[sdiy] standard filters from ladder topology?

[JH.]

Thank you, Don!

JH.


----- Original Message ----- 
From: "Don Tillman" <don at till.com>
To: <jhaible at debitel.net>
Cc: <synth-diy at dropmix.xs4all.nl>
Sent: Sunday, September 04, 2005 11:07 AM
Subject: Re: [sdiy] standard filters from ladder topology?


>    > 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
> 
>