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Author Topic: No FIR  (Read 6620 times)

Art Welter

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Re: No FIR
« Reply #10 on: April 20, 2018, 01:30:21 PM »

The only real disadvantage IMO, is latency.
 
I've kinda come up with a rule of thumb for effective FIR that goes: latency = 125% of the period of the lowest frequency you want to adjust.
So for example, if you wanted to use FIR down to 500Hz, which has a period of 2ms, it will probably take about 2.5ms of FIR time.
This rule of thumb works for up to about 48dB oct slopes. Less slope gets away with less FIR time, greater slope of course needs more.

I mention this rule of thumb because it might help to evaluate what FIR can actually be doing, given published latency specs.....
Maybe someone will knock down my formula, happy to learn a better one, or that I'm full of it !
Mark,

In the example of the SH100 compared to the HD1521 above, the HD1521 phase is basically flat to 150 Hz.
The HD1521 had only 2.87ms more latency than the SH100, 9.35ms compared to 6.48 ms.

From what I've seen of the latency in typical FIR use, taking it down to 100 Hz or so still would be less than time of flight from the back line to the usual down stage PA location.

Art
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Mark Wilkinson

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Re: No FIR
« Reply #11 on: April 20, 2018, 02:33:59 PM »

Mark,

In the example of the SH100 compared to the HD1521 above, the HD1521 phase is basically flat to 150 Hz.
The HD1521 had only 2.87ms more latency than the SH100, 9.35ms compared to 6.48 ms.

From what I've seen of the latency in typical FIR use, taking it down to 100 Hz or so still would be less than time of flight from the back line to the usual down stage PA location.

Art

Hi Art,  not to quibble, but I'd say phase is holding to maybe 300Hz. 
The breakup below 300 looks like maybe they were trying to push too low....

Also, my formula is against time zero..... I'm not sure if anything needs to be accounted for in your box to box comparison.
I should add the formula doesn't have anything to do with fixed DSP processing time. (Which seems a little high on those boxes.)

The best way I've found to get a feel for FIR vs frequency vs latency, is to grab freeware rePhase or purchase FirDesigner, and watch error from ideal grow as you cut taps , lower frequency, or steepen filters.
 
I've found their error simulations correlate well with an audible 'something ain't right'
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John L Nobile

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Re: No FIR
« Reply #12 on: April 20, 2018, 02:40:10 PM »

Here is a photo of a 3 way SH50 (on the left) and a 2way 8" coax (on the right)

They appear larger in real life. I took the back off my SH96HO and was pleasantly surprised by the size and layout of the xover in there.

Not what I'm used to seeing in a passive xover.
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Dave Guilford

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Re: No FIR
« Reply #13 on: April 21, 2018, 08:35:19 AM »

So I just upgraded to itech5000 for my Srx728/725 rig. 

If I’m using the built in processing for xover, should I switch to FIR / FII filters?  Currently using what jbl recommended, which was BW and LR
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David Sturzenbecher

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Re: No FIR
« Reply #14 on: April 21, 2018, 12:14:13 PM »

So I just upgraded to itech5000 for my Srx728/725 rig. 

If I’m using the built in processing for xover, should I switch to FIR / FII filters?  Currently using what jbl recommended, which was BW and LR

Use what JBL recommends unless you have the tools and chops to measure the result of implementing the FIRs.


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Tim McCulloch

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Re: No FIR
« Reply #15 on: April 21, 2018, 02:25:23 PM »

So I just upgraded to itech5000 for my Srx728/725 rig. 

If I’m using the built in processing for xover, should I switch to FIR / FII filters?  Currently using what jbl recommended, which was BW and LR

The processing for the SRX700 series extends only to V4, which is pre-FIR (based on dbx DR4800; V5 tunings use the BSS London processing in the HD amps).  I do not know if It doesn't appear JBL ever created V5 tunings for the SRX700 series.  Contact JBL and ask.

EDIT additions - device files that I can find in Audio Architect 2,15 for SRX700 series are for XTi and XTi2 amplifiers only.  I *thought* I remembered seeing device files for the original I-Techs, too, but so far I'm not finding them. V4 passive files (tops over subs) and V5 bi-amp files exist for the STX800s in I-Tech HD.

Rolling your own FIR filters is not impossible but it's not the relatively simple exercise of IIR filters.
« Last Edit: April 21, 2018, 03:28:00 PM by Tim McCulloch »
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Art Welter

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Re: No FIR
« Reply #16 on: April 21, 2018, 03:03:22 PM »


Also, my formula is against time zero..... I'm not sure if anything needs to be accounted for in your box to box comparison.
I should add the formula doesn't have anything to do with fixed DSP processing time. (Which seems a little high on those boxes.)
The total latency included the same 2 meter time of flight for each cabinet, the SH100 was powered by an analog amp with no DSP, so the HD1521 FIR and DSP processing time combined was 2.87ms longer than the passive crossover used in the SH100.
 
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David Sturzenbecher

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Re: No FIR
« Reply #17 on: April 21, 2018, 03:21:57 PM »



Rolling your own FIR filters is not impossible but it's not the relatively simple exercise of IIR filters.

I believe the poster you quoted is referencing the non-coefficient based linear phase FIR crossovers found natively inside the iTechHD. These are just as simple to apply as the IIR crossovers, as it is just a different pull down.

The manufactures delay settings will almost absolutely be wrong though if you change JBLs recommended crossover to the stock iTechHD FIR crossovers.



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Mark Wilkinson

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Re: No FIR
« Reply #18 on: April 22, 2018, 08:25:15 AM »

The total latency included the same 2 meter time of flight for each cabinet, the SH100 was powered by an analog amp with no DSP, so the HD1521 FIR and DSP processing time combined was 2.87ms longer than the passive crossover used in the SH100.
 

Hi Art, first let me apologize.....I never caught you were comparing such different loudspeakers, the Mackie HD1521 vs the old Danley SH-100.
The idea of comparing two such different speakers to make a case for FIR seems impossible to me...and i would have said so from the beginning.
(I quickly read/thought the SH-100 was a very similar Mackie model without FIR.)

Who can divide up what gives the HD1521 it's traces?  IMO, without a passive model, or a model with conventional DSP, its impossible to say what incremental mag and phase correction, FIR might have made.

I looked up the manual of the HD1521 and was impressed !  Mackie touts the 'acoustic corrections algorithms' developed by EAW. This was before the days of touting FIR I guess?  Anyway, what I've read of Dave G's work, suggests the alorithms were probably used for knocking out horn reflections, and may not have had anything to due with additional phase flattening, other than using embeded IIR eq's like I mentioned earlier.  Just speculating....   
Oh, and Mackie even shows a measured max SPL 10dB lower than calculated...I'm IMPRESSED !
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Mark Wilkinson

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Re: No FIR
« Reply #19 on: April 22, 2018, 08:40:26 AM »

I-TechHD's..........I read in a brochure that they have 3000 taps available.
 
Is it 3000 taps per channel, or total to be allocated between channels?

Does Audio Architect allow 3rd party FIR files to be put into the IT-HD's?  (ie .wav or .bin)

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Re: No FIR
« Reply #19 on: April 22, 2018, 08:40:26 AM »


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