Dear Ge & Rebecca, The previous version of ChucK had a /examples/analysis/tracking directory with a great example called "tracking.ck" in it. In the latest download that's gone. It's not so clear to me why that example should be gone because that happened to be the one that makes it clear how the numbers in the array that FFT yields relate to frequencies in the most clear way. On top of that it's fun and practical. As interesting as the examples with the current release might be they don't really make it clear what they are trying to accomplish and how. They run... they computate some numbers and print them but it's not explained what those numbers represent or why we would be interested in them. Because of a recent question on the forum about pitch tracking/detection I dug up the old example to post as a explanation and had to wonder about the how and why of the current range of examples. Much of it is quite baffeling to me; I'd be hard pressed to explain what goes on in the fft examples and the feedback they use. The code of the tracking one may be far longer and more involved but it's purpose is clear, it invites "poking around" and so I think it makes for a practically educational example to include. Yours, Kas.