Is 'Bessel' digital lowpass filter standard?

Hi,

I put an old IIR filter design script here:
https://drive.google.com/file/d/0B1gLUU8hXL7vN0ZQZHFjUlo3VG8/edit?usp=sharing
I make no promises on this algorithm...

The intention is to design a minimum-phase (=>low group delay) filter for a
given magnitude response. As it doesn't know the phase, it designs for some
starting value, then takes the phase of the result as input to the
least-squares solver and tries again several times.

You can control the result by
- specifying the frequency response you want (i.e. "1" for passband, "0"
for stopband
- setting a weight for different regions, trading passband ripple against
stopband rejection. Leaving "don't-care" regions with "0" weight will
improve performance in other areas.

You could also have a look here:
http://www.dsprelated.com/showcode/20.php
Slightly different approach - it creates the phase from the amplitude
spectrum via Hilbert transform.	 

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.. but if I use an elliptic (Cauer) or Chebyshev filter, it won't be that
much different from the above, and I get it with standard design methods.

These filters have significant group delay ripple (clearly asymmetric
impulse response) but are fast overall (the IR "kicks in" rather quickly).
I understood that is the requirement, otherwise this would be a little out
of place under the "Bessel" title...	 

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>.. but if I use an elliptic (Cauer) or Chebyshev filter, it won't be that
>much different from the above, and I get it with standard design methods.
>
>These filters have significant group delay ripple (clearly asymmetric
>impulse response) but are fast overall (the IR "kicks in" rather
quickly).
>I understood that is the requirement, otherwise this would be a little
out
>of place under the "Bessel" title...	 
>
>_____________________________		
>Posted through www.DSPRelated.com
>

mnentwig, thanks for your assistance here. I haven't looked through your
script yet as I had parked this for a while, but will definitely have a
look through it as soon as this issue comes back to bite me in the ass in
the not so distant future!	 

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Posted through www.DSPRelated.com

> I'm just wondering why I had such a hard time to find reference to a Bessel
> filter in the digital domain.

Google for Thiran-Filter instead.

See: Jean-Pierre Thiran: Recursive Digital Filters with Maximally Flat Group Delay. IEEE Transactions On Circuit Theory, Vol. CT-18, No. 6, November 1971.

On 5.3.18 14:49, tobiaszawada@gmail.com wrote:
> 
>> I'm just wondering why I had such a hard time to find reference to a Bessel
>> filter in the digital domain.
> 
> Google for Thiran-Filter instead.
> 
> See: Jean-Pierre Thiran: Recursive Digital Filters with Maximally Flat Group Delay. IEEE Transactions On Circuit Theory, Vol. CT-18, No. 6, November 1971.


You do not usually need them. A Bessel filter is an approximation
to a constant-delay filter, but in the digital domain, it is
possible to construct FIR (finite impulse response) filters with
constant delay.

(For completeness, it is possible to contruct a FIR filter in
the analog domain, if suitable delay elements are available).

-- 

-TV