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How do I use both FIR and IIR filters in NAJDA setup?
How do I use both FIR and IIR filters in NAJDA setup?
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Old 12th August 2017, 01:12 PM   #1
lowpoke is offline lowpoke  Australia
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How do I use both FIR and IIR filters in NAJDA setup?
Default How do I use both FIR and IIR filters in NAJDA setup?

I read in the main NAJDA thread that it's possible to utilise both FIR and IIR filters in the same setup, using IIR crossovers on the outputs, then adding a FIR filter on the inputs to align the phase.
However, only the FIR filter setup allows you to load filters on the inputs. So how does one go about specifying IIR filters on the outputs in the FIR setup section? Is it simply a matter of loading IIR coefficients instead of FIR? Or is there something I'm missing?
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Old 12th August 2017, 06:29 PM   #2
tommus is offline tommus  United States
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Not possible, unfortunately. You either run it as all FIR or all IIR.
A lot of NAJDA users have been waiting for this feature to be added (in fact I bought the board expecting this to happen years ago) but it is probably never going to happen...
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Old 12th August 2017, 11:10 PM   #3
lowpoke is offline lowpoke  Australia
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How do I use both FIR and IIR filters in NAJDA setup?
Damn. Thanks for that confirmation tommus. It seems to be a little short on taps to run a four way setup using FIR only. Now I understand why my emails to Nick were getting no reply. 😳
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Old 13th August 2017, 12:15 AM   #4
tommus is offline tommus  United States
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Lowpoke, if you are using a computer as a source you could create an FIR filter with rePhase software (free!) and use Foobar2000, JRiver or a similarly capable player to convolve the signal before it hits the Najda. This is what I am currently figuring out how to implement, wading through 200+ posts on the rePhase thread.

Last edited by tommus; 13th August 2017 at 12:15 AM. Reason: grammar
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Old 13th August 2017, 12:36 AM   #5
lowpoke is offline lowpoke  Australia
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How do I use both FIR and IIR filters in NAJDA setup?
I do use a Mac Mini as a source most of the time. (But like to listen to my turntable as well.) And I almost opted to handle the DSP in software with a USB audio interface, as I knew this would give me serious processing power. But after reading SEVERAL posts from Nick as far back as 2013 stating that FIR and IIR would be possible with the next software upgrade, I didn't even consider that four years later this wouldn't have happened! So I purchased the NAJDA. But I'll likely explore your suggested option of seeing what can be done to align phase BEFORE signal gets to NAJDA. I appreciate your input. 👍
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Old 13th August 2017, 09:29 AM   #6
fluid is offline fluid  Australia
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Even if the mixed FIR and IIR ever did emerge which seems unlikely the Najda has a terribly low tap count if you want to do low frequency FIR filters. I tried to generate a few test filters in rephase that would load in Najda and the response was not very good below 100Hz.

In rephase there is a Filters Linearization tab, there you can select the order of Linkwitz Riley crossover you are using and frequency, then when you generate a filter the phase turn associated with that filter will be "undone" for Jriver or most other convolvers generate the file as a 32bit stereo wav file at your sample rate and it should work without issue. It also has a box selection for port phase wrap and subsonic for a undoing the phase of a Subsonic High Pass filter. You can generate the filter and see how close to ideal it is in the main window and then use more or less taps in the filter to get the response you need. Very low frequency correction will need more taps if you want fine resolution.

You can use the other tabs for EQ and paragraphic phase EQ.

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Old 13th August 2017, 10:24 AM   #7
lowpoke is offline lowpoke  Australia
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How do I use both FIR and IIR filters in NAJDA setup?
Thanks for your input fluid, even though it's gone over my head somewhat. Are you saying this rephase file should be used in the way tommus suggested, in a convolution engine upstream of NAJDA, so it adds the required delay to the two channel signal BEFORE the crossover?
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Old 13th August 2017, 10:30 AM   #8
fluid is offline fluid  Australia
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Quote:
Originally Posted by lowpoke View Post
Thanks for your input fluid, even though it's gone over my head somewhat. Are you saying this rephase file should be used in the way tommus suggested, in a convolution engine upstream of NAJDA, so it adds the required delay to the two channel signal BEFORE the crossover?
Yes you would apply it via a convolution plugin so it applies the opposite phase distortion that the Linkwitz Filter does to even the two out. That is the simplest use of rephase and what it was originally intended for to undo the phase distortion of crossover slopes.

It's not really delay as that is slightly different, this is frequency dependant delay (phase).

Does that make more sense?
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Old 13th August 2017, 10:48 AM   #9
lowpoke is offline lowpoke  Australia
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How do I use both FIR and IIR filters in NAJDA setup?
Yes I get it now. I'm assuming the single FIR filter required for this 'phase unwrapping' is not as intensive as implementing a four way linear phase crossover using FIR filters exclusively?
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Old 13th August 2017, 10:56 AM   #10
fluid is offline fluid  Australia
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That depends on the length of the filter, the number of taps that are used. A longer filter has higher resolution but is more resource intensive and has a longer latency. It doesn't matter to the computer whether it is changing phase, frequency or both. Convolution in the time domain is equivalent to multiplication in the frequency domain. Most convolvers use FFT multiplication as it uses less resources.

For each way of the crossover the length of the filters could be tailored to the resolution needed for that frequency band.

That being said any reasonable computer will have no issue running quite a number of high tap count FIR filters without breaking a sweat. It is only when the filters need to be run in hardware like Najda or MiniDSP that the tap count limitations apply.

There should no measurable difference between an IIR based crossover that has been linearized in phase to one that was created as linear phase in the first place. As you already have the Najda to do the crossover duties and works pretty well in that role it is easy enough to add the phase linearization on the computer.

Once you've got that sorted you can then venture off into the land of DRC and really go wild

Last edited by fluid; 13th August 2017 at 11:07 AM.
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