5. A transmitting station for use in a system as claimed in claim 1 or 2, wherein the transmitting station is suitable for the transmission of several, different code words in order to selectively activate a receiving station which is identified by an associated code word.
7. A receiving station for use in conjunction with a transmitting station as claimed in claim 5 wherein said decoder comprises means for forming an internal activation signal under the control of said message indicating a valid code word.