We introduce the synchronization of a pushdown automaton by a sequential
transducer associating an integer to each input word.
The visibly pushdown automata are the automata synchronized by an one state transducer
whose output labels are -1,0,1.
For each transducer, we can decide whether a pushdown automaton is synchronized.
The pushdown automata synchronized by a given transducer accept languages which form
an effective boolean algebra containing the regular languages and included in the deterministic
real-time context-free languages.
