[PD] bandpass or resonant?

Tue Jan 13 16:16:55 CET 2015

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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