[PD] bandpass or resonant?

Alexandre Torres Porres porres at gmail.com
Tue Jan 13 03:09:17 CET 2015

Nice I give an impression to be an expert, but filters is just something
I've actually recently started studying :)

> I'm wondering if by "resonant" filter you mean the
> same thing as "resonator" filter?

Now you got me... good question, and I'm not sure, haha. The link looks
nice btw, will definitely check it. Thanks.

So now I'm even more confused. Is "resonant filter" and "resonator" two
different concepts? Maybe I'm having trouble with the english nomenclature
and everything.

To be honest and more detailed about the issues I'm encountering, I ask
this based on another topic I was discussing with Julius Smith in the
Supercollider list, but it went dead and I got no replies. In it I was
asking if the object "Resonz" should really be called a "Resonant filter",
because it was just a bandpass filter in my opinion. Then Julius was
mentioning how "*A resonator is a special case of a passband filter having
a nearly zero-width passband.*"

I see he used the term "resonator" and not "Resonant Filter" (as Resonz is
described). So yeah, now I'm more confused... is resonator the same as
resonant or what?

But anyway, we can bring the discussion into the Pd world, and talk about
the [reson~] object, as I will do later on.

I was googling and saw how the term resonant filter could be used to
describe a regular bandpass filter. And how the bandpass' center frequency
could also be called "resonant frequency". So they might be used in the
same way... (accurately or not).

Now here is my opinion. Just like a resonant low pass filter (the [lores~]
object in Pd), the concept of resonance in a filter relates to how it adds
gain around the resonant frequency.

In the Audio-EQ-Cookbook (link:
http://www.musicdsp.org/files/Audio-EQ-Cookbook.txt ) that presents
formulas for biquad coeficients you have two different bandpass filters,
lets call them BPF1 and BPF2. So, BPF2 has "constant 0 dB peak gain",
meaning it doesn't affect anything arounf the center frequency. Now BPF1
says it has "constant skirt gain, peak gain = Q", meaning that the Q or
bandwidth controls the gain of the filter. I consider BPF2 to be a regular
bandpass filter, whereas BPF1, which adds gain for narrower bandwidths,
seems to be a "resonant" one... (which makes me think Resonz shouldn't be
described as resonant filter, as it's just a bandpass, or "BPF2").

Oh, there's another term around, the "ringing" filter, which seems to be
another term for resonant filter. In SuperCollider they have Ringz, which
was supposed to be the same as Resonz object (or a resonant filter for that
matter), but they are different like the two different kinds of bandpass in
the EQ Cookbook (Ringz = BPF1 / Resonz = BPF2).

Coming into the Pd world we can talk about the [reson~] object. As the name
implies, it is a resonant filter. But the helpfile says it is a "Bandpass
filter" (damn). Funny enough, in Max, the [reson~] object is said to be
indded a *Resonant Bandpass Filter*. So maybe we should update [reson~]'s
help file in Pd... But the deal is: [reson~] is actually a bandpass like
BFP2 or Resonz, but it has a separate parameter for the gain. Meaning it
works basically as a bandpass filter, where changing the Q doesn't affect
the gain. But you can also give it a boost or a cut with the gain
parameter. By giving it a boost it would behave in a way that I'm
considering to be an actual "resonant" filter.

Now let me go ahead and share a patch that I'm writing for my computer
music classes. It's about several filters that can be obtained with biquad.
So I present Pd's vanilla filters such as [lop~], [hip~] and [bp~]. I also
present externals like [lores~] and [reson~] and I do present all the
filters from the Audio Eq Cookbook as well. It's in portuguese, and part of
a big series of examples, but what the hell...

By the way, I was also able to implement Resonz and Ringz as [biquad~] in
Pd, but I don't have it on this example (but to hell with supercollider
already, hehe).

So there you can check the behaviour and differences that I've pointed. For
[reson~], I have two separate parameters, one is for "Q", and the other
one, which is originally "gain", I'm calling the "resonance" parameter.
Much like the resonance parameter in [lores~]...

As for the Cookbook filters, BPF2 is what I'm calling a regular "bandpass",
and it has a parameter of "Q". As for BPF1, I'm calling it a "resonant
filter", and it has a parameter of "resonance" instead of "Q" (but the idea
is that more Q gives more gain/resonance).

I hope I'm clear in the midst of so much detail and information. It's just
a stupid doubt on the nomenclature of filters, but this kind of issue can
be quite a pain in the ass, and the subject of many confusions.

So, in short, I still keep my original question: Are bandpass and resonant
filters the same? Or is there a difference between calling one a resonant
and not a bandpass? Moreover, what about a resonator? Is my assumption on
how to call one a bandpass and a resonant correct or it doesn't have
anything to do with the official literature? And how about what Julius
Smith had to say?


2015-01-12 20:03 GMT-02:00 Brian Fay <ovaltinevortex at gmail.com>:

> Based on your posts in this group, you definitely know more about filters
> than I do, so I don't really have an answer to this question - but I'm
> wondering if by "resonant" filter you mean the same thing as "resonator"
> filter?
> I saw something interesting earlier today about using resonator filters as
> control sources for FM synthesis:
> http://tai-studio.org/index.php/projects/sound-programming/complexres/
> The paper that is linked in the article has some details on the
> mathematical implementation of the filter, but that's mostly mumbo-jumbo to
> me.
> On Mon, Jan 12, 2015 at 4:24 PM, Alexandre Torres Porres <porres at gmail.com
> > wrote:
>> Hi, are bandpass and resonant filters the same? Or is there a difference
>> between calling one a resonant and not a bandpass?
>> thanks
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