Re: [Audacity-devel] Classic Filters status

[Gale <ga...@au...>]

Hi Norm,

I'm not strongly opposed but do see docs and use cases as an outstanding 
issue.   


Norm C wrote
> I've been out of touch for a few months but I gather there's been some
> rather polarized discussions regarding the Classic Filters and they're now
> out of the build. In my view that's unfortunate. I'll summarize what I see
> as the issues:
> 
> - badly named: I agree, that's why I called it Scientific Filters when I
> submitted it. There's ample justification for this name:
> CoolEditPro/Audition calls the same effect Scientific Filters, and Matlab
> (certainly a major scientific application) uses these same filters
> (butter, cheby1 and cheby2) as its workhorse filters

I always felt to justify "Scientific" moniker there ought to be more
esoteric
filters e.g. Elliptic, Gaussian and any others that could be useful for
modelling
or educational/research interests (which you give as a use case).

Do you or anyone buy into that? Even if you do, I guess new filters would 
have to wait until release after next. 


Norm C wrote
>    
> - the filters are "unorthodox": Certainly not - as I pointed out,
> CEP/Audition and Matlab use these same filters (with the same behaviour
> near the Nyquist frequency), as do many other audio applications (eg.
> Logic, Goldwave and Wavosaur). Yes, the high frequency response is
> different from the other filters in Audacity, but it's good to have
> choices.

Yes but the greater HF attenuation in your first order Low Pass compared 
to docs that most users will find or understand is a drawback if we're 
presenting these as "Classic". 

Alternatives aren't usually "classic" though I understand the sense in 
which these are "classic".   
  

Norm C wrote
> - documentation is inadequate: OK. CEP's Scientific Filter documentation
> would be a good place to start. And I provided a fair amount of commentary
> on the filter when I first submitted it. I'd be glad to provide more -
> just let me know exactly what needs to be addressed.

Yes this is the main problem (and the first order Low Pass question above 
would I think need a sentence in the docs).  

This is the page in the Manual as it stands:
http://manual.audacityteam.org/man/Classic_Filters .

Not at all ready. Being quite close to 2.1.0, I feel you would need to work 
directly on that page yourself (I can sign you up there). You would need to 
get it passed by someone on the Manual team who only had average
competence in filtering, to verify that it was written in basic enough
fashion 
and said why one would want to use these. 

I know quite a bit of information has been given already about "why" but 
it's quite scattered around in different threads.   


Norm C wrote
> - doesn't work properly on tracks with multiple sample rates (same problem
> as Equalization has, I think): True, and I don't think there's a simple
> consistent way to make it work, as the filter response varies with sample
> rate. At a minimum we'd need to display two different frequency responses
> on the same graph and I think that would be both complex and confusing to
> the user. In my view the solution is to prevent the user from applying the
> same filter simultaneously to tracks with multiple sample rates and
> explain what he needs to do.

That is already addressed so isn't an obstacle. There is an error message
that "all selected tracks must be the same sample rate."


Gale 


Norm C wrote
> - Chebyshev filters are too uncommon to be used here: Certainly they're
> less common than Butterworth but it's far from unheard of to find them in
> audio gear, both analog and digital. I've got lots of examples if anyone
> cares.
> 
> Steve, Gale: you may still have concerns that I've missed here. Let me
> know if so, I'll try to address them.
> 
> Norm





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