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Old 3rd December 2007, 04:36 AM   #1
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Default correcting for time delay with active x-over

Hello, now that drivers have been chosen for my center channel prodject and have designed baffle for edge deffraction problems, I was wondering about time delay of the seperate drivers? with a 24db per octave slope and being electronic, this still can't do anything for the offset of acoustical centers of the drivers themselves right??? If this "is" the case then how does one implement correcting for time differences created from seperate drivers when using an active setup. Or is this even worth the trouble?
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Dallaire
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Old 3rd December 2007, 03:10 PM   #2
MarkMcK is offline MarkMcK  United States
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You will have to decide if it is worth the trouble, but the optimal way to correct for differences in air path delay is with a delay line. The convenient way to create an adjustable delay line is in the digital domain. There are professional (sound reinforcement) equipment that include both adjustable crossover options and adjustable delay lines.

Mark
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Old 3rd December 2007, 03:28 PM   #3
sreten is offline sreten  United Kingdom
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Hi,

One way is assymetric crossover slopes to adjust relative phase
response such that the phase is aligned at the crossover point.

/sreten.
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Old 3rd December 2007, 04:29 PM   #4
MarkMcK is offline MarkMcK  United States
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Do you want a create a coherent source to reproduce transients? If you test with a transient signal, a multi-way system using asymmetrical electrical filters will still test as multiple discrete sources. Asymmetrical electrical filters will not correct for differences in air path delay in multi-way systems.

It would be wonderful if such a simple fix would work, but it doesn't. Sorry!

Be a diyer. Test it for yourself. Easy to do.

Mark
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Old 3rd December 2007, 04:48 PM   #5
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by MarkMcK
Do you want a create a coherent source to reproduce transients? If you test with a transient signal, a multi-way system using asymmetrical electrical filters will still test as multiple discrete sources. Asymmetrical electrical filters will not correct for differences in air path delay in multi-way systems.

It would be wonderful if such a simple fix would work, but it doesn't. Sorry!

Be a diyer. Test it for yourself. Easy to do.

Mark

Hmmm.....

rather good at being patronising ......

Since when is 4th order L/R (presumably) transiently coherent ?

/sreten.
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Old 3rd December 2007, 08:04 PM   #6
MarkMcK is offline MarkMcK  United States
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Quote:
Hmmm.....

rather good at being patronising ......

Since when is 4th order L/R (presumably) transiently coherent ?

/sreten.
Of course, if I was patronizing, then you could dismiss all that I say.

To continue the actual discussion I will mostly ignore the first two lines of sreten's reply.

I notice that you (sreten) became specific in the third line. This is a statement that can advance discussion.

Not all asymmetrical electrical crossovers are L/R crossovers. The L/R fourth order is a specific design for a specific purpose. The L/R fourth purposely induces on-axis errors to correct a specific off-axis problem. I believe this is fairly well accepted since the on-axis problem part was part of the original paper.

The question for this thread (as I read it) was not about this off-axis problem. In relation to the question posed, the L/R fourth will not correct for differences in apparent acoustic centers (or differences in air path delay). It will, however, cause problems on-axis.

For me, the reason to be worried about differences in apparent acoustic centers (differences in air path delay) is to achieve an on and off-axis response that is as close to coherent and flat as possible. The L/R fourth will not accomplish this.

While sreten accused me of being patronizing (whether I was or not), I also said how Dallaire (or anyone else) could test what I said. I did not expect Dallaire to believe what I said just because I said it. An interesting difference.

Mark
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Old 3rd December 2007, 08:14 PM   #7
Raka is offline Raka  Europe
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As far as I see, there are two things: phase and time. You can correct the phase "error" introduced by the time delay with an easy all pass net. Going analog you can not correct the time delay.
I'm not too much concerned about the time alignment per se, because if you look at the pulse through the filter, it's kind of Quasimodo
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Old 3rd December 2007, 08:47 PM   #8
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Quote:
One way is assymetric crossover slopes to adjust relative phase response such that the phase is aligned at the crossover point.
Provided that all drivers are measured correctly with both amplitide and phase, or the phase is generated by FFT and the acoustic centres are perfectly offset in simulation software, such as SpeakerWorkshop, which we use to model the crossover. Does "align the phase" mean at the crossover point the phases of both drivers need to be the same, such as both are at 30 degree?
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Old 3rd December 2007, 09:01 PM   #9
MarkMcK is offline MarkMcK  United States
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Hi Raka,

I agree with you about the look of an impulse at the output of an analogue filter. A transient test of an analogue filter will not look good.

What if the acoustic impulse is important? It is not a problem to have a crossover network that does not sum electrically if when added to the inherent slopes of the transducers, it sums acoustically. It is not a problem to have an impulse response that looks incoherent at the output of the filter if it becomes coherent when added to the "filters" implicit in the imperfect transducers.

One problem is that you cannot produce a coherent acoustic impulse if all you do is play with the electrical filters. A related problem is that if you then correct for air path delay when you test acoustically, your acoustic output will not sum. Heyser used to do this in his Audio magazine loudspeaker reviews. He produced some very nasty looking frequency response graphs for most of the loudspeakers of those days.

Another problem is that there is a problem or blind spot with FT testing. If you have a multi-way loudspeaker and the air path delay differences are large and one driver has stopped voicing before the next driver begins to voice and so on, all you are looking at in the FT is the magnitude sums. Most of the possible time relation information is lost. Any possible interference is ignored. This makes designing and claiming this or that response much easier. Still, there are lots of errors in the actual loudspeaker. Compensate for the differences in air path delay and you will then be able to see the interaction problems.

Back to Dallaire, Raka has stated that she is not too much concerned with time alignment per se. In contrast, I am for time alignment for the reasons given above (and many more).

If you go with the digital delay and crossover, once again, you will be able to test for yourself the differences between a loudspeaker system that is time aligned and one that is not, but that does use asymmetrical filters or all-pass filters to adjust only for phase.

Mark
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Old 4th December 2007, 02:38 AM   #10
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Wow, my head is spinning! you guy's obviously know alot more than myself, trust me!
Perhaps I can elaborate a bit more as to my setup. I am making a WMTMW array. My crossover setup is the Marchand XM1's, speaker will be all electronicly crossed. I am using, and I hate to say it cause I don't want to get audio rocks thrown at me, but I am using the Behringer 2496 units for all channels in my dedicated HT room, but they are just the parametric EQ's no x-overs or other bells/whistles.
I think alot of time when someone says "time correctiing" people start calling this "phase" are'nt the two different? When I sat "time" I am talking about the voice coils being in line in the physical sense.
So now knowing my x-over setup and the proccessors I have, it sounds like I "would" then benifit from alligning voice coils of drivers.
The only thing that I am concered with is I have spent alot of time placeing drivers on baffle using "Edge" program to get the baffle step to look nice with regard to indended cross points and such. I now I start building drivers forward on the baffle, I think this would "almost have to change the deffraction estimate"! don't you think...?
Anyway, this is where I'm currently at.
Vince
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