2/10/11
The electrical transmission and reception of transient visual images. Like motion pictures, television consists of a series of successive images, which are registered on the brain as a continuous picture because of the persistence of vision. Each visual image impressed on the eye persists for a fraction of a second. In television in the United States, 30 complete pictures are transmitted each second, which with the use of interlaced scanning is fast enough to avoid evident flicker.
At the television transmitter, minute portions of a scene are sampled individually for brightness (and color for color television), and the information for each portion is transmitted consecutively. At the receiver, each portion is synchronized and reproduced in its proper position and with correct brightness (and color) to reproduce the original scene.
The scene is focused through a lens on a photoelectric screen of a camera tube. Each portion of the screen is changed by the photoelectrons to a degree depending upon the brightness of the particular portion of the scene. The screen is scanned by an electron beam just as a reader scans a page of printed type, character by character, line by line. When so scanned, an electric current flows with an instantaneous magnitude proportional to the brightness of the portion scanned. See also Television camera; Television camera tube.
Variations in the current are transmitted to the receiver, where the process is reversed. An electron beam in the picture tube is varied in intensity (modulated) by the incoming signals as it scans the picture-tube screen insynchronism with the scanning at the transmitter. The photoelectric surface of the picture tube produces light in proportion to the intensity of the electron beam which strikes it. In this way the minute portions of the original scene are re-created in their proper positions, brightness, and (for color transmission) color values. See also Picture tube; Television receiver; Television transmitter.
In the Western Hemisphere and Japan, the NTSC (National Television System Committee) system is used, in which an individual picture (frame) is considered to be made up of 525 lines, each line containing several hundred picture elements. All these lines are scanned and the light values are sent to the receiver so that each second 30 pictures are received. The picture is blanked out at the end of each line while the scanning beam is directed to the next line. During these short intervals, synchronizing signals are transmitted to keep the scanning process at the receiver in step with that at the transmitter. To take full advantage of the persistence of vision, each frame is scanned twice, alternate lines being scanned in turn. This technique is called interlaced scanning. See also Television scanning;Television standards.
The band of frequencies assigned to a television station for the transmission of synchronized picture and sound signals is called a television channel. In the United States a television channel is 6 MHz wide, with the visual carrier frequency 1.25 MHz above the lower edge of the band and the aural carrier 0.25 MHz below the upper edge of the band.
In the United States the sound portion of the program is transmitted by frequency modulation at a carrier frequency 4.5 MHz above the picture carrier. Maximum frequency deviation (bandwidth) of the sound signals is 25 kHz. See also Frequency modulation.
In television broadcasting, videotape recorders are used not only for delayed playback but also for program distribution and, especially, for storage of program segments during postproduction editing. In the latter case, the program tape that is actually broadcast can be several generations removed from that originally recorded at the television camera or telecine film reader. Three or four generations are typically encountered with dramatic programs; whereas, for commercials or productions involving complex special effects, as many as eight to ten rerecording generations are not uncommon. The quality of the image, especially as measured by the signal-to-noise ratio, degrades with each generation, since the recorder itself adds a noncoherent noise in each pass. To minimize the degradation from multiple generations, there has been a long-term effort to replace analog video recorders with digital technology. Digital communications has the advantage that noise does not accumulate in cascade links; thus, digital recorders will not accumulate noise through multiple generations of recording.
A digital system is also useful in the worldwide exchange of television programs, where standards conversions are necessary because of the large number of different scanning and color-encoding standards used in various countries. A single world wide digital studio standard was adopted in 1981, incorporating efforts to maximize commonality between equipments used in different standards, and to base the digital standards on separate luminance and color difference signals rather than on any given country's composite system. In 1986, specifications were completed for the digital recorder named D-1, which can retain good quality after more than 50 generations.
High-definition television (HDTV) was originally conceived as a system for providing cinemalike viewing in the home. It was designed to provide much improved resolution with a wider aspect ratio of 16:9 (instead of 4:3 in standard television) and high-fidelity audio quality. High-definition television has twice the horizontal and twice the vertical resolution of standard television, with improved color resolution and multichannel high-fidelity sound. Digital processing offers greater accuracy and stability with a much better signal-to-noise ratio than analog processing can provide for video signals.
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