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Re: Designing a first-order IIR filter

From: Shaun Jackman
Subject: Re: Designing a first-order IIR filter
Date: Tue, 28 Nov 2006 16:20:56 -0700

On 11/6/06, Doug Stewart <address@hidden> wrote:
>> octave-forge has butter, cheby1, cheby2 and ellip
>> for designing IIR filters.
> Thank you. These are all very good suggestions.
> I am particularly interested in designing a first-order filter. I
> believe the above filters are exclusively higher-order -- please
> correct me if I'm wrong. Or can one (all?) of the above be used to
> design a first-order filter?
Just give it a try. It works.

Thanks! It does mostly work. The only caveat being that a first-order
Butterworth filter gave two taps in the numerator:

octave:1> [b, a] = butter(1, 0.01)
b =

 0.015466  0.015466

a =

  1.00000  -0.96907

I simply moved the weight from the second tap to the first tap:

octave:2> b = [b(1) + b(2) 0]
b =

 0.030933  0.000000

Thanks for the help!

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