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What is a FIR filter?


Coytee

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In practical terms, what is a FIR filter in an active crossover?

 

What does it do?

 

What does it do that an active without a FIR filter can't do?

 

It seems that it's a bell (or whistle) in the feature department....  is it worth the extra cost?

 

(I've just done a google on it but it's still a bit over my head.

 

" In signal processing, a finite impulse response (FIR) filter is a filter whose impulse response (or response to any finite length input) is of finite duration, because it settles to zero in finite time. "

 

 

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So you know the story on the first one....  we had some kind of (candy) sucker.  It was a round lolly-pop on one end and the lips on the other end.

 

Gave it to Hattie as a lark...  and just like an infant, she took right to it, sucking on it.  It was pretty funny.  Tried it with the other dogs and they didn't 'get' it...and it just fell out of their mouths.

 

Once the pictures were taken, we took it away (didn't feel comfortable with her having a sugar sucker)

 

 

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To answer your question fully would take a long time and a lot of math that would not really help you understand. What is really important is, "What can a FIR filter do that an IIR (infinite impulse response -- the other kind) cannot?"

 

I will start by saying that, in general, all analog filters (those made up of capacitors, inductors, and resistors) are IIR (infinite impulse response) filters. (There are exceptions, but they are very rare and exotic.) And many digital filters are IIR. All IIR filters cause phase shift that changes with frequency.

 

FIR (finite impulse response) filters, though, can trade phase shift for pure delay. That is to say, an FIR filter can be configured so that the only thing that it does, other than changing the frequency response magnitude, is delay the signal by a small amount. This is called a "linear phase" filter, for mathematical reasons that are not important here. There are circumstances where a pure delay in the signal is preferable to a phase shift in the signal.

 

With the exception of the very rare and exotic analog filters mentioned previously, FIR filters must be digital. And they require much more processing power to implement than IIR digital filters. Oh, also, not all FIR filters are linear-phase -- they have to be designed that way.

 

Which is better? That's like saying, "Which is better, a Corvette or a Jeep?" The answer depends upon the context. The Corvette will smoke the Jeep on the road, but the Jeep will smoke the Corvette off the road. Just as each vehicle must be chosen according to its intended use, so must FIR or IIR filters.

 

In general, FIR filters can make some very interesting and exotic crossover networks that IIR filters cannot. They also make some interesting-sounding EQ filters that some people prefer while others do not. On the other hand, IIR filters can perfectly compensate for some real world minimum-phase problems that linear phase FIR filters cannot. Choose the right tool for the job.

 

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Back to FIR filter...

 

"Job at hand" would be potentially replacing the Dx38 on the Jubilee's in a residential application.

 

If it matters....  using some Crown K2's and currently, the TAD-4002 in lieu of the Klipsch K69 driver.  (still have the 69's though)

 

Looking at the Xilica after reading some positive comments from Chris.

 

When I went to the site, I notice they have two versions of the same thing...one with FIR filters and a digital input and the other, without.

 

Being a plug & play kinda guy....I need someone to point me into "this" direction and say "go that way".

 

Once I get a choice of something (to Fir or not to Fir) I start to wonder....debate....and try to learn about it so I can make an informed decision.

 

(paralysis by analysis)

 

 

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3 minutes ago, Coytee said:

Being a plug & play kinda guy....I need someone to point me into "this" direction and say "go that way".

 

 

I'm torn. On the one hand, I am an electrical engineer who specializes in signal processing. On the other hand, I am an audiophile.

 

My engineer side wants to analyze your application and arrive at an optimal solution. My audiophile side wants you to listen to both and decide which sounds better to you.

 

Listen to my audiophile side. He makes better sense. :)

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8 minutes ago, Coytee said:

XP verses XD (XD has FIR and XP doesn't)

 

http://www.xilica.com.au/products/xd-xp-series/

 

 

 

It looks like the only FIR filters in the XD are crossovers. All of the other filters are (traditional) IIR.

I have (negative) opinions about brick-wall filters that are not necessarily shared by others. Again, only you can decide for yourself whether you like the sound. I don't know what the price difference is between the XD and the XP. If it's small, then it might be worth a few bucks just to experiment.

 

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I'll admit....  I'm not even sure I know how to ask (phrase) this question....  I say that because it seems to me that to be able to phrase a question intelligently.....one must know part of the answer...

 

I'll try.

 

You say the FIR filters are in the crossover.  Is that to say that they are invisible to the end user?  Meaning, if someone selects a L/R crossover for example, then irrespective of the unit (Electrovoice DX or Xilica unit WITHOUT FIR or Xilica unit WITH FIR) that you look on the dial for L/R and dial in your slope....the FIR architecture is invisible to the end user? (there is no FIR function to 'select' over & above the other, every day parameters....  like, I want a FIR function of 2.52 on the bass bins)

 

If I could try to use the car analogy....  both the Corvette and Jeep use pistons but, one might use a cast piston and the other might use a forged....  so they both have an item that does the same thing and to a person ignorant of cast/forged they might appear to be the same thing, (a piston is a piston, a crossover is a crossover) they both still have different qualities and benefits relative to each other.  (this is a IIR crossover and this is a FIR crossover??)

 

 

 

 

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6 hours ago, Coytee said:

You say the FIR filters are in the crossover.  Is that to say that they are invisible to the end user?

 

Richard,

 

Basically the answer to your question is "no".  To use the FIR crossover filters in an XD series Xilica loudspeaker processor (which doubles the price over the XP series, BTW), you'd have to use something like REW and a calibration microphone to enable the process of creating the digital filter parameters--which consists of many numbers in an array, each of which are called "taps" like taps in a transformer. 

 

If you're willing to pay the sticker price and go down that path, then you will be rewarded with loudspeaker performance that has not only controlled on-axis frequency response, but to a degree, controlled phase response.  The price that you pay--in addition to the added sticker price--is added delay.  Note that the EQ filters in the Xilica XD series processors are not FIR filters, but rather the type of EQ filters that you're familiar with, and that create by-product phase shifts when used.

 

There are some freeware applications that run on PCs/Macs/Linux OS computers that provide that FIR EQ filter capability upstream of your preamplifier, but they also require REW or something similar to measure in-room response of each loudspeaker in order to correct each loudspeaker's phase and SPL response--measured at some point in your room, usually about a metre in front of the loudspeaker, but it could also be measurements taken outside semi-anechoically.

 

Chris

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14 minutes ago, Chris A said:

The price that you pay--in addition to the added sticker price--is added delay.

 

Latency?

 

I only glanced at pricing....  I thought I saw only a couple hundred difference.

 

To see if I understand some of your above....

 

How to word this...  in the Dx for example, there are pre-defined parameters.  Would this stream of numbers, be in essence, creating that parameter rather than using the preconfigured one?  Thereby giving you a much more 'fitted' and customized parameter than the canned version?

 

 

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22 minutes ago, Coytee said:

Would this stream of numbers, be in essence, creating that parameter rather than using the preconfigured one?  Thereby giving you a much more 'fitted' and customized parameter than the canned version?

In a way, yes.  But note that FIR filters are different than IIR filters, the type that you've been using on digital crossovers for about decade. 

 

FIR filters work on controlled delay line response rather than on an algorithmic formula that repeats every time step.  That's a gross simplification, but one that is useful to think about how IIR and FIR digital filtering processes differ.

 

Note that passive (analog) filters used in passive/balancing networks are the analog equivalent of digital IIR filters.  IIR filters are the ones with men's names attached to them: Bessel, Butterworth, Linkwitz-Riley, etc., and come in both analog and digital formats.  For audio, FIR filters are digital filters  and custom-made for every application.  There are some applications of analog FIR filters, but they are practically in the high frequency RF domain only.

 

 

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FIRs can do many functions "at once". Amplitude and phase response can be adjusted independently, crossover points, EQ, delays and other adjustments can be written into one program. The tradeoff is pre-ringing on impulse response measurements. There are techniques for minimizing these artifacts. Here is a company that sells a program for writing FIRs:

 

https://eclipseaudio.com.au/fir-designer/

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It is a gross simplification, but here goes: IIR filters (the familiar Bessel/Butterworth/Linkwitz-Riley/PEQ filters, and others like them), because of the way that they are structured, follow certain "rules" for the relationship between magnitude and phase that in many ways mimic what happens with real things in the real world. FIR filters don't have to adhere to those rules. I often call FIR filter design "fun with numbers", because FIR filters offer independent control over magnitude, phase, delay, and/or impulse response (down to some fundamental relationships that cannot be violated under any circumstances).

 

The price for having this control is responsibility for using it wisely. A filter can be designed with just about any arbitrary response you can imagine, but that doesn't mean the response that you imagine is a good one.

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Okay... I have been looking online for an active also, and I think I want to delve into this "Active" XO while Bi-Ampping a three way (la Scala).

couple questions.

can I give this a try for around $300<?

I assume if I am bi-amping a three way that I am still using the factory Passive XO for the HF section?

Will I be able to play with driver alignment at that price point?

Do I still need a mic and computer to run REW?

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3 hours ago, Schu said:

can I give this a try for around $300<?

I assume if I am bi-amping a three way that I am still using the factory Passive XO for the HF section?

Will I be able to play with driver alignment at that price point?

Do I still need a mic and computer to run REW?

 

You might find something for $300, but why would you want to? Folks think nothing of spending thousands for a preamp, but want a crossover for cheap. The crossover's job is a whole lot harder.

 

You are better-off biamping between the woofer and the mid/tweet than you are biamping between the woofer/mid and the tweet. A direct connection between the woofer and its amplifier will bring a noticeable improvement.

 

Just about any digital crossover will let you play with driver alignment delay. Delay comes for free in digital.

 

You don't need a mic and computer, but it helps.

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