[PD] bandpass or resonant?

Alexandre Torres Porres porres at gmail.com
Wed Jan 14 00:13:37 CET 2015 

sound on sound's synth secrets backs me up when I say resonant filters
amplify bands of frequencies ;)

shttp://www.soundonsound.com/sos/oct99/articles/synthsecrets.htm

not alone after all

2015-01-13 13:48 GMT-02:00 Alexandre Torres Porres <porres at gmail.com>:

> This bit of the wikipedia article on resonance about "Q" (
> http://en.wikipedia.org/wiki/Resonance#Q_factor ) mentions a lot of what
> we read about "Ringing Filter", and how a higher Q will make it "ring"
> longer... this is what you get from that bandpass in the EQ Cookbook that
> says it has "constant skirt gain, peak gain = Q" - or what I'm assuming
> to be a "resonant filter by excellence" or whatever...
>
> cheers
>
>
>
> 2015-01-13 13:38 GMT-02:00 Alexandre Torres Porres <porres at gmail.com>:
>
> "*Ultimately I think "resonant" is a general descriptive term for
>> filters, that shouldn't be interpreted as a detail of implementation.*"
>>
>> I guess you have a point there, and I was also driving to this conclusion.
>>
>> "*It sounds like the Resonz UGen in supercollider is exactly what Julius
>> Smith is talking about in that description of the two-pole filter.*"
>>
>> Actually, the Resonz UGen is a Two Pole / Two Zero filter, which is very
>> more closely related to [cyclone/reson~].
>>
>> A Two-Pole only filter (no zeros) like the one Julius is describing is
>> actually what the [bp~] object is!
>>
>> "*But then there's the other supercollider filter UGens with "resonant"
>> in the name, which seem more like what Martin was describing - RLPF
>> (resonant low-pass filter) for example is a low-pass filter where you can
>> adjust the resonance near the cutoff.* "
>>
>> Yep, and this is also much like the [cyclone/lores~] object in Pd. Check
>> that patch I sent for more detailed info on these filters.
>>
>> I've always assumed that a filter, in order to be called a "resonant
>> filter" or a "resonator" - being it a low pass, a high pass or a band pass
>> -, needed to boost/add gain to a particular cutoff frequency (in the case
>> of lowpass and highpas - which is the case for [lores~] or RLPF) or add
>> gain to some center frequency (in the case of a bandpass) - which is also
>> called "resonant frequency".
>>
>> The quote from Julius in that link - where he says "*A resonator is a
>> recursive filter that boosts signal amplitude at a particular frequency*"
>> - is in line with my assumption.
>>
>> But the concept of resonance in physics, in its utmost purity according
>> to wikipedia, is that it "is the tendency of a system to oscillate with
>> greater amplitude at some frequencies than at others". Meaning that it
>> doesn't really have to add gain to something, but only favor a frequency
>> amongst others... In this context, a bandpass - in general - is a
>> "resonator"...
>>
>> But it'd be cool if I could find a definitive word about this in the
>> filter literature!
>>
>> Cheers
>>
>> 2015-01-13 13:16 GMT-02:00 Brian Fay <ovaltinevortex at gmail.com>:
>>
>> It sounds like the Resonz UGen in supercollider is exactly what Julius
>>> Smith is talking about in that description of the two-pole filter.
>>>
>>> But then there's the other supercollider filter UGens with "resonant" in
>>> the name, which seem more like what Martin was describing - RLPF (resonant
>>> low-pass filter) for example is a low-pass filter where you can adjust the
>>> resonance near the cutoff. I haven't played with this too much myself, but
>>> I'm guessing with the right Q value you could drive this to
>>> self-oscillation?
>>>
>>> Ultimately I think "resonant" is a general descriptive term for filters,
>>> that shouldn't be interpreted as a detail of implementation.
>>>
>>> On Tue, Jan 13, 2015 at 9:28 AM, Alexandre Torres Porres <
>>> porres at gmail.com> wrote:
>>>
>>>> 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
>>>>
>>>>
>>>> https://ccrma.stanford.edu/~jos/filters/First_Order_Complex_Resonators.html
>>>>
>>>> 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).
>>>>
>>>> cheers
>>>>
>>>> 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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>>>>>>
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>>>>>
>>>>
>>>
>>
>
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