[sdiy] 4-pole SVF theory

[karl dalen]

--- Den fre 2011-08-05 skrev Tom Wiltshire <tom at electricdruid.net>:
> Hi All,
> 
> Can anyone point me at some good resources for state
> variable filter theory? 

aren't they everywhere!

>I've spent a long time playing with
> the classic 2-pole, 12dB oct SVF filter, and I think I get
> that now. So I want to move along. How do I convert it into
> a 4-pole filter? (without just sticking a second one on the
> end of the first one!)

> Presumably there are good reasons why you don't see lots of
> 4-pole SVF filters around, otherwise I'd have thought it'd
> be the killer synth filter  - LP, HP, BP outputs,
> independent Q control - cool, no? So what's wrong with it?

Dont know if you meant analog or digital, but of analog then:

I tried that once, i designed a 3, 4, and 6 pole and they sounded
worser the more poles one added. And i tried every method i could
come up with, multi phase and multi feedback etc, non worked well,
the filter sounded muffled, sort of lame. The only one that sounded
good was the 2 pole. But im no engineer, just a gardener so i dont
know much. Perhaps there exist a 4pole somewhere?

If you look at current single Chip SVF's implemented with 4 pole capability, late CEM, late SSM and Roland, they are ALL 2pole x 2
serial. And those of more discrete design JP6 and others that i
cant remember the name of are also 2 pole x2 serial. I bet 
the designer of these had a good reason for this, particularly
the CEM since they where designed for CD applications.

In the case of analog i reason gain/stability to be the issue
and wouldn't be surprised if that's the with digital to so
most reasonable would be to brake down into 2 pole stages
then cascade.

Just my 2 cent!

KD