试题详情

　　Fiber-optic lines will form most of the backbone of the information highway, just as they do for the phone system today. Fiber-optic cable is made of long, thin strands of glass rather than wire, and it transmits information in the form of digitized pulses of laser light rather than the radio waves used by coaxial cable. Because light pulses have shorter wavelengths than radio waves, engineers can cram much more data into fiber-optic lines than into other kinds of cables and wires. A single fiber, for example, can handle a mind-boggling 5,000 video signals or more than 500,000 voice conversations simultaneously. This huge capacity allows it to transmit all signals digitally. So noise or static easily can be filtered out. Finally, because glass is an inherently more efficient medium for transmitting information than other materials, a fiber-optic line can transmit a signal thousands of miles without much “signal loss”. Fiber-optic cable, simply put, is the method of choice for transmitting massive quantities of information over long distances. 　　Another key is “digital compression”—a variety of methods for reducing the amount of digital code (streams of ones and zeros) needed to represent a piece of information—whether it is a document, a still picture, a movie or a sound. Digital compression is most critical for transmitting video, because digitized video consumes enormous amounts of space. Just four seconds of a digitized film, for example, would completely fill a 100-megabyte hard drive. A feature film of typical length, if uncompressed, would occupy more than 350 ordinary compact discs. 　　Compression techniques achieve their gins by recording only the changes from one frame to the next, The background image in a movie scene, for example, typically does not change much from one flame to another. In a digital compression scheme, the background would be recorded only once; after that, only the actors’ movements would be captured. 　　One result is more choices—hundreds of channels coming through your cable TV line instead of dozens. Digital compression also makes it easier to piggyback data onto media that were not designed with data in mind: in particular, phone lines.