Teletext Tutorial (5)
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Teletext Tutorial (5) |
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 Recently, the potential for using teletext as a means for distributing data nationally and internationally has been realised. Data broadcasting, like teletext, uses part of the television signal to provide point to multipoint transmission of data for commercial or public services. It can offer:
Data broadcast data is embedded as digital data in existing television signals in the same way that teletext services use spare lines at the top of television picture frames. It can be carried either by terrestrial broadcast, satellite or cable transmission. Because the data is digital it can represent text, graphics or control information. All that is needed at the receiving site is a good receiver (e.g. satellite, UHF/VHF tuner) and a data broadcast decoder costing little more than a computer modem. Unlike public teletext the data cannot normally be viewed on teletext receivers. The advantage of data broadcast is that data is transmitted instantaneously to anyone capable of receiving a television signal, and who has the appropriate decoding equipment. Unlike telephone, telex or fax services, users only pay to send a message once regardless of the number of outlets or their geographical location. Broadcast data can be grouped into services which can be sent or addressed to specific decoders. For added security the data can be encrypted. The service typically becomes cost effective where there are more than 100 receiving sites and the data would normally be sent over a dial up telephone line.
A service provider or input provider (IP) sends data to the broadcaster using a standard telecommunications connection e.g. phone line, leased line, etc. Data is supplies in a form suitable for use by the subscriber's equipment; it does not need to be preformatted, since data broadcasting offers data transparency. The broadcaster formats the data, adding er5ror and data-loss protection, then combines the data stream with the television signal, which is distributed in the normal way, either nationally or internationally. The TV data is received using a standard TV antenna, and decoded using authorised receivers. Teletext-based data-broadcasting systems offer very high data rates (greater than 15 kilobits per sec per TV line), total byte-transparency and freedom to use any appropriate encryption technique for security.
A special decoder at the reception point receives the television signal from an antenna, satellite dish or cable system. The specific user data is stripped from the picture and any other teletext information. The data at this point should be identical to that sent by the input provider. The data could be displayed on a computer terminal, sent to a printer or input into another computer for further processing.
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Data Broadcasting