[PD] [lop~] coefficient calculation

[Alexandre Torres Porres via Pd-list]

hi all, I've been working on filter patches for my courses and I'm still
failing ti get biquad coeficients from the [vcf~] code. Maybe anyone out
there could help?

I wanted this to plot the frequency response in realtime...

the [vcf~] filters aren't in the audio Audio-EQ-Cookbook, and the code
looks a bit too complicated

thanks


2014-05-26 10:23 GMT-03:00 Joe White <white.joe4 at gmail.com>:

> Ahh yes of course thanks Frank!
>
> Have you guys checked out this paper on 'High-Order Digital Parametric
> Equalizer Design <http://www.aes.org/e-lib/browse.cfm?elib=13397>'?
> Apparently it reduces the need to cascade filter implementations to achieve
> high orders.
>
> Cheers,
> Joe
>
>
> On 24 May 2014 09:53, Frank Barknecht <fbar at footils.org> wrote:
>
>> Hi Joe,
>>
>> versions of these calculations without [expr] are also part of the
>> rj-library as u_lowpass, u_lowpassq etc. These have been taken straight
>> from the Audio-EQ-Cookbook.
>>
>> Ciao
>> --
>> Frank
>>
>> On Fri, May 23, 2014 at 12:06:45PM +0100, Joe White wrote:
>> > Thanks for the abstractions Chris. Am I correct in thinking the
>> licensing
>> > issues for [expr] have been resolved now?
>> >
>> > Cheers,
>> > Joe
>> >
>> >
>> > On 21 May 2014 23:22, Chris Clepper <cgclepper at gmail.com> wrote:
>> >
>> > > On Wed, May 21, 2014 at 5:31 PM, Joe White <white.joe4 at gmail.com>
>> wrote:
>> > >
>> > >>
>> > >>
>> > >> Is it intentional to not a bank of go-to filters? [biquad~] is the
>> next
>> > >> one I would go to, but generating your own coefficients isn't
>> that... err..
>> > >> efficient when you're wanting some that just 'works' :)
>> > >>
>> > >>
>> > > Attached are a set of abstractions wrapping most of the 'Audio EQ
>> > > Cookbook' formulae around biquad~.  It would be nice for Pd to include
>> > > something like this.
>> > >
>> > > The only drawback to [biquad~] is it doesn't take audio rate
>> coefficients.
>> > >  There are of course externals that do audio rate for cutoff, Q, etc.
>> > >
>> > > Chris
>> > >
>> > >
>> > >>
>> > >>
>> > >> On 21 May 2014 17:31, Miller Puckette <msp at ucsd.edu> wrote:
>> > >>
>> > >>> Hi Joe -
>> > >>>
>> > >>> That code is an approximation that works well for low cutoff
>> > >>> frequencies but badly for high ones.  (I should probably warn
>> > >>> about this in the help window... that'll go on my dolist)
>> > >>>
>> > >>> cheers
>> > >>> M
>> > >>>
>> > >>>
>> > >>> On Fri, May 16, 2014 at 12:58:31PM +0100, Joe White wrote:
>> > >>> > Hi,
>> > >>> >
>> > >>> > I've been looking at the [lop~] implementation (Pd-0.45-4) and
>> noticed
>> > >>> > something that seem weird to me.
>> > >>> >
>> > >>> > In d_filter, line 176:
>> > >>> >
>> > >>> > static void siglop_ft1(t_siglop *x, t_floatarg f)
>> > >>> > {
>> > >>> >     if (f < 0) f = 0;
>> > >>> >     x->x_hz = f;
>> > >>> >     x->x_ctl->c_coef = f * (2 * 3.14159) / x->x_sr;
>> > >>> >     if (x->x_ctl->c_coef > 1)
>> > >>> >         x->x_ctl->c_coef = 1;
>> > >>> >     else if (x->x_ctl->c_coef < 0)
>> > >>> >         x->x_ctl->c_coef = 0;
>> > >>> > }
>> > >>> >
>> > >>> >
>> > >>> > Is it correct that for:
>> > >>> >
>> > >>> > y[n] = x[n] * a + y[n-1] * b
>> > >>> >
>> > >>> > *a = 2π * Fc / Fs*
>> > >>> > b = 1.0 - a
>> > >>> >
>> > >>> > where Fc is the cut-off frequency and Fs the sampling frequency.
>> > >>> >
>> > >>> > I appreciate the a coefficient is bounded afterwards but wouldn't
>> that
>> > >>> mean
>> > >>> > that Fc values greater than Fs / 2π will have no impact on the
>> sound
>> > >>> being
>> > >>> > processed.
>> > >>> >
>> > >>> > For example if Fs is 44100, then Fc values above ~7020Hz will not
>> > >>> affect
>> > >>> > the filter.
>> > >>> >
>> > >>> > Have I missed something crucial or could this a bug in the code?
>> > >>> >
>> > >>> > The simple IIR filter described in
>> > >>> > http://en.wikipedia.org/wiki/Low-pass_filter suggests that the
>> actual
>> > >>> > coefficient calculation should be more like:
>> > >>> >
>> > >>> > a = 2π*Fc / (2π*Fc + Fs)
>> > >>> >
>> > >>> > Looking forward to understand this more!
>> > >>> >
>> > >>> > Cheers,
>> > >>> > Joe
>> > >>> >
>> > >>> > --
>> > >>> > Follow me on Twitter @diplojocus
>> > >>>
>> > >>> > _______________________________________________
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>> > >>>
>> > >>
>> > >>
>> > >> --
>> > >> Follow me on Twitter @diplojocus
>> > >>
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>
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>
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