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  3. Radio and Television Radio and television were major agents of social change in the 20th century. Radio was once the center

Radio and Television Radio and television were major agents of social change in the 20th century. Radio was once the center

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问题                     Radio and Television
    Radio and television were major agents of social change in the 20th century. Radio was once the center for family entertainment and news and television enhanced this revolution by adding sight to sound. Both opened the windows to other lives, to remote areas of the world, and to history in the making. News coverage changed from early and late editions of newspapers to broadcast coverage from the scene. Play-by-play sports broadcasts and live concerts enhanced entertainment coverage. For many, the only cultural performances or sports events they would ever hear or see would come from the speakers or the screens in their living rooms. Each has engaged millions of people in the major historical events that have shaped the world.
    If people could look at the sky and see how it is organized into frequency" bands used for different purposes, they would be amazed. Radio waves crisscross (十字形) the atmosphere at the speed of tight, delivering incredible amounts of information—navigational data, radio signals, television pictures—using devices for transmission and reception designed, built, and refined by a century of engineers.
    Key figures in the late 1800s included Nikola Tesla, who developed the Tesla coil, and James Clerk Maxwell and Heinrich Hertz, who proved mathematically the possibility of transmitting electromagnetic signals between widely separated points. It was Guglielmo Marconi who was most responsible for taking the theories of radio waves out of the laboratory and applying them to practical devices. His "wireless" telegraph demonstrated its great potential for worldwide communication in 1901 by sending a signal—the letter "s"—in Morse code a distance of 2 000 miles across the Atlantic Ocean. Radio technology was just around the comer.
    Immediate engineering challenges addressed the means of transmitting and receiving coded messages, and developing a device that could convert a high-frequency oscillating (振荡的) signal into an electric current capable of registering as sound. The first significant development was "the Edison effect", the discovery that the carbon filament (灯丝) in the electric light bulb could send out a stream of electrons to a nearby test electrode if it had a positive charge. In 1904, Sir John Ambrose Fleming of Britain took this one step further by developing the diode (二极管) which allowed electric current to be detected by a telephone receiver. Two years later, American Lee De Forest developed the triode (三极管), introducing a third electrode (the grid) between the filament and the plate. It could amplify a signal to make live voice broadcasting possible, and was quickly added to Marconi’s wireless telegraph to produce the radio.
    Radio development was prevented by restrictions placed on airwaves during World War I. Technical limitations were also a problem. Few people had receivers, and those that did had to wear headphones. Radio was seen by many as a hobby for telegraphy fans. It would take a great deal of engineering before the radio would become the unifying symbol of family entertainment and the medium for news that was its destiny.
    In the mid-1920s, technical developments expanded transmission distances, radio stations were built across the country, and the performance and appearance of the radio were improved. With tuning circuits, capacitors, microphones, oscillators, and loudspeakers, the industry blossomed in just a decade. By the mid-1930s almost every American household had a radio. The appearance of the transistor in the 1950s completely transformed its size, style, and portability.
    Both television and radar were logical byproducts of the radio. Almost 50 years before television became a reality, its fundamental principles had been independently developed in Europe, Russia, and the United States. John Baird in England and Charles Jenkins in the United States worked independently to combine modulated light and a scanning wheel to reconstruct a scene. In 1925, Baird succeeded in transmitting a recognizable image.
    Philo T. Farnsworth, a 21-year-old inventor from Utah, patented a scanning ray tube, and Vladimir Zworykin of RCA devised a superior television camera in 1930. Regularly scheduled broadcasts started shortly thereafter, and by the early 1940s there were 23 television stations in operation throughout the United States.
    Shortly after World War Ⅱ, televisions began to appear on the market. The first pictures were faded and flickering, but more than a million sets were sold before the end of the decade. An average set cost $500 at a time when the average salary was less than $3 000 a year. In 1950 engineers perfected the process of production and prices dropped to $200 per set. Within 10 years 45 million units were sold.
    A study of how human vision works enabled engineers to develop television technology. Images are retained in a viewer’s eye for only a fraction of a second after they strike it. By displaying images piece by piece at sufficient speed, the illusion of a complete picture can be created. By changing the image on the screen 25 to 30 times per second, movement can be realistically represented. Early scanning wheels slowly built a picture line by line. In contrast, each image on a modern color television screen is comprised of more than 100 000 pixels (像素), arranged in several hundred lines. The image displayed changes every few hundredths of a second. For a 15-minute newscast, the television must accurately process more than 1 billion units of information. Technical innovations that made this possible included a screen coated with millions of tiny dots of fluorescent compounds that emit light when struck by high: speed electrons.
    Today this technology is in transition again, moving away from conventional television waves and on to separate digital signals. This holds the potential for making television interactive—allowing a viewer to play a game or order action replays. Cathode ray tubes with power-hungry electron guns are giving way to liquid crystal display (LCD) panels. Movie-style wide screens and flat screens are readily available. Digital signals enable High Definition Television (HDTV) to have almost doubled the usual number of pixels, giving a much sharper picture. The appearance of cable television and advances in fiber-optic technology will also help lift the present bandwidth (带宽) restrictions and increase image quality.
Movement can be indeed showed only if the images on the screen are changed at the frequency of ______ .
选项
答案25 to 30 times per second
解析 根据题干中的关键词images on the screen定位到文章第十段第四句：By changing the image on the screen 25 to 30 times per second, movement can be realistically represented。通过在屏幕上以每秒25到30次的频率变换图像，运动就能够得到真实地呈现。题干中的be indeed showed是对原文中的be realistically represented的同义转述，而且题干把原文中的by短语转换成了一个由only if引导的从句。题干提问突破变换的速度，这样就可以得出答案为25 to 30 times per second。
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