Needed: introduction to FFT I am looking for a great introduction to FFTs for my job. I studied math, but never heard of FFT until I got this job. There is not a lot of documentation at my place of work and the articles I have found online have been somewhat confusing, even the wikipedia entry.

So I am looking for a recomendation for a higher level introduction to FFT. I would perfer free resorces over paid ones, but I can't be too picky here.

Thanks in advance!
I found the following review paper very illuminating:

Fast Fourier Transforms: A Tutorial Review and a State of the Art
by P. Duhamel and M. Vetterli.

It covers a lot of things, from history to implementations.