Why is the Window in the Frequency Analyzer not constant?

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Why is the Window in the Frequency Analyzer not constant?

Permanent link to this post Posted by Maltodextrino » Wed Aug 17, 2016 1:26 am

Hello, and sorry for my english, but i found no help in german Audacity Forums..
I use the Audacity Frequency Analyzer (Frequenanalyse) very often, but to be true: Often it does not help me because i cannot compare the results. The Window changes every time. When i analyze Audio with much different Frequencies the Window goes from -24dB to -36dB for example. When i analyse Audio with great changes in Frequencie, the window goes from -12dB to -84dB and so on. So, the result ist: Very different Audio looks the same way in the window. I want a analyser where every time the Window goes from -0dB to -100dB (or -90dB...) and only the grafic of the Audio Frequencies changes, so that i can compare the result. So now, the Analyzer is a very Funny tool, but in 50% in all cases useless because i cannot compare the results. Is there any possibility to solve the problem?
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Re: Why is the Window in the Frequency Analyzer not constant

Permanent link to this post Posted by kozikowski » Wed Aug 17, 2016 3:04 am

I thought by adding a 0dB audio tone into the show, the tool would default to a standard amplitude scale each time. It didn't work.

Then I tried comparing two simple known audio tones to each other. It got the frequencies correct, but both the relative and absolute amplitude values seem magic.

Different window settings just result in different magic.

???

Koz
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Re: Why is the Window in the Frequency Analyzer not constant

Permanent link to this post Posted by steve » Wed Aug 17, 2016 8:41 am

The results can be compared, but it is often not easy to compare the graphs easily by just looking at the "shape".

For as long as I've known, Plot Spectrum has used auto-scaling. In early versions, this was worse than now as there were no options to zoom or scroll the vertical scale. In more recent versions, there are two vertical sliders for zoom and scroll, but these only zoom and scroll within the range that has been determined by the auto-scale algorithm, so they may "sometimes" help, but are much less useful than they could be. Unfortunately the developer that was working on this is no longer active in Audacity development due to work commitments (and greatly missed). Fortunately, because Audacity is "open source software", someone else may pick up the challenge of improving this feature.

In order to compare spectrum plots, you need to look at the numbers on the scales. Assuming that the settings (Algorithm, Size, Function) are the same, and the same sample rate is used, then dB figures mean exactly the same each time.

What happens with auto-scaling is that the top end of the dB scale is set close to (above) the maximum level measured. The bottom end of the scale is set close to the lowest level that is measured, but ignores measurements that are considered "insignificant". I'm not sure how exactly it determines what is or is not "significant", but this is probably the most common cause of problems.

Personally I think that auto-scaling should be off by default, and I think this would resolve the problem that you describe. If you agree, then we can log your "vote" for this option (or you are free to make other suggestions / requests).

I'll move this topic to the "Adding Features" part of the forum.
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Re: Why is the Window in the Frequency Analyzer not constant

Permanent link to this post Posted by steve » Wed Aug 17, 2016 8:49 am

kozikowski wrote:Different window settings just result in different magic.

To compare apples with apples (or oranges with oranges), the same windows settings must be used for each measurement.

The simple way to think of this is that if you weigh one apple in ounces, and another apple in grammes, then you will get measurements that cannot be easily compared - you would need to know the mathematical formula to convert from one to the other. The mathematical formula to convert between different window settings is complicated (bordering on "magic") - much more so than converting grammes <-> ounces.

More information is provided on this page of the manual: http://manual.audacityteam.org/man/plot_spectrum.html
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Re: Why is the Window in the Frequency Analyzer not constant

Permanent link to this post Posted by Maltodextrino » Wed Aug 17, 2016 6:06 pm

Look here what i mean:
Image

http://img5.fotos-hochladen.net/uploads ... mdygje.jpg

On the left you the Spectrum of Browne Noise (more deep frequencies), on the right pink noise (more higher frequencies). Because of the window it looks almost the same. you must watch on the numbers bur you cannot see visualy, that the right one is the one with more higher frequencies.

I think it would be better like here (i made it with photoshop...)

Image

http://img5.fotos-hochladen.net/uploads ... jwr1yt.jpg

The window is always from 6dB to -90dB (you can zoom in of course) and you see visualy, that the right one (pink noise) has more higher Frequencies, compare to the left one. that would be easier i think. I know, Audacity is not a "much money project", but its great and it would be great when it goes like this.

PS: I have Version 2.1.1.
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Re: Why is the Window in the Frequency Analyzer not constant

Permanent link to this post Posted by Maltodextrino » Wed Aug 17, 2016 6:21 pm

steve wrote:The results can ... If you agree, then we can log your "vote" for this option (or you are free to make other suggestions / requests).

I'll move this topic to the "Adding Features" part of the forum.


Yes, of course i agree. I dont know what to do, so you can do what nessesarrly...
I see you understood my "problem", but i described it, with screenshots, in my formerly post.
Thanks!

PS: And it would be great when i can use a future version on Audacity also on mac os 10.6.8. ^^
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Re: Why is the Window in the Frequency Analyzer not constant

Permanent link to this post Posted by kozikowski » Wed Aug 17, 2016 11:36 pm

then dB figures mean exactly the same each time.

I think you're describing a good deal more consistency and rationality than actually exists.

I made a tone at 1 or 100% or 0dB. I measured exactly that in Plot Spectrum. That spectrum measurement wanders depending on whatever else is included in the measurement. I don't mean the range displayed wanders, that's old news, I mean the actual measurement is taken as far off as -12dB depending on its neighbors in the measurement.

The "size" is maxed out and it doesn't seem to matter which "window" I use.

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Re: Why is the Window in the Frequency Analyzer not constant

Permanent link to this post Posted by steve » Thu Aug 18, 2016 12:38 am

kozikowski wrote:The "size" is maxed out

That's the problem. With the size maxed out and sine tones, the points of the peaks are less than 1 pixel wide. If you stretch the window as wide as your screen (or wider), then you will probably see the peaks reappear.

What it should probably do is, if the peak is less than 1 pixel, round up the width to 1 pixel (but it does not currently do that).
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Re: Why is the Window in the Frequency Analyzer not constant

Permanent link to this post Posted by Gale Andrews » Thu Aug 18, 2016 4:32 pm

Maltodextrino wrote:it would be great when i can use a future version on Audacity also on mac os 10.6.8. ^^

Please explain what you mean in more detail. Audacity 2.1.2 should run on 10.6.8.


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Re: Why is the Window in the Frequency Analyzer not constant

Permanent link to this post Posted by waxcylinder » Mon Sep 19, 2016 10:42 am

And answer came there none ...
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