Different filter types : examples of uses

[pheeleep]

Curious about when you would typically use one filter type of another. I unfortunately don't have the time to research this.

"Filters include: Coincident, Butterworth, Chebyshev, Bessel, Critical, Legendre, Elliptic, Allpass types".

Thank you

[realmarco]

most people have never heard of what u just said. they only know what they like by using what is available.

i seriously doubt anyone except DSP programmers would be able to answer that question

[wesleyt]

The best way to learn is by doing. Ddmf's IIEQ Pro has a number of those; give them a try and come back to us with questions.

Also, wikipedia is faster than posting here.

Finally, Coursera has some great classes on sound design and production that cover EQ. Have a look.

[aciddose]

These are all different configurations of the same filter.

For example butterworth is set up to keep the pass-band maximally flat regardless of the transition width.

See here:
http://en.wikipedia.org/wiki/Butterworth_filter
http://en.wikipedia.org/wiki/Chebyshev_filter
http://en.wikipedia.org/wiki/Bessel_filter
http://en.wikipedia.org/wiki/Optimum_%22L%22_filter
http://en.wikipedia.org/wiki/Elliptic_filter

Coincident, critical and allpass are a little more complicated and the name alone doesn't give enough information to say exactly what sort of filter they are in the software you're using.

[nix808]

A good way to understand what you are dealing with in a filter type,
is to look at a frequency response graph.
This aids in learning the types.

I can help with Chebyshev.
They are very steep, but have ripples in the passband.

[eidenk]

Check out EngineersFilter plugin: http://www.rs-met.com/freebies.html

[Aroused by JarJar]

Curious about when you would typically use one filter type of another. I unfortunately don't have the time to research this.

"Filters include: Coincident, Butterworth, Chebyshev, Bessel, Critical, Legendre, Elliptic, Allpass types".

Thank you

You're referring to the DMG Equilibrium plugin, I assume. I'll try to give a very simple musician's introduction. There are a number of people here who can give you the reasons why these musical effects happen, and also explain why what I'm saying is just "rule of thumb for musicians".

Broadly and simply speaking, and from a musical point of view, Butterworth filters are just what you'd expect a classic non-resonant filter to be: they reduce highs or lows (both for a bandpass) and leave what's left over sounding almost unchanged. Legendre does the same but more steeply (on a LP the highs are going to be more reduced for example).

Chebyshev filters (I and II) can be much steeper (for example, you can slice lows right off with type I) but change the sound more because they "ripple". The steeper they get, the more they ripple. With a type I, for example if you have a steep lowpass it will cut off the highs above where the filter is set but it will emphasize the cutoff point itself, plus put little dips and valleys in the spectrum around that point. With type II you'll get ripples in the cuttoff band, so a steep lowpass for example will actually "leak" specific narrow bands of high frequencies. Chebyshev filters can be very musical and excellent for shaping sounds because these kinds of lumps-and-gaps are often characteristic of "natural" sounds. The same features that make them "musical" can also make the problematic: weird formant-like characteristics, a kind of whistling sound, funky phasing stuff that can make things "pale" or rob "punch", etc. You might find that you've made a nice "wooden" sound, but you might also find that you've created a killer sound color but "smallified" the sound.

Bessel and Elliptic are what you might call "scientific". They might be just what you need to slice out some very specific frequencies, but they can also tend toward sounding "hard" and if you get carried away, "harsh".

"Allpass filter" is too broad like aciddose said. Allpass filters alter phase and are something I've never used outside of reverberation and sound design/effects. No idea how they apply to an EQ! Have to try the demo of the DMG...

Someone else would have to tell you about "coincident" and "critical", because I don't know them at all and the kind of musician's-experience descriptions I'm giving aren't something I can look up on wikipedia or wherever.

I'm sure I'll get jumped on by someone, but these in my experience are fair generalizations.

[DaveGamble]

Heya,

I should probably jump in here to lend a hand

These are indeed largely engineering filters.

For a regular "audio" filter, use Coincident, or the DMG ones which offer resonance.

This is all covered in the manual here: http://dmgaudio.com/dl/DMGAudio%20EQuil ... Manual.pdf
in far more detail than is really interesting.

From an engineering perspective, designing filters is about wanting something, and being prepared to pay for it with something else.
Mostly, engineers designing filters want a LOT of attenuation.

Well, suppose you're not prepared to pay ANYTHING for your filter, you just want maximum attenuation. Well, that's a Butterworth. As steep as can be without any side-effects.

Ok, suppose you want an even sharper slope, and more cutoff, and you're prepared to pay with a little bit of ripple in the pass-band (i.e. you REALLY want to get rid of something, and you don't mind a little bit of wonky EQ over what you want to keep). Well, that's the Chebyshev (type I strictly).
Chebyshev type 2 allows you to have ripple in the stop-band, but is completely flat in the audio region. I didn't implement Cheb2 in EQuilibrium, and no-one has asked for it. Cheb1 is kinda musically interesting, Cheb2 is just a bit odd.

Ok, suppose you want the sharpest slope that can be achieved? You're prepared to pay with ripple EVERYWHERE across the spectrum, but you GOTTA have that slope! That's the Elliptic (or Cauer, or Equiripple) design.

Allowing, like, 0.1dB of ripple with a chebyshev1 or an elliptic can be a LOT sharper than a butterworth, but do bear in mind you will need 24dB/oct or higher to see any of this stuff. Try with a 48db/oct.

Ok, ok, who cares about sharpness of curve? What if you wanted a REALLY smooth phase response - no wobbles in the phase response. You're going to lose a lot of steepness, but this is the Bessel design. This is REALLY good for preserving phase relationships between parts of the spectrum.

Or what if you're obsessed with ripple? What if your impulse response MUST NOT ring?
That would be the "Critical" (for Critically damped) design.

Ok, suppose you don't need the filter shape to be flat, and you DO want sharpness, as long as it doesn't wobble up and down - only down! That's the Laguerre filter.

In audio, we don't usually go after ANY of these criterion - we want something smooth and predictable. That's the coincident design. As filter order goes up, we just add more filters at the same cutoff (and compensate the frequency to keep the -3dB point in the same place).

The DMG is a regular coincident design, with resonance. It covers about 98% of all filters you'll find in the wild, in audio gear. (Edit for accuracy: The DMG ditches the frequency compensation from the coincident, because that's more typical behaviour "out there". As you switch from 12->24 db/oct, you have a -6dB point at cutoff, rather than a -3dB.)

Harmonics is a set of resonant filters with a harmonic relationship. It's not based on hard science particularly, it's just characterful and fun.

The all-pass does NOTHING to the frequency magnitude spectrum at all. It's PURELY adjusting phase. With a high enough order, you can use it to push parts of the spectrum later (i.e. delay the bass, leave the tops roughly where they are). It's a frequency-selective delay. Except you won't hear much unless you add a few. Stick 4 48db/oct all-pass with Q about 3 on the graph at about the same frequency, and then drag them up and down the spectrum over the full mix. Very odd.

Hope this helps,

Dave.

[DaveClark]

Hi Dave,

The all-pass does NOTHING to the frequency spectrum at all. It's PURELY adjusting phase.

Would you please rephrase this?

As I'm sure you know, each frequency component is characterized by magnitude and phase. It's bad enough that many folks never even consider phase without implying as you have here that phase has nothing whatsoever to do with the frequency spectrum.

Thanks,
Dave Clark

[DaveGamble]

Hi Dave,

The all-pass does NOTHING to the frequency spectrum at all. It's PURELY adjusting phase.

Would you please rephrase this?

As I'm sure you know, each frequency component is characterized by magnitude and phase. It's bad enough that many folks never even consider phase without implying as you have here that phase has nothing whatsoever to do with the frequency spectrum.

Thanks,
Dave Clark

Edited.

[Oden]

Is this thread about equilibrium?

Because if it is, don't forget about the linear phase and FIR modes! You can even customize the length of the impulse and the windowing. Dave clearly went overboard with this one!

Reality is, you can't probably tell the difference. In normal use at least...

In mastering the filter with no overshoot could be useful (bessel). I like the bass to be a bit behind so use the one where the impulse response is skewed the most, coupled with allpass filter (Butterworth?). Want violin like frequency response wobbles? Elliptic and chebyshev are probably the best choices (I guess you should also shorten the FIR response to min and leave the rectangular window on).

The DMG/coincident filters are designed to be the most transparent all-rounders I guess. Dave any chance you could talk about these a bit more? Thanks for the lengthy post by the way, thumbs up!

[pheeleep]

most people have never heard of what u just said. they only know what they like by using what is available.

i seriously doubt anyone except DSP programmers would be able to answer that question

Hopefully this thread is also useful to others. The developer himself ended up providing a lot of the info I wanted. It's my lucky day

Thanks for the super detailed and useful info guys. A lot of useful info. Just what I was hoping for. Every post helped a bit, not just the 2 awesome novels.

Equilibrium, based on the little time I had reading up on it, seems absolutely awesome. I can't fork the money down for it, but cudos for coming up with such a great customizable design! I can see how it's the creme de la creme of EQ's. I was heartbroken when I researched that there are never sales. Sniffles.

But perhaps the price will go down one day, in which case I say "never say never".

I am under the impression that it's worth every penny, though.

[pheeleep]

Check out EngineersFilter plugin: http://www.rs-met.com/freebies.html

Yup, I have that one. Great recommendation. Also Rubber Filter (butterworth) has been in my arsenal for ages. I totally get it now. These are all surgical EQ's basically with different "depths" that affect the curve in different ways which can or can't be sideeffects depending on what type of sound you are going for.