Talk:Linear filter

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Wouldn't it be nice to have some circuit diagrams for each type of filter?

The high-pass filter entry describes the 'simplest' circuit, but perhaps someone could illustrate it?

I can make circuit diagrams for a few cases if we want. Which cases to show, though? First-order LP HP BP BS? I made an article for Butterworth filter. Chebyshev filter already has a short article. I'm not sure if they should each have their own article or if they should just be combined into this one. How much detail should we go into? - Omegatron

Contents

[edit] This could use expansion

Each type of filter should have their own article with equations, schematics, pole-zero plots, etc. But I am too lazy/busy to do it.  :-) I started Butterworth filter. We need articles for:

- Omegatron 14:55, Jul 27, 2004 (UTC)

[edit] Digital signal processing

I started some cleanup. The section looked rather out-of-date and did describe only one implementation method of one algorithm. The specialised article Digital filter should give the details, but this will some work. -- Pjacobi 16:44, 5 Aug 2004 (UTC)

[edit] No treatment of complex analysis for filter design

This is a very important field that has been completely missed out from this article. Also, the z-transform for discrete-time filters.

[edit] Ideal filters

Ok, we can't filter something ideally (rectangular frequency response) because the corresponding convolution in the time domain would require prediction of the future, right? But what if we already know the entire pre-recorded signal? (Like a piece of audio surrounded by zeros to infinity) Then can we apply an ideal filter? This occurred to me because of FFT filtering, but that is not perfectly ideal. It is limited to the effective bandwidth of the FFT bins, right? - Omegatron 18:24, Jan 26, 2005 (UTC)

Ideal filtering would require infinite (and no less) length to filter upon (both data and the time-domain sinc function). You can better approximate it by lengthening the filter but at some point your coefficients will be less than the quantization resulting in filtering of zeros. Cburnett 20:06, 26 Jan 2005 (UTC)
I was just talking mathematically/theoretically. So it can be done realistically, too? Neat. - Omegatron 20:14, Jan 26, 2005 (UTC)

[edit] The title is unfortunate

Filters are not confined to electronics. For instance, one might have a software filter that does the same thing. This article should have a title that reflects the broad field of signal processing. --Smack (talk) 05:11, 4 May 2005 (UTC)

Well, that is why we have the more generic Category:Filters to accommodate things like Mail filter. I suppose if we get more software-filter related articles we could create a separate category for them too.--Hooperbloob 05:17, 4 May 2005 (UTC)

Sorry if I was unclear, but that's not at all what I meant to say. The discipline of signal processing originated in electronics, but like a creature from one of M. C. Escher's tesselations, it has broken free of the matrix where it was spawned and become a separate entity. At any rate, that's what they tell us at Berkeley, though I understand that it's a bit of an innovation and that not everyone sees things this way.

We define a "system" as an abstract object that operates on signals. The set of filters is a loosely defined subset of the set of systems. It's completely abstract, so much so that the topic almost becomes a branch of mathematics. As a result, it's implementation-independent. For instance, consider a simple lowpass filter. It can be implemented electrically (for instance, as a capacitor to ground), in software (in any number of sophisticated ways), or even mechanically (for instance, as a damped oscillator). From this perspective, it's parochial to speak of electronic filters as anything other than a special case. --Smack (talk) 04:25, 6 May 2005 (UTC)

OK, I see where you're coming from. Yes, filters can be implemented in a variety of forms/mediums from dampners to DSPs and I'm open to suggestions if you can come up with an alternate naming scheme. Articles about the electronic signal ones seem to be in the majority at the moment though.--Hooperbloob 05:16, 6 May 2005 (UTC)

I don't get the issue here. This article is specifically about electronic filters, not all filters. So you're intentionally making this article to be larger than what it is. The intro makes it very clear to me that this article applies to electronic signals and, last I knew, sequences of values in a DSP constitutes a signal.......otherwise I need to hand back my MS in DSP.

So what *specifically* is your "complaint"? That an article on electronic filters doesn't cover mechanical filters???? Cburnett 07:36, May 6, 2005 (UTC)

That is not my complaint, though I will admit that the difference between my meaning and your interpretation is subtle. This is not actually an article on electronic filters. It's an article on filters, with no qualifying adjective, as abstract and implementation-independent objects, that claims to be an article on electronic filters specifically.
Now, you ask, shouldn't there be an article on electronic filters? I say that yes, indeed there should, but its content should be limited to details of implementation. Much of what is written here is applicable to filters in general, and much of the remainder also has a place here as a cursory survey of the technology. --Smack (talk) 04:57, 7 May 2005 (UTC)
Excellent point. Specifically the Frequency response and Mathematics of filter design. Perhaps filter needs to become an article covering filtering in general (chemistry, optics, computer/email/whatever, electronic, etc. Basically that filtering is a process of removing unwanted information. Cburnett 06:21, May 7, 2005 (UTC)
Perhaps there should be some general description at Filter, but I believe that that article should still be primarily disambiguatory in nature. IMO, most of the distinctions it draws are valid. Filters that act on real-number signals, or on digital signals representing real numbers, share some common ground with signals that act on "data streams" such as email, but I'd say that even these two categories are distinct. The unification that I propose is limited to signal processing, where a signal is a function mapping one numerical set (time, position, etc.) to another. --Smack (talk) 05:23, 8 May 2005 (UTC)
I guess I'm confused as to exactly what you are proposing here.... Care to make a bulleted list or something? Cburnett 06:48, May 8, 2005 (UTC)
I propose to generalize signal-processing articles to eliminate bias toward electronics. However, I believe that filters from outside the realm of signal processing are fundamentally unrelated and can be left alone. --Smack (talk) 01:42, 9 May 2005 (UTC)
Still not very clear to me. First, you're saying that signal filters in software...are not electronic? Secondly, what would you call this if not "electronic filter"? Cburnett 04:55, May 9, 2005 (UTC)
Carriage return.
←

First, I wasn't referring to all signal filters in software. An image-processing filter, for example, is a perfectly good filter from the realm of signal processing. In particular, many interesting signal filters are LTI, or can be (blurrers and color shifters, to name a few). I wanted to distinguish these kinds of software filters, which operate on streams of numbers, from software filters that operate on streams of data and perform semantic analysis. Yes, the latter can be said to lie within the field of signal processing because of the trivial fact that they process signals, but (as far as I can tell) they yield to none of the powerful analytical techniques of signal processing. An email spam filter doesn't have a frequency response. We can model it as a state machine, but I don't see how that would shed any light on the sophisticated algorithms governing its operation.

Furthermore, I want to remove the intellectual barrier that separates the former variety of software filters from electronic filters. Consider a simple causal blur filter, such that its impluse response is a decaying exponential. We can implement this as a computer program, or we can implement it as a lossy integrator. From an abstract standpoint, they're exactly the same.

My complaint is not just that the title of this article is poorly chosen. My complaint is that it fails to make the leap of abstraction that unifies all signal-processing devices - irrespective of implementation - within the analytical framework of information theory. --Smack (talk) 04:02, 10 May 2005 (UTC)

Perhaps signal filter might be a better name? Or linear filter for the very common case of such filters? -- The Anome 08:46, May 10, 2005 (UTC)
OK, the article is now called linear filter -- however, the category it belongs to is still called Category:Electronic filters. We should probably have two new categories: Category:Linear filters and Category:Non-linear filters, with both being sub-categories of Category:Filter theory, but I haven't the time to make the changes right now. -- The Anome 08:58, May 10, 2005 (UTC)

[edit] Article refactored into two

I've now refactored the article completely into two articles: linear filter (this one) and electronic filter (the technology bits removed from this one, with an added intro). We should now address the glaring lack of detailed discussion of the details of pole/zero filter design theory. -- The Anome 10:46, May 10, 2005 (UTC)

I think Filter (signal processing) or Filter (information theory) would be better choices, though each is a bit of a mouthful. --Smack (talk) 04:58, 11 May 2005 (UTC)
Umm, there are non-linear filters so I can't agree with the current split.... I also think electronic filter should go to filter (signal processing). Cburnett 06:07, May 11, 2005 (UTC)
Ok, so I just found non-linear filter. I think we need just a filter article since the basic concept of filtering is not unique to linear or non-linear. And electronic filter doesn't cut it as Smach pointed out above. Cburnett 06:15, May 11, 2005 (UTC)

[edit] Propose merging "Analogue filter" into this article

The Analogue filter article has been a stub for a long time and does not contain any significant information that is not here. Analogue filter/linear filter seem to be the same beast to me. A non-linear analogue filter might be theoretically possible but I don't know of any actual examples. The non-linear filter article seems to be describing signal processing of a different sort altogether from frequency filtering. I don't see the analogue filter article ever going anywhere so we may as well merge. SpinningSpark 23:39, 8 May 2008 (UTC)

Linear filter describes any filter which is linear, which includes non-analogue filters (i.e. digital filters). And as you've already pointed out, there's nothing that says that an analogue filter has to be linear (although in the vast majority of cases, it will be). Therefore, I would be tempted to avoid this merge. Oli Filth(talk) 00:04, 9 May 2008 (UTC)
It's about time that somebody did something with it then. The trouble as I see it, is that any expansion of that article would mostly be duplicating information here and the information here already has a large amount of overlap with Electronic filter. This is because most of the material is on linear analogue filters (for which there is no article - it re). Any suggestions on the way forward? Could just leave it alone, but I don't really like the idea of an article that is going to be permanently a stub. SpinningSpark 09:25, 9 May 2008 (UTC)
Good question. IMO, this article needs some serious work. There are several mistakes, but more importantly, the majority of the article is actually discussing LTI filters, and not linear filters in general (for instance, the concepts of transfer function, impulse response and frequency response really only make sense if the filter is time-invariant). I'll see about rectifying this over the weekend.
The overriding problem here is that the set of filter articles on Wiki seem to be very confused. We have Linear filter, Electronic filter, Digital filter, Analogue filter, FIR filter, IIR filter (and probably others) that all confuse and overlap various aspects of linear vs. non-linear, time-variant vs time-invariant, analogue vs. digital, continuous-time vs. discrete-time. I'm not sure how best to tackle this problem. Oli Filth(talk) 19:01, 9 May 2008 (UTC)
I agree, it is very confused. My immediate concern is that I am in the process of writing a series of articles on image designed filters and was unsure how to dove-tail these in to the existing material. However, it has soon become apparent that there are bigger problems than that. Starting from the top, the first problem as I see it is that there is no top-level article to help navigate around the rest. Filter is a disambiguation page covering a lot more than our meaning of filter so the first question is what should such a page be called. Do you think that we should start a filter project page (or maybe signal processing project page) to co-ordinate and discuss these activities? SpinningSpark 19:23, 9 May 2008 (UTC)