>A Gaussian transition filter is Gaussian to (typically) 6dB or 12dB down,
>then it falls off faster.

A true Gaussian drops off very quickly once beyond about -20 dB. For example,
it changes from 20 dB attenuation to 40 dB attenuation in half an octave, and
from -20 dB to -80 dB in one full octave. I'm curious as to what would require
an even faster attenuation rate.

>> Do you seek an analog implementation, or a digital implementation?
>
>Digital.

I actually determined that from re-reading your original message, but forgot to
remove that question from my response.

>Thanks I'll have a look at FDLS.

Pretty much anything that you can actually implement in analog, you can
approximate pretty well in digital with FDLS. It's when you start using
arbitrary magnitude and phase responses that it can fall apart.
Greg

Reply by Vladimir Vassilevsky●September 29, 20102010-09-29

Pete Fraser wrote:

>>So, what do you need exactly? Linear phase approximation? Gaussian impulse
>>response? Step response without overshoot?
>
>
> Step response with mild (~5% overshoot) but having a frequency
> response that has substantially better stop-band attenuation than
> a Gaussian (or Bessel).

Interestingly enough, from pole-only filters of any kind, the Bessel
filter of the second order has the fastest settling of the step response
for the given efficient noise bandwidth.

> I'll try your combined Bessel-Butterworth approach.

Bessel-Butterworth is trivial idea; it's only merit is that it could be
done as a closed form solutuon. If you need to design a filter only
once, the best approach would be brute force optimization.
Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

Reply by Pete Fraser●September 29, 20102010-09-29

"gretzteam" <gretzteam@n_o_s_p_a_m.yahoo.com> wrote in message
news:T42dnZzhyOsXFz7RnZ2dnUVZ_jSdnZ2d@giganews.com...

> If you want to quickly get some Bessel lowpass filters, you can use the
> online tool here:
> http://www-users.cs.york.ac.uk/~fisher/mkfilter/trad.html
>
> Nice thing of the Bessel filter approx is that the feedfordward
> coefficient
> are quite trivial to implement, and it gives you better attenuation than
> cascading first order allpole filters.

Thanks for the link.
Pete

Reply by Pete Fraser●September 29, 20102010-09-29

"Vladimir Vassilevsky" <nospam@nowhere.com> wrote in message
news:TeqdnVXH36raHD7RnZ2dnUVZ_gadnZ2d@giganews.com...

> Take a Bessel filter and add a Butterworth with cutoff frequency of an
> octave or two higher?

That sounds like a good approach.
Thanks.

> Engineers don't use words like "good", "well", "decent", "reasonable".

Most engineers that I know do.
You don't?

> Engineers use numbers.

I use numbers also.

> So, what do you need exactly? Linear phase approximation? Gaussian impulse
> response? Step response without overshoot?

Step response with mild (~5% overshoot) but having a frequency
response that has substantially better stop-band attenuation than
a Gaussian (or Bessel).
I'll try your combined Bessel-Butterworth approach.
Pete

Reply by gretzteam●September 29, 20102010-09-29

>Pete Fraser wrote:
>
>
>> A Gaussian transition filter is Gaussian to (typically) 6dB or 12dB

down,

>> then it falls off faster.
>
>Take a Bessel filter and add a Butterworth with cutoff frequency of an
>octave or two higher?
>
>> I have used analog versions in the past. They give
>> a reasonable compromise between a well-controlled step response, and
>> decent stop-band attenuation.

If you want to quickly get some Bessel lowpass filters, you can use the
online tool here:
http://www-users.cs.york.ac.uk/~fisher/mkfilter/trad.html
Nice thing of the Bessel filter approx is that the feedfordward coefficient
are quite trivial to implement, and it gives you better attenuation than
cascading first order allpole filters.
Dave

Reply by Vladimir Vassilevsky●September 29, 20102010-09-29

Pete Fraser wrote:

> A Gaussian transition filter is Gaussian to (typically) 6dB or 12dB down,
> then it falls off faster.

Take a Bessel filter and add a Butterworth with cutoff frequency of an
octave or two higher?

> I have used analog versions in the past. They give
> a reasonable compromise between a well-controlled step response, and
> decent stop-band attenuation.

Engineers don't use words like "good", "well", "decent", "reasonable".
Engineers use numbers.
So, what do you need exactly? Linear phase approximation? Gaussian
impulse response? Step response without overshoot?
Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

Reply by Pete Fraser●September 29, 20102010-09-29

"Greg Berchin" <gjberchin@chatter.net.invalid> wrote in message
news:6po6a6t9fkpo6p8b68q7vrhdc6sqrerkt7@4ax.com...

> What is the difference between a Gaussian filter and a Gaussian Transition
> filter? A Google search turns up only US Patent 4051458 for both
> "Gaussian
> Transition filter" and its citation, "'Filter Synthesis' by Ziev".

A Gaussian transition filter is Gaussian to (typically) 6dB or 12dB down,
then it falls off faster. I have used analog versions in the past. They give
a
reasonable compromise between a well-controlled step response, and
decent stop-band attenuation. I thought it was a fairly standard term,
as it's one of the standard filters given in Zverev and its derivatives
(e.g., Williams).

>
> Do you seek an analog implementation, or a digital implementation?

Digital.

>
> An analog Bessel filter is an approximation to a Gaussian filter, and the
> approximation improves as the filter order increases.
>
> Since you asked for an IIR approximation, you could either approximate a
> Bessel
> filter by traditional means, such as step invariance (from context I
> suspect
> that step invariance will be better than impulse invariance for your
> application) or bilinear transform, or by nontraditional means, such as
> FDLS.

Thanks I'll have a look at FDLS.
Pete

Reply by Greg Berchin●September 29, 20102010-09-29

>Is there any software around that designs Gaussian transition
>filters? ScopeIIR and Matlab both deal with most of the
>Zverev standards, but neither deals with GT.

What is the difference between a Gaussian filter and a Gaussian Transition
filter? A Google search turns up only US Patent 4051458 for both "Gaussian
Transition filter" and its citation, "'Filter Synthesis' by Ziev".
Do you seek an analog implementation, or a digital implementation?
An analog Bessel filter is an approximation to a Gaussian filter, and the
approximation improves as the filter order increases.
Since you asked for an IIR approximation, you could either approximate a Bessel
filter by traditional means, such as step invariance (from context I suspect
that step invariance will be better than impulse invariance for your
application) or bilinear transform, or by nontraditional means, such as FDLS.
Greg

Reply by Pete Fraser●September 29, 20102010-09-29

Is there any software around that designs Gaussian transition
filters? ScopeIIR and Matlab both deal with most of the
Zverev standards, but neither deals with GT.
Any suggestions?
I'm looking to do 6dB and 12dB, up to 12th order,
and implementing as DF1 bi-quads.
Thanks
Pete