Evolution of the Scanning Process
The idea of
seeing by telegraph engrossed many inventors after the discovery in 1873 of variation in the electrical conductivity of selenium when exposed to light. Selenium cells were used in early television devices; the results were unsatisfactory, however, chiefly because the response of selenium to light-intensity variations was not rapid enough. Moreover, until the development of the electron tube there was no way of sufficiently amplifying the weak output signals. These limitations precluded the success of a television method for which Paul Nipkow in Germany received (1884) a patent.
His system employed a selenium photocell and a scanning disk; it embodied the essential features of later successful devices. A scanning disk has a single row of holes arranged so that they spiral inward toward the center from a point near the edge. The disk revolves in front of a light-sensitive plate on which a lens forms an image; each hole passes across, or
scans, a narrow, ring-shaped sector of the image. Thus the holes trace contiguous concentric sectors, so that in one revolution of the disk the entire image is scanned. When the light-sensitive cell is connected in an electric circuit, the variations in light cause corresponding fluctuations in the electric current. The image can be reproduced by a receiver whose luminous area is scanned by a similar disk synchronized with the disk of the transmitter.
Although selenium cells proved inadequate, the development of the phototube (see photoelectric cell) made the mechanical disk-scanning method practicable. In 1926, J. L. Baird in Great Britain and C. F. Jenkins in the United States successfully demonstrated television systems using mechanical scanning disks. While research remained at producing pictures made up of 60 to 100 scanned lines, mechanical systems were competitive. These were soon superseded, however, by electronic scanning methods; a television system employing electronic scanning was patented by V. K. Zworykin in 1928. The 1930s saw the laboratory perfection of television equipment, and some programming became available in the United States beginning in 1939, but World War II almost entirely halted television programming and broadcasting. The television industry began to grow again only after 1945.
The television scanning process, used both to record and reproduce an image, operates as do the eyes in reading a page of printed matter, i.e., line by line. Prior to the introduction of television cameras using charge-coupled devices (see below), a complex circuit of horizontal and vertical deflection coils caused an electronic beam to scan the back of a mosaic of photoelectric cells in a 483-line zigzag 30 times each second, though the actual viewing area when the image was reproduced was typically 440 lines and 480 lines were used later by DVDs (digital versatile discs). (The standard was in fact a 525-line one, but not all the lines were used for the picture. The 525-line, 30-frame-per-second system was used in the United States, Japan, and elsewhere; many other countries used similar but incompatible systems.) Because of persistence of vision only about 16 pictures need be viewed each second to give the effect of motion. The development of interlaced scanning resulted in alternate lines being scanned each 1/60 sec. Half the lines were scanned in the first 1/60 sec, and the remaining lines, each one between two lines scanned during the first pass, covered in the next 1/60 sec.
Sections in this article:
- Evolution of the Scanning Process
- Development of the Television Camera and Receiver
- Development of Color Television
- Broadcast, Cable, and Satellite Television Transmission
- Television Technology Innovations
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