DESIGN OF INTERFACE CIRCUIT FOR TELETEXT DECODER

Abstract

In teletext the information is fed to a computer installed at the broadcasting station. This system trans-mits the information in the form of packets, called magazines. First the information is arranged in the form of magazines and then each magazine and page is defined by a number. All the magazines are then transmitted through a multiplexer in cylic order. In this way a lot of infor-mation can be broadcasted on the existing channel. The information is transmitted in the form of digital codes. Coding is done according to different teletext standards. India follows,. the Antiope teletext standard. The teletext transmission on the existing channels without interfering the normal transmission is made possible because the broadcast T.V. signal is not completely occupied with picture information. A television picture is transmitted as a rappied series of still pictures with gaps (of around 1.333 m. sec ) betweenstills. This gap is called vertical blanking interval ( V.B.I.. Teletext data is transmitted during V.B.I. only i.e. the vertical retrace period of the electron beam during which there is no information. So it does not disturbs the preceding and following information. T.V. sets functioning without decoder interface continue to receive the normal television picture as usual without any disturbance. vii. To view the information at the receiving end the, user has to install a teletext decoder attached to his television set. The decoder is already programmed to receive the requested information in the form of magazine and page number) and to store and display it on the T.V. monitor, The existing decoder gives R G B outputs. There is a need to modify the existing decoder as most of the T.V. ' sets do not take R G B input but are designed only to take R.F. or Composite Video Signal k C.V.S.) as input. So to interface the decoder with a T.V. set the output should be in the form of R.F. output. This project involved interfacing the antiope decoder with all (coloured/black and white) makes of T.V.s and monitors.