Re: [Audacity-devel] Classic Filters status

[Gary N. <por...@gm...>]

All
I've been kibitzing but perhaps it would be helpful to consider the FIR filter as a reference. We get linear phase => constant Group Delay. That is as good as it gets for filters. 

Analog Devices has an online analog filter designer that shows group delay of any design. Butterworths  can be crafted to have nearly constant group delay in pass band. Chebychevs have ripple in the group delay. Given a no cost choice, Butterworth is preferred because it is closer to FIR ideal. 
Gary Nelson


Sent from my iPhone

> On Jan 2, 2015, at 6:09 AM, Steve the Fiddle <ste...@gm...> wrote:
> 
>> On 2 January 2015 at 00:50, Martyn Shaw <mar...@gm...> wrote:
>> 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.
> 
> That makes sense where component count, physical size and
> manufacturing costs of analog filters are relevant, but surely it is a
> spurious trade off for digital filters. Yes a 2nd order Chebyshev
> filter may have a slightly steeper roll-off compared with a second
> order Butterworth, (at the expense of ripple in either the pass band
> or stop band), but for digital filters there is absolutely no need for
> that compromise - if you want a steeper roll-off, use a higher order
> Butterworth filter and then you get a steeper roll-off with no ripple.
> 
> 
>> 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.
> 
> Yes, indeed.
> 
>> 
>> 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 "educational value" of this example would appear to be the
> opportunity to discover that the Chebyshev filters are the worst
> choice of tool for the job.
> 
> 
>> 
>> 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.
> 
> Yes, I agree that would be "tutorial material".
> 
>> 
>> 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. :-)
> 
> For Low Pass and High Pass filters, I think that the documentation
> (http://manual.audacityteam.org/man/Low_Pass_Filter) is quite clear
> that:
> "Low Pass Filter passes frequencies below its cutoff frequency and
> attenuates frequencies above its cutoff frequency. This effect can
> therefore be used to reduce high pitched noise." (and the
> complimentary equivalent for High Pass).
> 
> I don't see the need to go into more detail than that, until we add in
> the complication of Chebyshev vs. Butterworth.
> 
> My impression with this effect is that it has been created as a "good
> idea" (yes I agree that it is a good idea, but with some reservations)
> that Audacity should have these filters, rather than to meet a real
> demand. In other words, a solution looking for a problem to solve.
> 
> The question that I would pose is; what would this effect be like if
> it were designed from the view point of fulfilling audio production
> needs?
> 
> If that were the design criteria I suspect that we would abandon
> "classic" analog modelling and produce a variable slope high pass/ low
> pass filter with zero ripple and probably with linear phase response.
> Such an effect could be more powerful than the current high/low pass
> filters, yet simpler (and less intimidating) than the full featured
> Equalization effect. The graphical interface could easily be extended
> to provide additional filter types, such as band pass/ band stop
> filters, shelf filters, and yes could even include "classic analog"
> modelled filters (Butterworth, Chebyshev type I, Chebyshev type II).
> 
> As Martyn said, just a few thoughts here. I'm not arguing against this
> effect per se, but I am questioning whether its current form is in the
> best interests of Audacity users. My view on that at present is that
> the Chebyshev filters introduce unnecessary complication that is not
> in the best interests of the vast majority of user, for very limited
> benefit for a very small minority of users, and due to bug 660 is
> inferior to what we already ship.
> 
> I would very much like to see this effect progress - it's been
> standing on the side-lines too long. A suggestion for moving this
> forward would be to initially introduce it with just low pass / high
> pass options (Butterworth, but no need for the manual to go into
> detail on that point), preferably with bug 660 resolved (though that
> could be raised to P3 and release noted).
> 
> 
> Steve
> 
>> 
>> 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
>> 
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