From help-request at octave dot org Mon Dec 20 11:06:17 2004
Subject: Re: Source of <butter.m>?
From: "Robert A. Macy" <macy at california dot com>
To: edA-qa mort-ora-y <eda-qa at disemia dot com>
Cc: help at octave dot org
Date: Mon, 20 Dec 2004 09:04:31 -0800

You will note the Butterworth is very simple and has evenly
placed poles around a circle.  You'll find the locations
are easy to calculate.  Octave is a breeze here.  

HOWEVER, if you start looking at Tchebyshev; be cautious.
 The poles are indeed in the smae locations, but start to
move in and out forming an ellipse.  The gotcha here is
that everything for the Butterworth is referenced to the
3dB rolloff point, BUT Tchebyshev is referenced to it's
"ripple" down point.  For some applications you then end up
not comparing apples to apples for performance.  

Simply adjust one so your reference point (rolloff?) is the
same and you will avoid the week of confusion I went
through.  

                   - Robert -

On Mon, 20 Dec 2004 15:59:12 +0100
 edA-qa mort-ora-y <eda-qa at disemia dot com> wrote:
> A little bit off-topic but: Can anybody here direct me to
> a good book or tutorial that would be able to recreate
> the "butter" function from scratch?
> 
> I've implemented an IIR filter to use the result of these
> Butterworth values, but I'd ideally like to have the
> calculation directly in the program (it is C++) -- also
> just for the purpose of knowing myself I want to learn
> it.
> 
> P.S. I've already gone through several DSP references and
> have a good idea on how to analyze filters, how to use
> the H(s) /z-transform, but I'm just getting confused
> trying to pull it all together into producing usable IIR
> filters... :(
> 
> 
> -- 
> edA-qa mort-ora-y
> Idea Architect
> http://disemia.com/
> 
> 
> 
>
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Octave is freely available under the terms of the GNU GPL.

Octave's home on the web:  http://www.octave.org
How to fund new projects:  http://www.octave.org/funding.html
Subscription information:  http://www.octave.org/archive.html
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