Re: [Audacity-devel] Renaming the "1/f" frequency scale
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From: Steve t. F. <ste...@gm...> - 2015-08-20 14:55:36
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On 20 August 2015 at 15:27, Roger Dannenberg <rb...@cs...> wrote: > On 8/20/15 9:42 AM, Federico Miyara wrote: > > > Roger, > > Finally, a minor point is that dB has no units, it's just a ratio (well > actually the log of the ratio) with respect to some amplitude we > arbitrarily call "1.0". Indicating "dB" is important to communicate that > it's a log scale. Similarly, the autocorrelation plots show numbers that > measure "degree of autocorrelation," but the units are arbitrary. Since the > scale is linear, I think just putting numbers alone makes sense. > > > The decibel (dB) *is* a unit. Being a unit doesn't mean that it has a > physical dimension. The radian is another dimensionless unit. See on page > 127 of the International System of Units brochure: > > http://www.bipm.org/utils/common/pdf/si_brochure_8.pdf > > "The decibel *is* a unit" seems a little strong. Did you read page 127? > "some scientists consider that they should not even be called units" and > "The units neper, bel, and decibel ... are not considered as SI units." But > OK, I'll agree that *my* statement, "dB has no units" was questionable. I > stand corrected. > > > However, there are two kinds of dB, absolute and relative ones. Absolute > dB's refer to some reference, such as sound pressure level (reference = 20 > micropascal), or the digital audio dB (reference = maximum unclipped value); > relative dB's refer to quantities such as gain or signal to noise ratio. > > My point was that *in Audacity track displays*, "dB" does not mean time or > micropascals or frequency or any other particular dimension, and the > reference level is unknown. You might argue that there is at least some > unknown sound pressure level corresponding to 0dB in an Audacity track > display, but remember that signals are not necessarily audio signals. I've > loaded all kinds of data into Audacity, e.g. frequency estimates from a > monophonic pitch detector. "dB" then becomes something akin to semitones. We have recently had someone asking about how to load market share prices into Audacity. I guess if they were displayed as a waveform, the vertical scale should be dollars ;-) Steve > > -Roger > > > > Regards, > > Federico > > > > > -Roger > > On 8/19/15 11:43 PM, Paul Licameli wrote: > > I think you are speaking of Audacity's Plot Spectrum window, but I am > thinking of the Pitch(EAC) view of tracks. Your horizontal is my vertical, > and your vertical is my color scale. My horizontal is the center time of a > sliding analysis window. > > I take away that you agree that "period" is a natural enough name for my > vertical. > > In the Plot Spectrum window, the Spectrum algorithm labels the vertical > scale in dB, but I don't know what units make sense for the vertical of the > other algorithms. You tell me there is no particular unit. > > PRL > > > On Wed, Aug 19, 2015 at 11:28 PM, Roger Dannenberg <rb...@cs...> wrote: >> >> Whether you call it "1/f" or "period", the name applies to the horizontal >> axis, which is measured in seconds. In fact, you could also call it "time" >> or "duration" but since the objective is to get a sense of pitch, it's more >> intuitive to use "1/f" or "period". In fact, the position on the horizontal >> axis is the lag (measured in units of time) used in the autocorrelation. >> E.g. at the 10ms point on the horizontal axis, the height is based on the >> correlation between the signal and a copy of the signal delayed by 10ms. If >> the signal is periodic with a 10ms period, the correlation will be perfect >> and the value of the EAC function will be high at 10ms. The correlation is >> less good and the value of the EAC function will be lower at other points on >> the horizontal axis, which correspond to different lags or periods. >> Your description refers to "height on the scale", but height is a >> different axis with different units. Just as a spectrum plot is typically >> magnitude (in no particular units) as a function of frequency (= 1/time), >> EAC is magnitude (in no particular units) as a function of lag or period >> (time = 1/frequency). >> You can plot any function of frequency f, F(f), as a function of >> period p = 1/f simply by plotting F(1/p), so we should be careful to >> distinguish the computation of EAC from the way it is displayed. In the case >> of EAC, the "natural" parameter is lag or period, i.e. EAC(p), so if you >> wanted to make a function of frequency, you'd compute EAC(1/f). >> >> -Roger >> >> >> On 8/19/15 10:42 PM, Paul Licameli wrote: >> >> I don't like the name "1/f" I gave to the last of the new frequency scales >> -- what at first I called "undertone" which Roger didn't like -- so how >> about "period" instead? >> >> Why is it there? Historical reasons, because it is the scale that old >> Pitch(EAC) view was plotted on (though I never saw where the documentation >> explained that). If you really liked the old Pitch view (and figured out >> that you needed to fully zoom out the spectrogram view scale first), you can >> still get the same displays. >> >> 1 / f does not really describe it -- really, pixel height as a function of >> f is more like a - b / f for certain positive constants a and b. >> >> "Period" is easier to explain, period varies linearly with height on that >> scale, though decreasing as height increases. >> >> PRL >> >> >> >> >> ------------------------------------------------------------------------------ >> >> >> >> _______________________________________________ >> audacity-devel mailing list >> aud...@li... >> https://lists.sourceforge.net/lists/listinfo/audacity-devel >> >> >> >> ------------------------------------------------------------------------------ >> >> _______________________________________________ >> audacity-devel mailing list >> aud...@li... >> https://lists.sourceforge.net/lists/listinfo/audacity-devel >> > > > > ------------------------------------------------------------------------------ > > > > _______________________________________________ > audacity-devel mailing list > aud...@li... > https://lists.sourceforge.net/lists/listinfo/audacity-devel > > > > ------------------------------------------------------------------------------ > > > > _______________________________________________ > audacity-devel mailing list > aud...@li... > https://lists.sourceforge.net/lists/listinfo/audacity-devel > > > > > ------------------------------------------------------------------------------ > > > > _______________________________________________ > audacity-devel mailing list > aud...@li... > https://lists.sourceforge.net/lists/listinfo/audacity-devel > > > ------------------------------------------------------------------------------ > > _______________________________________________ > audacity-devel mailing list > aud...@li... > https://lists.sourceforge.net/lists/listinfo/audacity-devel > |