[sdiy] standard filters from ladder topology?

[Don Tillman]

   > Date: 03 Sep 2005 22:23:30 -0700
   > From: Don Tillman <don at till.com>
   > 
   > The coefficients for a Butterworth filter are:
   >   1.000, 2.613, 3.414, 2.613, 1.000

[A little more thought...]

There is an optimization you can do.  Tapping off the ladder rungs is
not cheap, so you can save the first tap by frequency scaling your
filter coefficients to match the first two ladder coefficients.

So... if you frequency scale the above Butterworth coefficients by a
factor of 1.531, they become:

  1.000, 4.000, 8.000, 9.375, 5.491

Then the 'B1' tap is no longer needed.

  B1 = 0
  B2 = -2.000
  B3 = -1.375
  B4 = -1.116

(Again, I haven't checked my math.)

  -- Don

-- 
Don Tillman
Palo Alto, California
don at till.com
http://www.till.com