which only affects one digital channel, where it erroneously takes on the Main instead of Aux delay switch polarity? That would be very weird. This problem can't have anything to do with the actual polarity (etc) of the subs etc because that is not what is being varied. Only thing being varied is the DEQ and the type of connections to it. (I double checked the volume settings on the DAC and they are identical for AES/EBU and OPT.) And it only affects one channel. Weird.
I need to be vigilent if some future change or power cycle or something changes this back to the expected behavior, then my hack today would introduce the problem it's fixing now.
Another rule probably is to never do anything permanent without measuring it, to be sure you aren't locking in some kind of error (yours or the units).
Over time (and over this two month so far episode) I've seen lots of things change inexplicably. I previously didn't measure carefully enough to know it wasn't measurement error, but now I use a stand for my iphone.
I can't imagine keeping up with all these variables without measurement. Few serious speaker designers design without measuring, they measure while designing then listen and then decide if it needs to change, etc., just as I'm doing (now at least).
*** September 5
Sadly, this wasn't the end of this. When I was testing the updated panel DEQ balance with the Stereophile Test CD 2, using the phasing test (which is finer than the channel identification) I noticed that in-phase sounded out-of-phase and out-of-phase sounded like outer space.
Funny I hadn't noticed that all night long in background music listening yesterday. (Eisenson's Revenge.)
I rolled back both panel channels to the exact same designed delay (10ms) and my system was back to normal. Except that that the pesky crossover depression in the left channel is back.
It looks like it's just going to have to be fixed with another EQ adjustment. Trying to adjust it with delay changes was a hack that has run it's course.
While I was here, though, I tried to get the balance more right. I've been applying a -2.3 db attenuation to the left channel (in the Tact, the only place I can easily do this) to make the channels sound balanced, but A-weighted measurement show only -1.7 or so should be needed to correct for the level imbalance, so I've always wondered, why do I have to give it more attenuation than -1.7dB?
So I tried setting the level to the correct -1.7 dB, and applying various delays to the left channel also. Less than 0.02ms made no easily detectable difference. (That would be like moving the speaker a couple inches.) I could complete "fix" the balance error inconsistency a delay between 0.2 and 0.3 ms. But then I wasn't sure if it sounded right.
I delayed the left sub by a corresponding amount. I still wasn't sure it sounded right.
After some fussing with different levels and different delays, I gave up and went back to the default delays and the -1.3dB adjustment. At least it's not messing with the sound much.
But adding such tiny delays would have no effect on the bass crossover, since the cycle time of 125 Hz is 8 ms, it's going to be way out of phase by that point just like I noticed when I sat down to listen to the phasing test.
But it might be worth trying some of the smaller delays, up to the perhaps beneficial 0.3ms, on the panels to see if the flat-ish response can be restored by such small changes anyway, because they might be correct anyway.
Another possibility...use optical connections on the front panel DEQ. As to why that would fix one channel and not break the other I have no idea. What else could explain the differences between the two DEQ's?
*****
Thursday night I was thinking of removing all the old PEQ's from the front DEQ, and then entering them in the exact same order as in the chairside unit and see if that fixes the problem.
(I still ponder the math and don't quite believe that a difference in order would affect the results. But it's still the best explanation for the difference between DEQ's.)
But instead I wondered if I could fix the problem by lowering the left channel boost from 182 Hz to 133 Hz or something like that as used in the right channel.
Since I already had the PEQ's dialed in, and they're going to be reset anyway, why not just try messing with them a bit.
So I swept the left channel 100-300 Hz with the 182 Hz boost turned off, and indeed 133 Hz was the depression. So I put a boost there.
The result was very disappointed. It was still depressed at 125 Hz, the crossover frequency despite the very close boost.
I moved the boost right to 125 Hz and that didn't create a nice looking spectrum either.
Since the boost was already at maximum, I then tried removing the huge notches in the sub at 129 Hz and 144 Hz. Then I tried changing their depths to get the system response flatter (which meant keeping a bit of the narrow cut at 129 Hz).
Fiddling with those 3 things, the sub notches and the 133-ish panel boost (which I ultimately moved to 162 Hz), I was able to get more reasonable looking response than before. This would be OK if I absolutely needed to use the front panel DEQ and didn't care anymore that it didn't match the chairside DEQ.
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| Left after readjusting sub and panel filters |
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| Left Sub and Panel filters |
But I don't think it is as nice as what I've dialed into the chairside DEQ, if that would only work in the front. And I do care that the chairside and front DEQ's should be the same, otherwise I can't do very good experiments with the chairside DEQ.
Remaining theories about why the two DEQ's don't match are now down to:
a) the order of PEQ's does matter and is very different. Notably I put the boost on top of the front DEQ, and it's on the bottom fo the chairside DEQ.
b) Using the optical connections introduces a significant delay* in both channels (otherwise they'd sound out-of-phase) that has a terrible effect on the left channel, but virtually no effect on the right. (It's weird that it would have no effect on one channel, but sometimes things are like that.)
(Which has to be on the order of 10ms or so...artificially adding 10ms of relative anti-delay fixes it completely, as I previously discovered. But the 10ms of delay needs to be added to both channels.)
c) Using the optical OUTPUT of the miniDSP is different than the AES output possibly because of some setting in the miniDSP
d) One of the DEQ's has some sort of digital bug
Thinking about (b) a bit more... Remember I "fixed" the problematic left channel by going to Main delay, which removes all the delays from the Aux (and presumably digital) delay. BUT, when I did that, it DID make the Right channel worse, and in exactly the same way--a depression at the crossover frequency. So there is evidence that making the DEQ delay different would have that kind of effect, not fixing both channels at the same time as now happens with the chairside DEQ. But the change is in the different direction. The chairside DEQ does have the 10msec designed delay dialed in. If there were a delay for using optical connections, it would be on top of that, not subtracted from it. So to test this theory, what I need to do is dial in MORE delay, like another 10msec but possibly not exactly that. Perhaps unlike removing delay, adding more delay wouldn't mess up the right channel, but would fix the left channel.
A flurry of tests on Friday morning was very interesting.
Starting with the front DEQ, I tried adding more delay to the 10ms design delay in the range 2-10ms to the left channel. 10ms eliminated the crossover depression, and 6 and 4ms did almost but not quite as well. At 2ms the crossover depression was back.
In the right channel, adding 4ms delay introduced a significant crossover depression, but 3ms looked OK. 3ms looked OK in the left channel too, so we have a "winner," I could make chairside and front panel DEQ's match pretty well in both channels with a 3ms delay added to each in the front panel DEQ.
So this is consistent with the idea that the optical I/O is adding about 3ms latency (because of the optical conversion boards, not the speed of light).
However, then I hooked up the chairside DEQ with all AES connections. It still looked fine at the 10ms design delay in both channels. Adding more delay as I had with the front DEQ messed it up.
Conclusion: it doesn't seem to be related to the optical I/O, which makes no difference for the chairside DEQ.
So what about the PEQ order? I carefully copied all the chairside PEQ's into the front panel DEQ. The result was the same as before this copying: a depression at 125 Hz.
Conclusion: it doesn't seem to be related to the PEQ ordering either.
Final tentative conclusion: there is a vintage difference in the two DEQ's. The chairside DEQ has more or less latency in the +/- 4ms range. About +3ms seems to fit the existing data pretty well, although it could be less latency by a comparable amount instead of more.
Anyway, fine tuning the latency in the front panel DEQ is basically the only thing I can do to bring the two DEQ's into alignment, and it now looks like I may be able to do it reasonably well (for +/- 2dB differences at most, in both channels--which is the tricky part).
As I've already determined, making the PEQ's different in the panel DEQ isn't enough to help, I'd also need to modify PEQ's in the sub DEQ, which would make all future testing with the chairside DEQ impossible or very difficult.
Q: Wasn't the delay originally tuned for the front panel DEQ?
Yes, when I tuned the delays it was with the front panel DEQ (though I don't remember exactly if it was the current unit). But since then a lot of EQ and other things have changed. Large amounts of EQ can apparently change delays in the 3ms range.
As of right now, the (greater???) delay in the chairside DEQ works better, there is little or no crossover depression despite many other factors. In principle I could try to match the chairside with the front, but since the chairside DEQ is working better, I now think I want to match the front to the chairside, which somehow landed on a somewhat better delay.
It was actually very very hard to set the bass/panel time alignment. It was very ambiguous, probably because the majority of the bass isn't being seen from the sub driver but from reflections from the front wall which is 2 feet behind it. I may have previously matched to the driver, but matching to the wall behind may be more important for the frequency response.
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| Left panel, front DEQ, 10ms (default) delay |
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| Left, front DEQ, 12ms delay |
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| Left, front DEQ, 14ms delay |
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| Left, front DEQ, 16ms delay |
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| Left, front DEQ, 20ms delay |
14ms is clearly the best here, and very similar (or better!) than my chairside fine tuning. I'm shocked it makes so much difference. But now I think like this: 8ms is one full cycle of 125 Hz, 4ms one half cycle, and 2 is 1/4 cycle. To go from best (full add) to worst (full cancellation) it takes 4ms. So my default position was (pretty close to if not) the worst, and 14ms is (pretty close to) the best.
One problem is that there will be another "best" point at 6ms, 4ms less than my default 10ms value. Random noise measurements alone are probably not good enough to determine whether 6ms or 14ms is the true best choice. Listening might nail it, or some kind of impulse testing.
There are two things I need to figure out. 1) What is the true best optimal panel delay (relative to the current 8.x ms sub delays, it all could be renormalized around a 10ms panel delay all over again afterwards). 2) What is the latency difference between the chairside and front panel DEQ's.
As long as I'm doing "optimal crossover addition" (which may depend a lot on PEQ's) as the test for optimal delay, should I optimize both channels? I need to keep panel delays identical, but the sub delays possibly need adjustment relative to each other wrt the new EQ's anyway.
Of the above curves, 14ms looks closest to what I was getting with the chairside DEQ. But 14ms sucked out the crossover frequency in the right channel, so that can't be it.
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| Right, DEQ ?, 10ms delay |
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| Right, front DEQ, 14ms delay |
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| Right, front DEQ, 16ms delay |
13ms looked pretty good in both channels.
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| Right channel, front DEQ, 13ms delay |
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| Left, front DEQ, 13ms delay |
At 13ms, the left looks closer to the chairside than the right, so perhaps slighly smaller delay would bring closer agreement (since the right gets worse after leaving 10ms, backing up to that would make it better).
It would be reasonable to try 12.5 ms and maybe 12.75. We know 12ms is not good in the left at all so we shouldn't go there.
Once that is figured out, I can easily determine the other listening test case by simply subtracting 8ms. At this point, I don't even know if what the chairside unit is doing is correct. It's just a coincidence that the difference in delay turned out to work better in my pink noise testing, but it could be that it's actually being delayed either too much or to little by some multiple of 8ms and that could still be the case.
Testing the chairside vs the panel DEQ's at plus and minus conditions, it should be possible to tell which one matches the chairside better AND which one sounds better (which might not be the same), and once I've developed that skill, I might try 8ms further...
8ms delay is enough to seriously upset balance, among other things (so I could try split DEQ testing too).
I've been pondering whether I could do some kind of technical testing, say with impulses and digital recording from the electronics...but it still sounds easier/better to work acoustically if I can now. I don't need microsecond matching here, 0.2ms is good for practical use of the chairside DAC though 0.02ms would be wonderful, that's the finest adjustment available on the DEQ. (0.2ms is already down to about 3 inches, 0.2ms is 0.3 inches of sound travel say if you were mechanically adjusting driver distances. At low frequencies it's hard to adjust things that finely with typical impulses, which look very rounded coming out of subs, using digital recording or analysis. I think I ultimately relied heavily on the measured distances, which I could easily measure to high accuracy but they're fraught with issues such as the planar speaker being so big there's not just one "spot" where they add. Plus things may have moved since then without me updating the adjustments.)
It occurs to me that the optimal adding at the crossover frequency should be true regardless of the bass and panel relative levels, and especially because I use a Linkwitz-Riley crossover which has highest adding in the listening axis, and further because it is a phase-corrected Linkwitz-Riley crossover. Basically I could do a pretty good job tuning the relative sub/panel delay just by optimizing the crossover frequency response.
There are some issues though. The first is that there is more than direct sound involved. I do need to optimize the delay for the direct response, but not necessarily the reverberant response. The only way to really test for delays is with impulses (which is what I did last time, around 2022 IIRC).
For now, I'll just assume since I'm measuring at one spot, that's what counts, I'll ultimately try to optimize bass by adjusting the sub delays. But I also have to get the DEQ compensation delay.
I can't read the date code on my front panel DEQ, but it's not a paper label it's printed. I believe that was a later vintage when they just stamped them out in big batches. The date code on my chairside must be one of the earliest at 0203. So it stands to reason the front panel DEQ might have less latency, and require some to be added in to match the chairside. Meanwhile, the sub DEQ is an earlier vintage than the panel one, a factor I'd never thought about before. It may have been swapped in to replace an even earlier unit at some point, but it looks like the panel DAC was the last one swapped in. The sub time alignment may have been done prior to that swap, or not, I don't recall.
It would be less likely for the newer unit to have more latency, and therefore require delay to be subtracted. (Though, right now I don't know if the front panel DEQ is actually earlier.)
****
I tried gridding (delays of 13.0, 13.1, 13.2, etc) and searching several times on Saturday. It did not work out. Usually I gave up or got confused before too long. I took dozens of spectra, some looked like they were going in the right direction, but I never found anything overall better than the 3.06 delay difference (10 with chairside DEQ vs 13.06 with front DEQ). Those only seemed a little different, but still significantly different. I was going to write it all up but got confused over which spectrum was which.
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| Chairside DEQ at 10ms delay |
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| Front DEQ at 13.06ms delay |
The front DEQ has a little more upturn just before the 125 Hz crossover (or maybe it's that 125 is slightly more depressed), and a bit more downturn afterwards. Varying the front delay it was easy to get the relative pre-125Hz upturn bigger, but not smaller. When it was made smaller, it tended to go with the lagging part turning down even more.
I was stuck in something like gambler's fallacy as I was doing this. It seemed the big break was just around the corner, but never was.
So today I decided to go the entirely other approach. I decided to hook the two DEQ's to an oscilloscope (which I can do from their analog outputs) and run a polarity test signal (all positive pulses).
Of course I also needed to set both "delays" to Main since I was using the "Main" (analog) output.
It was a very sensitive test to small timing differences. I found a near perfect match closest to 15.16 on the front panel DEQ, and at 15.14 it was significantly different.
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| Delay match at +5.16ms at front DEQ |
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| Delay mismatch at +5.14ms at front DEQ |
This kind of delay match however can't distinguish between leading delay and following delay because it's a set of evenly spaced pulses. You can't tell whether which pulses to match! (Note that the wiggling of the impulse has to do with all the PEQ's, when I tested w/o those PEQ's it was a very basic looking pulse.) So we don't know if any pulse matches the forwards one or the backwards one in the other series. (If I had been smarter, I'd have made the test <pulse> <space> <pulse> <2 x space>.)
But since the crossover frequency is 125 Hz, that repeats every 8ms, which means that 15.16-8 or 7.16 should be another matching point (which might be better). Which one could be determined by a listening test or better measurement test.
But when I ran the pulses, I noticed something else that that I neglected to make clear in these photos. The two DEQ's are out of polarity with one another!!! To make these photos I had to invert the polarity of one of the signals at the scope. It is the chairside unit which appears to be out-of-polarity (pulses going downwards without inversion) while the front unit appears to be correct.
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| Chairside DEQ, all AES connections, default preset, Emotiva DAC, scope inversion off |
I figured this had to be a measurement error of some kind, and tried to isolate it.
I tried changing both DEQ's to the default clear preset (no PEQ's). The pulses got more square but the inversion remained.
I tried connecting the chairside DEQ via AES like the front instead of optical. The inversion remained.
I tried swapping the XLR to RCA adapters being used. No change.
I tried switching the 20dB attenuator on the back panel of the DEQ. The inversion remained.
This is extremely concerning in many ways. There is no control in the Behringer 2496 DEQ that controls polarity, either for one or both channels. I don't understand how this could happen except for:
1) a bug in some vintages of the Behringer DEQ (notably the chairside unit appears to be a very early vintage).
2) Somebody altered the analog stage of the Behringer DEQ swapping the XLR pins (or maybe one of the units was sold in EU or UK, where such difference was standard). I bought the chairside unit used so I don't know anything about its history.
This all could be (and probably is I think) limited to the analog outputs, which I never use anyway. What really counts is what happens at the digital outputs. And the only way I can check that is by connecting a DAC to the chairside unit. (The main panel DAC is currently muted for safety, I don't want to blow up my speakers with a test signal.)
So I hooked up a spare DAC, and sadly it also made no difference. The inversion of the signal occurs in the digital domain in the chairside unit regardless of AES or Optical input or the preset chosen.
(Note the AES and optical inputs originate from miniDSP which might have inversion associated with either output. But apparently not, they are the same.)
The broader disturbing thing is that it appears I cannot trust any random DEQ to not do polarity inversion. I'd always assumed they were all correct, but some (and I don't know how many) are not.
A less disturbing thing is that apparently I cannot use the current Chairside DEQ except for adjustments where polarity inversion is not an issue. It is a very serious issue for the sub/panel crossover, however the differences may be limited to a narrow region around the crossover frequency.
I also cannot use Chairside DEQ for serious listening, only for temporary adjustment purposes, and at lest a half octave away from the crossover frequency.
Now it's perfectly clear that no adjustment of the delays will get the two DEQ's to match. In fact, where it shows the greatest match on the DEQ's, it might actually be the worst match, because the delay is partially compensating for the polarity inversion.
But the results I get on the front panel DEQ are the results I get. That DAC has the correct polarity. And the current results are already pretty good in both channels when the delay is set in the range 13-15ms.
****
One trick I could do to use the chairside DEQ is to swap it with whatever crossover way of the system is NOT being adjusted during that adjustment, and compensate that way by reversing the polarity again at it's output. Oh boy. If I were to permanently swap the sub DEQ with the chairside, assuming sub DEQ is correct, and then compensate at the sub controls, that would make the chairside match the panel DEQ but no longer match the sub DEQ.
A benefit of having (at least?) one DEQ that's polarity inverted in the digital domain is that I could use it for on-the-fly inversion by simply selecting which DEQ is being used. However, that's useless for me because I have 2 ways, panels and subs, which BOTH need to be simultaneously polarity inverted. I can't just polarity invert one of them, unless I had two such out-of-polarity DEQ's not already in use.
****
So making the DEQ's match is a hopeless endeavor. This spare DEQ can only be used in certain ways, just not for "chairside" purposes without large reservations (which I am now staring at, the final adjustments around the crossover frequency including the delay itself cannot be done on the chairside because it has mismatching polarity).
To avoid all the kneeling necessary to try each little change to the DEQ, I would have to move that DEQ to the chairside position, which is dooable but also problematic.
Possible improvements:
Raise front DEQ temporarily on stand (interferes with stereo image)
Relocate front DEQ to chairside (must be moved back for parties)
Use midi (still may have to use controls a lot)
Get new(er) DEQ with correct polarity.
Figure things out with less trial and error (haha)
****
Just working with the front DEQ (and kneeling on my 3inch foam pad) I can wrap the adjustment project up for now with just a few more adjustments.
The primary one of these is setting the sub/panel delay correctly.
My current tests can't even for sure tell me if I'm in the correct 8ms ballpark. I need a polarity test that allows me to be sure of things like that, using asymmetrical delays. One must then match up no only the pulse directions, but the pattern of delays between them.
Now a single impulse would be just great, if it could happen at the moment you want and be stored using storage oscilloscope mode. But that's hard to do. It's easier just to have a test as I just described, which can be freely running just like the freely running oscilloscope.
All you need is pulses that are widely enough spaced that the width between pulses (or shorter series of pulses) is greater than the unknown latency. So if I want to decode an unknown latency with 8ms repeats, I need merely make a larger gap.
20 ms might work great, or even 200ms
It turns out my existing polarity test has about a 5ms gap between pulses. Therefore there should be matches every 5ms. The matching on this test has nothing to do with my crossover frequency (as I was just thinking).
It's good enough if the unknown latency is < 5ms, but it isn't.
I didn't look for one...but there might be a match right around 10ms...which might mean there isn't any delay difference at all! I need a bigger gap to be sure. It might only be the sad polarity difference that was causing all the trouble, and I was trying to offset it by just enough delay, which would be exactly 4ms, which is why I seemed to be ending up close to 14ms in both channels. THAT mis-matching did have to do with the crossover frequency.
The matching with my pulse test is related to the pulse gap in that test, not the crossover frequency!
People say the latency of DEQ 2496 is about 1ms. It's likely then that even early units did not vary by 4ms, and that varying latency wasn't the issue for me at all, only the polarity difference.
I need to re-check my results above and see if there isn't a match right at 0ms delay difference rather than almost 5 (which happens to match my test too well).
And then check latency/delay of sub unit.
But I've also wanted to know for a long time about the maximum output level of the Emotive Stealth dac. So I checked that first, using a test signal with a sine wave (200 Hz) generated at maximum level (which I re-checked in Audacity, it showed clipping on every peak when I set the clipping threshold to 1, but no clipping at all when I set the clipping threshold to 2, meaning at every peak it just barely touches the maximum digital level, this test had been generated in either Audacity or Sox).
Playing this signal, I set the Emotiva DAC to +12dB gain. That would be about 8V output. And it showed no clipping whatsoever.
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| Emotiva at +12dB output level |
I wondered how sensitive this test was. Might there be clipping and I just wasn't seeing it? So I artificially created digital clipping using digital gain in my Tact preamp. I was able to detect the clipping at a level of +0.2dB. I took this picture at +0.5dB:
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| +0.5dB digital clipping |
Note that this second test is not showing any kind of clipping in the actual DAC. There is no way to know with these tests how much additional analog headroom the Emotiva DAC has above 8V output. It may have some additional headroom to handle inter-sample-overs (ISOs) produced by its own digital filter. I'd need to run a test which produces ISOs to be sure. J-Test does that pretty well. Or 90's recordings by Steely Dan. The maximum theoretically possible ISO would be at +6dB, but many digital filters might not ever go that high.
This doesn't matter for me because I don't really need the 8V output anyway. My Aragon amp reaches rated power at 1.7V and reaches the end of its headroom well before 3.4V. But I may run the DAC at +12dB of gain anyway, and I just wanted to be sure it can output to at least the nominal 8V level, in case there are ISOs up to that level.
Then I proceeded to re-compare the latencies between the chairside and front panel DEQ's, but this time looking for alignment of the two outputs near 0ms instead of near 4ms. In fact I found such an alignment with the panel DAC set only 0.1ms lower. For some reason, the older chairside DAC actually has less latency rather than more as I had been assuming. Perhaps in fixing the output polarity problem in later DEQ's, Behringer added one more stage to the output line amplifier which increased its latency by 0.1ms.
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| Panel DEQ at alignment with chairside DEQ set to 10ms |
This 0.1ms is effectively inconsequential at the subwoofer crossover frequency of 125 Hz. 0.1ms corresponds to a frequency of 10,000 Hz and might have tiny effects down to 1khz at most. Still I generally at least try to set delay times closer than that. It would be the same about as about 1.5 inches of speaker distance.
So I was wrong about different delays causing the differences around the sub crossover frequency. The only thing that is doing that is that the chairside DEQ is out of polarity while the front panel DEQ (much newer) has correct polarity.
What about the front sub DEQ, which is an intermediate vintage. The date code on the sub DEQ is 1411 vs the 0203 for the chairside DEQ. The panel DEQ appears to be the ultimate vintage in which they didn't bother with date codes anymore. It could be that the chairside DEQ is the 2nd year of production whereas the sub DEQ is from the 14th year of production. The DEQ was still being sold for something like 20 years after its introduction (and now there isn't any kind of replacement).
The front sub DEQ was exactly like the front panel DEQ. It required the exact same 0.1ms difference in delay times to match the chairside DEQ. And it had the correct polarity too.
I'm very happy that my two front DEQ's match perfectly despite having somewhat different vintages. And of course I'm very disappointed that my chairside DEQ has the wrong polarity, and mildly disappointed that it has 0.1ms less latency.
Sadly because of the differences, I cannot use the chairside DEQ for the final adjustments around the crossover frequency.
Speaking of which, the first thing is to determine the correct sub polarity. Since I was getting better looking results with the chairside DEQ which inverts polarity, and I struggled and failed to produce as good looking results with the panel DEQ which doesn't invert polarity, it suggest the subs have been in the wrong polarity all along.
Only after I've figured out the correct polarity, and then the correct delay, should I get into the final tuning of the notches and boosts I've added around the crossover frequency, and only then can I get to the balancing questions.
***** End
Addendum: It seems I can still buy a DEQ 2496 brand new (!!!) for $279 from Sweetwater. They project expected delivery in January 2025. Many ebay sellers are selling parts-only units for more than that, and quite a few of the pre-owned units have date codes with leading 0 and are therefore might still have the polarity inversion bug. One ebay seller is selling a "new in box" unit for over $800 collectible price.
I have about a half dozen broken ones myself (currently I have 4 in use not counting the chairside one, and I'd be using 5 except the fifth one broke a few years ago...it was doing the sub EQ in the kitchen which is now approximated by my HTR. I started to repair one last year but got hung up on the lead free solder, which I wasn't experienced enough with yet. I could fix that one soon. But I have no idea how many of my broken ones have the polarity inversion bug, which seems to have been fixed in the newer ones.
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