四六级考试

【单选题】Cable Television
In the 1940s, there were four networks in the United States.Because of the frequencies allotted to television, the signals could only be received in a "line of sight" from the transmitting antennA、People living in remote areas couldn’t see the programs that were already becoming an important part of U. S. culture.
In 1948, people living in remote valleys in Pennsylvania solved their reception problems by putting antennas on hills and running cables to their houses. These days, the same technology once used by remote villages and select cities allows viewers all over the country to access a wide variety of programs and channels that meet their individual needs and desires.By the early 1990s, cable television had reached nearly half the homes in the United States.
Today, U. S. cable systems deliver hundreds of channels to some 60 million homes, while also providing a growing number of people with high-speed Internet access. Some cable systems even let you make telephone calls and receive new programming technologies!
The earliest cable systems were, in effect, strategically placed antennas with very long cables connecting them to subscribers’ television sets.Because the signal from the antenna became Weaker as it traveled through the length of cable, cable providers had to insert amplifiers at regular intervals to boost the strength of the signal and make it acceptable for viewing.
"In a cable system, the signal might have gone through 30 or 40 amplifiers before reaching your house, one every 1,000 feet or so," Wall says, "With each amplifier, you would get noise and distortion. Plus, if one of the amplifiers failed, you lost the picture.Cable got a reputation for not having the best quality picture and for not being reliable." In the late 1970s, cable television would find a solution to the amplifier problem.By then, they had also developed technology that allowed them to add more programming to cable service.
In the early 1950s, cable systems began experimenting with ways to use microwave transmitting and receiving towers to capture the signals from distant stations. In some cases, this made television available to people who lived outside the range of standard broadcasts. In other cases, especially in the northeastern United States, it meant that cable customers might have access to several broadcast stations of the same network. For the first time, cable was used to enrich television viewing, not just make ordinary viewing possible.
The addition of community antenna television stations and the spread of cable systems ultimately led manufacturers to add a switch to most new television sets. People could set their televisions to tune to channels, or they could set them for the plan used by most cable systems.
In both tuning systems, each television station was given a 6-megahertz (MHz) slice of the radio spectrum. The FCC、(FederalCommunicationsCommission) had originally devoted parts of the very high frequency (VHF) spectrum to 12 television channels. The channels weren’t put into a single block of frequencies, but were instead broken into two groups to avoid interfering with existing services.
Later, when the growing popularity of television necessitated additional channels, the FCC、allocated frequencies in the ultra-high frequency (UHF) portion of the spectrum. They established channels 14 to 69 using a block of frequencies between 470 MHz and 812 MHz.
Because they used cable instead of antennas, cable television systems didn’t have to worry about existing services.Engineers could use the mid-band, those frequencies passed over by broadcast TV due to other signals, for channels 14-22.Channels 1 through 6 are at lower frequencies and the rest are higher. The "CATV \Antenna" switch tells the television’s tuner whether to tune around the mid-band or to tune straight through it.
While we’re on the subject of tuning, i
网考网参考答案：D
网考网解析：
[线索词] nearly fifty percent of the homes [定位处] 第二段末句 [解析] 本题涉及到美国有线电视的用户数问题。该段末句提到，截止到二十世纪九十年代早期，美国的有线电视用户数已经接近一半，故答案为[D]。 [设题处] 时间处设题 document.getElementById("warp").style.display="none"; document.getElementById("content").style.display="block"; 查看试题解析出处>>