making LP filters into HP

[Don Tillman]

   From: Christopher List <Christopher_List at sonymusic.com>
   Date: 13 May 96 12:27:15 

   I built one of those filters Gene was talking about the other
   week. The Electronotes one with the 4 x (3080->cap->3140) stages. It's
   4-pole low-pass.  It works very well (as Gene described) and it's very
   easy to layout and put together.

   Anyway, I was thinking (or maybe I read somewhere) that if you take
   the output of a LP filter, invert it and add it to the original,
   unfiltered signal you get a HP output. I played with this for a little
   while, and sure enough, it worked.  

Well, maybe a little bit.

If this is the filter I'm thinking of, it's output phase goes from
0-degrees at low frequencies, through 180-degrees at the resonant
frequency to 360-degrees at high frequencies.  So yeah, it'll work,
but you'll get this interesting little anomaly in the middle

Not that that's a bad thing in a musical context, just don't assume
it's a simple high pass filter.

				       The problem is that when the Q
   changes you have to change the mix of the two signals to continue
   getting a proper HP response.

Oh yes.

   Is there any trick you experienced fellows have run across to avoid
   this? 

None that I can think of right now.  

	     I guess what I'm really wondering is do state-variable
   models have to be designed from the ground up as state variable or can
   you "enhance" any LP filter to get a high pass response <<with>>
   adjustable Q?

Generally state variable filters need to be designed that way from the
ground up. 

  -- Don