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Old 10th July 2007, 06:49 PM   #11
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Alas, no. It was by Steve Stokes.

It is however, a good article. I would post it, but fear that
it would annoy the copyright holders.

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Old 24th January 2013, 01:42 PM   #12
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I have noticed that an increasing number of people are now building FAST systems using one of the readily available DSP xover platforms. Some of these folks might be interested in a crossover topology offering improved transient response.

While it is of course possible to build almost any kind of "wild" xover topology with most DSPs, things don't look as easy if one wants to use the convenient SW tools that come with many of these DSP modules. These usually don't allow the use of subtraction and each way has to be configured as a "straight chain of filters".

The transfer functions of asymmetrical 2nd/1st and symmetrical 2nd/2nd order subtractive crossovers can still be modelled using these tools however.

Within the following few posts I will show how this could be achieved.

There is no free lunch however. The quite shallow slopes are one of the downsides, the other one is the lobing behaviour. The latter can be eased up a little by using close driver spacing or D'Appolito arrangements.


Regards

Charles
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Old 24th January 2013, 02:17 PM   #13
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by phase_accurate View Post
There are two problems that remain: 1) There is not much protection at the low end (though still a big improvement over real fullrange usage) and 2.) the phase-shift of the woofer can play havoc with proper SPL summation.
A simple and effective means for 2.) is the use of a highpass filter Hw in the fullrange branch that mimics the low-end response of the woofer and that also intrinsically improves 1.). For a closed box this would be a 2nd order highpass and for a reflex box a 4th order highpass respectively.
Though I am not sure if one wants to use a reflex box for a “transient-perfect” loudspeaker !
Hi,

I fail to see how adding to the high bass roll-off of the fullrange
with additional high pass filters would help, surely the phase
shifts would be made worse.

No wait a minute, perhaps your really do mean the woofers alignment
being applied to the FR, but that runs into the rather serious problem
of the FR's own bass alignment well before you get to the woofers.

You would need to LT the FR to the woofers alignment, and
that is no recipe for improving its protection at the low end.

I guess it depends on the two drivers in the FAST and the
x/o frequency, but it doesn't strike me as a general way
of doing things for all combinations of FAST drivers.

rgds, sreten.
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Old 24th January 2013, 02:18 PM   #14
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Tuned in !

In your 2007 posts you assumed baffle step is eq'ed out to make this work. Can you expand on this? Thanks.
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Old 24th January 2013, 02:29 PM   #15
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You don't LT the FR to the woofer's alignment. You LT the FR to the desired acoustical highpass slipe that you want to target. You then add another highpass filter to the FR in order to add the same phase-shift as the woofer to to the FRs (out of band) response and at the same you increase protection at those frequencies.

In some cases you might simply use the FRs natural rolloff as acoustical crossover, sometimes you would need a parametricc EQ for lowering the Qtc to 0.5. Each case has to be looked at separately. And there are cases where it simply wouldn't work.

Regards

Charles

Last edited by phase_accurate; 24th January 2013 at 02:31 PM.
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Old 24th January 2013, 02:52 PM   #16
sreten is offline sreten  United Kingdom
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Hi,

You effectively do, and then add the required filter function,
not necessarily in that order. You are clearly describing the
the high pass functions of both drivers being the same
without the x/o slopes.

rgds, sreten.
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Old 24th January 2013, 02:52 PM   #17
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In the schematics you can see how the baffle-step correction can be included in the feedback path of the woofer xover. For the FR this has to be added in one of the known fashions. And yes - it will further take something away from the steepnes of the achievable highpass but that's how life is. With the FR driver (Manger) that I used I could do without BS correction on the FR because it has a rising response towards the lower end which can be abused for BSC.

If a third-order highpass function was called for I can do the maths as well but be aware that this would mean some more overlap in the XOVER area.

Regards

Charles
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Old 24th January 2013, 03:07 PM   #18
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Quote:
You effectively do, and then add the required filter function,
not necessarily in that order. You are clearly describing the
the high pass functions of both drivers being the same
without the x/o slopes.
I will have to re-read the "old part" because it wasn't meant that way but maybe I was unclear with what I wrote.

What I basically did was "building" a 2nd order highpass using EQing of the FRs natural low-end response as the highpass part of the crossover. It is clear that this works much better if the EQed response has a higher cutoff frequency than the natural response of the FR than vice-versa.

The next step was to build a transfer function that approximates the lowpass function of the subtractive crossover and at the same time taking into consideration the woofer's non-idealities at its upper end to some degree.

The third step was to add the woofer's phase response to the FR by the use of a highpass filter. This is admittedly taking place at a frequency range where the FR doesn't contribute to total SPL that much anymore but it would still be contributing in a undesirable fashion du to taking place quite close to the xover frequency. This highpass would not even have to be a very exact replica of the woofer response. And in some cases it might be done by dimensioning some coupling caps accordingly.

Regards

Charles
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Old 24th January 2013, 03:13 PM   #19
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See post 3084470 and onward in that thread.
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Old 25th January 2013, 07:11 AM   #20
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The thread that you link to is dealing with another beast of crossover than what is dealt with here.
Here we are dealing with the classic constant-voltage crossovers that don't use any form of subtractive delay simply because of two reasons: They can't be built easily using analog methods (although I know at least one practical example which is a "mass grave" of allpass filters) or digitally using these DSP crossovers that are configured by the use of a convenient but restricted GUI.
Last year I was playing around with a crossover that is an approximation of a subtractive-delay crossover which is generating the highpass without subtraction and therefore needs less component accuracy while always guaranteeing a 3rd order rolloff in the stopband. The way I did this guarantees flat group dealy only until about one octave above the crossover frequency. But total group delay is smaller than LR2 or LR4 and both drivers are connected in the same polarity. If flat group delay is desired up to higher frequencies then this could be achieved with an allpass based group-delay EQ. But this would only make sense for the usual mid/tweeter crossover frequencies. Otherwise the EQ would be too long for practical purposes.

The xovers shown here are useful for orders up to three IMO. That would allow 2nd/1st, 3rd/1st and 2nd/2nd. Otherwise overlap etc would become too large.

Edit: Forgot to get into the crossover order discussion. I am totally aware that a second order acoustical rolloff doesn't exactly mean perfect driver protection. But a speaker built this way would still offer more protection for an FR than running it full-range without crossover - let alone the much increased LF capabilities of a FAST compared to a FR alone.

Regards

Charles

Last edited by phase_accurate; 25th January 2013 at 07:18 AM.
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