Chord detection algorithms?

Question

I am developing software that depends on musical chords detection. I know some algorithms for pitch detection, with techniques based on cepstral analysis or autocorrelation

Answer 1

This is a very difficult pattern matching problem, probably suitable for an AI technique such as training neural nets or genetic algorithms.

Basically, at every point in time, you guess the number of notes being play, the notes, the instruments that played the notes, the amplitudes, and the duration of the note. Then you sum the magnitudes of all the harmonics and overtones that all those instruments would generate when played at that volume at that point in thier envelope (attack, decay, etc.). Subtract the sum of all those harmonics from the spectrum of you signal, then minimize the difference over all possibilities. Pattern recognition of the thump/squeak/pluck transient noise/etc. at the very onset of the note might also be important. Then do some decision analysis to make sure your choices make sense (e.g. a clarinet didn't suddenly change into a trumpet playing another note and back again 80 mS later), to minimize the error probability.

If you can constrain your choices (e.g. only 2 flutes playing only quarter notes, etc.), especially to instruments with very limited overtone energy, it makes the problem a lot easier.