[sdiy] Digital 4 Pole Multimode Filter

[Richie Burnett]

Synth filters are always fun.

---- paula at synth.net wrote ----

> Just for the record, I'm loving this thread :)
> 
> 
> On 2018-05-03 08:37, Richie Burnett wrote:
> > If you already know how to implement the 2-pole digital (chamberlain)
> > state variable filter in code, then the easiest route to a 4-pole
> > filter is to cascade two discrete digital SVFs. For the purpose of
> > clarity: pass the audio samples through one digital SVF then pass the
> > output of that filter through another digital SVF.
> > 
> > You get some flexibility in the choice of overall response as each SVF
> > can be LP, BP, HP or notch, and you can independently set the fc and q
> > for each. You can also cascade even more stages for 6-pole, 8-pole,
> > etc, if the desire is there and resources permit.
> > 
> > From a programming point of view, each SVF has two internal integrator
> > states, and most DSPs have dual accumulators, so it fits nicely to
> > process blocks of audio through one 2-pole SVF at a time keeping the
> > integrator states at maximum-precision in the MAC accumulators. This
> > is harder to do with a moog 4-pole cascade that has 4 internal states.
> > 
> > -Richie,
> > 
> > Sent from my Xperia SP on O2
> > 
> > ---- 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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