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# Discussion Groups | Comp.DSP | design a digital butterworth filter using matlab??

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# design a digital butterworth filter using matlab?? - 2005-10-29 12:37:00

```Dear members:
In the demo in my class, my prof uses matlab to design a digital
butterworth filter. He first designs an analog filter using funtion
butter(, , 's') and then uses bilinear transformation to convert the
analog filter to a digital filter.

Since I know MATLAB function butter()allows us to directly design a
digital butterworth filter, I am wondering what are the advantages of
my prof's method over the direct design method?

I am wondering if prof uses this method just  to show how to design an
digital filter from an analog filter, and in practical cases, we can
use the MALAB butter to directly design digital filters, don't need to
use he bilinear transformation method. The result is the same.

Plz confirm my thinking.

Thanks```

# Re: design a digital butterworth filter using matlab?? - John E. Hadstate - 2005-10-29 13:39:00

```<V...@gmail.com> wrote in message
> Dear members:
> In the demo in my class, my prof uses matlab to design a
> digital
> butterworth filter. He first designs an analog filter
> using funtion
> butter(, , 's') and then uses bilinear transformation to
> convert the
> analog filter to a digital filter.
>
> Since I know MATLAB function butter()allows us to directly
> design a
> digital butterworth filter, I am wondering what are the
> my prof's method over the direct design method?

You might not always have MATLAB available, but presumably
you'll always be able to look up the technique your
professor is trying to teach you.

Having just done exactly what you describe without benefit
of MATLAB, I can appreciate your professor's diligence.

# Re: design a digital butterworth filter using matlab?? - Rune Allnor - 2005-10-30 09:51:00

```V...@gmail.com wrote:
> Dear members:
> In the demo in my class, my prof uses matlab to design a digital
> butterworth filter. He first designs an analog filter using funtion
> butter(, , 's') and then uses bilinear transformation to convert the
> analog filter to a digital filter.

I assume he first designs the butterworth prototype in s domain
and then calls a function "bilinear" to tranform it to discrete
time domain?

> Since I know MATLAB function butter()allows us to directly design a
> digital butterworth filter, I am wondering what are the advantages of
> my prof's method over the direct design method?

There are several advantages. In an educational setting, the
students have a chance of seeing what is really going on.
All too often somebody calls a function or uses aprogram
without knowing what it does or why. Second, splitting things
like that leaves you as a designer with far better control of
the details. I just implemented a filter design program in C++
(got it compiled as mex files under matlab today, actually)
for butterworth and Cheb 1 filters, and it let me do things
somewhat differently than matlab.

> I am wondering if prof uses this method just  to show how to design an
> digital filter from an analog filter, and in practical cases, we can
> use the MALAB butter to directly design digital filters, don't need to
> use he bilinear transformation method. The result is the same.

Well... I'll split some hairs here and say that the BLT is just
one of several possible ways to design discrete-time filters
from analog prototypes. It is useful, though, and it is well
worth the effort to understand.

If you want a programming challenge, implement the thing in
C++ where you try to re-use as much code as possible.

Rune```

# Re: design a digital butterworth filter using matlab?? - 2005-10-30 19:54:00

`Thank you very much, the group is very useful to me :)`