Abstract: A television system for broadcasting data simultaneously with but independently of television programs, in which digitally coded information carrying signals are superimposed or otherwise added to a video system in the field blanking interval. The local receiver is fitted with a decoder by means of which a viewer is able to select a particular page of supplementary information which is then superimposed over the broadcast program. Generally, this information is available to any member of the public who has a requisite decoder. In this system the digitally coded information carrying signals are enciphered prior to transmission in accordance with a predetermined key and the receiving terminal has a cipher circuit operable in accordance with the key to decipher the enciphered signals.