yolande_yj,
For a nice discussion of windows, see the following:
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h**p://www-nt.e-technik.uni-erlangen.de/WORLD/LNT/lehre/dsv/DSP_p0-591bw.pdf
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Window selection always involves a tradeoff between passband ripple and transition band steepness. The Kaiser window is convenient because it provides a continuum of ripple specifications by the selection of a single paramter. As ripple improves, transition steepnes gets worse, and vice-versa.
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Regards,
Kral