[sdiy] Digital 4 Pole Multimode Filter

[paula at synth.net]

Scott,

  I did this for the Monowave II on the Chameleon way back, in 2003'ish - 
https://www.chameleon.synth.net/english/skins/monowave2/

  And then for the Phoenix Polysynth in 2004'ish -
https://www.chameleon.synth.net/english/skins/phoenix/

I'll see if I can dig out the code for the filter (which I GPL'd), but 
it was based on the SoundArt example code for their monosynth which you 
can download here - 
https://www.chameleon.synth.net/english/developers/resources.shtml

 From the feedback I got everyone loved the sound of the filter, I did 
change their example one in a few places to give it some character, but 
the basics are there in the example code.

Paula


On 2018-05-02 19:51, Scott Gravenhorst wrote:
> Hi List,
> 
> I'm looking for information on digital 4 pole filters.  I need to be 
> able
> to control Q and Fc.  Among the many documents I found with google was 
> this
> document:
> 
> http://www.rane.com/pdf/linriley.pdf
> 
> which describes an analog version of a 4th order state variable filter, 
> but
> my math lessons were so many years ago, that it's not entirely helpful. 
>  On
> page 4 the document shows how the resistors are to be given values 
> (right
> side) for a Linkwitz-Riley crossover.  I have a feeling this is close 
> to
> what I want, but
> 
> 1) I'm unable to determine how to change the filter so that I can 
> control Q
> and Fc.
> 2) I'm unable to convert this to a block diagram representation of the 
> same
> circuit as a digital model.
> 3) Information on how to tap the structure for modes other than lowpass
> would be very helpful.
> 
> I'm familiar with the 2nd order state variable filter, but expanding 
> that
> by adding two more integrators is not all that simple to my eyes.  I 
> can
> see from the Linkwitz-Riley circuit that each integrator has a negative
> feedback path.
> 
> I would also be interested in other 4 pole filter topolgies where 
> access is
> given for Q and Fc control parameters.
> 
> I would be extremely grateful for pointers to readings and information
> helpful in this quest.
> 
> -- ScottG
> ________________________________________________________________________
> -- Scott Gravenhorst
> -- http://scott.joviansynth.com/
> -- When the going gets tough, the tough use the command line.
> -- Matt 21:22
> 
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