2nd-order allpass VCFs?

Don Tillman don at till.com
Tue Nov 23 17:18:59 CET 1999


   Date: Mon, 22 Nov 1999 23:31:56 -0800
   From: Sean Costello <costello at seanet.com>

   It seems like it is fairly easily to modify a state-variable
   filter to produce an allpass output - Bernie Hutchins describes this in
   some of the Application Notes. 

Yes indeed, simply sum the LP, BP and HP outputs, making sure the BP
is out of phase with the others. 

At very low Qs, 0.5 and below, this is like a pair of standard
opamp-style phase shift stages.  At higher Qs the phase shift effect
is compressed over a smaller range, something standard opamp-style
phase shift stages can't do.

   However, now I am wondering if it is really necessary to create
   allpass filters out of the state-variable filters in order to get
   the precise control over the notches.

If it's notches you want, you can also get notches out of a
state-variable directly.  Just sum the LP and HP outputs, in phase.
(At the resonant frequency they're 180-degrees apart and cancel.)

This is a little different from the notches you get from a normal
phase shift circuit.

   Perhaps mixing the bandpass outputs of several state-variable
   filters in parallel, and subtracting this from the original input
   signal, will produce similar results.

Not as practical -- here the levels have to be accurate for a good
notch.

  -- Don



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