One of the challenge was to find out when a note is played, and for how long.

For example, on the following simple piece of music,

the program must understand that:

1. at the very beginning of the song, that is at \( t=0s \), a pianist must press the la key (aka a).
2. at \( t=0.6s \), a pianist must release the la key and simultaneously push the sol key down (g)
3. at \( t=1.2s \), a pianist must release the sol key.

To extract these pieces of information I used two features provided by Lilypond: the include-settings parameter, and the event listener.

include-settings let the user include a file to set some global settings. That file is included before the score is processed. The event listener saves music events encountered during the music processing.

For this simple music sheet , the generated music events are:

0.00000000	tempo	400.00000000
0.00000000	note	69	4	0.25000000	point-and-click 4 36
0.25000000	note	67	4	0.25000000	point-and-click 9 360.00000000	tempo	400.00000000
0.00000000	note	69	4	0.25000000	point-and-click 4 36
0.25000000	note	67	4	0.25000000	point-and-click 9 36

That is with the default event-listener. Lilypond provides a way for the user to change that so more informations can be extracted.

Out of this, we get that the tempo is 400. This 400 is almost the tempo we wrote. This 400 comes from the fact that we defined a tempo based on quarter note.

The next line says that at \( t = 0 \) the key 69 (computer code for la) is played for a quarter note. And a quarter note after the beginning, the note 67 (key code for sol) is also played for a quarter note.

There are some basic maths involved to get from these tempo and quarter timing into time a computer can use to understand when to start playing a note, and when to stop.

One important thing to keep in mind though, is that the tempo can change during the music. These new tempos need to be taken into account when computing the time a note is played released.