Re: [Audacity-devel] Classic Filters status

[Steve t. F. <ste...@gm...>]

On 30 December 2014 at 13:45, Federico Miyara <fm...@fc...> wrote:
>
> Steve,
>
>
> but other than Butterworth, they are not commonly used in audio production
> software.
>
>
> Probably not even Butterworth is actually used, since it doesn't ensure
> linear phase response as FIR (finite impulse response) does.

Nevertheless, digital implementations of Butterworth filters are still
very widely used in audio production (for example, for rolling off low
bass in microphone recordings).

>
> I think that we all agree that the primary purpose of Audacity software is
> for audio production,
>
>
> When any software reaches certain level of sophistication its usage range
> widens a lot. I think it is the case of Audacity. Many people around the
> world use it for teaching rather than just audio production. Goldwave, for
> instance, more than a decade ago, included means to implement any kind of
> digital filter that can be specified in terms of its coefficients.
> Flexibility is a very appreciated feature by the one-percent people that
> will use that kind of feature.
>
> so it would seem strange to me if we shipped "obscure" (in audio production)
> filters (such as the Chebyshev type II) and not include much more common (in
> audio production) ones (such as band pass, band stop, sinc,
> Linkwitz-Riley...)
>
>
> Band pass may also be Butterworth or even Chebyshev (they are used in
> spectral analyzers). Sinc is used for signal reconstruction and for signal
> oscillogram rendering, mainly in digital oscilloscopes (audio software uses
> some kind of spline, since the image doesn't need to be that accurate).
> Linkwitz-Riley is used in speaker crossovers, not in audio processing. They
> cpould be used to simulate the result of crossover.

Sinc filters are useful in audio production to avoid aliasing
distortion (for example, when speeding up a recording).
Digital implementations of Linkwitz-Riley filters may be used for
splitting out an LF channel for multi-channel audio.

It seems a simple and obvious question, but so far no-one has provided
an answer - why would I choose to use a Chebyshev filter rather than
any other type of filter? (clearly, component count for hardware
implementation is irrelevant in Audacity). It bothers me that we are
considering adding a feature for which there is apparently no
practical use.


>
> As I wrote previously, I can completely see why we would want to include the
> Chebyshev filters in SPICE software, but I still don't know the answer to
> the audio production question "when and why should I use a Chebyshev
> filter?" For most effects the manual says why/when an effect would be used -
> a typical job - we still don't have that for Chebyshev filters.
>
>
> Chebyshev has been used in vintage and low-cost antialiasing filters. I
> think it is not frequently used nowadays since sigma-delta converters are
> much mor common. They could be used for demonstrating the effect of analog
> Chebyshev implementations, i.e., for filter simulation.

Surely there is much better software for analog circuit simulation, for example:
http://en.wikipedia.org/wiki/List_of_free_electronics_circuit_simulators

Steve


>
> Regards,
>
> Federico
>
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