Television system with access control

A television system with access control constituted by a transmitter and receivers, the transmitter comprising a circuit for the formation of a video signal organized in lines and fields and a sound signal, a transmission means for said signals and a jamming circuit using a service key K, said jamming circuit receiving the video and/sound signal and supplying jammed video and/sound signals which are then transmitted to the transmission means, each receiver comprising a reception means for the transmitted signals, means for the display of a picture and for the restoration of sound and an unjamming circuit connected to the reception means and working with said service key K, said unjamming circuit supplying unjammed video and optionally sound signals which are then transmitted to the means for the display of the picture and the restoration of the sound, wherein it also comprises:

• (A) a subscription management center comprising means for generating a plurality of digital signals constituting subscription keys C.sub.i changing in random manner at relatively long intervals of approximately one month;
• (B) in the transmitter:
• (a) a generator of a digital signal constituting the the service key K which changes in random manner at relatively short intervals of approximately a few minutes,
• (b) a circuit for the formation of digital messages M.sub.i obtained from the subscription keys C.sub.i and the service key K by an algorithm parametered by the subscription keys, i.e. Mi=F.sub.C.sbsb.i (K), said circuit supplying the same number of messages as there are subscription keys and these messages change with the service key,
• (c) a data broadcasting system able to insert messages Mi in the field blanking intervals of the video signal, jamming not affecting the messages contained in the field blanking interval;
• (C) in each receiver:
• (a) a subscription card on which is entered a signal corresponding to at least one of the subscription keys C.sub.i,
• (b) a circuit for the extraction of the data contained in the signal received and which restores the messages Mi,
• (c) a circuit for the restoration of the service key receiving on the one hand messages Mi supplied by the preceding circuit and on the other hand the subscription key Ci supplied by the subscription card, said circuit performing an algorithm K=G.sub.C.sbsb.i (Mi) which makes it possible to restore the signal corresponding to the service key K used in the transmitter;
• (d) at least one subscription card loading station which is connected to the subscription management center from which it receives the signals corresponding to the different subscription keys Ci generated by the center, each station being able to temporarily receive the subscription cards and write therein one of the subscription keys Ci.