http://music.columbia.edu/cmc/MusicAndComputers/chapter1/01_02.php
http://mirlab.org/jang/books/audiosignalprocessing/index.asp
https://fftguitartuner.files.wordpress.com/2011/03/ee113d-final-report2.pdf
http://digitalcommons.calpoly.edu/cgi/viewcontent.cgi?article=1853&context=theses
http://stackoverflow.com/users/3833092/user39991
机器学习实践：被嗶哩嗶哩抛弃的人
http://stackoverflow.com/questions/21631374/ios-instrument-tuner
https://github.com/catch-twenty-two/iOS-Guitar-Tuner
https://github.com/tfrank64/Pure-Guitar-Tuner
http://isource.bloter.org/content/pitchdetector