PURPOSE: To prevent wiretapping on double prevention basis, i.e. by scrambling and permitting or inhibiting image reception and to improve secrecy, by superposing a key, an address, and a data signal on a video signal in a CATV system and allowing the image reception only when the address signal coincides with the address number given to every terminal device.
CONSTITUTION: A video signal inputted from an IF modulating circuit 20 to an encoder 30 is scrambled and modulated by an up converter 22 to the frequency of a specific channel. A terminal device 28 demodulates and analyzes the key signal by the decoder in the main box 33 for descrambling and the scrambled video signal is descrambled according to the key signal to be demodulated into the normal video signal. Further, a computer 31 outputs an address code for indicating whether image reception is permitted or not together with a scrambled key signal of a random number and the address code is transmitted to the terminal device 28 by being superposed on the video signal by an encoder 30. The main box 33 separates and analyzes this address signal and when the address signal coincides with the preset address number of the device 28, the device 28 is permitted to receive an image, supplying the descrambled video signal to a television receiver 11.
WATANABE SHIGERU
TODOROKI RIYUUICHI
