Data windowing ?   Leave a comment

Link 1 to a simple explanation (Thanxs to arthur skudra for sending me this link)

Link 2 to a simple explanation (Thanxs to arthur skudra for sending me this link)

So what does this mean ? I did an experiment with SMAART7 with a sine wave as the reference signal .

First at 749Hz up to 751Hz no data window applied .

NO WINDOW sine wave @ 749Hz001 nw @749

NO WINDOW sine wave @ 750Hz002 nw @750

NO WINDOW sine wave @ 751Hz003 nw @751

Now with a HANN data window : sine wave @ 749Hz004 HANN @ 749

Now with a HANN data window : sine wave @ 750Hz005 HANN @ 750

Now with a HANN data window : sine wave @ 751Hz006 HANN @ 751

NO WINDOW sine wave @ 1499Hz007 nw @1499

NO WINDOW sine wave @ 1500Hz008 nw @1500

NO WINDOW sine wave @ 1501Hz009 nw @1501

Now with a HANN data window : sine wave @ 1499Hz010 HANN @1499

Now with a HANN data window : sine wave @ 1500Hz011 HANN @1500

Now with a HANN data window : sine wave @ 1501Hz012 HANN @1501

You can see the slight changes if you change the frequency by 1Hz steps at the 750Hz/1500Hz area .

So here are some more screenshots first .

Why did i choose these particular frequencies ? Well if you take the sample rate and divide that by the smallest FFT size possible within smaart you 48000/128=375Hz . The effect of windowing was more clear at 750Hz and 1500Hz which are multiples of 375Hz .

After that i thought it would be interesting to see what happened at different points so i divided the 48000 using a 1K fft so you add 48Hz to 750Hz = 798Hz and the same thing at 1500Hz=1548 . You can see the nw (no windowing applied) showing level differences and with han window the trace becoming more narrow at 797Hz and 1547Hz .

I’m guessing a bit here but if you read the file i suggests that at these points the sine wave starts exactly at the right frequency compared to the frequencies the FFT size “likes”.

So this was just a point of interest for me (also this might be a question i get during seminars :”what the heck does data windowing mean within the spectrum options menu.”)

If you do not care to much about this stuff use the Han window and let it be…..Glimlach.

Posted 12/11/2013 by timobeckmangeluid in Geen categorie

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