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post #1 of 21 Old 08-17-2014, 08:31 PM - Thread Starter
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MiniDSP & Delay

Is there a mathematical formula for figuring what my delays should be?

I have 4 subs, 2 towards the front of the room and 2 towards the rear. The rear subs are not equidistant to the MLP (the front pretty much are) so I would like some sort of formula to figure out what my delay on the rears should be.

I'm guessing it goes something like this: front subs are 17' away, rear sub #1 is 6 feet away - rear sub #1 needs the equivalent of 11' extra delay - is that anywhere near correct??

Also, is there a way to get the MiniDSP Advanced 2-way plug-in to do more than 7.5ms delay?
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post #2 of 21 Old 08-17-2014, 09:01 PM
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Not sure of there is a formula but I would probably start by making slight adjustments and measuring each change to see how it effects the response.
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post #3 of 21 Old 08-18-2014, 01:49 AM
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I'm not sure but I think that's what the adjustable polarity knobs are on the subs are there for.
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post #4 of 21 Old 08-18-2014, 05:30 AM
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distance in metres / speed of sound in air in metres per second = delay required in seconds

11' is 3.35m & speed of sound in air is 343m/s so that gives you 9.78ms

Having said that, what you really want to achieve is optimal phase alignment which, given the impact of the room, may not be exactly the same as the precise distances. To see this, you can use REW with an analogue mic & loopback & then learn how to manipulate the signal to line up the phase responses (example on another forum here but see assorted threads on HTS for lots of worked examples). If you do this then you can save yourself a lot of time measuring as you can also use REW trace arithmetic to do the calcs on paper & measurements for confirmation.
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post #5 of 21 Old 08-18-2014, 06:31 AM
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Roughly 1ms per foot. Exactly it is 0.88888888 ms per foot. You should introduce the delay to the port going to the sub that is closer.


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Last edited by ahmedreda; 08-18-2014 at 11:17 AM.
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post #6 of 21 Old 08-18-2014, 07:59 AM - Thread Starter
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Quote:
Originally Posted by 3ll3d00d View Post
distance in metres / speed of sound in air in metres per second = delay required in seconds

11' is 3.35m & speed of sound in air is 343m/s so that gives you 9.78ms

Having said that, what you really want to achieve is optimal phase alignment which, given the impact of the room, may not be exactly the same as the precise distances. To see this, you can use REW with an analogue mic & loopback & then learn how to manipulate the signal to line up the phase responses (example on another forum here but see assorted threads on HTS for lots of worked examples). If you do this then you can save yourself a lot of time measuring as you can also use REW trace arithmetic to do the calcs on paper & measurements for confirmation.
Thanks d00d, this seems to be the definitive answer....although those links kinda make my brain hurt.

I'll read 'em a few more times and then try to put it to use. One question though....is there any way to do the loopback when using REW with HDMI?
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post #7 of 21 Old 08-18-2014, 08:05 AM - Thread Starter
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Originally Posted by basshead81 View Post
Not sure of there is a formula but I would probably start by making slight adjustments and measuring each change to see how it effects the response.
That's exactly what I did bh, but I hit the limit (7.5ms) while the response was still improving, hence my question on how to get more delay out of the plug-in.
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post #8 of 21 Old 08-18-2014, 08:19 AM
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Quote:
Originally Posted by Alan P View Post
Thanks d00d, this seems to be the definitive answer....although those links kinda make my brain hurt.

I'll read 'em a few more times and then try to put it to use. One question though....is there any way to do the loopback when using REW with HDMI?
Afraid not, you could use holmimpulse instead though. It has a time lock feature for this purpose and similar trace arithmetic functionality.
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post #9 of 21 Old 08-18-2014, 08:50 AM - Thread Starter
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Originally Posted by 3ll3d00d View Post
Afraid not, you could use holmimpulse instead though. It has a time lock feature for this purpose and similar trace arithmetic functionality.
Hmmm...looks like a great option, thanks!
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post #10 of 21 Old 08-18-2014, 08:51 AM
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Originally Posted by charmerci View Post
I'm not sure but I think that's what the adjustable polarity knobs are on the subs are there for.
No that is not the same thing. Phase control only aligns around the crossover. Distance delay can align the entire FR.
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post #11 of 21 Old 08-18-2014, 10:42 AM
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If 7.5ms is a hard limit then....

You could try aligning them to be 360 degrees out of phase (ie in phase but a whole cycle out). Timing (snap, transients etc) would be off but at least frequency response should be OK.

You might also try inverting polarity on one set of and then aligning to be 180 degrees out. This might be easier to dial in without phase views as you would delay for the worst possible response and then flip the polarity.
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post #12 of 21 Old 08-18-2014, 11:46 AM - Thread Starter
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OK, maybe I don't need as much delay as I originally thought.....

Back to my OP; if all 4 subs are EQ'ed/delayed (sub distance setting) by Audyssey collectively at 18', but the rear subs are only 4-5 feet from the MLP, I just need to add delay to the rear subs equal to their actual physical distance....is this correct?

If that is the case, I would only need to add:

Sub #3 - 4' from MLP
4x12=48"
48/13.5=3.555ms

Would this be correct???

I still plan to measure the actual delay needed with the tools referenced above by d00d, but just wanted to know if I've got a good grasp of the theory.
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post #13 of 21 Old 08-18-2014, 11:49 AM - Thread Starter
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Quote:
Originally Posted by 3ll3d00d View Post
If 7.5ms is a hard limit then....

You could try aligning them to be 360 degrees out of phase (ie in phase but a whole cycle out). Timing (snap, transients etc) would be off but at least frequency response should be OK.

You might also try inverting polarity on one set of and then aligning to be 180 degrees out. This might be easier to dial in without phase views as you would delay for the worst possible response and then flip the polarity.
How do I align them to be 360 degrees out of phase?? My knobs only go to 180.
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post #14 of 21 Old 08-18-2014, 12:55 PM
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Quote:
Originally Posted by Alan P View Post
How do I align them to be 360 degrees out of phase?? My knobs only go to 180.
I believe he meant to say 180 out of phase...360 puts you back where you started.
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post #15 of 21 Old 08-18-2014, 03:17 PM
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IMO, you should delay them by the distance difference between the front subs and the rear subs.
If you have the front subs at say 10 feet and your rear subs at 4 feet, you should introduce a 6 feet delay to the rear subs. That would be 6 * .89 ~= 5ms.
The 18' foot is irrelevant as it is based on the circuitry processing delay as well as the physical delay.



Quote:
Originally Posted by Alan P View Post
OK, maybe I don't need as much delay as I originally thought.....

Back to my OP; if all 4 subs are EQ'ed/delayed (sub distance setting) by Audyssey collectively at 18', but the rear subs are only 4-5 feet from the MLP, I just need to add delay to the rear subs equal to their actual physical distance....is this correct?

If that is the case, I would only need to add:

Sub #3 - 4' from MLP
4x12=48"
48/13.5=3.555ms

Would this be correct???

I still plan to measure the actual delay needed with the tools referenced above by d00d, but just wanted to know if I've got a good grasp of the theory.


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post #16 of 21 Old 08-18-2014, 03:33 PM
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Quote:
Originally Posted by Alan P View Post
How do I align them to be 360 degrees out of phase?? My knobs only go to 180.
Quote:
Originally Posted by basshead81 View Post
I believe he meant to say 180 out of phase...360 puts you back where you started.
I did mean 360 however I was thinking of something else entirely (so ignore that bit!).

Anyway I think your original post was correct, you want to delay the nearby subs to meet the further away subs and that present that as a mono signal to audyssey. If that means more delay than the minidsp can add then you either need to use a different device (or chain 2 minidsp's together?). However the room effect will dominate anyway so it's possible that a precise time alignment is not the best response, I know I've seen that in my own room when I have had multiple subs in there (in my case, ~2ms difference was best).
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post #17 of 21 Old 08-18-2014, 03:38 PM - Thread Starter
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Quote:
Originally Posted by ahmedreda View Post
IMO, you should delay them by the distance difference between the front subs and the rear subs.
If you have the front subs at say 10 feet and your rear subs at 4 feet, you should introduce a 6 feet delay to the rear subs. That would be 6 * .89 ~= 5ms.
The 18' foot is irrelevant as it is based on the circuitry processing delay as well as the physical delay.
I figured that my figuring was wrong. Thanks ahmedreda.

So, with that formula; my front subs are ~17 from the MLP, my rear subs are ~4' & 6' from the MLP respectively.

Rear sub #1
17-4=13'
13x12=156"
156/13.5=11.5ms

Rear sub #2
17-6=11'
11x12=132"
132/13.5=9.8ms

Both settings are over the upper 7.5ms limit on the MiniDSP. Looks like I'm going to have to try the phase trick that d00d mentioned above if I want to get them properly time aligned.

That, or I could just stack both rear subs directly behind my chair.
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post #18 of 21 Old 08-18-2014, 04:14 PM
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Quote:
Originally Posted by Alan P View Post

Both settings are over the upper 7.5ms limit on the MiniDSP. Looks like I'm going to have to try the phase trick that d00d mentioned above if I want to get them properly time aligned.
Just to be clear, i would ignore that suggestion (I was thinking of aligning sub to mains which means aligning through a small frequency range so it can be possible to use delay as a rough proxy).
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post #19 of 21 Old 08-18-2014, 04:53 PM - Thread Starter
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Originally Posted by 3ll3d00d View Post
Just to be clear, i would ignore that suggestion (I was thinking of aligning sub to mains which means aligning through a small frequency range so it can be possible to use delay as a rough proxy).
Hmm....so I need to find another way to add more delay then?
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post #20 of 21 Old 08-19-2014, 02:12 AM
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Hmm....so I need to find another way to add more delay then?
quite possibly yes though personally I would get the measurements sorted first and then see what is required. Remember that if you get accurate timing data in your measurements that you can then shift the impulse by as many millis as you want and/or invert polarity & then use trace arithmetic to add them together to see what the result will be. Once you work out the ideal alignment on paper then you will know how much delay you really need & then you can decide what the most cost effective way to get there is.
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post #21 of 21 Old 08-19-2014, 07:19 AM - Thread Starter
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The point (for me at least) may be moot. I did some measurements last night and may be putting all 4 of my subs in the back of the room....essentially solving my problem with delay.

See this post.
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