電氣信息工程專業英語（簡體書）

The discovery in 1873 by Joseph May, a telegraph operator at the Irish end of thetransatlantic cable, that when a selenium resistor was exposed to sunlight its resistance decreased,led to the development of a light-to-current transducer. Subsequently, various schemes for imagetransmission based on this discovery were devised by George Carey, William Ayrton(1847-1908), John Perry and others. None of these was successful because they lacked anadequate scanning system and each element of the picture had to be sent on a separate circuit,making them quite impractical.
In 1884, Paul Nipkow (1860-1940) was granted a patent in Germany for what becameknown as the Nipkow Disc. This consisted of a series of holes drilled in the form of spirals in adisc. When an image is viewed through a second disc with similar holes driven in synchronismwith the first, the observed effect was scanning point-to-point to form a complete line andline-by-line to cover the complete picture. This was a practical scheme since the point-to-pointbrightness of the picture could be transmitted and received serially on a single circuit. Thepersistence of an image on the human eye could be relied on to create the impression of acomplete scene when, in fact, the information is presented point-by-point. Nipkow s schemecould not be exploited until 1927 when photosensitive cells, photomultipliers, electron tubeamplifiers and the cathode ray tube had been invented and had attained sufficient maturity toprocess the signals at an acceptable speed for television. Several people made significantcontributions to the development of the components as well as to the system. However, twopeople, Charles Jenkins (1867-1934) and John Baird (1888-1946), are credited with thesuccessful transmission of images at about the same time. They both used the Nipkow disc.
Mechanical scanning methods of various forms were used with reasonable success until about1930 when Vladimir Zworykin (1889-1982) invented the "iconoscope" and Philo Farnsworth(1906-1971) invented the electronic camera tube, which he called the "image dissector". Theseinventions finally removed all the moving parts from television scanning systems and replacedthem with electronic scanning. The application of very-high-frequency carriers and the use ofcoaxial cables have contributed significantly to the quality of the pictures. The use of color intelevision had been shown to be feasible in 1930, but it did not be available to the general publicuntil the mid-1960s. By the 1980s, satellite communication systems brought a large number oftelevision programs to viewers who could afford the cost of the dish antenna. By the beginningof the 21st century, the dish antennas had shrunk in size from over 3m to less than 70 cm and thesignal had changed into digital form.