Technology: High-definition TV gives way to clearer pictures

日期:2019-02-28 05:14:01 作者:叶庇筮 阅读:

By BARRY FOX in TOKYO JAPAN’S television stations have started to use a new broadcasting system that could undermine plans to introduce high-definition television in the 1990s. Televisions equipped to receive transmissions by this system, called Clear Vision, produce far clearer pictures than ever before. The new approach came about because some engineers believed that high-definition television would be unnecessary if the potential of the present technology was fully exploited. The commercial stations took up their idea and produced Clear Vision which, like conventional broadcasting systems, scans 525 lines across the screen. The more advanced high-definition technique scans 1125 lines; Japan’s state broadcasting company, NHK, has been developing such a system, called Hi Vision. To produce Clear Vision, the TV companies use a high-definition camera, which records on 1125 lines, in the studio, instead of a conventional camera that records on 525 lines. Equipment in the studio converts the picture electronically to the 525-line format for transmission. In practice, the pictures leaving the television station are already of better quality. A television that can receive the new system stores each incoming picture of 525 lines and displays it twice. The result for the viewer is a remarkably clear picture which approaches the quality of Hi Vision for a fraction of the price. Clear Vision worked so well that this month, all television stations in Japan, including NHK, started using it to broadcast five hours of their daily programmes. The quality of the picture is so good that viewers seem likely to prefer it to NHK’s more expensive system, scheduled to start operating in 1991. Anyone who wants to receive Hi Vision will have to buy either a new television costing at least Pounds sterling 5000, or an expensive converter. Meanwhile, some manufacturers are starting to sell televisions with Clear Vision for about Pounds sterling 1600. Existing sets will work as before, ignoring the additional information received. Fifteen years ago, NHK started work on the technology of high-definition television aiming to improve pictures. The company has already started testing Hi Vision for one hour a day from Japan’s BS2 satellite. A full service is to begin in 1991 following the launch of the BS3 satellite. As well as sharpening the image, Clear Vision also avoids the ghost shadows that frequently appear on the screen. People see these when their receiver aerials do not have a clear line of sight to the transmitter aerial; the receiver picks up reflected signals which have bounced off tall objects. The reflected signals interfere with each other and produce the ghost images. An object like a human head will appear to have faint replicas staggered across the screen. Clear Vision transmits a signal called a ghost cancel reference on one of the 525 lines. The GCR signal is a brief pulse with a clearly defined shape. Conventional receivers ignore it, but new receivers will generate a pure pulse internally and compare it with the incoming pulse. The receiver analyses any mismatch between the two pulses and alters the incoming picture signal to remove ghosts. Hi Vision eliminates them too because it is broadcast by satellite. If Clear Vision catches on, the Japanese broadcasting stations will experiment with second-generation technology which adds panels at the side of the pictures. The next generation of receivers will then be able to display wide pictures. Broadcasters in the US want to use the ‘ghost busting’ system as soon as possible. They believe it will greatly improve the quality of reception in cities. European companies that broadcast from the ground are watching the progress of Clear Vision with special interest. Their system transmits 625 lines,