Case Study: Sub Alignment using REW v5 - Page 2 - Home Theater Forum and Systems - HomeTheaterShack.com
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post #11 of 29 Old 05-10-10, 04:30 AM
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Re: Case Study: Sub Alignment using REW v5

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What would you recommend for time aligning a subwoofer to mains?
I like Bill's idea of band limited measurements of sub and mains over the range both operate. Another option would be to make measurements at the line level outputs to sub and mains to accurately characterise the delays in processors etc, free of any room influences, then add in the delays from the relative distances of the speakers/sub to the listening position. An option for fine tuning is to set the sub to opposite polarity and tweak the delay adjustment to get the largest dip at the crossover frequency, then flip the sub back to correct polarity, though that is subject to room influences on the measurement which may make the evaluation difficult.
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post #12 of 29 Old 05-10-10, 05:29 AM
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Re: Case Study: Sub Alignment using REW v5

I am wandering how adjusting the time delay in your AVR, after Audyssey is run, affects what Audyssey is trying to do. I'm all for improving your system, and your results sound positive, but if you manually change settings after Audyssey stores its settings, your then basically altering the response Audyssey is trying to base its calculations on.

This is one of those things that would niggle away at me, even if I cant hear the difference, at least for a while. Ive always been an advocate of helping Audyssey as much as possible to get the very best results, so I really wander how this figures into the equation.
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post #13 of 29 Old 05-10-10, 07:31 AM
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Re: Case Study: Sub Alignment using REW v5

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I like Bill's idea of band limited measurements of sub and mains over the range both operate. Another option would be to make measurements at the line level outputs to sub and mains to accurately characterise the delays in processors etc, free of any room influences, then add in the delays from the relative distances of the speakers/sub to the listening position. An option for fine tuning is to set the sub to opposite polarity and tweak the delay adjustment to get the largest dip at the crossover frequency, then flip the sub back to correct polarity, though that is subject to room influences on the measurement which may make the evaluation difficult.

I've basically been adjusting distance in my prepro to peak out the crossover region also. Inverting the subwoofer signal gave me the same results.

One tweak that I'd like to see in REW is the ability to send out a DD 5.1 signal where the user can select which speakers to turn on/off so we can tweak distances in the other channels. For example, the user could turn off all but the left and right main, run the signal generator and adjust distances to peak spl. Then turn off the left channel and turn on the center and repeat the test. So on and so forth with the other speakers.
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post #14 of 29 Old 05-10-10, 09:03 AM Thread Starter
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Re: Case Study: Sub Alignment using REW v5

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I am wandering how adjusting the time delay in your AVR, after Audyssey is run, affects what Audyssey is trying to do. I'm all for improving your system, and your results sound positive, but if you manually change settings after Audyssey stores its settings, your then basically altering the response Audyssey is trying to base its calculations on.

This is one of those things that would niggle away at me, even if I cant hear the difference, at least for a while. Ive always been an advocate of helping Audyssey as much as possible to get the very best results, so I really wander how this figures into the equation.
Unless you are using an Audyssey Pro system, that actually looks at how the satellites merge with the sub and calculates the results of each choice of crossover, this doesn't impact the Audyssey did it's equalization at all. Audyssey generates filters to equalize each speaker individually to 75dB up to the points where the speaker rolls off. Time/distance alignment of the speakers doesn't affect that, Audyssey still runs, although it might make the little square or green light turn off that indicates you are using the original Audyssey values. Also, especially for the sub, Chris recognizes that Audyssey's distance calculation is often off, hence the advice when it's too low to set it to the physical distance. With REW v5 we now have a measurement tool to make it right whether the Audyssey calculated distance was too low or too high.

I actually took advantage of this in the band limited impulse plot. Because Audyssey equalized the sub and the mains, the impulse curve, generated just over the range where both can respond, should look similar. With Audyssey equalization, the sub's range is extended as high as possible, to give the best available resolution. I intend to add a plot showing how I picked 200Hz for the upper end, and why I should have used a higher lower end, e.g., 30Hz or 40Hz for the sub impulse to match the main's range.

It is important to have Audyssey On when taking any of the measures for estimating distance. With the left channel as timing, one can see the delay from enabling the Audyssey processing. In my Denon, I'm seeing about 9 msec (10') difference between the impulse curves of Audyssey Off and On.

Last edited by laser188139; 05-10-10 at 09:27 AM.
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post #15 of 29 Old 05-11-10, 12:07 AM Thread Starter
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Re: Case Study: Sub Alignment using REW v5

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I am wandering how adjusting the time delay in your AVR, after Audyssey is run, affects what Audyssey is trying to do. ...
As promised, I updated the description to include a freq resp curve showing why I stopped the impulse sweep at 200Hz.

Just for you, Dan, I also included a comparison showing the initial Audyssey chosen distance.
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post #16 of 29 Old 05-11-10, 03:09 PM
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Re: Case Study: Sub Alignment using REW v5

Cheers Laser

Ive just got in from work an hour ago, and I'm just settling down for the night, I'll sink my teeth into into it in a little while and have a think.
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post #17 of 29 Old 05-20-10, 03:59 PM Thread Starter
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Re: Case Study: Sub Alignment using REW v5

I've expanded my description to reflect later experiments I made investigating the effect of the crossover in the AVR.

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I've basically been adjusting distance in my prepro to peak out the crossover region also. Inverting the subwoofer signal gave me the same results.
...
Of course you are right, had I started with the physical distance to the sub and stepped its distance upward, I would have eventually found a distance setting that gave the best frequency response.

I've seen comments elsewhere in the forum, perhaps yours, which described how someone adjusted the sub distance over a wide range looking for the best frequency response. I was always reluctant to do this, both because of the time involved and because frequency response is only one criteria, the absolute time relationship matters too. So I did not follow this route until v5 gave us tools to look at the timing.

I did some more experiments to see if a narrow sweep range around the crossover could give a starting distance as good or better than I determined with my first technique. This proved not as satisfactory as using a range 50Hz-200Hz while moving the crossover filter out of the way.

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... Another option would be to make measurements at the line level outputs to sub and mains to accurately characterise the delays in processors etc, free of any room influences, then add in the delays from the relative distances of the speakers/sub to the listening position. ...
I have made some more measurements of the line level outputs from the AVR, some of which I've included in the article. For me, the physical distance of the speakers to the listening position is itself problematic when considering an all-horn floor standing speaker, where the woofer faces backward to use the back of the cabinet to extend the horn. Add to that, there is some delay in the custom crossovers in the front speakers.

Bill
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post #18 of 29 Old 05-23-10, 12:59 PM Thread Starter
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Re: Case Study: Sub Alignment using REW v5

After looking at impulse responses over narrow ranges to see the effect of the receiver's crossover at different frequencies, it occurred to me that one should see the same thing in the group delay curve. So I've added that graph, confirming what crossover design would lead one to expect.

Bill
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post #19 of 29 Old 01-11-11, 11:26 AM
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Re: Case Study: Sub Alignment using REW v5

Somehow I just found the time to read this thread fo rthe first time... thanks for the great work Bill.
But I'm almost thoroughly confused... so please correct me if I'm wrong...
The sweeps you took to generate the two IRs you used to measure your delay were both done from 50Hz-200Hz, right? One was the Left speaker set with a 40Hz Xover in place, and the other was the sub with a 250Hz xover in place.

Right?

-Greg

Don't worry... nothing new here, I've already made that mistake. Trust me.
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post #20 of 29 Old 01-11-11, 11:34 AM Thread Starter
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Re: Case Study: Sub Alignment using REW v5

Correct, Greg.

Because of how the range of the sweep affects the shape of the impulse curve, essentially the rise time at the start of the curve, looking at both speakers over the same frequency range lets one try an apples-and-apples comparison of the relative timing of the curves.

Last edited by laser188139; 01-11-11 at 02:04 PM.
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