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Old 24th September 2010, 12:50 PM   #11
hhenne is offline hhenne  Denmark
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The theory of cascading filters is:
Lowpass and highpass filter sums correctly in a 2 way system. In 3-ways they don't - unless the xover frequences are more than a decade apart - In the mid channel phase lag from one xover will affect the phase at the other xover. The solution is to make 2 perfect 2-ways systems and cascading them - consider mid/high as the high part in a 2-way system.

In miniDSP you can only do that in the advanced mode, where you can duplicate the HP part of the low freq xover in the LP and the HP part of the high freq xover - as mentioned by someone else.
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Old 24th September 2010, 05:34 PM   #12
macboy is offline macboy  Canada
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Quote:
Originally Posted by gainphile View Post
...
I was hoping that the algorithm of MiniDSP considers this. I'm not sure what it means by "Allow the output of the first filter to go to two different places, just dulplicate that first filter in the other path".... could you please elaborate further?
...
Note how in Linkwitz's crossover, the input for the high pass of the mid is not the input signal per se, but the output of the high pass for the woofer. That is because the woofer high pass filter will still cause altered magnitude and phase response even at the mid's high pass frequency.

Instead of using the output of the woofer high pass as the input to the mid high pass, you can duplicate the woofer high pass filter before the woofer high pass filter. That will have exactly the same effect. Like this:
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Old 24th September 2010, 07:30 PM   #13
Davey is offline Davey  United States
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macboy,

Well, yeah, I think that concept/implementation is understood by everyone. But the original question was does the miniDSP architecture support that. The question has been answered already by "minidsp."

Cheers,

Dave.
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Old 24th September 2010, 09:10 PM   #14
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Quote:
Originally Posted by hhenne View Post
In miniDSP you can only do that in the advanced mode, where you can duplicate the HP part of the low freq xover in the LP and the HP part of the high freq xover - as mentioned by someone else.
Yes. Can MiniDSP do this... and how? This is exactly what I want to hear! That is similar to cascaded filter stages.

That will make MiniDSP perfect (well maybe not yet .. I'll reserve that for hires ADC/DAC )

Thanks for the drawing also!



Edit: I just re-read the 4-Way advanced Plugin manual. Are you referring to using "PEQ" to achieve this?
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Last edited by gainphile; 24th September 2010 at 09:18 PM.
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Old 25th September 2010, 04:04 AM   #15
minidsp is offline minidsp  Hong Kong
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To further comment Macboy post, you can indeed build this exact configuration but only by using the miniDSP 4way Advanced plug-in allowing custom Biquad programming.

In advanced mode, you basically have access to 8 biquad per channel for the crossover only (see back of datasheet of 4way advanced plug-in for example). These biquad are typically used for the crossover section (LPF + HPF) in the basic mode. Each biquad is able to perform a 2nd order filter so that's how we can go up to 8th order on the LPF (4 biquad) and 8th order on the HPF (remaining 4). Makes sense?

Now to build the configuration shown by MacBoy, you simply need to calculate each filter biquad coeff separately and load the Biquad settings to each section as per the diagram.... Eventually allowing you to have that exact configuration.

I apologize if I confused a couple of you guys earlier on when saying that we couldn't do it. I meant we couldn't do it in the basic mode that is pre-configured for HPF + LPF per output. In the advanced mode (Biquad programming), you really have a lot more flexibility than people actually realize.

If 8 biquads (up to 16th order) are not enough filter cascading, you can always do the same with the remaining 6 biquads used for PEQ... Up to 14 x 2nd order filters should be enough for you guys, no ? ;-)

Finally, I'm sure that one of you will say: Calculating biquads is too complex... If you have high school math knowledge, you'll see that it's a real simple task. :-) Just google RBJ (Robert Bright Johnson) EQ cookbook and you'll find a wealth of knowledge.

Soon enough, we'll figure out a way to embed all this into a nice online calculator...

Hope this helps clarify the Advanced Biquad concepts,
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Old 25th September 2010, 07:24 AM   #16
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THANK YOU!

So the summary is .....

Slopes (obviously) ... ok
Phase alignment/allpass filters ... ok
Notch filters ... ok
Channel level ... ok
Linkwitz Transform ... ok (thanks to the netherlands DIYers!)
Shelving Lowpass filter ... ok
Master volume control ... ok
Cascaded filter stage topology ... ok


Amazing. I have placed an order.
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Old 25th September 2010, 03:30 PM   #17
Davey is offline Davey  United States
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gainphile,

I'm not sure why this (cascading filter topology) might have been a show-stopper for you. I don't really think it's a concern. As mentioned by hhenne, when xover frequencies are at least a decade apart, (which they are in the Orion) and then you factor in the acoustic responses of the drivers this effect disappears into the mud. (More so with higher slope crossovers.)

From a theoretical standpoint...analyzing just electrical responses on paper it looks good, but in the real world most probably a non-issue.

Linkwitz designs his ASP's using this approach more from a practical standpoint because it requires less components to implement and the analog topology makes it the obvious way to do it.

minidsp,

Thanks for the clarification/update. Good info.

Cheers,

Dave.

Last edited by Davey; 25th September 2010 at 03:38 PM.
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Old 25th September 2010, 08:12 PM   #18
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Hi Davey, indeed it would have been a showstopper for me, because I have such experience with my xo. Furthermore, in the future I want to develop a 4-way system in which separating frequency a decade apart would not be achievable.

At that level I believe it can be a real differentiator for MiniDSP product.

As for analog xo, it is actually a 'sacrifice' to implement cascaded filters because that means more noise is injected into the signal (signal passes more op-amps).
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Last edited by gainphile; 25th September 2010 at 08:16 PM.
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Old 26th September 2010, 03:56 AM   #19
Davey is offline Davey  United States
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Ah, you're one of those op-amps-are-horrible guys. I don't agree that it's a sacrifice (at least from a noise standpoint) to implement cascaded filters in an analog crossover.

Anyways, if noise is a concern you may want to re-think your decision. While the miniDSP outputs are less noisy than any DSP-based unit I've auditioned (quite a few) they are still a good deal more noisy than the outputs of a typical analog, active crossover.

My Phoenix ASP, for example, is inaudible (with my ear right next to the drivers) hooked via a power amp with 30db voltage gain and 93db sensitive speakers. That's low noise.

I'm not sure your priorities are the same as mine, but it doesn't matter. All good fun.

The miniDSP guys have done a nice job with these units.

Cheers,

Dave.
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Old 26th September 2010, 04:55 AM   #20
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Quote:
Originally Posted by gainphile View Post
As for analog xo, it is actually a 'sacrifice' to implement cascaded filters because that means more noise is injected into the signal (signal passes more op-amps).
I'm running a 4 way 4th order cascaded active crossover using AD825's. No additional noise with it in or out of the chain.
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