Simplified REW Setup and Use (USB Mic & HDMI Connection) Including Measurement Techniques and How To Interpret Graphs - Page 397 - AVS Forum
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post #11881 of 12323 Old 10-01-2014, 09:05 AM
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Originally Posted by Bluescale View Post
That's because there's so little output below 30Hz. If he dropped his volume 10-15 dB on the red graph, he'd see a similar decay from 30Hz down.
I'd measure that to verify. I know in my room, even though none of my subs are rated for much output below 35Hz, I still got a nasty resonance at 31Hz. I mean, like 20dB higher than adjacent frequencies.

Room modes are evil.
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post #11882 of 12323 Old 10-01-2014, 09:06 AM
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Originally Posted by artur9 View Post
Then below 30Hz the resonance is less for the green than it is for the red. The red doesn't seem to decay much. Probably because the green isn't exciting the room as much at those frequencies.

IME, I hear that lack of decay below 40Hz as if a bird were beating on my eardrums. Can't stand that.
Resonance below 20-30Hz has been discussed quite a bit in this thread. I believe the general consensus was that it's not that easy for most to hear the direct sound of this wavelength let alone the resonances. Therefore and also because it's very difficult to deal with these long wavelengths, we've pretty much limited our discussion of room treatments to above 30Hz. I think depending on your viewing habits, you're more likely to miss the null than notice the additional extension in the red response but that's just my opinion. For those down in the single digits with their extension, I'm sure they'd feel otherwise. Also, to what 3ll3d00d said, the bass decay is more similar below 30Hz than it appears. The major difference is the lower SPL on the green plot. The higher the amplitude (SPL) the more it will resonate for a given frequency.

EDIT: +1 to what Bluescale said above which was also the point I was trying to make. IMHO, the biggest improvement in the waterfall between the two is a result of eliminating the null in the 40-60Hz range.

Last edited by jkasanic; 10-01-2014 at 09:11 AM.
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post #11883 of 12323 Old 10-01-2014, 09:20 AM
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Originally Posted by jkasanic View Post
.. it's not that easy for most to hear the direct sound of this wavelength...
Maybe I shouldn't have used the word hear. I know I can't hear anything that low.

But I have felt that beating of the bird wings on my eardrums. That's what prompted me to do room treatments and learn REW.
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post #11884 of 12323 Old 10-01-2014, 09:36 AM
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Originally Posted by artur9 View Post
Maybe I shouldn't have used the word hear. I know I can't hear anything that low.

But I have felt that beating of the bird wings on my eardrums. That's what prompted me to do room treatments and learn REW.
Sorry, just to be clear, I wasn't directing my response towards your post even though mine was subsequent to yours. I got busy and didn't finish responding before you and Bluescale made your posts. Having recently added about 2' of pink fluffy behind my screen and measuring no real improvement in bass decay below 40Hz, I'd be interested to know how you tamed your resonance at 31Hz?
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post #11885 of 12323 Old 10-01-2014, 10:10 AM
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Originally Posted by AustinJerry View Post
I agree that the best solution would be to get rid of the 60Hz dip AND preserve the bass extension, but evidently that isn't as easy as it sounds.
Your telling me! Been screwing with these subs for over a year trying to get the most out of them I can.

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Originally Posted by 3ll3d00d View Post
what do you mean by that? the only visible difference in those waterfalls to my eye is that one has greater output, the rate of decay looks pretty much the same, i.e. down by ~10dB in the first ~100ms and then more slowly decays from there.
My thoughts exactly.

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I'm also assuming that the bit of resonance that never finishes is something like refrigerator hum.
Either the fridge or my A/C.

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Originally Posted by jkasanic View Post
I think depending on your viewing habits, you're more likely to miss the null than notice the additional extension in the red response but that's just my opinion.
Thanks for all the responses guys!

I've got a plan...I'm going to try the 2 front/2 back config again and attempt to get back the ULF with phase and time alignment adjustments with a MiniDSP. I had issues with the MiniDSP and clipping before, so I just took it out of the chain. I'm going to try and overcome that problem with gain/trim adjustments. I'll report back!
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post #11886 of 12323 Old 10-01-2014, 11:06 AM
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Alan,

I lean towards the green graph. But what is being mostly played through the system should be a prime considering factor.

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post #11887 of 12323 Old 10-01-2014, 11:07 AM
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I think you are splitting hairs (not you, Jim). Above 30Hz, both waterfalls look fine. Below 30Hz, one would expect the level of ringing to be slightly higher on the red graph because the bass extension is greater. Regardless, IMO the dip at 60Hz poses the biggest problem, which is why I prefer the green response curve (if nothing else can be done to improve the dip).
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post #11888 of 12323 Old 10-01-2014, 11:11 AM
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Originally Posted by jkasanic View Post
Having recently added about 2' of pink fluffy behind my screen and measuring no real improvement in bass decay below 40Hz, I'd be interested to know how you tamed your resonance at 31Hz?
My room treatments are
  • almost 12 linear feet of 4" Roxul SafeNSound behind the MLP, for something like 48 ft^2 of panels.
  • I also filled the wall cavities with more Roxul. I had placed the mic into the wall cavities through some access panels and it picked up a fair amount of sound. One wall cavity has 12" of Roxul and the other has 4".
  • The ceiling has 1" of high density stuff but I don't know exactly which one (OC7??).
  • 2 other 2x4x4 panels in a kind of nook above the MLP.
  • A really large cushy sofa.

But using 3 subs and adjusting their gains and crossovers according to Geddes got rid of it ultimately. I think I could have gotten away with 2, though.

Current placement with my 4 subs are:
  • 2 up front flanking my bookshelf speakers. I like the effect of stereo subs crossed over fairly high @ 100Hz. The LHS sub has a pretty nice FR but the RHS sub could use some EQ.
  • One at the end of a half wall. The room is oddly shaped so that ends up being something like 1/3 of the way into the room and maybe 5/8 of the way along the back wall. When I had only one sub this was the best place for it.
  • One more or less opposite the previous one along the RHS wall. This is the second best place for a single sub.

WAF works out because the 2 flanking subs "appear" as part of the bookshelves. The half-wall sub is a slim model (external amp) and seems invisible and the last sub is hidden by the sofa.
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post #11889 of 12323 Old 10-01-2014, 11:30 AM
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I find the sub 30Hz response of the green trace slightly odd. It has the same notch in the mid teens which is presumably room induced yet falls off at about 18dB/octave. It is not obvious to be why putting 2 subs at the back would do that given the effect seems to extend beyond the modal region. Where is your 1st mode?

Similarly I wonder whether the 60Hz dip is room induced or results from the way the subs interact in those positions. Do you know? Do your mains help out at all in that area?

I do think you are a good candidate for doing the delay experiments offline with time aware measurements. You may want to look back at bluescale's recent attempt to use Holm for this.

Last edited by 3ll3d00d; 10-01-2014 at 11:32 AM.
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post #11890 of 12323 Old 10-01-2014, 11:35 AM
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Originally Posted by artur9 View Post
But using 3 subs and adjusting their gains and crossovers according to Geddes got rid of it ultimately. I think I could have gotten away with 2, though.
I suspected this to be the case and likely what I'll need to do in my space (i.e. add more subs to even the response which has the byproduct of reducing the ringing). Thanks.
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post #11891 of 12323 Old 10-01-2014, 11:48 AM
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Quote:
Originally Posted by 3ll3d00d View Post
I find the sub 30Hz response of the green trace slightly odd. It has the same notch in the mid teens which is presumably room induced yet falls off at about 18dB/octave. It is not obvious to be why putting 2 subs at the back would do that given the effect seems to extend beyond the modal region. Where is your 1st mode?
Yes, I agree that the FRs are very similar but the slope on the green just drops off a cliff...I have no idea why (except for phase/delay issues).

Where is my 1st mode? I also have no idea.

Here's a diagram of my room (with all possible sub placements, I have measurements for all placements in the thread I linked to before). Subs are currently stacked at positions 1/2 (red trace). The front/rear config (green trace) is positions 1/2/5/6.



Quote:
Similarly I wonder whether the 60Hz dip is room induced or results from the way the subs interact in those positions. Do you know? Do your mains help out at all in that area?
I know it's room/MLP location induced (which I really don't want to move). And, no, the mains most certainly do not help out...in fact they are causing a lot of harm. Unfortunately they are corner horns and cannot be moved.

Here's a measurement of my mains:



Quote:
I do think you are a good candidate for doing the delay experiments offline with time aware measurements. You may want to look back at bluescale's recent attempt to use Holm for this.
Could you explain what is bolded? I don't understand what you mean.
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post #11892 of 12323 Old 10-01-2014, 12:20 PM
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Originally Posted by 3ll3d00d View Post
I do think you are a good candidate for doing the delay experiments offline with time aware measurements. You may want to look back at bluescale's recent attempt to use Holm for this.
I was thinking the same thing.

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Could you explain what is bolded? I don't understand what you mean.
Go back and take a look at pages 393 and 394. There was quite a bit of discussion around using HolmImpulse to try to time align speakers. It was a bit over complicated in my scenario, although I kept playing with it after the conversation in this thread ended, and was able to get it very close to correct. The biggest problem I faced was the amount of time it took me between measurements. If you're not using a USB mic, that won't be an issue.

Last edited by Bluescale; 10-01-2014 at 12:21 PM. Reason: Fixed page numbers
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post #11893 of 12323 Old 10-01-2014, 12:23 PM
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Could you explain what is bolded? I don't understand what you mean.
if you have measurements that reference a known time source (loopback) then it is possible to take a measurement at each locations and then use trace arithmetic to combine them into an accurate projection of the combined response. You can then apply delays to each one individually offline, sum them again and you still end up with a valid projection of the expected response if you applied those delays. The benefit to you is twofold; firstly you get to try out all possible combinations, even those your minidsp can't implement, without having to go through the tedious process of moving subs, changing delays and measuring a million times & secondly you could share those measurements and perhaps you'd get some different ideas from people playing with them themselves (or maybe you won't, who knows!).

The problem is you have a USB mic and it seems this means the timelock feature (http://www.diysubwoofers.org/misc/holmimpulse/ta.html) is fallible over time (http://techtalk.parts-express.com/sh...reeping-offset), this would render the calculations void. However it seems that the offset is small initially so perhaps it will still work ok as long as you get the measurements done within x seconds. Alternatively, if you have the means to use an SPL meter for measurements, then that would suffice for this purpose.
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post #11894 of 12323 Old 10-01-2014, 12:43 PM
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Originally Posted by jkasanic View Post
I suspected this to be the case and likely what I'll need to do in my space (i.e. add more subs to even the response which has the byproduct of reducing the ringing). Thanks.
This page has a wealth of information for when you get to that point.
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post #11895 of 12323 Old 10-02-2014, 06:27 AM
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I came across this piece of software posted by member mtbdudex in a DIY thread for acoustic treatments. It calculates reflection points based on room dimensions, speaker location and MLP using a Win PC. Having trigged out my ipsilateral and contralateral reflections manually, I thought others might find it useful.
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post #11896 of 12323 Old 10-02-2014, 03:30 PM
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Originally Posted by jkasanic View Post
I came across this piece of software posted by member mtbdudex in a DIY thread for acoustic treatments. It calculates reflection points based on room dimensions, speaker location and MLP using a Win PC. Having trigged out my ipsilateral and contralateral reflections manually, I thought others might find it useful.
I went to the first post on that thread and tried to download but got the equivalent of a 404. Is the zip hosted elsewhere?
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post #11897 of 12323 Old 10-02-2014, 04:28 PM
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Originally Posted by artur9 View Post
I went to the first post on that thread and tried to download but got the equivalent of a 404. Is the zip hosted elsewhere?
The link in post #215 worked for me. It was an archive within an archive but extracted fine. If you can't get it to work then PM me your e-mail and I'll send it.
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post #11898 of 12323 Old 10-03-2014, 11:15 AM
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OK guys, what would you do?

I've been messing with sub placement for what seem like forever. See this thread if you want to know the details.

Anyways, I've basically narrowed it down to 2 different configurations:

- 2 up front / 2 in back
- all 4 up front / 2 stacks of 2

This is with 4 XS15s (original, non-SE).

Here's the measurements:



Here's the waterfall comparison:





Green is front/back, red is stacked up front. As you can see, I'm getting much better extension with all 4 up front (+20db at 10hz!), but I have a huge suck-out from 55-67hz. I prefer the sound with all 4 up front, but seat-to-seat variance suffers...heck, if I just lean forward in my chair the response changes significantly. That being said, I rarely move around when watching a movie or doing critical music listening...and my wife couldn't care less. It could just be in my head, but I swear I miss that narrow band where the null is....it is pretty narrow though...prolly in my head.

Oh, and when I have all 4 up front, I only have to run the subs 4db hot. When they are in the rear I bump them up 7db.

I want to get a couple more subs to put in the back of the room fill in that null and smooth across seats, but until funds become available....which set up would you want to live with?? Do you think upgrading all 4 to SE would be worthwhile or no?
OK guys,

Did some more tweaking last night on the "2 up front/2 in back" configuration. Stuck the MiniDSP back in the chain and played with the delay. My front subs are 17' away, each rear sub is 7.5' from the MLP. I ended up with 15ms delay on the rear subs. I just did sweeps and changed the delay by 1ms at a time until I got the flattest response. Can someone take a look at my MDAT file and tell me if I'm doing something wrong? The impulse graphs look very different, but I have no idea what I'm looking for there.

https://www.dropbox.com/s/afh01iofpc...pare.mdat?dl=0

All the graphs look great! I've got the flat response and ULF extension that I was getting with the "4 stacked up front" config, but no huge dip at 60hz.

Here's a before/after comparison:



Looks good, eh? WHY DOES IT NOT SOUND GOOD?!

Doesn't sound near as good as the "4 stacked up front" did....if I move about 4' in front of my MLP, it sounds great.

Thinking tonight I'm gonna try reversing phase on the rear subs (green trace in quoted post above) and mess with the delay to see if it improves. Any other suggestions?
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post #11899 of 12323 Old 10-03-2014, 11:16 AM
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Looks good, eh? WHY DOES IT NOT SOUND GOOD?!
in what way does it sound bad?
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post #11900 of 12323 Old 10-03-2014, 11:17 AM
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Bass sounds "thin", not much tactile feel. But, like I said, if I move 4' out into the room it sounds like it should.
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post #11901 of 12323 Old 10-03-2014, 11:21 AM
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Originally Posted by Alan P View Post
Bass sounds "thin", not much tactile feel. But, like I said, if I move 4' out into the room it sounds like it should.
So you have less tactile feel with 2 near field subs than with all subs 17' away? Have you tried some sweeps at higher levels to check for compression artefacts?
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post #11902 of 12323 Old 10-03-2014, 11:27 AM
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Haven't tried any compression sweeps yet, was too late last night when I finished up.

I'm guessing that there is some sort of cancellation going on, but it's not showing up in the FR sweeps....strange, right?
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post #11903 of 12323 Old 10-03-2014, 11:33 AM
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Just looking at the measurements again and noticed there is quite a difference in the phase plots....again, don't really know what I'm looking for here.



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post #11904 of 12323 Old 10-03-2014, 11:36 AM
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And here is the phase when I had "2 up front/2 in back" without time aligning (just turning the phase on rears to 180):

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post #11905 of 12323 Old 10-03-2014, 11:40 AM
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the current distance setting means you've gone about ~7' too far right? I mean you have a 9.5' delta physically but have added ~16.5' so have overshot by 7'. You can see this in the decay charts, certainly in the first 60-80ms anyway as it seems the delayed one decays more slowly. I believe I've read that that can have an audible effect in terms of making a fuller/fatter sound (has less snap) but whether that applies in this case is another question. It certainly doesn't seem to correspond to a description of it sounding thin.

You might be better off flip the polarity on one pair of subs and then adding the delay. You might find they align without needing so much delay which might correct for the problem suggested above.

Actually OTOH you have been running without delay so far so perhaps you've got used to that sort of fuller/fatter sound and are now missing it? (Edit: mind you, you have gone from 9' one way to 7' feet the other way so there is no real difference in that regard).

Last edited by 3ll3d00d; 10-03-2014 at 12:24 PM.
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post #11906 of 12323 Old 10-03-2014, 12:10 PM
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Quote:
Originally Posted by Alan P View Post
Just looking at the measurements again and noticed there is quite a difference in the phase plots....again, don't really know what I'm looking for here.



that phase wrap shows up as a big mid bass hump in the excess group delay chart, I'm not sure whether that is to be expected though as the improved frequency response is precisely because there are multiple devices combining in that range.
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post #11907 of 12323 Old 10-03-2014, 12:17 PM
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Quote:
Originally Posted by Alan P View Post
Bass sounds "thin", not much tactile feel. But, like I said, if I move 4' out into the room it sounds like it should.
Your description of the bass sounds like you're not getting enough extension but the green trace above would seem to indicate otherwise. Have you tried running sweeps around the MLP (especially the 4' in front of the MLP you described) to see if there's a lot of seat to seat variance?

EDIT: I was a little slow responding so missed your back and forth with 3ll3d00d. I'd be interested as well to understand more about the excess group delay chart.

Last edited by jkasanic; 10-03-2014 at 12:21 PM.
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post #11908 of 12323 Old 10-03-2014, 12:21 PM
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Originally Posted by Alan P View Post
Bass sounds "thin", not much tactile feel. But, like I said, if I move 4' out into the room it sounds like it should.
Why don't you try placing the mic 4' out into the room and taking a measurement? You may not be used to bass that measures flat. Perhaps there is a peak at the 4' point that boosts the bass, making it sound more like what you are used to.

I don't know why you are focusing on the phase graphs. Look back in this thread--has anyone paid any attention to those graphs before? I would focus on adjusting delay with the MiniDSP until the frequency response is as smooth as possible at the MLP. Then take some measurements at spots outside of the MLP to see how consistent the bass response is. If the bass continues to sound thin at the MLP, raise the bass trim in the AVR until it sounds like what you are looking for.
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post #11909 of 12323 Old 10-03-2014, 12:29 PM
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Originally Posted by AustinJerry View Post
You may not be used to bass that measures flat.
That is a good point. Remember that changing the spectral balance can really change how it sounds. Diagnosing what thin means here means knowing (though measurement or because you have a good handle on how what you hear relates to frequency, I often overestimate how low something is) the spectral content of what you are listening to and then understanding how that relates to your before and after measurements. For example, with the right content, greater extension can make something sound less punchy as the mid range is no longer so dominant.
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post #11910 of 12323 Old 10-03-2014, 12:42 PM
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I remember someone warning me that "flat bass sounds like no bass". Until you get used to a flat bass frequency response, especially when the ringing has been removed as well, it can definitely sound thin. But over time, when you realize you can now hear the subtle variances in bass notes, rather than hearing everything as "one-note bass", you begin to appreciate what all the hard work was about. That's when it becomes sheer joy.
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