[sdiy] [OT] Analog synths with 2 pole filters

[Tom Wiltshire]

On 22 Feb 2013, at 11:47, rburnett at richieburnett.co.uk wrote:

>  In the case of the state-variable filter just taking the damping term down to zero is not enough.  The damping has to actually go slightly below zero, and this requires increased complication that most filter designs don't include.  That's my take on it anyway, I hope it helps the discussion :-)

Yes, Richie is quite right. Re-reading my post, I made it sound like just removing the damping is enough, but actually you need to tweak the other feedback path to give you more gain. Once you've done that, you've got more feedback than you need, so you need more damping to cancel it. Getting the balance of these two right is the sum total of the work in an SVF design, I reckon. The rest of it is just putting the right bits in the right places, but sorting out the feedback so that it oscillates cleanly over the whole range is the tricky bit for sure.

Tom