[PD] [bp~] really equal to [vcf~]?

[Alexandre Torres Porres]

> whether a [cpole~] filter like [vcf~] could be obtained with [biquad~].

I meant at least the real output of [vcf~]


2014-07-24 14:28 GMT-03:00 Alexandre Torres Porres <porres at gmail.com>:

> yep, totally makes perfect sense to me now that it is a [cpole~].
>
> I guess you can consider (ampcorrect * oneminusr) as a gain factor and
> multiply the input signal by this much before getting it to a [cpole~] with
> the same coefficients, and voilá... although getting all these
> coefficients to come up in a vanilla patch seems a lot of trouble...
>
> But what I'm really and still dying to learn/figure out is whether a
> [cpole~] filter like [vcf~] could be obtained with [biquad~]. My intuition
> said: Yes. But after a few experiments I'm changing my mind... Can anyone
> just confirm me that?
>
> What drove my intuition is the attached patch, which is a biquad~ filter
> made of raw filters I derived from mmb's work. If it made any sense is
> because it is implemented with [cpole~] and [czero~] objects, so it seemed
> that a set of biquad coefficients could result in just a couple of
> coefficients for one [cpole~]. But then, by checking it out and trying to
> do it, it doens't seem possible to get a coordinate pair for a [cpole~]
> with biquad coefficients... hmmm, bummer.
>
> Any thoughts?
>
> thanks
>
>
> 2014-07-24 13:33 GMT-03:00 Miller Puckette <msp at ucsd.edu>:
>
> Yep, vcf~ isn't a 2-pole real filter but a 1-pole complex one (although
>> its second outlet -- I think -- should be similar to a 2-pole real
>> filter's output in theory, but in reality should be numerically more
>> precise.)
>>
>> I'll change the comment to something vaguer and more descriptive in the
>> code..
>>
>> Unless I'm badly mistaken, vcf~ is simply a cpole~ with extra stuff to
>> compute
>> the coefficient built in.  There's no 'x2' because the input is taken to
>> be
>> real-valued.
>>
>> cheers
>> M
>>
>> On Thu, Jul 24, 2014 at 01:18:28PM -0300, Alexandre Torres Porres wrote:
>> > >> The real part may be used as a resonant
>> > >> bandpass filter and the imaginary as a
>> > >> resonant low-pass filter, but they can be
>> > >> combined to make other possibilities."
>> >
>> > > what sort of possibilities???
>> >
>> > I wouldn't expect much about this. I guess it's like any other filters
>> that
>> > can be combined in many ways.
>> >
>> > Another thing I wanted to inquire is about bp~ having a frequency
>> response
>> > that is not symmetric. The lower part of the spectrum has more energy.
>> > Could one consider it more of a "resonant low-pass filter", similar to
>> > [vcf~]'s second outlet?
>> >
>> > By the way, while we're at it, I'd like to share something that may not
>> be
>> > too important, but it's related. I was able to implement [cpole~] in
>> > [fexpr~]. Here it goes:
>> >
>> > expr 1:
>> >
>> > [fexpr~ $x1 + ($x3 * $y1) - ($x4 * $y2);
>> > $x2 + ($x4 * $y1) + ($x3 * $y2)]
>> >
>> > if we consider $x3 and $x4 as variables named, respectively, coefr
>> > (coeficient for the real part) and coefi (coeficient for the imaginary
>> > part), we then have:
>> >
>> > expr 2:
>> >
>> > *[fexpr~ $x1 + (coefr * $y1) - (coefi * $y2);*
>> > *$x2** + (*coefi * $y1) + (coefr * $y2)]
>> >
>> > Now, by checking the code of [vcf~] I was able to narrow down to its
>> core
>> > formula, which is something like this
>> >
>> > expr 3:
>> >
>> > *ampcorrect * oneminusr* * *f1 + (coefr * re2) - (coefi * im)*;
>> > *(coefi * re2) + (coefr * im)*
>> >
>> > the bolded letters in "expr 3" seem to match perfectly to "expr 2". The
>> > differences are underlined ($x2 in expr 2 and ampcorrect/oneminusr in
>> expr
>> > 3).
>> >
>> > That gets me closer to being able to implement [vcf~] with a [cpole~] I
>> > guess, but I find it weird that the imaginary output does not have the
>> $x2
>> > signal input to be added to the rest of the expression. I worry that
>> > actually prevents it from being successfully implemented with [cpole~].
>> Am
>> > I missing something?
>> >
>> > Anyway, the thing is that I'm still really curious to learn wether vcf~
>> is
>> > a "two pole" filter or a "one complex pole" filter, and the reason
>> behind
>> > it is because I believe we could make a biquad~ version of [vcf~] (at
>> least
>> > for its real output). That's the bottom line.
>> >
>> > cheers
>> >
>> >
>> >
>> >
>> > 2014-07-24 6:45 GMT-03:00 i go bananas <hard.off at gmail.com>:
>> >
>> > > just chipping in with my 2 cents that it would be fantastic to get
>> more
>> > > documented info on all this.
>> > >
>> > > especially curious about:  "but they can be combined to make other
>> > > possibilities."
>> > >
>> > > what sort of possibilities???
>> > >
>> > >
>> > > On Thu, Jul 24, 2014 at 5:07 PM, Alexandre Torres Porres <
>> porres at gmail.com
>> > > > wrote:
>> > >
>> > >> Hi Miller, still trying to get my head around this. But anyway, one
>> thing
>> > >> I have to note is that the source code of [vcf~] says it is a "two
>> pole
>> > >> filter", not a "one complex pole" filter.
>> > >>
>> > >> Should that description be changed? If not, why?
>> > >>
>> > >> thanks
>> > >>
>> > >>
>> > >> 2014-04-12 14:13 GMT-03:00 Miller Puckette <msp at ucsd.edu>:
>> > >>
>> > >> Yep - vcf~ is essentially a wrapper for cpole~.
>> > >>>
>> > >>> cheers
>> > >>> M
>> > >>>
>> > >>> On Sat, Apr 12, 2014 at 02:10:19PM -0300, Alexandre Torres Porres
>> wrote:
>> > >>> > that's great to know, thanks!
>> > >>> >
>> > >>> > Let me just see if I get a bit of the theory. Can I get [vcf~]
>> with
>> > >>> just
>> > >>> > one [cpole~] object and the right coeficients?
>> > >>> >
>> > >>> > Cheers
>> > >>> >
>> > >>> >
>> > >>> > 2014-04-12 13:36 GMT-03:00 Miller Puckette <msp at ucsd.edu>:
>> > >>> >
>> > >>> > > They're quite different.  bp~ is the cheapest possible bandpass
>> > >>> filter
>> > >>> > > (as far as I know).  vcf~ is a one-pole complex filter whose
>> outputs
>> > >>> are
>> > >>> > > the real and imaginry parts.  The real part may be used as a
>> resonant
>> > >>> > > bandpass filter and the imaginary as a resonant low-pass
>> filter, but
>> > >>> > > they can be combined to make other possibilities.
>> > >>> > >
>> > >>> > > It's possible to graph their frequency responses using the help
>> patch
>> > >>> > > "H10.measurement.pd' in 3.audio.examples.
>> > >>> > >
>> > >>> > > cheers
>> > >>> > > Miller
>> > >>> > >
>> > >>> > > On Sat, Apr 12, 2014 at 04:28:11AM -0300, Alexandre Torres
>> Porres
>> > >>> wrote:
>> > >>> > > > Hi there, who can confirm that both [bp~] and [vcf~] are
>> exactly
>> > >>> the
>> > >>> > > really
>> > >>> > > > same thingy? The code looks quite different...
>> > >>> > > >
>> > >>> > > > Moreover, why the two outlets for vcf~? Help doesn't say
>> anything.
>> > >>> > > >
>> > >>> > > > Thanks
>> > >>> > >
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>> > >>>
>> > >>
>> > >>
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>
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