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Satellite TV Today, you see compact satellite

游客2024-05-04  16
问题                               Satellite TV
    Today, you see compact satellite dishes perched on rooftops all over the United States. Drive through rural areas beyond the reach of the cable companies and you’ll find dishes on just about every house. The major satellite television companies are bringing in more customers every day with the lure of movies, sporting events and news from around the world.
The Broadcast TV Problem
    Conceptually, satellite television is, a lot like broadcast television. It’s a wireless system for delivering television programming directly to a viewer’s house. Both broadcast television and satellite stations transmit programming via a radio signal. Broadcast stations use a powerful antenna to transmit radio waves to the surrounding area. Viewers can pick up the signal with a much smaller antenna.
    The main limitation of broadcast television is range. The radio signals used to broadcast television shoot out from the broadcast antenna in a straight line. In order to receive these signals, you have to be in the direct "line of sight" of the antenna. One problem is that the Earth is curved, so it eventually breaks the signal’s line of site. The other problem with broadcast television is that the signal is often distorted even in the viewing area. To get a perfectly clear signal like you find on cable, you have to be pretty close to the broadcast antenna without too many obstacles in the way.
The Satellite TV Solution
    Satellite television solves the problems of range and distortion by transmitting broadcast signals from satellites orbiting the Earth. Since satellites are high in the sky, there are a lot more customers in the line of site. Satellite television systems transmit and receive radio signals using specialized antennas called satellite dishes.
    The television satellites are all in geosynchronous orbit, meaning that they stay in one place in the sky relative to the Earth. Each satellite is launched into space at about 7,000 mph(11,000 kph), reaching approximately 22,200 miles(35,700 km)above the Earth. At this speed and altitude, the satellite will revolve around the planet once every 24 hours—the same period of time it takes the Earth to make one full rotation. In other words, the satellite keeps pace with our moving planet exactly. This way, you only have to direct the dish at the satellite once, and from then on it picks up the signal without adjustment, at least when everything works right.
The Overall System
    Early satellite TV viewers were explorers of sorts. They used their expensive dishes to discover unique programming that wasn’t necessarily intended for mass audiences. The dish and receiving equipment gave viewers the tools to pick up foreign stations, live feeding between different broadcast stations, NASA activities and a lot of other stuff transmitted using satellites.
    Some satellite owners still seek out this sort of programming on their own, but today, most satellite TV customers get their programming through a direct broadcast satellite(DBS)provider, such as Direct TV or the Dish Network. The provider selects programs and broadcasts them to subscribers as a set package. Basically, the provider’s goal is to bring dozens or even hundreds of channels to your television in a form that approximates the competition, cable TV. Unlike earlier programming, the provider’s broadcast is completely digital, which means it has much better picture and sound quality. Early satellite television was broadcast in C-band radio—radio in the 3. 4-gigabertz(GHz)to 7-GHz frequency range. Digital broadcast satellite transmits programming in the Ku frequency range(12 GHz to 14 GHz).
The Programming
    Satellite TV providers get programming from two major sources: national turnaround channels(such as HBO, ESPN and CNN)and various local channels(NBC, CBS, ABC, PBS and Fox affiliates in a particular area). Most of the turnaround channels also provide programming for cable television, and the local channels typically broadcast their programming over the airwaves.
Turnaround channels usually have a distribution centre that beams their programming to a geostationary satellite. The broadcast centre uses large satellite dishes to pick up these analogs and digital signals from several sources.
    The broadcast centre converts all of this programming into a high-quality, uncompressed digital stream. At this point, the stream contains a vast quantity of data—about 270 megabits per second(Mbps)for each channel. In order to transmit the signal from there, the broadcast centre has to compress it. Otherwise, it would be too big for the satellite to handle.
Compression, Encryption and Transmission
    The two major providers in the United States use the MPEG-2 compressed video format—the same format used to store movies on DVDs. With MPEG-2 compression, the provider can reduce the 270-Mbps stream to about 5 or 10 Mbps(depending on the type of programming). This is the crucial step that has made DBS service a success. With digital compression, a typical satellite can transmit about 200 channels. Without digital compression, it can transmit about 30 channels.
    At the broadcast centre, the high-quality digital stream of video goes through an MPEG-2 encoder, which converts the programming to MPEG-2 video of the correct size and format for the satellite receiver in your house.
    After the video is compressed, the provider needs to encrypt it in order to keep people from accessing it for free. Encryption scrambles the digital data in such a way that it can only be decrypted(converted back into usable data)if the receiver has the correct decryption algorithm and security keys.
    Once the signal is compressed and encrypted, the broadcast centre beams it directly to one of its satellites. The satellite picks up the signal with an onboard dish, amplifies the signal and uses another dish to beam the signal back to the Earth, where viewers can pick it up.
The Dish and the Receiver
    A satellite dish is just a special kind of antenna designed to focus on a specific broadcast source. The standard dish consists of a parabolic(bowl-shaped)surface and a central feed horn. To transmit a signal, a controller sends it through the horn, and the dish focuses the signal into a relatively narrow beam.
    The end component in the entire satellite TV system is the receiver. The receiver has four essential jobs:
    It de-scrambles the encrypted signal. In order to unlock the signal, the receiver needs the proper decoder chip for that programming package.
    It takes the digital MPEG-2 signal and converts it into an analog format that a standard television can recognize. In the United States, receivers convert the digital signal to the analog NTSC format. Some dish and receiver setups can also output an HDTV signal.
    It extracts the individual channels from the larger satellite signal. When you change the channel on the receiver, it sends just the signal for that channel to your TV.
    It keeps track of pay-per-view programs and periodically phones a computer at the provider’s headquarters to communicate billing information.
    While digital broadcast satellite service is still lacking some of the basic features of conventional cable(the ability to easily split signals between different TVs and VCRs. for example), its high-quality picture, varied programming selection and extended service areas make it a good alternative for some. With the rise of digital cable, which also has improved picture quality and extended channel selection, the TV war is really heating up. [br] Satellite television providers can bring dozens or even hundreds of channels to TV viewers because______.
选项 A、satellite can send more programs
B、the television signals sent to satellites are compressed
C、the signals are encrypted
D、the signals converted back into usable data
答案 B
解析 推理判断题。由题目中的dozens or even hundreds of channels找到小标题Compression，Encryption andTransmission下第一段最后两句With digital compression，a typical satellite can transmit about 200channels．Without digital compression，it can transmit about 30 channels．(通过数字压缩，一个典型的卫星可以传播200个左右的频道，而没有经过压缩的，就只能传播30个左右的频道。)由此可知有线电视节目供应商之所以能提供如此多的节目给用户，是因为电视信号可以被压缩，之后传送到卫星上。所以答案是B。
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