Determines the probability that a particular instrument dominates the song; this can also be used to assess whether there is a distinct melody. A value greater than 0.45 indicates its presence, and a value greater than 0.7 is considered significant.

If 1, usually means the song is monophonic, and can't be uesd to show a valid Mid-Focus Level.

Return:
{
'mid_focus_level': [ 0， 1 ]
}