Modern video technology now encompasses a wide range of imaging and display techniques that make it possible to convert optical images into electrical signals and vice versa. These video signals span both distance and time, and can be used to create an accurate optical reproduction of the original color image on a television screen. The imaging process depends on a number of photosensitive devices that convert light into electrical signals. These include vacuum-type camera tubes that are scanned by an electron beam, flying spot scanners that read photographic film and translate it into video images, and \Lcharge-coupled device\ls (CCDs), which are solid-state light sensors in chip form. On the receiving side, the televised image can be re-created by a direct-view or projection \Tcathode ray tube\t, by a \Tliquid crystal\t display, by large-screen luminescent panels created by gas-plasma devices, or by light-valve projectors, which employ a fixed light source and special optical systems to project video images. Video technology also includes complex analog and digital signal processing at the source, to mix images or create special effects. A similar, although much simpler, video-signal processing can take place in the monitor or home receiver, to display multiple images on the screen or to extract data from the video signal that provides useful information, including \Tteletext\t and \Tvideotex\t. The distance factor is overcome by sending video images in real time from one location to another over a variety of transmission systems including electromagnetic propagation (radio waves) and hard-wire connections (cables). The time domain is overcome by video-recording devices that can capture images on magnetic tape, magnetic discs, or optical discs, and play them back at any time. Television depends on two fundamentals of physics, the conversion of light (photons) into commensurate electrical impulses (scanning and transmission), and the electroluminescence principle that generates light output in a chemical substance in proportion to an electrical input (display). In between these two extremities of the original and the reproduced image, there lies a vast array of signal-processing techniques that permit the video signal to be manipulated and distributed to the end viewer. The video manipulation can be done in either the analog or digital domains. Up until the early 1970s, television signals were almost all analog, that is, the video signal representing a given optical scene was a continuously variable voltage, whose value at any point reflected the luminance (brightness) and chrominance (color hue and saturation) of the picture element being transmitted. As digital techniques developed, much of the analog video-processing circuitry was replaced by high-speed sampling methods that convert the continuously changing gradients of the video signal into binary digital bits, each representing a discrete signal level. (See \Tdigital technology\t.) THE PICTURE ELEMENT The retina of the eye has an estimated 6-7 million light-sensitive receptors, which can simultaneously transfer a color image to the brain over a nerve bundle with approximately one million fibers. No practical color TV system can transfer that much information instantaneously from camera to TV screen. Instead, television depends on the methodical sequential scanning of a color image, in order to relay it over a single channel from the source to its final destinations. To do so, the original image must be broken down into picture elements called pixels. A pixel is the smallest area of a TV image that can be reproduced by an electrical signal. For the television system to transmit properly, an exploring spot scans progressively across each picture element from the top left to the bottom right corner of the image. In scanning, the image is broken down into a series of horizontal lines (in the United States, 525 lines is the standard). In a process called interlaced scanning, the scanner "reads" every even line on one scan, producing one "field." The second scan reads the odd-numbered lines left empty during the first scan, producing the second field. The scanning process is repeated continuously, producing 30 frames, or complete scenes, per second. The continuous video signal generated by this action is interspersed by synchronizing pulses, and is relayed via a communications system to the home receiver. There, a reproducing spot, scanning the picture tube in synchronism with the scanning element in the studio, will re-create the image. The smaller the picture elements that the system can handle, the sharper the reproduced image will look. Improved cameras, using more picture elements, create color television images rivaling the quality of 35-mm film. In existing TV broadcasting systems, the picture elements are defined by the system parameters, such as line structure and channel bandwidth. High Definition TV (\THDTV\t), the most advanced TV transmission system, uses line structures ranging above 1,000 and bandwidths up to 30 MHz, thus allowing for sharper resolution, both horizontally and vertically. Japan is already broadcasting in \THDTV\t, and Europe is developing its own system. It is hoped that if the United States can agree on \THDTV\t standards--probably with Japan--U. S. transmission systems will also convert to \THDTV\t. THE CAMERA TUBE The invention of the Iconoscope by Vladimir \TZworykin\t and of the Image Dissector by Philo T. \TFarnsworth\t in the early 1920s led to electronic television as it is known today. Subsequent improvements in imaging-tube technology produced the Orthicon and Vidicon--which first transmitted images in black and white--and, as color television developed, the Plumbicon and Saticon were added. More recently, new imaging devices have been invented that use solid-state charge-coupled devices (CCDs) or metal oxide semiconductors (MOS) as image sensors. Photoemission The conversion of light into a series of electrical signals is accomplished by a variety of photosensitive devices that function as camera elements on which the scene to be televised is optically focused. The energy in the photons of light coming through the camera lens and falling on the photosensitive surface is converted into free electrons in a process called photoemission. The most common photosensitive devices used in TV cameras are vacuum-tube types, such as Vidicons, Plumbicons, and Saticons. Here, a thin layer of a complex metal alloy is placed on a substrate and located at the front end of a glass vacuum tube. An electron gun scans the rear of the substrate with an electron beam. Electrons freed from the photosensitive layer modulate the current flow from the electron gun in proportion to the light falling on the front surface of the light-sensitive metal-alloy layer. The readout of this variable current is the video signal, representing the actual brightness at each point in the scene. Photoconduction Another type of camera tube uses the process of photoconduction. Here the light falling on a layer of semiconducting material changes the conductivity of the material in proportion to the light intensity at a given point. A CCD is a photoconductor, a light sensor whose electrical conductivity is changed by the absorption of photons. In the CCD camera, three small solid-state chips (about 1/3 X 1/2 in/0.84 X 1.27 cm) replace the camera tubes and act as imagers for the red, green, and blue components of the color image. There are several varieties of semiconductor, solid-state imagers. In cameras where they are used, they have some inherent advantages over tubes: a better signal-to-noise ratio (the power of the signal relative to the internal noise in a channel); no image retention between scans; no overloading due to high light levels; and a virtually limitless life span. Whether the image sensor is the front "window" of a camera vacuum tube, or the exposed surface of a CCD chip, the method of producing a color television image is the same. The original image is picked up by the camera lens and relayed through an optical system that includes red and blue dichroic mirrors. These mirrors essentially split light into its primary colors: the red mirror reflects red, but allows blue, and green to pass through; the blue mirror reflects blue, but allows the passage of red and green. In this way, the reflected light from the scene is divided into its primary color elements, and each separate color is directed to its appropriate imaging device. The red, blue, and green video signals generated by the image sensors are then matrixed--that is, they are cross-mixed--and signals conveying luminance (brightness) and chrominance (color hue and saturation) are added. The composite color signal now also includes horizontal and vertical synchronizing pulses. THE PICTURE TUBE On the display side, the vast majority of color television sets use a direct-view picture tube, ranging in size from 2 to 40 in (5 to 92 cm) measured diagonally. These tubes have one of two basic fluorescent-screen structures, known as the dot phosphor shadow mask, and the strip phosphor Trinitron (a trademark of the Sony Corp.). In both cases, chemical compounds (phosphors) that convert electron-beam energy into radiant light, forming the additive primary colors (red, green, blue), are deposited on the inner face of the glass picture tube in precision arrays of dots or stripes with alternate colors. A power supply and scanning circuits provide the voltages that energize the picture tube. Processing circuitry decodes the video signal into red, green, and blue components that drive the picture tube's electron guns. The video signal controls the intensity and position of the electron stream from the guns. As the beams scan the light-emitting phosphors in the tube, the color image is reconstructed on the face of the tube. The lag effect in the tube, and the retentivity of the human retina both contribute to the illusion of a continuous image on the screen, even with rapid motion in the picture. Finally, a separate sound detector and amplifier feed the speaker--or speakers, if the receiver has stereo components. Sophisticated television receivers may also have teletext decoders, hard-copy printers, and direct video inputs for VCRs and home computers. Receivers with internal digital processing can add picture within picture, freeze frame, image zoom, and sequential stills. The latest improvement in such receivers is the use of noninterlaced scanning to give the appearance of doubling the horizontal line rate. The resulting image looks sharper because the space between the scanning lines is filled. Specially designed CCD chips can also be used to cancel "ghosts" or multipath interference in television receivers, rendering much better images in mountainous terrain. In the future, the use of better encoding and decoding techniques, the use of comb filters--special circuits that sharpen resolution--and the incorporation of digital processing will produce home television images approaching studio picture quality. Flat-Screen TV The ultimate television set will hang on the living room wall like a picture, and be only a few inches thick. The most promising technology in this field is the liquid crystal display (LCD), which has produced pocket-size portable TV sets with up to 4-in (10.2 cm) diagonal screens. While screen diagonals up to 12 in (30.5 cm) have been demonstrated as prototypes, none have the brightness, resolution, or contrast range of a direct-view picture tube. At the other end of the scale, very large flat screens have been built for installation at large athletic parks, where they serve as giant scoreboards or huge television screens. The largest such screen by far was built by Sony at the Tsukuba Expo 85 in Japan. Using individual triads of red, green and blue "trini-lites" (color phosphors arrayed together inside a cell), the screen stood 14 stories high, had more than 10,000 sq ft (930 sq m) of luminous surface (82 X 131 ft; 25 X 40 m), and could easily be seen by 50,000 viewers. Projection Television Larger television images can be achieved through the use of a variety of projection systems that are designed for home or professional environments. Home projection TV systems are usually the enclosed cabinet variety, with screens ranging from 40 to 50 in (92 to 127 cm) diagonally. The image comes from three high-brightness cathode ray tubes, each providing a primary color. These are projected through a folded mirror system, on the rear of a translucent screen. Larger screens normally employ front projection, using either CRT's or light-valve techniques, and can produce television images on theater-size screens. DIGITAL AND ANALOG IN VIDEO TECHNOLOGY Both the sensing of a video image and its subsequent display are basically analog, created as they are by a continuously varying signal. However, the processing of video signals in their transition from the camera to the receiver screen can often be improved by converting them into digital form. The major advantage of handling a video signal in digital form is that it can be manipulated with special effects, can be recorded on videotape, or used for image overlays or multiple-imaging mixing without any degradation in image quality. The digital signal is almost impervious to degradation. In addition, several digital signals can be interleaved (multiplexed), expediting the transmission and processing of auxiliary information. At the present time, most television studios are analog, with "islands: of digital equipment dedicated to creating special-effects and computer-assisted video graphics. Within the home receiver, conversion to digital will improve the visual display by adding features not normally possible with analog circuits. CCD memories within the receiver will permit a "picture in picture" display, allowing the viewer to watch one channel on the full screen, with the video from another channel or from a VCR displayed in a small box in the corner of the screen. With digital circuits, the video image can be frozen or zoomed, or the screen can show up to nine sequenced still images. Digital video techniques will continue to take over more of the TV image chain. As digital components become more plentiful and more cost effective, both TV studio equipment and consumer electronic devices will become more digitized. TELEVISION RESOLUTION The quality of a television image is usually rated in lines of resolution and may range from 200 to 400 depending on the sophistication of the receiver circuitry. Even the best TV monitor or receiver, however, cannot display a better picture than the TV camera can produce on a given scanning system. To improve television images beyond their fundamental limits and to create a high-definition television (\THDTV\t) image will require a change of the basic scanning methods now in use. Several proposals have been made in the United States, Japan, and Europe for high-definition television systems using horizontal-line rates of 1,125 to 1,250, and expanding the picture to a 5:3 aspect ratio to give it a wide-screen look, thus emulating the cinema screen. These \THDTV\t systems will produce television images with 4 to 6 times the visual information content of a normal TV picture, increase the contrast ratio, and remove the visible line structure common to standard TV images. \THDTV\t may also be used to create original programs that can be recorded on videotape, then transferred to film for motion picture theater projection. TELEVISION TRANSMISSION The television channel that carries pictures and sound into the home receiver is part of a very rigid system of frequency allocations made by the International Telecommunications Union (ITU) to various regions around the world. Space in the electromagnetic spectrum is so scarce and valuable that periodic global meetings are held to reexamine the needs of member nations and to maintain or change the allocations. In addition to the world body, the Federal Communications Commission (FCC) in the United States is constantly under pressure to reallocate spectrum space for emerging or growing communications services. In North America the standard television channel is 6.0 MHz wide. Video signals are transmitted in a single sideband mode (sideband signals are present immediately above and below the carrier frequency), with the picture carrier at 1.25 MHz above the lower frequency boundary, and the sound carrier at 0.25 MHz below the upper boundary. This provides the video signal a 4.2 MHz channel, in which the color subcarrier is placed at 3.58 MHz above the picture carrier. (See also \Ttelevision\t \Ttransmission\t.) Television signals must be transmitted in accordance with FCC regulations, with regard to the signal format, radiated power, modulation depth, and a variety of other parameters that adhere to the rules of good practice. These standards assure that the viewer will receive a stable, high-quality image and clear sound. VIDEO RECORDING TECHNOLOGIES A permanent record of a television image or program can be made on film, on magnetic tape or discs, on optical discs, or in a solid-state memory called a frame buffer. Up until the development of the first practical videotape recorder by Ampex Corporation in 1956, all television programs were recorded on film by kinescope recorders. Old black-and-white reruns of early TV shows made in the 1950s come from such film and attest to the limited quality of that medium. While color kinescope recorders of much better quality are in use today, videotape has become the preferred medium for almost every variety of programming applications. Electronic cinematography has become the all-encompassing term for the use of television cameras to create program material, which is recorded, edited, and distributed on reels of videotape. In motion pictures there are three basic formats--35-mm professional, 16-mm industrial/educational, and Super 8-mm home movies. In the videotape world, things are not that simple. Professional videotape comes in 2-inch, 1-inch, 3/4-inch, 1/2-inch, 1/4-inch, and 8-mm widths, with enough different recording methods to create at least 15 non-interchangeable formats. In home video there are two half-inch formats, VHS and Beta, and the new 8-mm video format, none of which can play each other's tape cassettes. Videotape Recording VIDEOTAPE consists of a strip of plastic backing coated with a permanent layer of tiny metal particles imbedded in a durable resin. The particles are made of magnetic materials such as iron or cobalt and are capable of holding a magnetic charge imparted to them by the video recording head. When the tape is passed over the playback head, the resident magnetic charges are reproduced as the original video signal and the accompanying sound. In broadcast television, videotape recorders can now reproduce television programs that are virtually indistinguishable from a live show, although such machines are relatively complex and expensive. Because videotaping a TV show permits correcting any mistakes made, and allows cost-effective scheduling of TV studio facilities, most major programs are taped, then replayed at the appropriate time slot in each time zone. \TNBC\t's "Tonight Show" and "CBS News" are each recorded at least 3 hours earlier, sent by microwave or satellite to the network stations across the country, and rerecorded for local insertion. (See also \Ttelevision production\t.) Camcorders Television crews who go out in mobile vehicles to cover the news usually carry electronic cameras that incorporate a video recorder either as a separate portable unit or as a built-in cassette recorder on the camera. These self-contained battery-operated camcorders, weighing about 7 kg (15 lb), permit very flexible coverage in tight places and make possible pictorial results of very high visual impact. The imaging and videotape recording techniques developed for broadcasting have also resulted in the production of consumer-oriented devices. Videocassette recorders are now both compact and affordable, and they are used in nearly 50% of U. S. households. By adding equally compact color cameras, electronic home movies can now be made by anyone. Videocassette Recorders For the last few years, home VCRs have come in two incompatible formats, the Beta system developed by Sony and the VHS made by several manufacturers. Although both systems produced comparable images, the VHS configuration has now taken over the bulk of the market and is currently the leading format in sales, availability of prerecorded movies, and variety of accessories. Extended Definition Beta and Super VHS are the latest technological improvements in the field. Both use digital circuitry to produce special effects and enhance picture quality. Camcorders are also available in these formats. Several manufacturers are also offering components that use 8-mm cassettes. (See \Tvideo recording\t.) Optical Recorders Video or audio signals can also be stored on optical discs by using lasers to record and play back the spiral tracks. The larger size \Tvideodisc\t (30.5 cm/12 inches in diameter) can hold over one hour of normal television programming, one hour of a movie, or 15 minutes of high-definition TV, all with stereo sound. The \Tcompact disc\t can also be used for storing computer data, in which capacity it is called a CD-ROM (Read Only Memory). The advantage of the optical, or laser-read, disc over videotape is that it is virtually indestructible, since noncontact optical readout gives it an unlimited life. It also provides rapid random access to the information on the disc. The disadvantage, at least at present, is that the user cannot record his or her own program but must buy a prerecorded disc. New, recordable discs may soon be available, however. Optical discs have a very high information-packing density. For example, the entire Academic American Encyclopedia is stored on a single CD-ROM, with room to spare. Video Still Cameras Although they are still new and relatively untried, color cameras that record still pictures on magnetic discs have reached the market. These cameras convert the optical image into an electrical signal, then deposit the signal on circular tracks on a small magnetic disc inside the camera. up to 50 individual video shots can be taken, and they may be displayed on a home TV receiver through a video player. Because there is no processing necessary, the pictures may be viewed immediately. GRAPHICS TECHNOLOGY Video Art The development of \Tcomputer graphics\t systems permit graphic artists to create images directly in the television medium, greatly influenced daily programming. Electronic "paint systems" use a palette on which the artist renders the image with a stylus or light pen while watching a color television monitor to see the actual image being produced. The color monitor will also show the artist a wide selection of colors, a variety of print fonts, and various shapes such as circles, squares, and triangles (see PAINTBOX). External images can also be entered through a video camera and digitized, so that they may be altered in size, position, or coloration. The more-advanced computer graphic systems will also permit animation of the images to form a moving sequence. The TV station identification, the logos, commercial spots, and news inserts are usually created on computer graphics systems. The graphics can also be combined with live images. Major sporting events such as the Olympics or the Super Bowl will have computer graphic artists who create event-oriented graphics that are inserted into the show for "color." Computer graphic systems have steadily come down in size and price. These smaller systems are capable of three-dimensional rendering of television images and can be programmed to create animated segments for assembly on videotape recorder. Computerized Animation The commercials and cartoons that are being produced today for television broadcasting are often made on computer graphic systems with extended capabilities. While the individual images may be created by a system similar to the computer graphics system just described, the animation is dependent on larger computers with more memory and more capabilities. The key to generating the animation is to create a start "key frame" and an end "key frame." By telling the computer the rate at which the beginning and end images should progress, the computer will do all of the calculations, and produce the intermediate images to go smoothly from start to finish. Short animated sequences of this nature (up to 30 sec) can be stored on digital discs, from which they are transferred to tape and sequenced with past and future segments. Video Special Effects To achieve a high visual impact during a television show, it is necessary to manipulate images in a way that will make them appear more interesting. Because video signals can be digitized and put into a frame buffer ( a color-information storage device) in pixel form, it is also possible to read out these pixels in a nonlinear form. The original image may have been rectangular, but with the proper software it can be read out as a trapezoid, circle, or some other shape. Similarly, the image may be zoomed, tumbled, overlaid with other images, or made to disappear into the horizon. These effects can be done in real time by a "joystick" and a series of function keys that select the type of effect desired. In many cases these digital video effects are recorded, and the tape is used over and over for opening a new show or for creating a promotional insert. It is also possible to use these effects during live programming to enhance the appearance of the home viewers' image. Joseph Roizen Bibliography: Ayers, Ralph, Graphics for TV (1984); Benson, K. Blair, ed., Television Engineering Handbook (1985); Blank, Ben, and Garcia, Mario, Professional Graphic Video Design (1985); Fletcher, James, ed., Handbook of Radio and TV Broadcasting (1981); McCavitt, William E., Television and Technology: Alternative Communication Systems (1983); Oakey, Virginia, Dictionary of Film and Television Terms (1982); Oringel, Robert, Television Operation Handbook (1984); Rzeszewski, T., ed., Television Technology Today (1985).