EAVESDROPPING PREVENTING CIRCUIT OF CATV SYSTEM

摘要

PURPOSE:To prevent the eavesdropping by providing a device, which detects a change of contract contents, in an ROM block for contract in the CATV system. CONSTITUTION:In case of a contract for reception of a specified channel in the CATV system, a descrambler part is so provided that the horizontal synchronism is operated normally only for the specified channel, and an ROM consisting of a resistance block is mounted in the descrambler. A terminal of the ROM is connected to a pulse output circuit, to make the reception of the corresponding channel possible; but it is opened, to make this reception impossible. Terminals (n) and (m) of the ROM are used for preventing the eaves dropping and resistances R1 and R2 are adjusted. If a different ROM is mounted for a wrong use, an excessive current is flowed to the earth and the normal operation is not performed when the resistance is too small. When the resistance is larger, a transistor 13 is turned on, and the normal operation is not performed.