PURPOSE: To prevent data error and failure due to noise, by superimposing a key signal for descrambling on a vertical synchronizing signal and a horizontal synchronizing signal.
CONSTITUTION: An encoder 30 to superimpose the key signal is inserted between an IF modulation circuit 20 and an up converter circuit 22 in a CATV system. Further, a main box 31 incorporating a decoder is inserted between a branch cable 9 and a television receiver 11 and a control box 12 is connected to the main box 31. A video signal inputted to the encoder 30 is supperimposed on the key signal at the horizontal synchronizing signal and modulated into a specified frequency at the converter 22. The decoder of the main box 31 separates the key signal and demodulates the scrambled video signal with the expansion of level according to the designation of the key signal.
WATANABE SHIGERU
TODOROKI RIYUUICHI
