[sdiy] Digital 4 Pole Multimode Filter

[Scott Gravenhorst]

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