[PD] bandpass or resonant?

Martin Peach chakekatzil at gmail.com
Tue Jan 13 04:20:10 CET 2015

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.


On Mon, Jan 12, 2015 at 9:09 PM, Alexandre Torres Porres <porres at gmail.com>

> 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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