[Audacity-devel] Classic Filters status

[Norm C <n_b...@ho...>]

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
- 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.
- I thought I saw that someone had complained the first order lowpass is
different from the other orders. It's not, they're all completely
consistent. I'll explain further if anyone still thinks this is an issue.
- 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.
- 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.
- 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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