[PD] [lop~] coefficient calculation

[Joe White]

Ahhh.... cool :)


On 23 May 2014 13:12, i go bananas <hard.off at gmail.com> wrote:

> as far as i know, [expr] went from charcoal to grey, didn't it?
>
>
> On Fri, May 23, 2014 at 8:06 PM, Joe White <white.joe4 at gmail.com> 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
>>>>>
>>>>> > _______________________________________________
>>>>> > Pd-list at lists.iem.at mailing list
>>>>> > UNSUBSCRIBE and account-management ->
>>>>> http://lists.puredata.info/listinfo/pd-list
>>>>>
>>>>>
>>>>
>>>>
>>>> --
>>>> Follow me on Twitter @diplojocus
>>>>
>>>> _______________________________________________
>>>> Pd-list at lists.iem.at mailing list
>>>> UNSUBSCRIBE and account-management ->
>>>> http://lists.puredata.info/listinfo/pd-list
>>>>
>>>>
>>>
>>
>>
>> --
>> Follow me on Twitter @diplojocus
>>
>> _______________________________________________
>> Pd-list at lists.iem.at mailing list
>> UNSUBSCRIBE and account-management ->
>> http://lists.puredata.info/listinfo/pd-list
>>
>>
>


-- 
Follow me on Twitter @diplojocus
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.puredata.info/pipermail/pd-list/attachments/20140523/7a891c83/attachment-0001.html>