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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