Key signal conversion device for CATV system

摘要

A key signal conversion device for use in a CATV television system in which more than one different scrambling process is performed, the conversion device receiving one of a scrambled high-frequency signal in a television system and a sound intermediate frequency signal in the television system, extracting a key signal for unscrambling the scrambling in the television system and adding the key signal to a nonscrambled television signal which is to be subsequently scrambled by a process other than a process used to scramble the received signal, the key signal conversion device including: a first detecting circuit, for receiving the one of a scrambled television signal and a sound intermediate frequency signal, and for detecting one of a synchronizing signal of the television signal and a synchronizing signal superimposed on the sound intermediate frequency signal; a gate circuit, for receiving the one of a scrambled television signal and a sound intermediate-frequency signal, and for extracting the key signal therefrom using the synchronizing signal detected by the first detecting circuit; a second detecting circuit, for receiving the nonscrambled television signal, and for detecting a synchronizing signal thereof; and an inserting circuit for superimposing the key signal on the nonscrambled television signal in a predetermined horizontal interval in the vertical blanking interval of the nonscrambled television signal by the gate circuit using the signal detected by the second detecting circuit.