Re: [Audacity-devel] Classic Filters status

[Martyn S. <mar...@gm...>]

Hi. I guess Gale is where this is aimed primarily...

On 28/12/2014 15:28, Gale wrote:
> Hi Norm,
>
> I'm not strongly opposed but do see docs and use cases as an outstanding
> issue.

'Classic Filters' (that is, traditional analog IIR filters implemented 
in the digital domain) are just as valid filters, to a user, as the 
rather crude pre-sets available in the current Equalization effect. 
But read on.

I do not agree that documentation is an issue for this new effect.

> 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

Agreed, workhorse filters.  Should be in the build.

> 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).

Well, Gale, I don't think you have the background to discuss Elliptic 
filters, or any others.  The ones selected here are the most common by 
a long way, I think.

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

Why would it have to wait?  More esoteric filters could, obviously, be 
added.  Maybe you could find yet another word on wikipedia and hold 
that up longer, making the effect more complex and documentation 
demands higher.  You say that you are "not strongly opposed", so still 
opposed to this being released, and making the barrier still higher.

> 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".

I disagree, as I have before.  Just because you, Gale, don't have a 
thorough understanding of DSP, it does not mean that users won't find 
these filters useful.

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

No, I don't think you do understand Gale.  Please read, for example,
http://web.mit.edu/~cjoseph/Public/Discrete-Time_Signal_Processing_-_Oppenheim_-_2nd_Edition.pdf
especially chapter 7 on 'Filter Design Techniques', or trust that 
people who do understand that stuff know what they are doing.

> 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.

I disagree.  That page isn't too bad for a new effect.  Somebody has 
put a lot of useful effort into that.

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.

That sounds fair enough.  Make sure that the manual page is 
understandable by the people who usually write those pages.  It looks 
OK to me.

TTFN
Martyn

> 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
>
>
>
>
>
> --
> View this message in context: http://audacity.238276.n2.nabble.com/Classic-Filters-status-tp7565152p7566434.html
> Sent from the audacity-devel mailing list archive at Nabble.com.
>
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