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From: Pete Fraser on 30 Jun 2010 21:21
I'm new to Matlab, so this is probably a really dumb question.
I'm trying to design an FIR where I can set the points.
I want a symmetric FIR.
I'm assuming that an N tap FIR gives me N/2 degrees of freedom
for an even number of taps, and (N + 1)/2 degrees of freedom
for an odd number of taps.
This should allow me to set the frequency response at a
number of points equal to the degrees of freedom, and
have the frequency response travel through these points
exactly. If I don't specify transitions that are too sharp
relative to the number of taps, then the frequency response
should be reasonably well controlled between the set points.

I had assumed that I could use the filter toolbox's freqsamp
method to do this, but it doesn't seem to work as I expected.
What am I doing wrong? I'd normally do this in Mathematica
or C, but I'm trying to get into the spirit of Matlab.

I would want to solve the point setting problem anyway, but
perhaps there's an easier way to do the immediate task that
I wanted it for. I want to implement a Butterworth amplitude
response in an FIR filter (to emulate some legacy equipment)
but I can't find an easy way of doing that in Matlab (hence the
point setting experiments).

Thanks

Pete


From: Vladimir Vassilevsky on 30 Jun 2010 21:35


Pete Fraser wrote:

> I'm new to Matlab, so this is probably a really dumb question.
> I'm trying to design an FIR where I can set the points.
> I want a symmetric FIR.
> I'm assuming that an N tap FIR gives me N/2 degrees of freedom
> for an even number of taps, and (N + 1)/2 degrees of freedom
> for an odd number of taps.
> This should allow me to set the frequency response at a
> number of points equal to the degrees of freedom, and
> have the frequency response travel through these points
> exactly. If I don't specify transitions that are too sharp
> relative to the number of taps, then the frequency response
> should be reasonably well controlled between the set points.
>
> I had assumed that I could use the filter toolbox's freqsamp
> method to do this, but it doesn't seem to work as I expected.
> What am I doing wrong? I'd normally do this in Mathematica
> or C, but I'm trying to get into the spirit of Matlab.
>
> I would want to solve the point setting problem anyway, but
> perhaps there's an easier way to do the immediate task that
> I wanted it for. I want to implement a Butterworth amplitude
> response in an FIR filter (to emulate some legacy equipment)
> but I can't find an easy way of doing that in Matlab (hence the
> point setting experiments).

But why would anyone want to emulate Butterworth magnitude response in a
linear phase FIR filter whereas it is very straightforward to do the
(**exact) Butterworth as IIR filter?

** frequency warping



Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com


From: Pete Fraser on 30 Jun 2010 22:06

"Vladimir Vassilevsky" <nospam(a)nowhere.com> wrote in message
news:0Jidnc3knONVbbbRnZ2dnUVZ_oSdnZ2d(a)giganews.com...

> But why would anyone want to emulate Butterworth magnitude response in a
> linear phase FIR filter whereas it is very straightforward to do the
> (**exact) Butterworth as IIR filter?

Butterworth will be one of several possible responses
from the same hardware.

I'd rather not deal with Butterworth's phase issues.

Pete


From: Jerry Avins on 30 Jun 2010 22:39
On 6/30/2010 10:06 PM, Pete Fraser wrote:
> "Vladimir Vassilevsky"<nospam(a)nowhere.com> wrote in message
> news:0Jidnc3knONVbbbRnZ2dnUVZ_oSdnZ2d(a)giganews.com...
>
>> But why would anyone want to emulate Butterworth magnitude response in a
>> linear phase FIR filter whereas it is very straightforward to do the
>> (**exact) Butterworth as IIR filter?
>
> Butterworth will be one of several possible responses
> from the same hardware.
>
> I'd rather not deal with Butterworth's phase issues.

What is so good about a Butterworth amplitude response that you want to
imitate it?

Jerry
--
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������
From: Tim Wescott on 30 Jun 2010 22:39
On 06/30/2010 06:21 PM, Pete Fraser wrote:
> I'm new to Matlab, so this is probably a really dumb question.
> I'm trying to design an FIR where I can set the points.
> I want a symmetric FIR.
> I'm assuming that an N tap FIR gives me N/2 degrees of freedom
> for an even number of taps, and (N + 1)/2 degrees of freedom
> for an odd number of taps.
> This should allow me to set the frequency response at a
> number of points equal to the degrees of freedom, and
> have the frequency response travel through these points
> exactly. If I don't specify transitions that are too sharp
> relative to the number of taps, then the frequency response
> should be reasonably well controlled between the set points.
>
> I had assumed that I could use the filter toolbox's freqsamp
> method to do this, but it doesn't seem to work as I expected.
> What am I doing wrong? I'd normally do this in Mathematica
> or C, but I'm trying to get into the spirit of Matlab.
>
> I would want to solve the point setting problem anyway, but
> perhaps there's an easier way to do the immediate task that
> I wanted it for. I want to implement a Butterworth amplitude
> response in an FIR filter (to emulate some legacy equipment)
> but I can't find an easy way of doing that in Matlab (hence the
> point setting experiments).

Make a vector that "draws" the frequency response that you want, then
take its FFT.

It may not be the entirely kosher way to do it, but it certainly works.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html
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