  Covariant phase difference observables are determined in two different ways,
by a direct computation and by a group theoretical method. A characterization
of phase difference observables which can be expressed as difference of two
phase observables is given. Classical limit of such phase difference
observables are determined and the Pegg-Barnett phase difference distribution
is obtained from the phase difference representation. The relation of Ban's
theory to the covariant phase theories is exhibited.
