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 February 1, 2006 More Video Initials There aren’t many places in electronics that the old-style vacuum tube (aka valve) still holds sway. Some high-end audiophiles are prepared to pay big bucks for exotic tube gear, but other than that, television is about the only place you’ll find this venerable technology, and then only in the picture tubes themselves.The TV picture tube, or cathode-ray tube (CRT), has many drawbacks: it’s large, it’s heavy, and it runs hot. There is a definite limit to the size of a direct-view CRT screen, and CRT-based rear-projectors suffer from relatively narrow viewing angles. Increasingly, therefore, video buyers have become infatuated with the flat-panel plasma and liquid crystal displays (LCD), and the relatively slim LCD and Digital Light Processing (DLP) projection units. These newer screen types have come a long way in recent years, and there are many excellent examples on the market, but the good ol’ direct-view CRT still offers the best picture quality, primarily because it’s particularly good at reproducing blacks. So far, the newer technologies can usually manage only a dark gray. In a CRT, an electron gun at the back of the tube fires a constant stream of electrons at the front of the tube, the inner surface of which is coated by phosphors that light up when struck by electrons. Electromagnets in the neck of the tube deflect the electron beam to make it sweep back and forth over the phosphors, and the varying intensity of the constantly moving beam "paints" the picture as it scans. The horizontal resolution is determined by the bandwidth of the electronic circuits that modulate the beam. All other types of video display divide the screen into discrete picture elements, or pixels. These are arranged in rows that correspond to a CRT’s scanning lines, and the horizontal resolution is determined by the number of pixels in each row, which is determined by the pixel size. The pixels are switched on and off to paint an image sequentially, just as the moving CRT beam does. LCD pixels let light pass through in proportion to the voltage fed them; the light itself comes from an illuminated panel behind the screen (in the case of a flat-panel display), or from a bulb (in the case of a projector). DLP projectors also use a bulb, but its light is reflected off a series of tiny mirror-pixels and then onto the screen. Plasma pixels produce light in much the way a fluorescent bulb does. In both cases, the pixels are strictly on/off devices; the degree of brightness is determined by how often they are switched on and off during a given period of time: the more flashes, the brighter the image. Now there’s a new kid on the block that shares many of the virtues of the newer pixel-based technologies but retains some of the advantages of the CRT. It’s called a surface-conduction electron-emitter display (SED). The first research into SED technology was conducted in 1986, by Canon, who formed a joint venture with Toshiba in 1999. Prototype displays were shown at the 2005 and 2006 Consumer Electronics Shows. The companies expect the debut product -- a 55" HDTV set -- to be available in Japan this summer, and elsewhere by the end of the year. As in other panel technologies, an SED screen is divided into pixels, but each SED pixel is like a microscopic CRT. The back of each pixel contains three low-voltage electron emitters, all aimed at the pixel’s front panel, which is divided into three color subpixels. The pixel’s brightness is proportional to the voltage supplied to it; when there is no voltage, there is true black, rather than the light spill that characterizes other flat-panel technologies. An SED panel can be as thin as the other flat-panel types, and, like them, doesn’t suffer from CRT’s problems of geometric distortion and convergence. But in such areas as overall brightness and contrast ratio, SED should outdo all other imaging systems. When they arrive, SED panels will undoubtedly be pricey, but if they live up to their promise, they could revolutionize video -- even if that simply means putting the 70-year-old CRT technology to rest for good. ...Ian G. Masters
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