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Old 23rd September 2010, 03:14 PM   #1
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Default MiniDSP as Linkwitz Orion ASP

While I'm not building the Orions, Linkwitz' ASP is a great basis for Open Baffle Speakers XO. Looking at the stages I can see that MiniDSP satisfies most of its requirements, except for a few which I would like to confirm:

Here I assume the requirement is 4-channel Loudspeakers per side... so I would need:

2x MiniDSP kit Rev B
1x 4-Way Advanced Plugin
15v DC power source


The XO itself must be able to do:

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 ... not sure how to do this ?

The other important question is what are the effects of 4-way DSP XO without staging/cascading the XO slopes as SL describes as "common crossover topology mistakes" here ?

linkwitzlab.com/frontiers_5.htm#V
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Old 23rd September 2010, 03:47 PM   #2
Davey is offline Davey  United States
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What is your objective? To try and improve the Linkwitz design? Or duplicate and then have the option to bring other features to the table......like room correction, etc?

The miniDSP doesn't support an equivalent allpass filter, but it will implement a pure delay that should satisfy the requirement for phase-coherence....in a different way.
Shelving filters are easy to achieve with the miniDSP. Set frequency to the center of the shelf range, set attenuation/boost, and set Q=1 and you have first-order shelf with exactly the same characteristics as an analog filter.

I currently have my miniDSP combo setup to implement a close equivalent of my Phoenix ASP. Programming/testing took me an hour or so. There are some potential pitfalls regarding clipping, cascading of EQ sections, etc, etc, but it seems to be working fine.

An Orion ASP (DSP) with Thor sub-woofers should be realizable in much the same way.......but I haven't done it yet.

Cheers,

Dave.
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Old 23rd September 2010, 03:52 PM   #3
ra7 is offline ra7  United States
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Has anybody attested to the sound of the mindsp? Just curious.
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Old 23rd September 2010, 06:21 PM   #4
macboy is offline macboy  Canada
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Why wouldn't you cascade the sections? I am not familiar with the miniDSP but if it doesn't have flexibility to allow the output of the first filter to go to two different places, just dulplicate that first filter in the other path. Because it is digital, two identical filters are truly identical (unlike two "identical" analog filters).

A shelving filter is also used for bass and treble controls. A bass control should be a shelving low-pass.
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Old 23rd September 2010, 08:55 PM   #5
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Thank you for the answer regarding shelving lowpass filter. Then that section is answered.

Shelving Lowpass filter ... ok

The second question which is as important, is because in a multi-way situation the highpass of previous section has phase shift effect for the next stage (see SL's explanation). This is both logical and audible in practice ie. the cascaded filter is coherent-sounding. The only exception is where the xo point is a decade apart (sometimes possible in a 3-way, but not possible/practical in 4-way). In a 2-way this is normally not an issue.

I was hoping that the algorithm of MiniDSP considers this. I'm not sure what it means by "low 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?

My objective is simply to have an xo similar to Linkwitz ASP. But different xo point, notch, etc. I have done it on a breadboard and am happy, but simply do not have the energy to transform it to a PCB.
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Last edited by gainphile; 23rd September 2010 at 09:01 PM.
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Old 23rd September 2010, 09:17 PM   #6
cuibono is offline cuibono  United States
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You know, I've wondered about the cascading/phase issue SL points out. On the one hand, he definitely knows what he is talking about. But in my current DSP setup, where I do XO and EQ in the computer, I don't cascade my filters. I measure/develop each section separately, and then combine them. When I've measured it all as a whole, it sums just as one would hope. So I don't know what to think about the issue.

Luckily, it would be easy for me to change it to a cascaded setup - I just haven't yet. Hmmm...
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Old 23rd September 2010, 10:02 PM   #7
Davey is offline Davey  United States
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I only have the 4-way PEQ plug-in so can only speak to that one. You can't "re-route" internally the output of one filter back to the input of another. With this plug-in, once the signal is split, it stays split.

In the case of an analog signal processor you can implement an all-pass delay circuit in one branch and then a further delay in another branch when you split the signal again. In the case of the miniDSP you can't do that, so you'd have to add up all the delays required in each particular branch.

I'm not sure it's really an issue.


Here's a copy of the architecture for each of the miniDSP plug-ins:

MiniDSP - MiniDSP Pluggins

Cheers,

Dave.
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Old 24th September 2010, 06:04 AM   #8
minidsp is offline minidsp  Hong Kong
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Hey Guys,

In answer to some of the general questions being asked:
- Shelving Lowpass filter: It's indeed been there for a while now (check plug-in datasheet/manual). Have a look at the video here and it will show you how you can toggle each EQ band from a Peak/Low Shelf/High shelf.

- As for the 4Way crossover, current implementation is as per diagram Davey pointed out on our website. it isn't the configuration as per Linkwitz. Maybe one day we'll come up with a configuration like this one. It's quite a redesign actually since it breaks all current structure we have in the code/UI so no idea if when we would have it... Would be worth to see the advantages though.. I can't answer that question since I've never tried myself this arrangement. It may mean nothing at all, but most of Live/Studio DSP 4 way processor I know off wouldn't have such configuration though... By no mean a reference to say that Linkwitz isn't right (he's a reference by all means), but just stating that 4way crossovers do work on a lot of products with a normal configuration... As anything, any products out there, there is always something else you can add/modify on a product...

My 2cts.
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Old 24th September 2010, 06:31 AM   #9
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Thank you. To summarise:

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 ... no


As explained previously the last feature I mentioned above is very rarely used. In fact I have never seen any commercial product doing it (except SL's ASP).
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Old 24th September 2010, 09:09 AM   #10
minidsp is offline minidsp  Hong Kong
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Yes, your summary is correct. Just one detail though, we don't have all pass filters implemented. Only delay and polarity.

All pass filters can however easily be implemented with a simple Biquad filter in the Advanced mode... Just need to punch up the numbers...

Hope this makes sense,
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