Videotex system

A videotex system comprising a transmission station and receiver stations, whereby the transmission station comprises:

• means for composing or setting a magazine constituted by pages organised in rows of characters,
• a data processing circuit incorporating means for coding the characters as digital signals grouped in data octets (8 bits) and means for forming control octets with 6th and 7th zero bits, constituted by codes indicating more particularly the headings and ends of pages, the headings and ends of rows, as well as visually display attributes, all the control and data octets incorporating a heavy weight bit which is an imparity bit, the circuit finally having a magazine store in the form of data octets inserted between the control octets,
• a junction connected to the data processing circuit
• a transmission multiplexer used for the insertion of data into the lines of television signals,
• a modulation member,
• each receiver station comprises:
• a circuit for the reception and demodulation of the television signal,
• a digital signal processing channel containing a digital junction followed by a digital signal decoder,
• a videopicture signal processing channel
• a visual display member
• wherein
• (A) in the transmission station:
• the data processing circuit comprises control signal formation means constituted, for each control, by a redundant sequence of control codes, each sequence being formed by at least two consecutive codes, whereof at least one is the conventional code corresponding to the control in question and the other or others having no significance liable to interfere with the conventional receivers and
• (B) in certain of the receiver stations:
• an identification circuit is provided in the digital signal processing channel and between the digital junction and the decoder which utilises the redundancy of the control sequences
• (a) for recognising in the sequences a particular control in spite of minor errors which can occur in this sequence and to restore a corrected control code corresponding to the conventional code which is then taken into account by the decoder
• (b) and to recognise and indicate to the decoder that certain control code sequences are seriously erroneous and must not be taken into account,
• (C) the receiver stations which are not provided with an identification circuit and which process in a conventional manner the digital signals passing directly from the digital junction to the decoder.