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Eclipse Audio FIR Designer

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Mark Wilkinson:

--- Quote from: Helge A Bentsen on January 24, 2022, 03:16:33 AM ---Another thing that surfaced here.

I've tried a few things, and to my ears it seems like using IIR filters to adjust the response of each frequency band sounds better than using auto mag, even if the result looks similar in supplementary responses. My best sounding preset to date is done using IIR to correct as much as possible before I apply a LF/HF linear phase crossover, a couple of manual linear phase eq to adjust summation and a couple of manual filters to adjust the phase response as needed.
I then apply a tiny bit of auto mag/phase to the HF, leaving the LF manually adjusted.


Does this sound familiar to others using FIR Designer?
I still have a couple of things I need to work out, there is a bit of "jumpyness" in the horn, some part of the spectrum stick out a bit om some recordings. I suspect I might have overcorrected a bit on the HF, going to do a A/B with a version without auto mag correction.
BTW that is an andvantage of sticking to IIR filters, it's really easy to fine-tune a filter directly in the DSP, no need to load a new FIR filter on the outputs.

--- End quote ---

Hi Helge, very nice work/observations imo.   
And i owe you a small apology....one of the first questions i should have asked before offering any auto-EQ advice, is how many taps at what sample rate are you using?

Apart from delay, there are two shortcomings/dangers with FIR, or at least two I've found.
First is the frequency resolution.  Like Pat Brown illustrates in this article you may have seen......   www.prosoundtraining.com
An example he gives shows that that 1024 taps at 48kHz, used as linear phase with impulse centering, only has a frequency resolution of 95Hz. Fine for high frequency work, but clearly insufficient for low frequency work. Even if the 1024 tap FIR filter is used for IIR replication by moving impulse peak to the front of the filter, the resolution only improves to 47.5Hz....still insufficient, and no phase correction at all, so why bother...
IIR filters on the other hand, have theoretically infinite resolution.

You may/probably already know all this...i'm writing as much for general knowledge's sake as anything.
So lot's of taps (and delay) are needed for any kind of EQ resolution capable of handling full range....which is why FIR is really only applicable for live sound, maybe stating at around 500Hz ime.

The second issue with FIR, becomes more true as tap count increases when using any form of auto EQ.....it demands good data, good measurements, to keep from correcting what should not be corrected.  I personally believe, the only valid EQ corrections occur at the individual driver level, and are always IIR / minimum phase. Once drivers sum together in acoustic space, they quit acting as minimum phase, and auto EQ FIR will do a beautiful job correcting to a single point in space, but at the expense of everywhere else.
Which makes your use of IIR first, such a great strategy.

It took me a long time, learning how to make spatially average measurements that are suited for auto EQ...and still learning. i also must stress that this is at the individual driver section level.
I'd never apply FIR auto-EQ to an entire speaker, unless it really was for perfect sound for one small spot.

Where i think FIR and linear phase really shine, is when used for complementarity linear phase xovers.  All gain, no loss, if the delay can be tolerated.  Bye bye unnecessary xover induced phase rotation.  The jury is out on how audible that is, but if nothing else, lord knows it helps nailing down timing alignments between drivers. Getting flat phase traces to overlap is easy peasy.

I think the best tuning practice i know, is to first tune each driver section with IIR, flattening both in-band response, and out-of-band response through the critical summation region (i like to achieve that to at least -15dB for each driver).
Then apply FIR and complementarity linear phase xovers. (the complementary part is important, at least to measurements.  If not complementary, crap showing pre-ringing shows up in measurements away from the reference tuning axis.)
You can apply lin phase steeper xovers, to undo the out-of-band IIR flattening that could be potentially dangerous to the drivers lower end response. And the steeper filters also reduce the width of the critical summation range, reducing the task of out-of-band flattening. Best benefit of all, is by reducing the range of summation, lobing range goes goes down.  This all works (steep) simply because of the lack of phase rotation. I think it may be the biggest factor in why i like what i hear so much from my DIYs.

I get away with using firD's auto-EQ essentially imbedding the IIR EQ's into a linear phase file, because i do a lot of spatial averaging work to acquire a reference measurement i'm willing to auto-correct, and i use as the saying goes here, a metric shit ton, of taps.  16,384 at 48 kHz to be exact...170ms delay....not exactly good for live, huh?

Hope i've been of some help.

Helge A Bentsen:
Thanks, lot of good points :)

I ended up with this method by studying what manufacturers do in their tunings and looking a lot at supplementary responses.
Pretty sure my LF needs more refinement in the measured data I put in to FIR D, but I can't get outdoors to get better measurements right now (winter...).
Did a few measurement sessions before I was able to capture data that I could EQ in such a way that what I heard correlated to what I measured after EQing.

As an example, I tried a linear phase brick wall for LF/MF crossover, but it introduced some variables in the response off-axis.
Noticed that EAW uses Butterworth 24dB/oct for a speaker with similar xo points, so I tried that. It sums better off axis, good on axis, and I used a out of band EQ point to nudge it a bit into place.

I'm using a fairly short FIR filter (maybe too short?) filter delay of 200 samples and filter length of 400 samples @48K.
My LF/MF xo is right now at 600hz.
I think my next step is to load the HF FIR without auto mag and have a listen to it while I wait for warmer weather.



Chris Grimshaw:

--- Quote from: Mark Wilkinson on January 24, 2022, 09:12:46 AM ---First is the frequency resolution.  Like Pat Brown illustrates in this article you may have seen......   www.prosoundtraining.com
An example he gives shows that that 1024 taps at 48kHz, used as linear phase with impulse centering, only has a frequency resolution of 95Hz. Fine for high frequency work, but clearly insufficient for low frequency work. Even if the 1024 tap FIR filter is used for IIR replication by moving impulse peak to the front of the filter, the resolution only improves to 47.5Hz....still insufficient, and no phase correction at all, so why bother...
IIR filters on the other hand, have theoretically infinite resolution.

--- End quote ---

Mark,

What about using a larger number of taps, but moving the impulse centre to be nearer the start of the filter? Would that gain back the frequency resolution?

RePhase lets you choose the delay in terms of taps (number), delay (ms), or as a fraction of the filter length. ie, it can be chosen independently of the number of taps. Of course, filter performance will vary.

Chris

Mark Wilkinson:

Glad to have helped Helge, and hi Chris,

I'll try to reply to both of your posts, with regard to frequency resolution.
Helge, your filter with 400 samples @ 48k has a time span of 400/48000, or 0.0083 sec, or 8.33ms.
Using the F=1/T rule, 1/.0083 = 120Hz.  Since you are using the file as linear phase with impulse centering, effectively only 1/2 of the samples, 200 of them, make frequency response corrections.
So the resolution drops by half, to 240 Hz.  A rule of thumb Pat Brown gave in an article i tried to link, is that 3X the resolution of the FIR file, is about the lowest frequency we should try to EQ.   So your file becomes effective somewhere around/above 720Hz.
 
(i don't know why the SynAudCon article link got changed to a home page link, but the great article that explains all this is "FIR-ward Thinking Part 5 Are More Taps Better?")

And Chris, yes, exactly....... more taps and moving the impulse peak to the front of the file for IIR replication, increases frequency resolution (as also very well explained in the article)
I say 'IIR replication' because it is still not true IIR, which has theoretically infinite resolution.  It is 'IIR replication' with the resolution inherent in the size and structure of the FIR file.

So using my 16,384 taps @ 48k for example.  With linear phase and impulse centering i get a resolution of 8,192/48000 or about 6Hz which should be good for work down to 18Hz, or rather all i need ime/imo. 
If i found i needed more resolution for IIR replication, because of maybe a particular high-Q EQ that is needed, and doesn't quite replicate with impulse centered, I could move impulse peak closer to start....all the way to start for 3Hz resolution if desired..... or only as far toward start as needed, and continue to maintain a linear phase xover.

Hope that made sense! I'm not near as good an explainer as Mr Brown !

Helge A Bentsen:

--- Quote from: Mark Wilkinson on January 26, 2022, 08:39:03 AM ---Glad to have helped Helge, and hi Chris,

I'll try to reply to both of your posts, with regard to frequency resolution.
Helge, your filter with 400 samples @ 48k has a time span of 400/48000, or 0.0083 sec, or 8.33ms.
Using the F=1/T rule, 1/.0083 = 120Hz.  Since you are using the file as linear phase with impulse centering, effectively only 1/2 of the samples, 200 of them, make frequency response corrections.
So the resolution drops by half, to 240 Hz.  A rule of thumb Pat Brown gave in an article i tried to link, is that 3X the resolution of the FIR file, is about the lowest frequency we should try to EQ.   So your file becomes effective somewhere around/above 720Hz.
 
(i don't know why the SynAudCon article link got changed to a home page link, but the great article that explains all this is "FIR-ward Thinking Part 5 Are More Taps Better?")

And Chris, yes, exactly....... more taps and moving the impulse peak to the front of the file for IIR replication, increases frequency resolution (as also very well explained in the article)
I say 'IIR replication' because it is still not true IIR, which has theoretically infinite resolution.  It is 'IIR replication' with the resolution inherent in the size and structure of the FIR file.

So using my 16,384 taps @ 48k for example.  With linear phase and impulse centering i get a resolution of 8,192/48000 or about 6Hz which should be good for work down to 18Hz, or rather all i need ime/imo. 
If i found i needed more resolution for IIR replication, because of maybe a particular high-Q EQ that is needed, and doesn't quite replicate with impulse centered, I could move impulse peak closer to start....all the way to start for 3Hz resolution if desired..... or only as far toward start as needed, and continue to maintain a linear phase xover.

Hope that made sense! I'm not near as good an explainer as Mr Brown !

--- End quote ---

Thank you :)

This probably explains why I like the IIR filtered result better than some of my first versions with a lot of FIR EQ.

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