(TV), transmission of visual images of moving and stationary objects and generally simultaneous transmission of accompanying sounds by means of electrical signals. Since the introduction of commercial television broadcasts in the early 1940s, the scope of television has grown enormously. Long accepted as the primary source of entertainment and information in the home, by the late 1990s the impact of television had expanded in important nonbroadcast areas, and digital video had become an integral part of the broad field of information technology.

TELEVISION PICTURES

Television pictures are formed from a pattern of tone elements that blend to form a complete picture. Unlike the dots of a halftone engraving (see PHOTOENGRAVING,) or FACSIMILE TRANSMISSION, (q.v.), which are all present simultaneously on a paper surface, the individual tone elements of the television image appear on the receiving surface one after another in temporal succession. They appear to blend into a complete image because of the characteristic of the human visual system known as persistence of vision, whereby the human eye retains an image for a fraction of a second longer than the image is actually present. The principles used in television technology are based on this and other characteristics of the human visual system. See also VISION,.

TELEVISION SYSTEMS

The three most important components of a television system are (1) a television camera in which a picture focused on the light-sensitive surface of an imager device is converted into an electrical signal; (2) a display device that converts this electrical signal into a picture; and (3) a generator that transmits timing signals to the camera and the display to ensure that both synchronously scan the picture along horizontal lines, from left to right and top to bottom, in such a way that the pictorial information picked up by the camera is shown in proper sequence in time and position on the surface of the display device. The electrical signal is delivered from the television camera to the display device via a cascade of processing, storage, and transmission systems, which may include transmission by electromagnetic waves, as in terrestrial (“over-the-air”) and satellite broadcast systems, or by cable and optical fibers, as in cable systems. See also RADIO,.

The Camera.

Like an ordinary photographic camera, a television camera uses a lens to focus an image onto a photosensitive flat target. In a procedure known as scanning, the camera's scanner sweeps over the entire image in much the same way as the eye of a reader sweeps over a page of print, word by word and line by line. The scanner generates an electrical signal proportional to the brightness of the scanned spot. At the receiver, a second scanner re-creates an image of the object by displacing a spot of light, modulated by the signal, in exact synchronism with the transmitter scanner.

In the past, the sensitive surfaces made up part of electronic tubes called camera tubes, which had the ability to transform variations in light intensity into variations in electric charge or current. The original camera tube was the iconoscope, a type long used for televising films. Later, the highly sensitive image orthicon and the vidicon were developed. By the late 1990s, vidicons had largely been supplanted by solid-state cameras that were more compact, more sensitive, and less costly. See also LENS,.

CCD cameras employ an imager composed of charge-coupled devices (CCDs), which are small light-sensitive elements arrayed in parallel rows. In the scanning process, these picture elements, or pixels, act as “buckets” that get filled by electrons in proportion to the intensity (number of photons) of visible light impinging on them. After a brief exposure, the charges in all the buckets are dumped into associated readout buckets. The charges in the column of readout buckets are subsequently transferred “bucket-brigade” style upward into an empty row of buckets at the top of the array, which acts like a gutter. The charges are again transferred the same way out of the gutter of buckets. This output from the imager is an electric current whose intensity varies with time as the light intensity varies along the rows. The sequential output from all pixels in the array constitutes a single frame (single static picture). The bucket-brigade process is repeated, frame after frame, generating a signal representing a sequence of successive pictures, as in motion pictures. Generally, circuits in the camera subject the output signal from the CCD imager to smoothing and amplification.

The Video Signal.

The television signal is composed of the following parts: (1) the output from the imager (as described above); (2) empty time slots between scanned lines (horizontal blanking interval or HBI) and between scanned frames (vertical blanking interval or VBI); and (3) synchronizing pulses (sync) from the timing generator, inserted in the blanking intervals, that lock the receiver to the same scanning rate as the generator. The video signal is conveyed together with audio signals to viewers by various transmission methods. See Broadcasting of Standard Analog Video, below.

The Display Device.

The display device in practically all television receivers is the picture tube, or CATHODE-RAY TUBE, (q.v.; CRT), sometimes called a kinescope. A beam of electrons is shot out from a gun in the back of the tube and is focused onto a glass faceplate at the front of the tube. The faceplate is covered by an electroluminescent material that emits light briefly where it has been hit by the electron beam. With the help of a magnetic field generated by electric currents in coils in a yoke around the neck of the tube, the beam can be deflected to form a raster of horizontal lines on the faceplate. A sync signal, extracted from the synchronizing pulses in the video signal, controls the deflection to form the same line structure as in the imager in the camera. The intensity of the beam emerging from the gun is controlled by the video signal. In this way visible pictures—replicas of the images seen by the television camera—appear on the faceplate. Generally, television viewers watch the faceplate directly, although some television sets project the images onto a screen.

BASICS OF COLOR TELEVISION

The scanning process for color television is similar to that for black-and-white television, except that a color camera separates an image into three color components: red, green, and blue. These are the three primary colors perceived by the human visual system, and a wide range of colors can be created by blending these three primary colors in various proportions (see COLOR,).

Many professional color television cameras use three imagers with optical filters (see OPTICS,) to produce red, green, and blue signals. Other modern cameras, in particular those used in consumer camcorders, deliver the red, green, and blue signals from a single imager in which every pixel consists of three tiny light sensors, each responding to one of the primary colors.

The red, green, and blue signals (R, G, and B, respectively) are carefully controlled to have normalized levels ranging from 0 to 1. The proportions of R, G, and B determine the color that appears on the screen. For example, when R = 1 (G and B = 0), a viewer sees the brightest saturated (undiluted) red that the system can reproduce; when G = 1, the viewer sees the brightest saturated green, and similarly for B. When R = G = B, a shade of gray appears, and when R = G = B = 1, the brightest white that the system can display appears.

For transmission purposes, the R, G, and B signals are combined to form a luminance signal Y (typically .3R + .59G + .11B) and two chrominance signals, R – Y and B – Y. The Y signal determines the brightness of a picture element, the proportion of the chrominance signals determines hue (tint), and the level determines saturation (color). The luminance signal and the chrominance signals are called video component signals. While a viewer can see fine luminance details in a picture, the human visual system tolerates much less detail in chrominance information—and television systems take advantage of this fact. A composite color video signal can be formed by inconspicuously inserting the chrominance signals into the luminance signal without increasing the required capacity of the transmission system, and with little picture degradation. See Specifications and Standards, below.

Broadcasting the composite video signal ensures compatible reception with both color and black-and-white receivers. In color receivers, the component signals are extracted from the composite signal, and the R, G, B signals are easily derived from the component signals and delivered to a color picture tube. When a black-and-white television set receives the signal, the presence of the added chrominance signals are inconspicuous in the displayed picture, so that it looks virtually the same as if it had received the luminance signal only.

Most color picture tubes use the shadow-mask principle developed by Radio Corp. of America (RCA). The interior surface of the faceplate is coated with small dots of electroluminescent phosphors, three dots per pixel, each radiating red, green, or blue light through the faceplate. Three guns—one each for red, green, and blue—in the back of the tube shoot electron beams at the respective dots. The intensity of the beams is controlled by R, G, and B signals to the respective guns. A metal “shadow mask” covers the interior of the faceplate. The mask contains three tiny holes for each pixel, each hole precisely aligned with one of the three electroluminescent dots for that pixel. It ensures that the beam of electrons from the red gun hits only red dots, the beam from the green gun hits only green dots, and the beam from the blue gun hits only blue dots.

BROADCASTING OF STANDARD ANALOG VIDEO

Various broadcast methods are used to deliver television signals to receivers.

Terrestrial Broadcasting.

In terrestrial color television broadcasting in the U.S., the composite video signal and stereo left and right audio signals (L and R) are delivered by a link from the studio to powerful television transmitters. The video signal amplitude-modulates the picture carrier so that the carrier level is low for white and high for black and reaches a maximum amplitude at the peak of the sync pulses. The audio signals frequency-modulate the sound carrier in essentially the same way as in stereo FM radio broadcasting. In the U.S. the sum of the modulated picture and sound carriers occupies a 6-MHz band of frequencies in the radio frequency (r.f.) spectrum. In other countries, up to 8 MHz of r.f. bandwidth may be required.

The combined audio and video signal travels over a transmission line to an ANTENNA, (q.v.), usually located on a high mast on a hilltop or on the roof of a high building. The power of the ELECTROMAGNETIC RADIATION, (q.v.) from the antenna is concentrated in the horizontal direction, in a pattern designed to cover the desired service area. In the U.S., the FEDERAL COMMUNICATIONS COMMISSION (q.v.; FCC) allocates a 6-MHz-wide channel to a broadcast station and specifies the maximum height of the antenna above the surrounding terrain, as well as the maximum effective radiated power in various horizontal directions. This ensures that a desired television signal can be received at most locations in the station's service area most of the time, with an acceptably low level of interference from undesired television signals operating on the same or adjacent channels. See Standards, below.

The 6-MHz-wide channels for television broadcasting in the U.S. are allocated to three bands in the electromagnetic spectrum: channels 2 to 6 in the low VHF (VERY HIGH FREQUENCY,; q.v.) band from 54 to 86 MHz, channels 7 to 13 in the high VHF band from 174 to 216 MHz, and channels 14 to 69 in the UHF (ULTRAHIGH FREQUENCY,; q.v.) band from 470 to 806 MHz. See also FREQUENCY,.

At a receiver site, radiation from many sources in the broadcast bands is picked up by an antenna, which may be an outdoor multielement type or indoor “rabbit ears.” The antenna delivers a broad spectrum of signals to the tuner of a television receiver, in which circuits select the channel chosen by the viewer and convert it to an intermediate frequency. Picture and sound carriers are subsequently demodulated to yield composite video and audio signals, which in turn are processed to yield R, G, and B color signals, to be delivered to the picture tube, and audio L and R signals, to be delivered to speakers.

As the distance from a selected broadcast station increases, the received signal becomes weaker and the picture becomes “snowy” because of unavoidable electric noise generated in the receiver. Hilly terrain, buildings, and trees between transmitter and receiver tend to make the signal weak. The levels of desired and undesired signals in a channel can vary greatly with location and time, even within a small neighborhood, and depend very much on the antenna installation. Particularly disturbing are “ghosts” (overlapping double images) which occur when the desired signal reaches the receiving antenna by different paths (multipath transmission). Some receivers may have de-ghosting circuits. Interference from an undesired television station operating on the same channel causes a disturbing “venetian blind” type of pattern in the picture. Adjacent channel interference and man-made (impulse) noise (from a hair dryer or electric drill, for instance) may also impair the picture.

Pictures with an acceptably low level of “snow” can be received with an outdoor antenna at distances up to about 120 kilometers (about 75 miles) from a high-power VHF transmitter. UHF coverage is usually less than about 80 kilometers (about 50 miles) and is more susceptible to shadowing effects by hills, trees, and buildings. Much poorer reception is obtained with indoor antennas.

Cable Broadcasting.

Cable television (CATV; formerly referred to as community antenna television) systems in the U.S. were launched about 1950 as small operations serving communities with poor over-the-air reception. Television signals received by antennas on a site with good reception were delivered to homes via a system of cables and amplifiers (see AMPLIFIER,). As more signals were imported by satellite links and improved amplifiers were used, cable systems grew to deliver more than 100 channels.

At the head end of a CATV system, television signals received over the air, by satellite, or by microwave (see MICROWAVES,)—or originated locally or from tape recorders—are converted into standard television r.f. broadcast signals and fed into one or more trunk lines, which are strung on utility poles or run underground along main roads in the community. Today, trunks are made of optical fibers (see FIBER OPTICS,). Older trunks are made of coaxial transmission lines (see CABLE, ELECTRIC,), up to 1 inch in diameter, interconnecting a cascade of 20 to 60 trunk amplifiers spaced about every 2000 feet along the trunk. “Bridger” amplifiers, which distribute the signals into neighborhoods via ¹/2-in. feeder cables, are also spaced along a trunk. The trunk cables carry low-voltage electric power for the amplifiers. Taps on the copper feeder cables connect to flexible drop cables, which in turn feed subscribers' television equipment. Such equipment may include a cable-ready television set or a set-top converter, which converts any channel selected by a viewer into either a channel 3 or a channel 4 signal, or into a video component signal for a television set, a videocassette recorder (VCR), or a computer. All subscribers receive a basic package of channels. With a variety of technologies, including signal scrambling, the cable operator can control a subscriber's access to premium and pay-per-view channels.

Cable systems can deliver more than 60 high-quality television channels per cable, in the frequency band from 50 to 450 MHz. A dual cable system may thus deliver 125 channels. The channel capacity and coverage of cable systems are limited primarily by the weakening of signal strength with frequency—high frequencies call for more amplification, which causes more noise and distortion.

Frequencies from 5 to 40 MHz are reserved for upstream transmission from homes to head end in two-way systems, most likely to be used primarily for transmission of digital signals. Not many such systems have yet been implemented, partly because of accumulation of excessive noise from many homes and amplifiers; however, techniques to improve upstream performance are being developed. Future CATV systems better suited to two-way transmission may use low-loss, large-bandwidth optical fiber lines extending to “smart” nodes serving as many as 500 homes. Eventually, broadband fiber optics may come directly into homes, without an intervening copper feeder. Moreover, with the development of more advanced CATV systems, digital signals may be conveyed, potentially increasing the delivery capacity per cable to several hundred digital television channels as well as data and audio. See Digital Television, below.

Direct Broadcast Satellite.

With advances in satellite communications technology, several direct broadcast satellite (DBS) systems were put into operation during the late 1990s, each delivering about 200 television channels to subscribers with an 18-in. (about 46-cm) dish antenna and a set-top converter, similar to those used for CATV. Video and audio signals, increasingly in digital format, originate at a ground station and are transmitted by microwave up to one or more geosynchronous satellites, which in turn transmit to individual antennas by microwave in a 12.2- to 12.7-GHz band to cover the entire contiguous U.S. A subscriber's dish antenna is oriented in a fixed direction for best reception, at an elevation angle of approximately 10 to 50°. The digital signal includes error-correcting codes to ensure acceptably reliable reception despite variations in signal level (fading) caused by rain. Access to premium channels is controlled by encrypting the signals for those channels. See also COMMUNICATIONS SATELLITE,.

DIGITAL TELEVISION

Digital television comprises a stream of data from which a sequence of intelligible pictures can be produced. In digital television, data representing visual information is derived by sampling the analog television signal at regular intervals (see ANALOG-TO-DIGITAL CONVERTER).

In the past, composite color television signals were commonly sampled and digitized for the purposes of processing, editing, recording, and distribution of video signals. Today, however, the three video component signals are generally digitized at the outset, for transmission as well as for various kinds of signal treatment. Sometimes the R, G, and B signals are digitized, but more commonly the luminance signal and the two chrominance signals are digitized.

The advantages of digital video are overwhelming: perfect reproducibility; precise level and time controls, digital storage, and signal processing; data compression; manipulation by computers and inclusion as part of interactive multimedia; digital transport and packet switching; and error control to ensure reliable and efficient delivery of unimpaired pictures and sound. In digital television, errors caused by transmission impairments can be managed to yield virtually error-free images, up to a maximum error rate, at which the picture falls apart all together—a phenomenon known as the “cliff effect.”

Video Compression.

A major feature of digital video is that redundant and irrelevant information can be removed by compressing data, which greatly reduces the bandwidth needed for quality transmission (see DATA COMPRESSION,). The most widely used method for video compression, MPEG-2, compresses video data by a ratio of more than 50 to 1 with hardly perceptible picture degradation, allowing high-quality images to be conveyed at a data rate of about 4 Mbps (megabits per second). The lower-data-rate digital video standard, MPEG-1, is used for lower-quality moving pictures, such as those in computer games. MPEG standards include formats for synchronization, data packetization, scanning and sampling, and rules (protocols) for editing, transport, and communication. See Standards, below.

Broadcasting.

Because of compression, digital television uses available broadcast channels more effectively than analog television. Up to five digital standard-definition television (SDTV) programs, or one digital high-definition television (HDTV) program with improved picture quality and compact disc–quality sound, may be fit into a 6-MHz-wide channel.

Starting in late 1998, each high-power broadcast station in the U.S. received an additional 6-MHz-wide channel for digital television broadcasting, with most channels in the UHF band. With the radiated power allowed by the FCC, it is expected that a broadcast station can cover about the same number of homes with digital television as are now covered with its analog transmitter. However, better home antenna systems may be needed for reliable digital television reception, particularly for UHF channels.

MMDS.

In addition to being distributed by terrestrial, cable, and satellite direct broadcasting, digital television (DTV) is also broadcast using MMDS, the multichannel multipoint distribution system, also called wireless cable, that emerged in the late 1990s. Terrestrial microwave signals carrying compressed digital television signals and data are broadcast from antennas located high above the community to be served. An MMDS may broadcast more than 100 digital television channels. The signals are received with a microwave antenna and converted to the same format that emerges from CATV and DBS set-top boxes.

SPECIFICATIONS AND STANDARDS

Because the television industry involves many different manufacturers and broadcasters, uniform specifications are essential so that products and services are compatible. Accordingly, national and international organizations have established precise standards to be used in television. The National Television System Committee (NTSC) began setting U.S. standards in 1941. A number of countries use NTSC standards, but other standards are more widely used.

Aspect Ratio.

The displayed area on the picture tube containing visual information is referred to as the active picture area. The active picture area plus the dark area of horizontal and vertical blanking intervals created by the retrace of the scanning beam is known as the total picture area. The ratio of the width to the height of the active picture area is the aspect ratio. The aspect ratio in analog television is 4/3.

Resolution.

The resolution, or sharpness, of a television system is defined as a function of aspect ratio and the height of the picture. Resolution is measured by exposing the camera to a pattern of successive bright crests separated by dark areas. The displayed distance between successive bright crests is called a cycle, and the largest number of cycles per picture height (cph) that the system can convey is defined as the resolution of the system. The higher the cph, the sharper the picture. In the U.S., analog television has a vertical resolution of 240 cph and the luminance component has a horizontal resolution of 165 cph.

Frame Rate.

The frequency with which an image is sampled to provide the illusion of continuous movement is known as the frame rate. Motion is perceived as smooth if at least 24 frames are shown every second; this is the frame rate used in motion pictures.

The standard frame rate for television in the U.S. and Japan is 30 frames per second (fps); in most other countries it is 25 fps. While these frame rates are adequate to produce the impression of motion as perceived by the human visual system, the picture would appear to flicker because of the blank periods between frames. In movies, this problem is solved simply by repeating the same frame several times before showing a new one.

In current broadcast television systems, flicker is greatly mitigated by displaying a frame as two successive fields of interlaced lines. In interlaced scanning (i), every other line is displayed in the first field and the remaining lines are displayed in the second field. The field rate is thus 60 fields per second, or 60 Hz, even though the frame rate is still 30 fps. Interlaced scan may still cause small area flicker in busy pictures, on alphanumeric characters, on horizontal edges, and in some types of motion. Some newer analog television receivers use sophisticated techniques, known as deinterlacing or line doubling, to eliminate these minor flicker effects.

These effects do not occur at all in noninterlaced, or progressive (p), scanning in which all lines in each frame is scanned; progressive scanning is used in computer displays and in some HDTV systems. This type of scanning, however, requires an actual 60-Hz frame rate, and the required bandwidth of the transmission channel is double that of interlaced scanning.

Standards.

In 1941 the NTSC set the monochrome (black-and-white) television standard for the U.S. at 525 lines per frame with interlaced scanning at a field rate of 60 Hz (30 fps). This standard is referred to as the 525/60 scanning standard. The highest frequency in the video signal was set at 4.2 MHz, a value that allows acceptable resolution without consuming excessive bandwidth. This choice led to the establishment of 6-MHz-wide television channels, a bandwidth that can accommodate a picture carrier modulated by the video signal and a sound carrier modulated by an audio signal.

Most of the rest of the world later adopted an interlaced standard with a 50-Hz field rate (25 fps), 625 lines per frame, and a video bandwidth ranging from 5 to 6 MHz. This standard is referred to as the 625/50 scanning standard. Like the U.S. standard, the international standard specifies an aspect ratio of 4/3.

Compatible color television video standards.

In 1953 the NTSC and the FCC adopted a standard for broadcasting color television signals that could also be received and displayed in monochrome by all existing black-and-white television sets. This was achieved by adding to the luminance signal a “subcarrier,” amplitude-modulated to carry the two chrominance signals. The frequency of the color subcarrier is chosen to be almost invisible in black-and-white as well as in color reception. It appears on the screen as an interlaced dot pattern moving slowly up the picture. On a blank screen, the pattern is hard to see unless a viewer consciously follows it with the eyes, and is indiscernible in a typical television program. A reference phase and amplitude of the color subcarrier is conveyed to receivers by bursts of the color subcarrier added during the horizontal blanking intervals. Japan, Canada, and several countries in Central and South America also use this system, called NTSC after the committee that developed it.

In Europe, a monochrome-compatible system called PAL (Phase Alternating Lines) was developed for the 625/50 system used in most of the world. PAL differs from NTSC primarily in that the phase of its color subcarrier is altered every line. France developed another system called SECAM (Séquentiel à Memoire) using two frequency-modulated color subcarriers to carry the two chrominance signals on alternate lines. SECAM is used in France, eastern Europe, and some African countries.

The vast proliferation of methods and standards to fit the needs, environments, politics, and broadcast organizations of different countries are classified by the International Telecommunications Union (ITU) and labeled B, G, H, I, K, KI, L, M, and N; only system M is NTSC.

Digital video standards.

In 1982, the ITU developed a standard for uncompressed standard digital video, known as ITU-R601. It applies to 525/60 and 626/50 interlaced scanning standards related to analog color television broadcast standards. It specifies a 13.5-MHz sample rate and 720 active luminance samples (pixels) on the active (visible) scanning lines. There are 480 active lines in 525/60 and 576 in 625/50. The pixels are aligned vertically. This very successful standard is used in professional VIDEO RECORDING, (q.v.) and post-production (see below). All but a few SDTV and HDTV digital standards now use frame, field, line, and sample rates related to a basic ITU-R601 frequency: 13.5/6=2.25 MHz.

In the U.S., based on tests and analyses made by the FCC Advisory Committee for Advanced Television Services (ACATS), digital standards were developed by the Advanced Television System Committee (ATSC). They were adopted by the FCC in December 1996 for both SDTV and HDTV.

SDTV offers resolution comparable to conventional analog television and the same narrow-screen aspect ratio (4/3) as analog television in its picture. HDTV, on the other hand, offers clear, finely detailed images. Its picture is wide, with an aspect ratio of 16/9, equal maximum vertical and horizontal luminance resolution, and 540 or 360 cph, depending on whether interlaced or progressive scanning is used.

Due to the flexibility of MPEG-2 standards, the ATSC standards allow for a number of digital video compression formats. All SDTV formats have 480 active lines and may have interlaced or progressive scan rasters. HDTV formats have either 720 active lines (720p) and frame rates of 30 to 60 fps with a progressive scan raster, or 1080 active lines (1080i) and 30 to 60 fps with a progressive scan raster or 30 fps with an interlaced scan raster.

Unlike the standards set for color television, which were designed to be compatible with black and white receivers, digital standards are not compatible with existing analog receivers. Digital television cannot be received with standard analog receivers unless a converter is attached; even then the picture quality deteriorates, especially for a converted HDTV image. To mitigate this problem, during the transition period to all digital broadcasting, the FCC decided that terrestrial broadcasts of analog and digital television would be simulcast; they are expected to coexist without interfering too much with each other or with other services in the television broadcast bands. In addition to terrestrial broadcasting, digital standards are expected to be used in CATV, DBS, and MMDS.

Canada and several other countries also adopted the ATSC standard. In Europe, digital video broadcast standards are still being considered.

NETWORK DISTRIBUTION SYSTEMS

Television networks used to distribute analog television signals by cable and terrestrial microwave radio to affiliated broadcasters and CATV systems. Today, however, fiber optics and satellite technology are used almost exclusively for point-to-point and point-to-multipoint distribution of television (see TELECOMMUNICATIONS,). These media offer ample bandwidth.

While some of these systems still use analog transmission, digital modulation techniques are generally used to transport compressed digital video and audio. The data rate for HDTV is about 20 Mbps, and only about 4 Mbps for SDTV, requiring roughly 6-MHz and 1.2-MHz bandwidths respectively. Thus, satellites and fiber optic cables can convey a huge number of digital television channels. Increasingly, digital video is transmitted as switchable packets of data, as in voice and data communication.

TELEVISION PRODUCTION AND POSTPRODUCTION

The sophisticated and expensive technology used in the production and postproduction of television programs continues to evolve with advances in digital video, video recording, computers, and telecommunications.

Film, an unwieldy format, is now rarely used in television production. Television programs originate in some video format. Programs recorded on film are converted in telecine machines to video, usually digital video, and recorded on tape or disc for postproduction processing. Digital video allows for excellent conversions of video from one standard to another, for example from the European PAL to the American NTSC.

In studio production, a number of cameras and sophisticated lighting techniques are used. The studio control room contains a production console and a number of monitors displaying video inputs. The program and technical directors select from these video inputs, some live from cameras and some from tapes or discs, to choose among a number of transition formats: a take is a simple switch in the vertical blanking interval; fades, wipes, and dissolves are softer transitions. Digital video makes possible more creative transitions, such as page turning, rotation, and zooming, and video inputs can be mixed to create superpositions and split screens. To avoid picture “rolls,” the various inputs must be synchronized, either by locking them to the studio's master synchronization (called gen locking), or by using a frame memory to assure proper frame-to-frame transitions.

Most programs, even those intended for almost immediate broadcasting (like sports events), are recorded on tape or disc for postproduction, the final stage of processing in which a number of recorded video and audio inputs are edited, video colors are adjusted, video effects and titles are added, and audio is mixed, enhanced, and integrated with video to yield a final product. In editing, the original sources are combined onto a master recording. The editor keeps an edit decision list showing time events, time codes, and durations of the insertions to keep track of what has been done. For special effects, part of a picture can be keyed into another picture (for example, the score of a sports event can be inserted into a picture of the game). Informative graphics can also be keyed onto slow motion replays from hard disks. In chroma keying, a person can be inserted into a background picture—for example, a weather forecaster (A) pointing to a large and changing weather map (B). The weather forecaster stands in front of a background of uniform color, usually blue; the video of the scene is processed by eliminating everything in picture A with the specific blue color and eliminating everything in picture B within the forecaster's contour. Pictures A and B are subsequently superimposed for broadcasting (and for display so that the forecaster knows where to point on the weather map). Even more sophisticated chroma keys can be made with, for example, shadows or soft contours.

NONBROADCAST APPLICATIONS

Major progress has been made since the mid-1970s in nonbroadcast television, or video communication, using satellites and optical fibers to serve broadcast stations and cable systems.

Consumer Applications.

Prerecorded video programs on videotape or DIGITAL VIDEO DISC (q.v.; DVD) provide consumers with a vast selection of entertainment and educational materials. Television programs may be recorded on home VCRs and original videos can be created with handheld camcorders. As the performance of personal computers and data communication systems improved, not only animations, but motion pictures of almost NTSC color television quality could be displayed on a high quality monitor (see PERSONAL COMPUTER,). Video games with joystick control have long been a simple popular form of interactive television; interactive multimedia programming now involves television, audio, animation, graphics, and text.

Teleconferencing.

Video teleconferencing—conducting business meetings with participants at different locations—has been quite successful; participants hear each other by audio link and simultaneously see each other by video link. Videophones may also prove possible using the INTERNET, (q.v.).

Education and Training.

Television, including prerecorded video, interactive television, and multimedia, is used extensively and effectively for education and training in schools, colleges, businesses, and government operations including the military. Most campuses and many large corporations have television studios for production and distribution of educational programs. Flight simulation is an advanced application of training with interactive television.

Surveillance.

Surveillance by closed-circuit television cameras is used extensively by businesses and governments for crime prevention and law enforcement. Military satellite surveillance uses high-resolution cameras and sophisticated television technology; infrared-sensitive cameras, which detect the heat of observed objects, can be used for night vision.

Medical Applications.

X rays, magnetic resonance imaging (see NUCLEAR MAGNETIC RESONANCE), and other medical images displayed on television monitors are used as diagnostic tools. Closed circuit television is used for surveillance of patients and training of health care personnel.

Meteorological and Earth Science Applications.

Pictures of moving clouds and storm patterns seen in television weather reports are taken from orbiting satellites with high-resolution cameras. Television pictures are a valuable aid in predicting weather and saving lives with storm warnings. Satellite-based television surveillance is also used for the prospecting of minerals on the earth and for estimating crop harvests.

Space Science Applications.

In a number of manned and unmanned space missions, spectacular television images have been delivered from the moon, Mars, Jupiter, Saturn, Venus, and Jupiter's moon Europa. Important scientific pictures have been transmitted from unmanned spacecrafts. The HUBBLE SPACE TELESCOPE, (q.v.), orbiting about 350 miles above the earth's surface, has sent remarkably clear pictures of galaxies billions of light-years away.

HISTORY OF TELEVISION TECHNOLOGY

For the business, programming, regulation, and politics of television broadcasting, see BROADCASTING, RADIO AND TELEVISION,.

Before World War I.

Television technology has been evolving since the end of the 19th century. The word télévision was coined by a Frenchman in 1900, at which time the basic concepts of television were understood. In fact, the German inventor Paul Gottlieb Nipkow (1860–1940) had implemented the first working system in 1884 and is generally credited with the invention of television. For a “camera,” Nipkow focused an image of a scene on the surface of a mechanical rotating disk; holes in the disk were spaced along a spiral to scan the image along as many lines as there were holes in one revolution of the disk. The light that passed through the holes was picked up by a photocell, a photosensitive device that converted the light variations to an electrical signal (see PHOTOELECTRIC CELL,.) For a display Nipkow used a similar synchronously rotating disk exposed to a light source whose intensity was controlled by the output signal from the photocell. Because of persistence of vision, a viewer observing the light from the holes in the disk sees an image. Thus Nipkow introduced the scanning process with lines and frames as it is used today; he also established the need for synchronization.

The CRT, invented in 1897, was used for the first time as a television display in 1907. Crucial to this development was the invention of the vacuum-tube amplifier the year before by the American inventor Lee De Forest; the vacuum tube made it possible to amplify weak signals such as those in television systems (see VACUUM TUBES,.) Meanwhile, work continued on suitable cameras; various mechanical scanners with mirrors were developed, and progress was made in photoelectric materials and devices.

Between the Wars.

After an interruption during World War I, intensive development resumed. In 1923 the Russian-born American physicist Vladimir K. Zworykin applied for a patent for an all-electronic television camera, the iconoscope. Zworykin demonstrated his invention in 1933. Meanwhile, the American engineer Philo T. Farnsworth (1906–71) applied for a patent for an electronic camera, the image dissector, in 1927, and demonstrated it the same year. (The U.S. Patent Office awarded priority of invention to Farnsworth in 1934.) The iconoscope could store light energy as electrical charges on the exposed target, which was a major breakthrough in the development of television. In 1932 RCA demonstrated an all-electronic television system that used an iconoscope and an improved CRT display called a kinescope. An all-electronic system was also demonstrated in Great Britain in 1935 by Electric and Musical Industries (EMI). At this time intensive engineering work toward practical television broadcast systems and television technical standards was in progress in Germany as well as in the U.S. and Great Britain.

Broadcasting Begins.

The first public television broadcasts were made in England in 1927 and in the U.S. in 1930, in both instances using mechanical systems. At the opening of the 1939 World's Fair in New York City, RCA demonstrated a practical all-electronic television system, marking the start of regular television broadcasting service in the U.S. An industry committee, the Radio Manufacturers of America (RMA), had begun development of television standards, and in 1940 formed the National Television System Committee (NTSC). The NTSC proposal for television technical standards for the U. S. was adopted by the FCC in May 1941. By July, several stations were on the air.

Expansion After World War II.

Although most broadcasting was interrupted by World War II, television was used during the war for military purposes and advances were made in the technology. Broadcasting resumed in 1946, reaching some 10,000 households from about 10 stations. The explosive growth that followed, with hundreds of applications to the FCC for each of the 12 VHF channels, led the agency to initiate a moratorium on new licenses. The FCC resumed licensing new stations in 1952 and opened the UHF spectrum for television broadcasting on 70 more channels (later reduced to 56). For many years, UHF stations found it difficult to compete with the VHF stations, partly because of more adverse transmission and reception conditions. To prevent interference, the FCC set up rules for distances between stations transmitting on the same or adjacent channels. By 1956 the number of broadcast stations in the U.S. had grown to almost 500 and households with televisions numbered about 35 million. That year the quadruplex video tape recorder, the first practical recorder of color video signals on magnetic tape, was introduced, revolutionizing the production of television programs.

Image quality was greatly improved with the commercial introduction of large, bright CRTs and improved cameras, such as the image orthicon, the vidicon and, in the 1970s, CCD imagers. Television also expanded quickly in Europe, where significant technical contributions were made, particularly in Great Britain. In the 1950s cable television emerged in primitive systems serving communities where the over-the-air reception was poor.

Color Television.

Research and development of color television also started in the early 1950s. An electromechanical system in which the display of a sequence of red, green, and blue pictures designed to merge perceptually in the viewer's eyes as a color picture was proposed by the Columbia Broadcasting System (CBS). Adopted by the FCC in 1951, this field sequential system was not compatible with the monochrome system—the color TV signal could not produce a black-and-white picture when received by black-and-white television sets. Because of the moratorium on new licenses, the system was not implemented; by the time the moratorium was lifted in 1952, a sophisticated, black-and-white compatible color system had been developed by RCA.

It took about 10 years for a significant market for color receivers to develop, partly because of the cost of the receivers and because stations were slow to broadcast in color. Since 1954, the price of color receivers has dropped steadily, and performance has greatly improved. Today, virtually all programs are broadcast in color.

The Digital Era.

Research into digital image processing began in the 1960s. By the early 1980s, the concept of digital television became a reality and a subject for international standards committees. An advanced, international video standard for digital television, developed by the ITU in 1982, allowed all analog color video signals to be converted to a digital data stream. The original goal was to achieve a standard for high-quality pictures that could be recorded, edited, and copied without degradation. Developers knew at the outset that most images contain a lot of redundant information, and that the frame-to-frame redundancy in motion picture scenes is considerable. Later, researchers sought to reduce the bandwidth needed to transmit video signals by eliminating much of the redundancy in images and motion pictures.

The Moving Pictures Expert Group (MPEG) was set up in 1988 by the International Organization for Standardization and the International Electrotechnical Commission to establish standards for encoding moving pictures and associated audio. MPEG developed techniques to eliminate redundant and, for the viewer, irrelevant information and for compressing the data required for transmission by a factor exceeding 50:1. While the MPEG standards are quite flexible, they established a detailed protocol for digital video transmission.

During the 1980s researchers in Japan and the U.S. mounted efforts to develop HDTV, with larger and wider displays and sharper resolution. Since it was assumed at the time that an all-digital HDTV system would require an excessive bandwidth for terrestrial broadcasting, a number of analog systems were proposed; the focus changed in 1990 with a bold proposal by General Instruments (GI) for an all-digital HDTV system based on compressed digital video that actually used bandwidth more effectively than analog television. An industry group, the Grand Alliance, joined forces with GI to develop digital HDTV for the U.S. using MPEG-2. In 1996, the FCC approved standards for digital HDTV and for digital SDTV as well.

The FCC allocated an additional channel for digital terrestrial television broadcasting to each U.S. broadcast station. Mandated by the FCC, digital terrestrial broadcast service began on Nov. 1, 1998, and analog service was to be phased out over several years.

Direct broadcasting by satellite to homes of digital SDTV started in the late 1990s, and distribution of digital television by cable systems was in development. At the same time, MMDS began carrying DTV and data.

Television Today.

In the U.S., about 1700 high-power stations broadcast to about 250 million receivers in almost 100 million television households—about one receiver per person, who watches an average of about 4 hours of television per day. A number of low-power stations also serve local communities and universities. Many commercial broadcasters receive programs, usually via satellite, from the major networks. Educational broadcasters receive most programs from the Public Broadcasting Service network.

With steadily improving technology, CATV has grown into a major industry serving 2 out of 3 homes in the U.S.—well over 60 million households. Some cable systems now deliver more than 100 channels. Viewers have now become accustomed to choosing from a vast selection of programs, including special programs for news, sports, and weather.

In 1998, about 8 million people subscribed to the four DBS services in the U.S., each operating with two or more satellites. Although DBS now must broadcast the same programming to all subscribers, the use of focused beams (spot beams) to selectively serve small areas with local programs is under development.

Using digital signals, all broadcast media can also deliver other digital communications, such as Internet data and digital audio.

For further information on this topic, see the Bibliography, sections 548. Television, 755. Radio and television broadcasting.