Re: [Audacity-devel] Classic Filters status

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

Hi

Sorry I have not got to this sooner.

I have read all the rest of this thread and see that there is 
confusion over when to use the different filter types in an audio 
production context, and a desire to document that.  I cannot offer any 
advice on when to use which for audio production I'm afraid, other 
than listening to the result.  I think Roger said the same.

Obviously the different 'classic' types were originally optimised for 
different things in the world of analogue (and then transferred into 
the digital world) as others have said.  I won't repeat what the 
optimisations were here, but to address the 'editor comments' on
http://manual.audacityteam.org/man/Classic_Filters
the ripple that you allow in the passband for Type 1 is traded off 
against the attenuation that you get in the stopband.  For Type 2 the 
'Minimum Stopband Attenuation' that you specify is traded off against 
the attenuation at the end of the passband.  You can see that in the 
graph if you try different values.  Some of the other comments are no 
longer valid as the boxes are invisible.

In terms of the filter 'order' (or 'Rolloff (dB per octave)'/6) in the 
case of the Nyquist ones), it may be worth noting that the Nyquist 
ones go to 8 (1, 2, 4, 6, 8), 'Classic Filters' go 1 to 10, whereas 
the Equalization effect defaults to 4001 and can be pushed to 8191; 
OK, not quite comparing like with like but you get the idea of how 
powerful the Eq is compared to the others.

Use case:
Say a user wanted to recover speech from a degraded source that had 
lots of interference, particularly at high frequencies (and they 
recognised this).  The easiest option would be the 'Low Pass 
Filter...' Nyquist effect (fewest parameters), then the 'Classic 
Filters...' where they could play with the different options to reduce 
the unwanted HF noise, then the 'Equalization...' where they have 
near-unlimited stuff to play with.  That is a situation that users may 
be faced with, and 'Classic' gives an extra option that could be 
educational.

The Classic's GUI has a similar level of complexity as the EQ (in 
'Draw Curves' mode) in terms of interpreting what they see in the 
graph, but they need some knowledge to use that.  They need to know 
how to use the 'Plot Spectrum...' tool and/or the Spectrogram view to 
make best use of it, but that is surely the purpose of a tutorial, 
rather than the manual.

I do not see any explanation in
http://manual.audacityteam.org/man/Low_Pass_Filter
or
http://manual.audacityteam.org/man/Equalization
of which we should use when and so do not really understand the 
insistence that
http://manual.audacityteam.org/man/Classic_Filters
should have the same. :-)

So, just a few thoughts here.  I'd like Classic to be in this release 
for further comment, as you know.

TTFN
Martyn


On 30/12/2014 09:54, Peter Sampson wrote:
> Martyn responded to Gale:
>  >> This is the page in the Manual as it stands:
>  >> http://manual.audacityteam.org/man/Classic_Filters
> <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.
>
> Your feedback as it being "not too bad" Martyn as someone with good
> experience
> in this area of expertise is useful to hear.
>
> I think that what we on the Manual team have always felt lacking on that
> page was some editorial input by the effect's author - which is why
> both I and Gale
> have recently written, inviting Norm to participate.
>
> In particular what we feel is missing is some description of where and
> why you
> might choose to deploy the filers in this effect - and some use cases
> or examples
> that can be included to aid the non-expert reader.
>
> Perhaps Martyn, if Norm cannot be persuaded to engage, we could
> persuade you
> to assist with the improvement of this page?
>
> None of us on the Manual team afaik has the necessary expertise to
> provide this,
> we've already taken it as far as we can with our limited knowledge.
>
> Cheers,
> Peter.
>
> Peter Sampson
> Tel: +44 (0)1625 524 780
>