[PD] bandpass or resonant?

Alexandre Torres Porres porres at gmail.com
Tue Jan 13 15:28:49 CET 2015

I'm pending to say that there is no real distinction between "Resonant
filter" and a "resonator", and a "bandpass" can be implicitly thought of as
a resonator. Here's what I also found in Julius' website


Pass the mouse cursor over the "Resonator" over the title "First-Order
Complex Resonators <https://ccrma.stanford.edu/~jos/filters/Two_Pole.html>"
to see the popup (also attached).


2015-01-13 1:20 GMT-02:00 Martin Peach <chakekatzil at gmail.com>:

> I was looking at circuit diagrams for analog synthesizers recently and
> noticed that the "resonance" control is nothing more than feeding some
> fraction of the output back to the input. With more feedback oscillation
> occurs at the cutoff frequency for any type of filter, highpass, bandpass
> or lowpass.
> Martin
> On Mon, Jan 12, 2015 at 9:09 PM, Alexandre Torres Porres <porres at gmail.com
> > wrote:
>> 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?
>> Cheers
>> 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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