1. A method of operating a man-machine interface unit included in a home network system, a home entertainment system, or a service robot, the method comprising: classifying at least one utterance of a speaker to be of a first type or of a second type, wherein if said utterance is classified to be of said first type, said utterance belongs to a known speaker of a speaker data base, and if said utterance is classified to be of said second type, said utterance belongs to an unknown speaker that is not included in said speaker data base; storing a set of utterances of said second type; clustering said set of utterances into clusters, wherein each cluster comprises utterances having similar features; automatically adding a new speaker to said speaker data base based on utterances of one of said clusters; adapting a model of said known speaker based on said at least one utterance, if said utterance is of the first type; and splitting said model depending on an acoustic distance measure and on a user profile of a user.