Rubato in the strictist sense refers to piano practice, specifically
Chopin. It is a difficult concept for me to understand as it refers to
the left hand playing its part in a different tempo than the right hand.
Whatever give and take between the two hands, must equalize itself and
come out together. In relating to trumpet playing (solo playing of any
instrument) the term rubato has come to indicate freedom of rhythm/tempo.
Ideally it should balance itself as in whenever one speeds up in a phrase
there should be a corresponding slowing as well. --- Again this equalizes
out. "ad libitum" or as I learned "ad liberto" means "at ones liberty".
Again the "freedom" concept. To many the generic term rubato and ad
libitum would be the same, it just depends on how specific one wishes
to get in exact definitions.
Bill Dishman
Gainesville, Florida