# FFT and Windowing



## JayMB (Mar 28, 2012)

I was hoping to open up a discussion amongst you EE types to expound upon the advantages and disadvantages in different windows in different situations. Would anyone be kind enough to explain this in light of frequency response, THD and IMD measurements, burst measurements, and CSD? John K had advised to just stick with Blackmann Harris for SE while the ARTA manual suggests different windows for different circumstances. thnx.


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## lcaillo (May 2, 2006)

How familiar are you with the concept of FFT?

Here is a very crude and brief explanation...

The general idea is that any signal can, theoretically, be described as the sum of sin waves at various frequencies. Now to break down a signal into components at different frequencies requires filtering everything out except the signal at each frequency. If you break the signal into components that contain all of a particular frequency's signal you could use a rectangular window that cuts off everything else. Most signals are complex, however, and we often take "chunks" that contain components of other frequencies. The window function defines how much of those nearby frequencies is included in each chunk. The complexity and type of signal and the resolution of the FFT can impact how well each component represents the signal at a given frequency. Since music contains many frequencies that are many times larger than the shortest wavelengths, it is hard to parse them into perfect components with a consistent rectangular window. So we use a function that takes parts of adjacent frequencies, so that each component is a reasonable representation of its frequencies and the total sums up to the source signal.


Try this white paper from National Instruments for a better description and suggestions about when you would use what type of window.
http://www.ni.com/white-paper/4844/en/


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