Abstract: A first unit (10) comprises a first transmitter (12) for wireless transmission of a signal in the from of periodic frames comprising identity information encoded in varying form in each frame, and a second unit (20) comprises a first receiver (22) for receiving said signal and for causing an alarm (29) to operate in response to failure to receive or to recognize identity information. Means (16, 26, 15a, 25a, 15b, 25b) are provided for automatically deactivating at least one of the first and second units when they are physically close together, and for automatically activating said units when they are physically separate from each other.

Abstract: The switching matrix comprises two data bus each one with n channels for conveying video signals, one address bus, control conductors and an array of N switching points; each of said switching points has two output switches, two input switches, one circuit receiving address signals to select on each data bus a channel corresponding to the received address, one circuit receiving control signals designed to validate either the pair formed by the first output switch and the second input switch, or the pair formed by the second output switch and the first input switch, to validate the setting up of a connection between the video transmitting means of the associated subscriber terminal and the selected channel.