听力原文： A twenty-one-year-old American named Philo Farnsworth built the first working television receiver in nineteen twenty-seven. Many scientists around the world had made important discoveries that led to the development of television. But Philo Farnsworth had recognized as a boy that electrons could capture a picture sent as light and sound waves through the air. Over the years, the technology has changed and improved. But the idea behind the television broadcast is still the same. TV stations send a powerful signal from a transmitting antenna. An antenna connected to a television set receives the signal. The problem with this system is that the receiver antenna has to be in line with the transmitting antenna. Mountains or tall buildings can interfere. One solution is cable television. This system began in the nineteen forties in Pennsylvania. Only a few television stations existed then, and they were in large cities. People in small towns could not receive the signals. So a store owner put an antenna on top of a pole and placed it on a nearby mountain. This antenna received the television signal. Wires led from the antenna to the store. The cable brought clear pictures to the television sets inside. Later, the idea of cable television spread to cities, to provide people with more stations to watch. Today, people can watch hundreds of stations. And another way to receive them is with a satellite dish antenna. A small round device of the size of a pizza can receive signals from satellites .high above the Earth. The antenna is connected to a special receiver which connects to the television set. Some broadcasts over satellite can be watched free of charge. But the others cost money, just like cable service. The passage mainly focuses on ______.