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Mike Raiford wrote:
> Looking at dspguide.com (The first site that actually got me to make a
> bit of sense out of FFT in the first place) it seems the overlap is
> quite simple.
dspguide.com is the resource I used - and yet, it's really very good,
IMHO. The stuff about IIR filter design is quite fun. ;-)
> What to overlap seems to be the length of the filter
> kernel... But, how do you determine that if, say ... you present the
> user with an interface that allows them to create a graph of which
> frequencies to pass...?
If you have an N-point filter kernel, you can take X samples of input,
add on N-1 zeros at the end, FFT, multiply, inverse-FFT, overlap by N-1
samples with the previous window. This should produce the same results
as doing a normal convolution.
Using a larger N gives you more precise control over the frequency
response (because there are more points in it).
> I suppose this is where you'd want to use a Blackmann window to clean up
> the edges of the kernel before applying it to the input data.
You know how if you JPEG-compress something too much, you get ghosting?
Well if you aren't careful with your filter kernel, your frequency
response ends up with ghosting. The Blackmann window is a way to try to
get rid of that. (By blurring the whole thing, unfortunately.)
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