Teletext is the oldest form of digital broadcasting technology, having been used in the UK for over 30 years, during which the decoder (an optional extra in a TV set) has steadily shrunk in size and cost, though not in the complexity of the internal workings of its ICs or chips . Teletext uses spare lines in the broadcast field blanking interval to carry a digital pulse stream which when decoded can type out data on the TV screen in the forms of text and simple graphics like maps and diagrams. TV lines 7 18 in even fields and 320 331 in odd fields are used to carry the data, which can be seen as lines of twinkling dots above the live picture if its height is turned down.

Each text line carries a colour burst to maintain the colour decoder s sub-carrier oscillator synchronisation, along with the usual line sync pulse and blanking interval. The line time given to text pulses is the same for that of picture signal: 52 ?s. To prevent inter-carrier buzz with FM sound systems, the amplitude of the text pulses is limited to 66% of the peak white signal. In this two-state AM data transmission system, binary zero is signalled by the text signal hitting black level, and binary one by the signal rising to the 66% level, which represents 0.462 V with a standard 1 V peak-to-peak video signal. The pulses are not square: they are filtered before transmission and further rounded by the receiver s IF...