December 3, 2010

Time alignment with Behringer DCX

2 minutes with a mic and you can have a perfectly time aligned setup. This is about as easy as it gets.





Why should you time align

Short answer: you'll get the most accurate imaging
Detailed answer: see my post on the reason time alignment is needed

What you will need

Behringer DCX
Measurement mic that will interface with DCX (ECM8000 or similar)
XLR cable

Step 1: equipment setup

1. Place your mic in your listening position. For this test you don't require a mic tripod, since you can probably simply place it in your couch, where your ears are when listening.

2. Connect the mic to input C on Behringer DCX

Step 2: run Auto EQ

1. Press the setup button







2. With the page button navigate to page 2 - auto align

3. Press OK

All channels will be muted. Un-mute all the channels by pressing all of them. A series of pulses will play through each channel.

4. Set gain

Rotate the dial to set the gain on DCX. If you have a preamp upstream, it will not work since DCX is acting as a source. You will need to set the volume quite high or the routine will not work.

5. Click OK

DCX will play a series of pulses and measure the time delay. It will then assign settings to align.

Step 3: review settings

1. Press channel 1 then via the page button navigate to page 7 - polarity.

You may notice the polarity of some channels has been inverted. Click through all the channels to see what has been done.

2. Review delay settings on page 8.

If you have a subwoofer included, don't be surprised if the other channels have been delayed by a large amount like 8m or 20 ms. DCX is correcting for phase shift as well, not simply the physical location of the drivers.

Why does DCX do it differently each time?

If you run the routine again, with nothing altered, you will get different settings. At first, they may appear radically different, but on closer inspection  you will find the different settings are in fact doing the same thing in slightly different ways. It can correct differences with both polarity reversal and digital delay. If in a subsequent run it handles polarity differently, then it will compensate with different delay.

Confirming with measurements

You may wish to confirm with measurements. Investigate with frequency response where phase is also plotted. If you see a common 180 degrees phase shift at the crossover, DCX will simply invert polarity. If the shift isn't a neat fit, then digital delay may be used. If you know the phase shift at the crossover, you can work out the required digital delay.

First work out the time for one full cycle

Time = 1/frequency

eg. at 3k, time = 1/3000 = 0.3 ms

Delay = phase shift/180 x time for one cycle

So the delay = phase shift / 180 x 1/frequency

In this case, reversing the polarity at 3k is equivalent to 0.15 milliseconds.

Time alignment series

Why you should time align >
Is it really necessary?

Time alignment overview >
A bird's eye view of various ways that are used to time align speakers. Does physical offset really work?

Time alignment with Behringer DCX >
This is about as easy as it gets. You need a mic and a few minutes to run the auto routine.

Digital time alignment >
My preferred method. No need to build bizarre baffles that can introduce their own issues.

Subwoofer phase alignment
Yes, even subwoofers benefit, but not for the same reason

8 comments:

  1. Paul...thanks for puting this up. I have read through that DCX manual many times...it requires a bit of editing and you have done a fine job!
    In my system ,the volume had to be so loud before a "reading" would occur, that I feared some damage would evenutate(to my ears, if nothing else).
    You have given me confidence to tackle it again.
    The only drawback for me at the moment, is that I prefer to use the DCX to manage the sub drivers only, in a range below that which my "Golds/cabinets" do not go anyway. In order to delay the Golds, they would have to go through the DCX, which I had been hoping to avoid.

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  2. I'm glad you found it helpful. You might like to check out the recent listening test thread on StereoNET and my post here. My impression has been that DCX is reasonably transparent in a compatible system. There are some that won't be satisfied with it's sonics - small differences can be picked in an instant switching test if the conditions are right. It's worth trying in your system - try to keep an open mind. You might also try for yourself a digital conversion test. Simply switch a loop cable vs DCX working with null settings so that it does analogue > digital > analogue conversions. You will find out exactly what the digital conversions do to the sound. I'd contend that most would find it difficult to pick a difference and would have to listen carefully.

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  3. Hi Paul,

    Great column and great site. I just bought a DCX2496 and am preparing to install it in my home theater to crossover my two diy 3-way front L&R speakers.

    I gather I just install it as I would a regular crossover (I have a Rane AC23 in place at the moment) then follow your instructions.

    Does it test and align all the drivers in both L&R speakers at the same time?

    Or does it just align the drivers in, say the left speaker with each other, and then align the drivers in the right speaker with each other, leaving the alignment between the left and right speakers to the surround processor?

    Thank you for your consideration.

    Best wishes,
    Dave
    (I don't mean to be anonymous, I'm just not sure how to do the other choices)

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  4. Auto align will cover all channels. It depends how you have it set up. If you have stereo link for a 3 way then it seems to choose the same settings for left and right. In other words, it will not adjust for the fact that the left speakers is closer to the mic, but it will delay the mid and tweeter on the left to align with the woofer. I haven't tried it with link off, but I would guess it would change how the channels are aligned.

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  5. Hi Paul,

    Thanks to your great column, I was able to time align my two front speakers using the DCX. After reading your columns on the minidsp units, I'm thinking of getting a couple to use as crossovers and EQ's . It seems like for determining the delay times I could use the DCX as before, just write down the delay times and whether or not polarity is reversed, and use that information to set the minidsps. Do you see any problem with that?

    Thanks for the great blog.

    Best wishes,
    Dave

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  6. Dave, I find that for subwoofers you can require more delay to the mains than MiniDSP will allow - that is a problem! Also, the times calculated by DCX are not always accurate. You need to check. One way to do this is with excess group delay on REW - check the online help files for that one.

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  7. Hi Paul,

    Nice column and great job. I encountered a problem though which i didn't manage to pass through though when using the auto align . Here is what i did, i hooked up everything including the ecm, switched on the dcx and amps, ( i was running a 2way setup with sub). setup the crossover 2 way and proceeded to the auto align page. pressed ok and unmuted 4 channels then went ahead to crank up the gain on dcx outputs. I managed to hear the short sound bursts but they continued for sometime and seem to take forever to fnish so I opted to go manual.The environment was noisy so im wondering what I did wrong......which gain should be cranked up?

    Thanks
    Ibrahim

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  8. Noise can be a real problem and even in a very quiet room you have to make it ridiculously loud to work. The solution is to run it when and where background noise is low or you may not get it to work. DCX sends its signal direct to the power amps and there is only one gain control you can use. I should mention that it appears you can't trust the DCX measurement of time alignment entirely.

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