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  3. 　　Fiber-optic lines will form most of th

　　Fiber-optic lines will form most of th

考试题库2022-08-02  6
问题 　　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.
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解析 光纤电缆构成了今日电话系统的主干线，同样，它也将构成未来信息公路的主干线。光纤电缆的制造材料是细长的玻璃丝而不是金属导线，它的信息传输方式是激光数字脉冲而不是同轴电缆使用的无线电波。由于光脉冲的波长比无线电波的波长短，因而工程师可以输入光纤电缆的信息量之大令其他类型的电缆和导线望尘莫及。例如，一根光纤电缆可同时处理五千个电视信号或50多万次电话通话，简直令人惊奇。这种巨大的能力使它可以实现全部信号传输数字化，从而轻而易举地滤掉噪音或静电干扰。最后要指出的是，玻璃作为媒介，它本身的传输效率便高于其他材料，因而一条光纤电缆可将一个信号传输数千英里而几乎没有“信号流失”。简而言之，光纤电缆是远距离传输大量信息的最佳方式。
　　另一关键技术是“数字压缩”技术——及减少数码(一连串的l和0)数量的各种方法，这种数码用来表示某种信息，如一份文件、一张静止图片、一部电影或一种声音。数字压缩技术对影像传送来说至关重要，因为数字化影像会占据大量的存储空间。例如，仅仅四秒钟的数字化影片，便要占据一百兆的硬盘存储空间。一部普通长度的故事片，如果未经压缩，要占350多张普通光盘。数据压缩技术只记录一个画面与下一个画面之间的变化，从而实现其功能。例如，电影场景的背景图像在一个画面转为另一个画面时通常是不变的，在数字压缩系统中，背景图像只被记录一次，此后便只记录演员的动作。
　　数据压缩技术使可供选择的有线电视频道从几十个增加到几百个，数字压缩技术还能使数据更容易通过本不是用于传送数据的媒体线传送——特别是电话线。
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