Digital Signal Processing

For a basic introduction to Fourier Transforms, the theory behind signal decomposing signals into a combination of various single-frequency sinusoidal, I found thefouriertransform.com to be a good overview.

For an excellent, free, practical, and reasonably comprehensive overview of a lot of digital signal topics, see Steve Smith's online textbook (downloadable for personal use).

Topics include
* Sampling theory/fundamentals
* Analog to digital and digital to analog conversion
* Convolution and Fourier Transforms
* Digital Filters
* Various Applications examples
* Further advanced mathematics (e.g. Laplace transform, z-transform)

For most algorithm discussions (FFT etc.), he includes code (sadly it's BASIC) to illustrate, a nice touch.

ASSOCIATED LINKS
Fourier Transform
Discreet Fourier Transform
Fast Fourier Transform