MIDI Data

Unlike audio, MIDI data allow few, but very precise parameters: pitch, duration, velocity, time... Of course, in real-time played music, duration is only known a posteriori. But the NOTE-ON/NOTE-OFF mechanism allows us to detect very simply note's start and note's end.
Another great advantage can be found in polyphonic music: we have, with MIDI, all the informations, while polyphonic audio data are more difficult to use.

The greatest problem with MIDI is the limited number of instruments that can product it. Except Keyboard instruments (MIDI-Piano, Organ, MIDI-accordeon...), the produced MIDI is not very clean. There are mainly two ways to produce MIDI for non-keyboard instruments:

Pitch trackers:
The audio signal is acquired, analysed, and MIDI parameters are extracted and outputed. Well working on simple signal, but often produce errors with harmonics on attack, or with special musical features like vibrato, tremolo or tongue-flat for wind instruments.

Instrument "midification":
Captors are placed on the instrument to capture the mechanic features of the note without having to analyse it: for example MIDI-captors on guitar strings, mechanic captors on flute keys... The problem is always a lack of precision: for the guitar, note's beginning is easy to determine, but note's end is threshold depending. For the flute, attacks are detected by a microphone near the embouchure, and pitch with keys positions: the problem is we are not able to detect octaviations, since keys positions are the same for octaved notes, for example A4(440Hz) and A5(880Hz).
Furthermore, such instruments are rare, expensive, and very often heavy and hard to play because of all the "added mechanisms".