IIR vs FIR: opinons?

[cuibono]

I've been designing some multi-way OB speakers for a while, and looking to push them past the finish line. I use a calibrated mic with SoundEasy for measurement, and do all my XO and EQ in the computer - specifically, with Thuneau's Allocator.

I believe Allocator is an IIR type digital filter - it acts similarly to analog components, being able to define the frequency, order and Q. I've gotten my design to the point where I am very happy with it - and then I realized FIR filters offer a whole other world of filtering. Particularly, I was looking at Acourate, and the ability to completely flatten and define the bandpass of a driver (for a given point in space). That could be very interesting.

So here is the conundrum - I had planned on reproducing my IIR filters from Allocator 'outside the box' with analog, active line level circuits. Removing the computer from the equation is very desirable in terms of ease of use. But I'm wondering if FIR filters are a big enough improvement to keep the computer involved?

In another thread, DSPgeek suggested that FIR filters are most useful for the mids and highs, while IIR filters are good enough for the bass (for a couple of reasons). Anyone else here with experience with both types of filters? I suppose if the mid and high drivers were inherently flat, there would be no need for FIR filters.

[panson_hk]

Originally Posted by cuibono

In another thread, DSPgeek suggested that FIR filters are most useful for the mids and highs, while IIR filters are good enough for the bass (for a couple of reasons).

Can you direct me to the referred thread? Thank you.

[cuibono]

Enjoy: http://www.diyaudio.com/forums/showp...7&postcount=33

[DSP_Geek]

<SFX: cloud of sulfurous smoke>

You rang?

Actually, what I said was that IIRs are good for room EQ, since rooms tend to show minimum-phase response characteristics. HOWEVER, this does not mean EQ will make any lousy room sound wonderful. You still need to work on making a useful listening environment before doing anything else, just as you want to build a good speaker before trying EQ, otherwise you'd might as well join Bose.

And after two years of listening at various Burning Mans, I'm more convinced than ever that linear phase in the low->mid crossover is essential to get the last bit of fidelity from a system. At BM1, vocals through just about every full-range system were compelling enough that I had to drop a conversation and wander into the listening room, which I didn't expect at all because most of my experience is with large multiway systems. At BM2, Iain McNeill showed a system with linear phase, then (with the same frequency response!) 8th order allpass phase response. Percussion, particularly snare drums, through the former sounded more solid and less vague, less polite, than through the latter. I heard little difference with crossover frequencies above 2K, but I may have not known what to listen for.

I would be careful of using extreme FIR slopes, however, since they do tend to generate ringing. Of course, the ringing is negated by the converse ringing from the other driver, but that only applies on axis since differential time delays could make the off axis sum actually have worse ringing that from one filter alone. Moreover, steeper slopes also mean longer delays, which can become a factor if you want to watch a movie.

[cuibono]

Hmmm, well, it sounds like linear phase might be worth pursuing...

Thanks!!

[catapult]

If you're looking at FIR filters because of the linear-phase crossovers, you should already have that with the Thuneau package. Frequency Allocator does the crossovers and EQ with IIR filters and Phase Arbitrator unwraps the phase with inverse allpass filters. Some would argue that that's a better approach because it doesn't have the pre-ringing in the time domain that constant-delay (linear phase) FIR filters introduce. It's maybe a moot point if the FIR filters aren't too steep but the thing is you already have software that does what you're looking for -- square wave in, square wave out. The bad news, you can't do either approach with analog filters, it's gotta be a digital box.

http://www.thuneau.com/arbitrator.htm

[catapult]

The good news, you can switch Phase Arbitrator in and out and decide if it's worth it. If you can't hear any real difference, you can go ahead and reproduce your Allocator settings with analog components.

[cuibono]

Originally Posted by catapult

...but the thing is you already have software that does what you're looking for...

http://www.thuneau.com/arbitrator.htm

Sometimes it amazes me what I don't know (guess I forgot)

The other thing that is impressive with FIR filters (or just Acourate?) is the ability to define a bandpass and flatten the response within it, via convolution of an impulse response. Does that impress anyone else, or is linear phase the main claim to fame for FIR?

[catapult]

or is linear phase the main claim to fame for FIR?

Yeah, I think so. If you get too carried away with inverting the measurement, you end up 'correcting' measurement artifacts that should be left alone.

[breez]

Originally Posted by catapult

Yeah, I think so. If you get too carried away with inverting the measurement, you end up 'correcting' measurement artifacts that should be left alone.

Yep, that's why equalizing loudspeaker drivers is best done using broad, gentle minimum-phase filters rather than straight inversion. You just never know what's up with the data in compromised measurement conditions (living room pseudo-anechoic measurement). Small ripples are best left alone.

On the topic of DSP I find impulse response data very useful. Make one good set of measurements and you can play around with those tweaking the crossover and never need to measure again.

Regarding FIR filters and off-axis ringing, there was a perceptual study in a recent AES journal which should prove quite valuable to anyone interested in FIR filters for loudspeaker crossovers. While the article costs money you can download the more extensive Master's Thesis which it is based on, here: http://lib.tkk.fi/Dipl/list.html (Korhola, Henri: Perceptual Study of Loudspeaker Crossover Filters.)