A data line (12) and a synchronisation line (13) extend throughout the length of the antenna with connections to each circuit (10) at a site of one or more hydrophones (11). The clock generator (20) connected to the synchronisation line drives a time interval counter (21) for simultaneous scanning of all hydrophone sites and digitisation (23) of the received signals, and for serial transmission of the digitised signals over the data line. After the lapse of the scanning interval a channel counter (25) can be activated and its count compared (27) with a site-characteristic channel number (26).
