Re: bug in IDL's hanning() window-generating function

[bennetsc(at)NOSPAMucs.orst.edu]

In article <9kg7jb$bbt$1@news.orst.edu>,  I, <bennetsc@NOSPAMucs.orst.edu>, wrote:
>I have not filtered the data before applying the window and
>taking the transform because I want to see all of the resolvable
>spectrum.  A data window is still necessary to prevent generation
>of spurious spectral power that would result from the break in
>"continuity" at the data segment boundaries.

     I see I forgot to mention the other big reason for using a data
window, which is to reduce leakage.  Not using a data window is the
same thing as using a rectangle (i.e. boxcar) window, which results
in no leakage at all *iff* the only signals in the data are exactly
at one or more of the Fourier frequencies for the time period being
transformed and there is no noise in the data.  In the real world,
however, that is virtually never the case.  Energy in any frequency
band in a spectrum may lose and/or gain energy to/from any other
band in that spectrum by leakage, which kind of runs counter to the
whole point of analyzing a spectrum.  The amplitude of leakage is
worst for the rectangle window (i.e. no window) and is typically
very much reduced for other windows.


                                  Scott Bennett, Comm. ASMELG, CFIAG
                                  College of Oceanic and Atmospheric
					Sciences
                                  Oregon State University
                                  Corvallis, Oregon 97331
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