  We investigate cellular automata where some global quantity varies
periodically or quasiperiodically with time. We find that these systems are
highly predictable, and can be rather well described by a set of mean-field
variables. We conclude that this is not a collective phenomenon - where
different subsystems are supposed to synchronize - but rather like many very
weakly coupled oscillators fluctuating around one exact frequency. The global
quantity measured is a mean taken over all these subsystems, and gets more
precise the larger the system is.
