Re: [chuck-users] bandwidth in band reject filter

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 http://youtu.be/Z3uBNUfvih4 And sorry, no sound. I should learn how to use internets someday. michael On Mon, Oct 27, 2014 at 5:24 PM, Spencer Salazar 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 applicable.

spencer

On Mon, Oct 27, 2014 at 12:34 PM, Michael Heuer wrote:

...

Hello,

I'm currently using the C*APS Eq10 10-band equalizer

http://quitte.de/dsp/caps.html#CAPS_for_Electric_Guitar

via the LADSPA ChuGin

https://github.com/jwmatthys/chugins

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

http://chuck.cs.princeton.edu/doc/program/ugen_full.html#BRF

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

http://msp.ucsd.edu/techniques.htm

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. :)

michael _______________________________________________ chuck-users mailing list chuck-users@lists.cs.princeton.edu https://lists.cs.princeton.edu/mailman/listinfo/chuck-users

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