Talk:Classic Filters

Martyn wrote by email to -devel on 19Aug14
I know that 'Classic Filters' are off for 2.0.6, and don't want to revisit that.

Just a note really on Gale's comments / insistence that the algorithms in the 'Classic Filters' are 'unorthodox versus most standard documentation'. This is not true, as others have pointed out, and is a mis-reading of 'standard documentation'. Indeed the low-pass Butterworth filter in 'Classic Filters' is almost identical to the Nyquist plug-in 'Low Pass Filter', as well as being identical to the curves in Oppenheim and Schafer 'Digital Signal Processing' (my copy dating to 1975, so fairly 'classic'), all IIR filters designed with the bi-linear transform method.

The 'standard documentation' that Gale appears to be referring to is presumably http://en.wikipedia.org/wiki/Butterworth_filter which is about analogue filters, but perhaps he should be referring to something like http://en.wikipedia.org/wiki/Bilinear_transform where it talks about the effects of frequency warping for digital filters.

Further: As far as I know there is no signal-processing reason why 'Classic Filters' should not be included in a release of Audacity, but if there is a documentation issue (which there appears to be) then it shouldn't be in at this point. I am in agreement with Roger, that users should select a filter by the use of their ears.

Aside: On the manual, I don't think that we should be trying to explain the intricacies of filter design, any more than we do on http://manual.audacityteam.org/man/Equalization (which is outdated, but that is OT).


 * Gale 27Aug14: No-one is suggesting we should discuss detailed intricacies of filter design. I suggest you paste the responses to Martyn here then you will see what Steve and I think are the questions the article in the Manual should address (assuming the effect does not change). There must IMO be at least a sentence saying why the Classic Filters first order Butterworth low pass does not have a similar frequency response as the Nyquist first order low pass (despite both are digital IIR filters). We know what that sentence could say but we don't know "why" users should use Classic Filters rather than Nyquist High or Low Pass.
 * Peter 28Aug14: "I suggest you paste the responses to Martyn here" - sorry can't do that, I purged my email folders yesterday so no longer have that thread. By all means delete Martyn's comment if you think it is wrong, irrelevant or not of value.
 * Gale 28Aug14: Added Steve's reply and mine. Note there is a Nabble archive of -devel at http://audacity.238276.n2.nabble.com/audacity-devel-f238278.html.


 * Steve on -devel: First order Classic Filters Butterworth looks (in Plot Spectrum) very distinctly different from the Nyquist version (and similarly different from the vast majority of illustrations that can be found on the Internet). http://en.wikipedia.org/wiki/Butterworth_filter looks much like the frequency response observed in the Nyquist digital filters. http://en.wikipedia.org/wiki/Bilinear_transform may talk about the effects of frequency warping for digital filters but for > 99% of our users is totally meaningless.


 * Gale on -devel: The problems I see are the name of the effect, its choice of filters, and documenting
 * when a user should use Classic Filters instead of Nyquist High/Low Pass?
 * why is Classic Filters better than Nyquist, which might justify its 'apparently untypical' first order Low Pass? (I think we can already say why it's 'untypical')
 * When should user use Chebyshev II and why?

archived ednote
Some information about the filter types from Federico Miyara


 * 1) Roger Dannenberg pointed out that:
 * "For music/audio production, "better" is a strong word. I bet you'd be hard-pressed to hear the difference between these filters or to describe the difference. For a novice, I'd say "if one sounds better, use it; if not, don't worry, they're very similar." Engineers like to optimize things, and by choosing different things, you get different filters. But they're all doing more-or-less the same thing."