[chuck-users] bandwidth in band reject filter
heuermh at gmail.com
Mon Oct 27 23:40:52 EDT 2014
Thank you, Spencer.
It's still going to take me a bit to catch up with the math.
For a SawOsc at 220.0 Hz through a BRF at 880.0 Hz Q has rather
interesting behavior between 0.01 and 0.03, cuts out at 0.04, and then
doesn't seem to change much between 0.05 and 1.0. I made a
nearly-impossible-to-read screencast here
And sorry, no sound. I should learn how to use internets someday.
On Mon, Oct 27, 2014 at 5:24 PM, Spencer Salazar
<spencer at ccrma.stanford.edu> wrote:
> Hey Mike,
> I usually assume that mathematically Q = f / Δf and this hasn't gotten me in
> trouble yet, but I am not a real audio engineering guy so maybe this could
> be false in some cases.
> In ChucK's case, for brf, the code looks like this:
> // set_brf
> inline void set_brf( t_CKFLOAT freq, t_CKFLOAT Q )
> t_CKFLOAT pfreq = freq * g_radians_per_sample;
> t_CKFLOAT pbw = 1.0 / Q * pfreq * .5;
> so in that last line, Δω = ω/(2Q), so Q = ω/(2Δω) = f/(2Δf). I don't know
> for sure if the extra 2 in the denominator is a scaling factor or actually
> part of the Q-bandwidth relationship for this filter. I would be surprised
> if it was the latter though given the "standard" definition of Q one sees,
> given above.
> I can see how it would be confusing because the term Q is also used for the
> LPF and HPF filters as resonance, which is the gain of the peak at the
> cutoff frequency, in which case the f/Δf relation is not correct or even
> On Mon, Oct 27, 2014 at 12:34 PM, Michael Heuer <heuermh at gmail.com> wrote:
>> I'm currently using the C*APS Eq10 10-band equalizer
>> via the LADSPA ChuGin
>> as part of an effect but all I really need is something to scoop the
>> mids. I thought I might use the BRF band reject filter
>> I can't tell how to set the bandwidth (or described another way, the
>> range of frequencies around the center frequency that are rejected by
>> the filter). I went off into reading Miller Puckette's book Theory
>> and Techniques of Electronic Music
>> and am now lost somewhere in the imaginary plane.
>> The definition of the Q parameter seems to vary among the different
>> ChucK filters (resonance vs quality vs Q) and I suppose I should
>> understand what that means. Feel free to tell me to go back to my
>> reading. :)
>> chuck-users mailing list
>> chuck-users at lists.cs.princeton.edu
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