[sdiy] Digital 4 Pole Multimode Filter

Tom Wiltshire tom at electricdruid.net
Wed May 2 21:35:08 CEST 2018

You could have a look at Antti’s Powerwave synth paper from DAFX 05. It’s an FPGA-based synth that uses a Moog-model for the filter if I remember. That’s a much simpler structure than an SVF. Furthermore, the Powerwave uses pole-mixing (a la Xpander) to provide multiple different responses. After all, in the digital realm, there’s really no reason not to.

Aside from that, the Ear Level Engineering site is good:

http://www.earlevel.com/main/category/synthesizers/ <http://www.earlevel.com/main/category/synthesizers/>

..and that links to Vadim Zavalishin’s "THE ART OF VA FILTER DESIGN”:

https://www.native-instruments.com/fileadmin/ni_media/downloads/pdf/VAFilterDesign_1.1.1.pdf <https://www.native-instruments.com/fileadmin/ni_media/downloads/pdf/VAFilterDesign_1.1.1.pdf>

       Electric Druid
Synth & Stompbox DIY

> On 2 May 2018, at 19:51, Scott Gravenhorst <music.maker at gte.net> 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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