The Television Camera
The television camera is the first tool used to produce a television program. Most cameras have three basic elements an optical system for capturing an image, a pickup device for translating the image into electronic signals, and an encoder for encoding signals so they may be transmitted.
Optical System
The optical system of a television camera includes a fixed lens that is used to focus the scene onto the front of the pickup device. Color cameras also have a system of prisms and mirrors that separate incoming light from a scene into the three primary colors: red, green, and blue. Each beam of light is then directed to its own pickup device. Almost any color can be reproduced by combining these colors in the appropriate proportions. Most inexpensive consumer video cameras use a filter that breaks light from an image into the three primary colors.
Pickup Device
The pickup device takes light from a scene and translates it into electronic signals. The first pickup devices used in cameras were camera tubes. The first camera tube used in television was the iconoscope. Invented in the 1920s, it needed a great deal of light to produce a signal, so it was impractical to use in a low-light setting, such as an outdoor evening scene. The image-orthicon tube and the vidicon tube were invented in the 1940s and were a vast improvement on the iconoscope. They needed only about as much light to record a scene as human eyes need to see. Instead of camera tubes, most modern cameras now use light-sensitive integrated circuits called charge-coupled devices or CCDs.
When recording television images, the pickup device replaces the function of film used in making movies. In a camera tube pickup device, the front of the tube contains a layer of photosensitive material called a target. In the image-orthicon tube, the target material is photoemissive-that is, it emits electrons when it is struck by light. In the vidicon camera tube, the target material is photoconductive-that is, it conducts electricity when it is struck by light. In both cases, the lens of a camera focuses light from a scene onto the front of the camera tube, and this light causes changes in the target material. The light image is transformed into an electronic image, which can then be read from the back of the target by a beam of electrons which are tiny, negatively charged particles.
The beam of electrons is produced by an electron gun at the back of the camera tube. The beam is controlled by a system of electromagnets that make the beam systematically scan the target material. Whenever the electron beam hits the bright parts of the electronic image on the target material, the tube emits a high voltage, and when the beam hits a dark part of the image, the tube emits a low voltage. This varying voltage is the electronic television signal.
Scanning
Television cameras and television receivers use a procedure called scanning to record visual images and re-create them on a television screen. The television camera records an image, such as a scene in a television show, by breaking it up into a series of lines and scanning over each line with the beam or beams of electrons contained in the camera tube. The pattern is created in a CCD camera by the array of photodiodes. One scan of an image produces one static picture, like a single frame in a film. The camera must scan a scene many times per second to record a continuous image. In the television receiver, another electron beam-or set of electron beams, in the case of color television-uses the signals recorded by the camera to reproduce the original image on the receiver's screen. Just like the beam or beams in the camera, the electron beam in the receiver must scan the screen many times per second to reproduce a continuous image.
In order for television to work, television images must be scanned and recorded in the same manner as television receivers reproduce them. In the United States, broadcasters and television manufacturers have agreed on a standard of breaking images down into 525 horizontal lines, and scanning images 30 times per second. In Europe, most of Asia, and Australia, images are broken down into 625 lines, and they are scanned 25 times per second. Special equipment can be used to make television images that have been recorded in one standard fit a television system that uses a different standard. Telecine equipment (from the words television and cinema) is used to convert film and slide images to television signals. The images from film projectors or slides are directed by a system of mirrors toward the telecine camera, which records the images as video signals.
The scanning method that is most commonly used today is called interlaced scanning. It produces a clear picture that does not fade or flicker. When an image is scanned line by line from top to bottom, the top of the image on the screen will begin to fade by the time the electron beam reaches the bottom of the screen. With interlaced scanning, odd-numbered lines are scanned first, and the remaining even-numbered lines are scanned next. A full image is still produced 30 times a second, but the electron beam travels from the top of the screen to the bottom of the screentwice for every time a full image is produced.
Transmission of Television Signals
The audio and video signals of a television program are broadcast through the air by a transmitter. The transmitter superimposes the information in the camera's electronic signals onto carrier waves. The transmitter amplifies the carrier waves, making them much stronger, and sends them to a transmitting antenna. This transmitting antenna radiates the carrier waves in all directions, and the waves travel through the air to antennas connected to television sets or relay stations.
Cable Television Cable television was first developed in the late 1940s to serve shadow areas-that is, areas that are blocked from receiving signals from a station's transmitting antenna. In these areas, a community antenna receives the signal, and the signal is then redistributed to the shadow areas by coaxial cable a large cable with a wire core that can transmit the wide band of frequencies required for television or, more recently, by fiber-optic cable. Viewers in most areas can now subscribe to a cable television service, which provides a wide variety of television programs and films adapted for television that are transmitted by cable directly to the viewer's television set. Digital data-compression techniques, which convert televisio n signals to digital code in an efficient way, will eventually increase cable's capacity to 500 or more channels.
Television Receiver
The television receiver translates the pulses of electric current from the antenna or cable back into images and sound. It consists of a tuner, an audio system, and a picture tube.
Tuner
The tuner blocks all signals other than that of the desired channel. Blocking is done by the radio frequency or RF amplifier. The RF amplifier is set to amplify a frequency band, 6 MHz wide, transmitted by a television station; all other frequencies are blocked. The particular frequency band that is amplified is determined by a channel selector connected to the amplifier. When a new channel is selected, the amplifier is reset accordingly. In this way, the band, or channel, picked out by the home receiver is changed. Once the viewer selects a channel, the incoming signal is amplified, and the video, audio, and scanning signals are separated from the higher frequency carrier waves by a process called demodulation. The tuner amplifies the weak signal intercepted by the antenna and partially demodulates or decodes it by converting the carrier frequency to a lower frequency-the intermediate frequency. Intermediate frequency amplifiers further increase the strength of the signals received from the antenna. After the incoming signals have been amplified, audio, scanning, and video signals are separated.
Audio System
The audio system consists of a discriminator, which translates the audio portion of the carrier wave back into an electronic audio signal; an amplifier; and a speaker. The amplifier strengthens the audio signal from the discriminator and sends it to the speaker, which converts the electrical waves into sound waves that travel through the air to the listener.
Picture Tube
The television picture tube receives video signals from the tuner and translates the signals back into images. The images are created by an electron gun in the back of the picture tube, which shoots a beam of electrons toward the back of the television screen. A black and white picture tube contains just one electron gun, while a color picture tube contains three electron guns, one for each of the primary colors of light red, green, and blue. Part of the video signal goes to a magnetic coil that directs the beam and makes it scan the screen in the same manner as the camera originally scanned the scene. The rest of the signal directs the strength of the electron beam as it strikes the screen. The screen is coated with phosphor, a substance that glows when it is struck by electrons. The stronger the electron beam, the stronger the glow and the brighter that section of the scene appears.
In color television, a portion of the video signal is used to separate out the three color signals, which are then sent to their corresponding electron beams. The screen is coated by tiny phosphor strips or dots that are arranged in groups of three: one strip or dot that emits blue, one that emits green, and one that emits red. Before light from each beam hits the screen, it passes through a shadow mask located just behind the screen. The shadow mask is a layer of opaque material that is covered with slots or holes. It partially blocks the beam corresponding to one color and prevents it from hitting dots of another color. As a result, the electron beam directed by signals for the color blue can strike and light up only blue dots. The result is similar for the beams corresponding to red and green. Images in the three different colors are produced on the television screen. The eye automatically combines these images to produce a single image having the entire spectrum of colors formed by mixing the primary colors in various proportions.
Television's History
The scientific principles on which television is based were discovered in the course of basic research. Only much later were these concepts applied to television as it is known today. The first practical television system began operating in the 1940s.
In 1873 the Scottish scientist James Clerk Maxwell predicted the existence of the electromagnetic waves that make it possible to transmit ordinary television broadcasts. Also in 1873 the English scientist Willoughby Smith and his assistant Joseph May noticed that the electrical conductivity of the element selenium changes when light falls on it. This property, known as photoconductivity, is used in the vidicon television camera tube. In 1888 the German physicist Wilhelm Hallwachs noticed that certain substances emit electrons when exposed to light. This effect, called photoemission, was applied to the image orthicon television camera tube.
Although several methods of changing light into electric current were discovered, it was some time before the methods were applied to the construction of a television system. The main problem was that the currents produced were weak and no effective method of amplifying them was known. Then, in 1906, the American engineer Lee De Forest patented the triode vacuum tube. By 1920 the tube had been improved to the point where it could be used to amplify electric currents for television.
Nipkow Disk
Some of the earliest work on television began in 1884, when the German engineer Paul Nipkow designed the first true television mechanism. In front of a brightly lit picture, he placed a scanning disk called a Nipkow disk with a spiral pattern of holes punched in it. As the disk revolved, the first hole would cross the picture at the top. The second hole passed across the picture a little lower down, the third hole lower still, and so on. In effect, he designed a disk with its own form of scanning. With each complete revolution of the disk, all parts of the picture would be briefly exposed in turn. The disk revolved quickly, accomplishing the scanning within one-fifteenth of a second. Similar disks rotated in the camera and receiver. Light passing through these disks created crude television images.
Nipkow's mechanical scanner was used from 1923 to 1925 in experimental television systems developed in the United States by the inventor Charles F. Jenkins, and in England by the inventor John L. Baird. The pictures were crude but recognizable. The receiver also used a Nipkow disk placed in front of a lamp whose brightness was controlled by the signal from the light-sensitive tube behind the disk in the transmitter. In 1926 Baird demonstrated a system that used a 30-hole Nipkow disk.
Electronic Television
Simultaneous to the development of a mechanical scanning method, an electronic method of scanning was conceived in 1908 by the English inventor A. A. Campbell-Swinton. He proposed using a screen to collect a charge whose pattern would correspond to the scene, and an electron gun to neutralize this charge and create a varying electric current. This concept was used by the Russian-born American physicist Vladimir Kosma Zworykin in his iconoscope camera tube of the 1920s. A similar arrangement was later used in the image-orthicon tube.
The American inventor and engineer Philo Taylor Farnsworth also devised an electronic television system in the 1920s. He called his television camera, which converted each element of an image into an electrical signal, an image dissector. Farnsworth continued to improve his system in the 1930s, but his project lost its financial backing at the beginning of World War II 1939-1945. Many aspects of Farnsworth's image dissector were also used in Zworykin's more successful iconoscope camera.
Cathode rays, or beams of electrons in evacuated glass tubes, were first noted by the British chemist and physicist Sir William Crookes in 1878. By 1908 Campbell-Swinton and a Russian, Boris Rosing, had independently suggested that a cathode ray tube or CRT be used to reproduce the television picture on a phosphor-coated screen. The CRT was developed for use in television during the 1930s by the American electrical engineer Allen B. DuMont. DuMont's method of picture reproduction is essentially the same as the one used today.
The first home television receiver was demonstrated in Schenectady, New York, on January 13, 1928, by the American inventor Ernst F. W. Alexanderson. The images on the 3-in screen were poor and unsteady, but the set could be used in the home. A number of these receivers were built by the General Electric Company (GE) and distributed in Schenectady. On May 10, 1928, station WGY began regular broadcasting to this area.
Public Broadcasting
The first public broadcasting of television programs took place in London in 1936. Broadcasts from two competing firms were shown. Marconi-EMI produced a 405-line frame at 25 frames per second, and Baird Television produced a 240-line picture at 25 frames per second. In early 1937 the Marconi system, clearly superior, was chosen as the standard. In 1941 the United States adopted a 525-line, 30-image-per-second standard.
The first regular television broadcasts began in the United States in 1939, but after two years they were suspended until shortly after the end of World War II in 1945. A television broadcasting boom began just after the war in 1946, and the industry grew rapidly. The development of color television had always lagged a few steps behind that of black-and-white monochrome television. At first, this was because color television was technically more complex. Later, however, the growth of color television was delayed because it had to be compatible with monochrome-that is, color television would have to use the same channels as monochrome television and be receivable in black and white on monochrome sets.
Color Television
It was realized as early as 1904 that color television was possible using the three primary colors of light: red, green, and blue. In 1928 Baird demonstrated color television using a Nipkow disk in which three sets of openings scanned the scene. A fairly refined color television system was introduced in New York City in 1940 by the Hungarian-born American inventor Peter Goldmark. In 1951 public broadcasting of color television was begun using Goldmark's system. However, the system was incompatible with monochrome television, and the experiment was dropped at the end of the year. Compatible color television was perfected in 1953, and public broadcasting in color was revived a year later.
Other developments that improved the quality of television were larger screens and better technology for broadcasting and transmitting television signals. Early television screens were either 7 or 10 in diagonally across. Television screens now come in a range of sizes, but many of them measure as large as 32 or 35 in diagonally. Projection televisions were introduced in the 1970s with screens as large as 7 ft diagonally. Manufacturers have also developed very small, portable television sets with screens that are 3 in diagonally across.
Television in Space
Television evolved from an entertainment medium to a scientific medium during the exploration of outer space. Knowing that broadcast signals could be sent from transmitters in space, the National Aeronautics and Space Administration or NASA began developing satellites with television cameras. Unmanned spacecraft of the Ranger and Surveyor series relayed thousands of close-up pictures of the moon's surface back to earth for scientific analysis and preparation for lunar landings. The successful U.S. manned landing on the moon in July 1969 was documented with live broadcasts made from the surface of the moon. NASA's use of television helped in the development of photosensitive camera lenses and more-sophisticated transmitters that could send images from a quarter-million miles away.
Since 1960 television cameras have also been used extensively on orbiting weather satellites. Video cameras trained on the earth record pictures of cloud cover and weather patterns during the day, and infrared cameras cameras that record waves radiated by warm objects instead of light waves detect nighttime temperatures. The ten Television Infrared Observation Satellites TIROS launched by NASA paved the way for the operational satellites of the Environmental Science Services Administration or ESSA, which in 1970 became a part of the National Oceanic and Atmospheric Administration or NOAA. The pictures returned from these satellites aid not only weather prediction but also understanding of global weather systems. High-resolution cameras mounted in Landsat satellites have been successfully used to provide surveys of crop, mineral, and marine resources.
Home Videotape Recording
In time, the process of watching images on a television screen made people interested in either producing their own images or watching programming outside of standard broadcasting times. It became apparent that programming on videotape-which had been in use since the 1950s-could be adapted for use by the same people who were buying televisions. Affordable video recorders were introduced in the 1970s and have since become almost as common as television sets.