vcf~ isn't a 2-pole real filter but a 1-pole complex one
kinda relative to this, since biquad~ can be emulated with 2 complex poles
- 2 complex zeros, is it more accurate to say biquad is a
2-complex_pole-2-complex_zero-filter???
In its description it says it is a 2-pole-2-zero-filter...
cheers
2014-07-24 13:33 GMT-03:00 Miller Puckette msp@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@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@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@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@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 > > > _______________________________________________ > > Pd-list@iem.at mailing list > > UNSUBSCRIBE and account-management -> > http://lists.puredata.info/listinfo/pd-list > >
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