Ciphering and deciphering system for analogue signal - employing series connected delays individually switched from synchronised program control unit

摘要

A ciphering and deciphering process is applied to a radiophonic signal for a TV transmission. This allows transmissions to be made at night to a specific group such as doctors. These transmissions are not intended for the general public to see or hear. An audio signal is applied to a low pass filter from which it passes to a pre-accentuation and compression circuit. An analogue delay comprising a number of delays in series is connected via the individual outputs to a switching circuit. The switching circuit is addressed via an addressing circuit from a cipher control circuit to which a clock and synchronisation circuits are connected to provide synchronised timing periods. The switching circuit output is applied via a low pass filter to a mixer where the sync. signal is added for application to the transmission modulator.