Two-way television


By Barry Fox DIGITAL TV viewers in the Republic of Ireland will soon be able to transmit signals from their television aerials, as well as receiving them, as they explore new TV-based services ranging from teleshopping to participating in political debates. Transmission from a domestic TV aerial is not possible with analogue technology because high power is needed to make the continuously variable analogue signal readable above atmospheric noise. But clean digital signals—a stream of 0s or 1s—are more easily read amid noise and can be broadcast at much lower power. Ireland’s public broadcaster RTE is developing the technology, called the Wireless Interactive Network for Digital Services (WINDS), with cash from the European Union. Signals broadcast from the main transmitter follow the Digital Video Broadcasting standard used throughout Europe for terrestrial digital TV. The innovation is to make the set-top box that decodes incoming signals also work as a low-power transmitter, sending data signals to the normal roof-top or set-top aerial, which transmits them back to the broadcaster’s mast. Setting up a means for viewers to send a signal to a TV station is not a new idea. Methods proposed so far include using a cable TV line, a phone line or a cellphone link—but these are all expensive answers. Peter Branagan, director of digital planning at RTE, says WINDS is different. “WINDS can provide a wireless return path that is free to use and adds less than E30 to the cost of the receiver. And viewers will be able to interact even when their children are on the phone.” The Irish government has allocated 1-megahertz slices of the UHF spectrum to carry the return-path signals, and each slice is split into 1000 channels that are 1 kilohertz wide. The receiver hops between channels until it finds one in its area that is clear. WINDS return-path signals use very low power in very small cells, borrowing an idea from digital cellphone networks, which also keep transmission power to a minimum. This allows the system to reuse the same frequencies in neighbouring areas. The set-top box transmits and receives test signals, and adjusts its transmission strength to get the best connection with the broadcaster. In built-up areas with many viewers, the government has allocated more frequencies, so viewers should always be able to access one to send data. Ireland has spare UHF frequencies,
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