Mechanical scanning of an image involved a spinning disk, with a spiral grouping of holes, located at both the sending and receiving ends. At the sending end, a photocell-like device varied the strength of an electrical signal at a rate proportional to the amount of light hitting the cell through the holes in the disk. At the receiving end, a source of light correspondingly varied in intensity at the rate of the electrical signal it received and could be seen through the holes in the rotating disk, thereby recreating a crude copy of the image scanned at the sending end. Today, moving images are scanned electronically as described below and the varying electronic signal representing the scanned images can be transmitted or sent through wire to be re­ created at the receiver or monitor.

The earliest practical mechanical scanning and transmitting of moving images occurred in the mid- 1920s, and by the early 1930s electronic scanning had generally replaced the mechanical scanning methods. At first, the images were crude-little more than shadow-pictures-but as the potential  for television as a profit-making medium became apparent, more money and effort went into television experimentation, and improvements continued through the 1930s; regular transmissions by the BBC began in Britain in 1936, using a 405-line, electronic system developed by EMI. By 1941 technical standards for the scanning and transmission of television images in the United States had been agreed upon, and these standards have, in general. been maintained ever since. The U.S. standard, known as National Television System Committee (NTSC), utilizes 525-line, 60-field, 30-frame, interlaced scanning. This means that images are scanned in the television camera and reproduced in the television receiver or monitor 30 times each second. Each full image, or frame, is scanned by dividing the image into 525 horizontal lines and then sequentially scanning first all the even lines (every other line) from top to bottom, creating one field, and then scanning the odd-numbered lines in the same manner, creating a second field. The two fields, when combined (interlaced), create one frame. Therefore, 30 complete images or frames, each made up of two fields, are created each second. Because it is not possible to perceive individual changes in light and image happening so quickly, the 30-times-per-second scanned images are perceived as continuous movement, a trait known as "persistence of vision," similar to motion-picture viewing (which operates at 24 frames per second). The NTSC standard is used in Canada, parts of Asia, including Japan, and much of Latin America, as well as in the United States. There are two other "standards" in common use today. The PAL systems, a 25-frame-per­ second standard with a number of variants, are used throughout most of western Europe and India. as well as other areas. The SECAM 25-frame-per-second standard is used in many parts of the world, including France, Russia, and most of eastern Europe. Both PAL and SECAM use a 625-line picture, giving a sharper image resolution than NTSC. Countries that use 60- hertz (cycles per second: Hz) AC (alternating current) power have adopted a 30-frame-per-second television system. Countries that utilize a 50-Hz power system have a 25-frame-per-second television system. In all these television systems. Therefore the frame-per­ second rate is equal to half the AC power frequency.

     The aspect ratio of the television screen-the ratio of the horizontal dimension to the vertical dimension-is 4:3. For instance, if a TV receiver screen is 16-inches wide, the screen will be 12-inches high. (TV picture tubes are defined by their diagonal measurement, so in this example the screen would be described as a 20-inch TV.) Often,motion pictures are shown on television in a "letter-box" format. Because motion pictures are usually shot in an aspect ratio greater than 4:3, it is necessary to leave a black space at the top and bottom of the television screen so that the film can be viewed in a form resembling its theatrical dimensions, without cutting off the sides. High-definition television (HDTV) also utilizes a greater aspect ratio, generally 16:9.

     The television camera consists of a lens to focus an image onto the front surface of one or more pick-up-devices, and-within the camera housing-the pick­ up-device(s) and the electronics to make the camera work. A viewfinder to monitor the camera's images is normally mounted in or on the camera. The pick-up­ device, either a camera tube or charge-coupled device (CCD). reads the focused visual image and converts the image into a varying electronic signal that represents the image. On high-quality cameras, three pick­ up-devices are often utilized: one to pick up each of the three additive primary colors (blue, green, and red) that make up the color image.

     The face of the camera tube has a photoemissive material that gives off electrical energy when exposed to light. The stronger the light at any given point, the more energy is emitted by the tube. By reading the amount of energy on the surface of the camera tube at each point, an electronic representation of the visual image can be created. The camera tube "reads" the amount of energy that the focused image creates on its surface by scanning the image, both horizontally and vertically, with a moving electron beam. The scanning functions by means of precise magnetic deflection of the beam.

     The CCD replaces the camera tube in most modern cameras, commonly called "chip cameras." This solid­ state device measures the energy at each one of a grid of discrete points on its surface, known as pixels: converts this information into a numeric equivalent and stores this figure as binary in formation: and then sends out this varying electronic signal, which represents the image. CCD image pick-up devices are becoming more popular due to their small size, long life, greater sensitivity and light tolerance, minimal power requirements, less image distortion, and ruggedness.

     In the receiver's, or monitor's, the picture tube, the camera tube process is essentially reversed. The face of the picture tube is coated with a phosphor-like material that glows when struck by a beam of electrons. The glow lasts long enough to make the scanned image visible to the viewer. An electron gun shoots the thin beam of electrons at the face of the screen from within the picture tube. The beam's direction is varied in a precise manner by magnetic deflection in a way that matches or synchronizes with the original image scanned by the television camera. Color picture tubes can have one electron gun (such as in the Trinitron), or three guns, one for each primary color. One major difference between a receiver and a monitor should be mentioned here. A receiver (such as a domestic TV set) is able to tune in a television station frequency and show the images being transmitted. A monitor (such as those used to display CCTV pictures in a security control room) does not have a tuning component and can receive video signals by wire only.

     At a television station, the electronic signal from a television camera can be combined or mixed with video signals from other devices-such as video tape players, computers, film chains or telecines (motion­ picture and slide-projector units whose outputs have been converted to video  signals)-using  what  is known as a switcher. The switcher is also used to create various special visual effects electronically. The video output from the switcher can then be recorded, sent to another studio or master control room, or sent directly to a transmitter.

     The complete video signal sent to a transmitter or through wire to a monitor consists of signals representing the picture (luminance), color (chrominance}, and synchronization. Synchronizing signals force the receiver to lock onto (sync-up) and reproduce the original image correctly. Otherwise, for example, the receiver might begin to scan an image that starts halfway down the screen.

     Television stations are assigned a specific transmitting frequency and operating power. In the United States, VHF (very-high frequency) television, channels 2 through 13, occupies a portion of the electromagnetic spectrum between 54 and 216 MHz (million Hertz, or 1 million cycles-per-second). Channels 2-6 are located between 54 and 88 MHz. The FM radio band, 88-108 MHz, is located between television channels 6 and 7. Channels 7 through 13 are located between 174 and 216 MHz. UHF (ultra-high frequency) television, originally channels 14 to 83, was assigned the frequency range from 470 to 890 MHz. In 1966 the Federal Communications Commission (FCC) discontinued issuing licenses for UHF television stations above channel 69. In 1970 the FCC took away the frequency range from 807 to 890 MHz for other communication uses, and so the UHF band now consists of channels 14-69, from 470 to 806 MHz. The upper end of this current range, channels 52 through 69, is being covered for other frequency spectrum uses, and it appears that the number of channels in the UHF band available for use by television will continue to decrease. Each television channel has a frequency bandwidth of 6 MHz. So, for instance, channel 2 occupies the spectrum between 54 MHz and 60 MHz. Within its assigned band, each station transmits the video signal as described earlier, an audio signal, and specialized signals such as closed-captioning information.

     In the television transmitter, a carrier wave is created at an assigned frequency. This carrier wave travels at the speed of light through space with specific transmission or propagation characteristics determined by the individual frequency. The video signal is piggy-backed onto the much higher-frequency carrier wave using a process known as "modulation." Modulation, in the simplest terms, means that the carrier wave is modulated, or varied slightly, at the rate of the signal being piggy-backed. In a television transmission, the video signal varies the amplitude or strength of the carrier wave at the rate of the video signal. This is known as "amplitude modulation" (AM) and is similar to the method used to transmit the audio of an AM radio station. However, the television station audio signal is piggy-backed onto the carrier wave using frequency modulation (FM). With television audio, the carrier wave's frequency (instead of its amplitude) is varied slightly at the rate of the audio signal.

     The modulated carrier wave is sent from the transmitter to an antenna. The antenna then radiates the signal out into space in a pattern determined by the physical design of the transmitting antenna. Traditionally, the transmitter and antenna were terrestrially located, but now television signals can be radiated or delivered by transmitters and antennas located on satellites in orbit around Earth. In this case, the television signal is transmitted to the satellite at one frequency and then retransmitted at a different frequency by the satellite's transmitter back to Earth.

     Besides delivery by carrier-wave transmission, television is often sent through cable directly to homes and businesses. These signals are delivered by satellite, over-the-air from terrestrial antennas, and sometimes directly from video players to the distribution equipment of cable television (CATV) service providers for feeding directly into homes. The signals are sent at specific carrier-wave frequencies (sometimes called "radio frequencies" [RF]) as chosen by the cable service provider.

     A television receiver picks up the transmit television signals sent over the air or by cable or satellite, selects the necessary video and audio signals that have been piggy-backed on the carrier wave, discards the carrier wave, and amplifies and converts the video and audio signals into picture and sound. A television monitor accepts direct video signals to provide pictures and, sometimes, audio signals to provide sound. As mentioned above, a monitor cannot receive carrier waves.

     As computer and digital technologies are merged with traditional television, significant and positive changes are being witnessed in a number of areas. The utilization of digital storage equipment and methods is providing ever-more effective means of accessing, duplicating, archiving, and transferring traditional program materials. When such materials are stored on computer-like servers, the need for moving-part recording and playback equipment can sometimes be eliminated, thereby improving reliability. In addition, digital storage saves significant physical space. The FCC has mandated that all U.S. television stations must transmit digitally by 2006. By early 2002, 229 stations in 80 markets, representing 74 percent of U.S. TV households, were transmitting a digital signal. With a conversion cost of $2 million to $10 million per station (a not-insignificant expense), two results are being seen. First, a strong market is being created for companies offering digital transmitters and other digital equipment. Second, a large number of stations have begun requesting extensions from the FCC. putting into question the viability of the 2006 deadline. An additional factor is that consumers seem to have little interest in digital television. As of 2003, it remains to be seen whether or not the FCC will require that all future television sets be able to receive digital signals in order to strengthen the market for digital television. In the United Kingdom. where the government has announced that analogue transmissions will be turned off in a rolling program between 2006 and 2010. Both the BBC and commercial broadcasters have created several new channels that are free-to-air but only available digitally, as a way of encouraging and accelerating the take-up of new digital TV sets.

     High-definition television (HDTV) advances have been slow, owing to a continuing reluctance to agree standards, limited program material, and an accompanying lack of consumer confidence and a viable market. As more television stations and networks show letterbox-formatted programming, there should be increasing acceptance, and eventual consumer demand for HDTV. In the meantime, however, high-definition, as well as digital, technology is being used more and more in the production of programming material.

     As television technology continues to evolve, equipment quality is becoming more refined, weight and size are decreasing, and costs are becoming lower. An important example of this can be seen in videophone technology for television. First used commercially by CNN  in April  2001  while covering  the incident  of a U.S. spy plane forced to land  by  the Chinese.  Within six months this technology had been integrated into the standard equipment of international news correspondents. The videophone gear, slightly larger than a laptop computer, allows  field  reporters  in  remote locations to send  television  camera  images  via satellite to their bureaus across the world. As coalition military forces became engaged in Afghanistan in late 2001 and 2002, this equipment emerged as the standard way for correspondents to report from the field.

     From primitive experimentation in the 1920s and 1930s through the advent of commercial television in the late 1940s to the establishment of color television as the standard by the mid- 1960s, television has grown quickly to become perhaps the most important single influence on society today. From a source of information and entertainment to what some have dubbed the real "soma" of Aldous Huxley's Brave New World, television has become the present era's most influential medium. While the medium continues to evolve and change, its importance, influence, and pervasiveness appear to continue unabated. How will nanotechnology change the face of television? Once the realm of science fiction, we are now seeing new delivery systems. on-call access. a greater number of available channels. two-way interaction, and the coupling of television and the computer. We are in the process of experiencing better technical quality, including improved resolution. HDTV, the convenience of flatter and lighter television receivers, and digital processing and transmission. And yet, the basic standard for television broadcast technology in the United States has been with us, with only minor changes and improvements, for well over 50 years.