Note pitch (Note errors)

For a MIDI input, we don't have many difficulties to define what is a right note and what is a wrong one: The pitch of the played note is not exactely the same as the expected one, then we have a wrong note.

It's a little bit more difficult with an audio input: We must in this case define a frequency threshold $\delta$f and consider we have the right note if its frequency is between $(f - \delta f)$ and $(f + \delta f)$