C/C++/Obj-C Real-time algorithm to ascertain Note (not Pitch) from Vocal Input

Question

I want to detect not the pitch, but the pitch class of a sung note.

So, whether it is C4 or C5 is not important: they must both be detected as C.<

Answer 1

Pitch is a human psycho-perceptual phenomena. Peak frequency content is not the same as either pitch or pitch class. FFT and DFT methods will not directly provide pitch, only frequency. Neither will zero crossing measurements work well for human voice sources. Try AMDF, ASDF, autocorrelation or cepstral methods. There are also plenty of academic papers on the subject of pitch estimation.

There is another long list of pitch estimation algorithms here.

Edited addition: Apple's SpeakHere and aurioTouch sample apps (available from their iOS dev center) contain example source code for getting PCM sample blocks from the iPhone's mic.