[Audacity-devel] Reassigned spectrograms

[Paul L. <pau...@gm...>]

As of
https://github.com/audacity/audacity/commit/3ddbbd375d6963838b91fe2f54c91df96d90e79a

Now reassignment is a third algorithm choice in spectrograms preferences
and view settings.

This is an algorithm that sharpens the results of a spectrogram.  It works
by computing two extra FFTs using different windowing functions.  The
contributions that the usual FFT makes to pixels on the screen, are in
effect moved slightly using results of the other FFTs.  This can counteract
both the "time smearing" and "frequency smearing" effects.

But not only is this spectrogram about three times as costly to compute --
I also had to disable the caching that allows horizontal scrolling of
spectrograms to go faster.  This sort of caching is important for
scroll-scrub with spectrogram on.

The zero padding of spectrograms that I implemented in the last version was
meant to accompany this.

Try it out with tones and chirps -- you will find just a thin bright line,
even if your window size is small!

Try it with an impulse -- you will find a bright vertical line, even if
your window size is large!

Try it with real world examples -- and it is true there are strange
artifacts from "cross terms" if you have two or more sources of sound, or
inharmonic sounds, which make a sort of gauzy pattern as in this article.
https://en.wikipedia.org/wiki/Reassignment_method


PRL