[Stk] Configuring BiQuad

[Stephen Sinclair]

Ariel,

The BiQuad is a general purpose algorithm for a 2-pole filter.  To
design the actual coefficients is a fairly large topic.  However, some
"cookbook" examples are available, which you linked to.  (You should
ask a more specific question about one of these cookbook examples
perhaps.)  I have used the musicdsp.org Audio EQ cookbook in the past
and it works well.  Some similar BiQuad design algorithms are
available in the STK BiQuad object.

See functions setNotch() and setResonance().

https://ccrma.stanford.edu/software/stk/classstk_1_1BiQuad.html

Some other methods that provide "cooked" filters are

OnePole::setPole(), (low-pass and high-pass)
OneZero::setZero(), (low-pass and high-pass)
PoleZero::setAllpass(), (all-pass filter, useful for modifying phase)
TwoPole::setResonance()  (resonance filter)
TwoZero::setNotch()  (notch-filter)

For the picture you linked to, you would usually combine several
BiQuads, one to apply each feature you want.  Either cascaded (feeding
one into the other), or in parallel (adding the result of applying
each filter).

I'm afraid there aren't any shelving / EQ filters provided out of the
box, so you'll have to use a cookbook or figure it out :)

Steve

On Mon, Jul 25, 2011 at 6:29 PM, Ariel Elkin <arielelkin at gmail.com> wrote:
> I am trying to build a set of EQ presets with the STK's BiQuad class. My background is in electronic music production, so I know my way around a graphic EQ fairly well. But despite some fair amount of research (I have been using these pages: http://peabody.sapp.org/class/350.838/lab/biquad/ and http://www.musicdsp.org/files/Audio-EQ-Cookbook.txt), I'm still unable to figure out how some of the BiQuad's parameters relate to the variables in a graphical EQ…
>
> Say I want to implement an EQ preset like this one: http://img847.imageshack.us/img847/7273/screenshot20110725at101.png
>
> In the screenshot, I have a low shelving filter with a stop frequency at 70Hz , then I'm boosting the gains for 90Hz by 16.5dB with a Q of 8.2, and 205Hz by 24dB with a Q of 10.0.
>
> How can I thus configure a BiQuad/Filters from the STK?
>
> Cheers,
>
> Ariel
>
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