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From: Alan Kennedy on 21 Sep 2009 11:57
Hello all,

I'm trying to design a linear phase FIR filter to a certain set of specifications, in a manner similar to the function firpm, yet subtly different. Instead of linearly interpolating between the frequency/amplitude points as firpm does, I require specific exponential slopes between them (e.g 6db/oct).

Anyone point me in the right direction of the correct function that I might be able to use?

Thanks,
Alan
From: Rune Allnor on 21 Sep 2009 12:01
On 21 Sep, 17:57, "Alan Kennedy" <alan.ocinne...(a)gmail.com> wrote:
> Hello all,
>
> I'm trying to design a linear phase FIR filter to a certain set of specifications, in a manner similar to the function firpm, yet subtly different.  Instead of linearly interpolating between the frequency/amplitude points as firpm does, I require specific exponential slopes between them (e.g 6db/oct).

I can't remember to have heard of a FIR filter design
method that uses roll-off as design parameter. If that's
a design parameter, use an IIR.

Rune
From: Doug Schwarz on 21 Sep 2009 14:43
In article <h987p1$b7a$1(a)fred.mathworks.com>,
"Alan Kennedy" <alan.ocinneide(a)gmail.com> wrote:

> Hello all,
>
> I'm trying to design a linear phase FIR filter to a certain set of
> specifications, in a manner similar to the function firpm, yet subtly
> different. Instead of linearly interpolating between the frequency/amplitude
> points as firpm does, I require specific exponential slopes between them (e.g
> 6db/oct).
>
> Anyone point me in the right direction of the correct function that I might
> be able to use?
>
> Thanks,
> Alan

Can you use fir2?

--
Doug Schwarz
dmschwarz&ieee,org
Make obvious changes to get real email address.
From: Alan Kennedy on 22 Sep 2009 07:35
Doug Schwarz <see(a)sig.for.address.edu> wrote in message <see-20C21D.14432721092009(a)news.frontiernet.net>...
> In article <h987p1$b7a$1(a)fred.mathworks.com>,
> "Alan Kennedy" <alan.ocinneide(a)gmail.com> wrote:
>
> > Hello all,
> >
> > I'm trying to design a linear phase FIR filter to a certain set of
> > specifications, in a manner similar to the function firpm, yet subtly
> > different. Instead of linearly interpolating between the frequency/amplitude
> > points as firpm does, I require specific exponential slopes between them (e.g
> > 6db/oct).
> >
> > Anyone point me in the right direction of the correct function that I might
> > be able to use?
> >
> > Thanks,
> > Alan
>
> Can you use fir2?
>
> --
> Doug Schwarz
> dmschwarz&ieee,org
> Make obvious changes to get real email address.

Thank you, gentlemen, for your responses.

I specified FIR filters, but the truth is my filter doesn't require to be - perhaps IIR would be more appropriate.

What is required is that it is linear phase, and a specific exponential frequency drop between frequency/magnitude points. This seems to me to be a very unusual way to want to design a filter, and none of the usual functions that I've used before can be so specific. FIR2, as far as I can tell, does not ensure linear phase.

Can a filter be designed using Matlab in this fashion?

Best,
Alan
From: Rune Allnor on 22 Sep 2009 08:12
On 22 Sep, 13:35, "Alan Kennedy" <alan.ocinne...(a)gmail.com> wrote:
>  FIR2, as far as I can tell, does not ensure linear phase.

I can agree that the documentation for the function does
not specifically mention whether it does.

The documentation mentions the 3rd edition of Leland B.
Jackson's book. I don't have it, but I do have the 2nd
edition. In the 2nd edition the design goal behind the
frequency sampling technique is to come up with a linear
phase FIR filter.

So I think FIR2 could be useful for your task.

Rune
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