One example is audio visualisers, though you can buy chips like the MSGEQ7 that will output amplitudes at a range of fixed frequencies. See for example: http:/

One example is audio visualisers, though you can buy chips like the MSGEQ7 that will output amplitudes at a range of fixed frequencies. See for example: http://elm-chan.org/works/akilcd/report_e.html http://elm-chan.org/works/akilcd/report_e.html - Elm Chan's fftavr library has been around for ages now.