[sdiy] Digital 4 Pole Multimode Filter

[Andrew Simper]

Hi Scott,

I've derived all the equations for the 4 pole state variable filter and
have a working linear and non-linear version. The non-linear version I
still have grand plans of making a hardware unit at some stage with the
kind offer of help from Olivier from Mutable Instruments, but I still
haven't had much spare time to pursue it.

What do you want to use the 4 pole SVF for?

In terms of operation counts and numerical performance it's pretty hard to
beat cascaded 2 poles for most things, and you also get the benefit from
having independent cutoff so you can do more than a single 4 pole filter
can.

Also I recommend not using DF1 bi-quads (RBJ), at least use cascaded
digital SVFs or Sallen Keys.

Cheers,

Andy


On 2 May 2018 at 20:51, 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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