TV's easy rider

日期:2017-07-09 05:23:48 作者:万扃 阅读:

By Barry Fox in Amsterdam IT COULD soon be possible to watch television in a moving car or on a bus or train, thanks to a new European system that modifies terrestrial digital TV signals to eliminate the interference that normally makes TV unwatchable in moving vehicles. The constant changes in the signal picked up by a moving receiver—produced by the transmissions bouncing off buildings, vehicles and even aircraft—can wreck both analogue and digital TV reception. In May, after a year of feasibility tests by Deutsche Telekom, the European Commission funded a project, Motivate, to see if the international standard for digital video broadcasting could be adapted to eliminate this. Motivate has found a way to make reception possible in vehicles travelling at up to 300 kilometres per hour. The main problem was to design a mobile-friendly signal that existing digital set-top receivers in homes will also be able to pick up. In terrestrial digital broadcasting, the TV signal is spread over many narrow carrier bands. For mobile reception, the amount of information per carrier signal is reduced from the normal 64 bits per cycle to 16 bits. This makes reception more accurate. In addition, half the picture information is transmitted twice, to fill any gaps caused by reception glitches. For fixed digital reception only a third of the bits need to be sent twice. The downside is that the transmission rate falls from 24 to 12 megabits per second. So a range of frequencies that can carry, say, six normal digital stations will carry only three mobile channels. Also, the mobile signal must be transmitted at the same strength as analogue TV, which is more powerful than normal digital transmissions and could interfere with other services. To demonstrate the system, IBC delegates were driven around Amsterdam in a minibus equipped with an analogue TV set and a digital widescreen set. The analogue picture was unwatchable, while the digital set gave clear,