TELEVISION TECHNOLOGY

The technology that makes television work is a complex subject, explained here in a basic, introductory fashion. Though television seems a thoroughly modern invention, available only since mid-20th century, the concept of recreating moving images electrically was developed much earlier than is generally thought. It can be traced at least to 1884 when Paul G. Nipkow created the rotating scanning disk which provided a way of sending a representation of a moving image over a wire using varying electrical signals created by mechanically scanning that moving image.

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 representing the amount of light hitting the cell through the holes in the disk; the greater the amount of light, the greater the strength of the electrical signal. At the receiving end, a source of light varied in intensity at the rate of the electrical signal it received and could be seen through the holes in the rotating disc, 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 though wire to be recreated 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 throughout the 1930s.

By 1941, technical standards for the scanning and transmission of television images in America had been agreed upon and these standards have, in general, been maintained ever since. The American 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. The NTSC standard is used in Canada, parts of Asia including Japan, and much of Latin America as well as in the United States. But 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. As may or may not be obvious, countries that use 60 Hertz (cycle) A.C. (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 the frame-per-second rate is equal to half the A.C. 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 the example above 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 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 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 created. 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 created on its surface by the focused image by scanning the image, both horizontally and vertically, with a moving electronic beam. The scanning occurs because 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 the points on its surface, known as pixels, converts and stores this information and then sends out this varying electronic signal that 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, 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 is able to tune in a television station frequency and show the images being transmitted. A monitor 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 and/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 correctly lock onto (sync-up) and reproduce the original image correctly. Otherwise, for example, the receiver might begin to scan an image that begins half-way down the screen.

Television stations are assigned a specific transmitting frequency and operating power. In the United States, VHF (very-high frequency) television, channels 2-13, occupies a portion of the frequency spectrum from 54-216 MHz (million Hertz or 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 7-13 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 807-890 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 is being coveted for other frequency spectrum uses and it appears that the number of available channels in the UHF band will continue to decrease. Each television channel has a frequency bandwidth of 6 MHz. So, for instance, channel 2 has a frequency bandwidth from 54 MHz. to 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 (A.M.) and is similar to the method used to transmit the audio of an A.M. radio station. However, the television station audio signal is piggy-backed onto the carrier wave using frequency modulation (F.M.). With television audio the carrier wave's frequency (instead of its amplitude) is varied slightly at the rate of the audio signal.

The modulated television station 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 the earth. In this case the television signal is transmitted to the satellite on a specific frequency and then retransmitted at a different frequency by the satellite's transmitter back to the 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 R.F., or radio frequencies) as chosen by the cable service provider.

A television receiver picks up the transmitted television signals sent over-the-air or by cable or satellite, removes the necessary video and audio signals that had 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 and audio signals to provide pictures and sound. As mentioned above, a monitor cannot receive carrier waves.

From primitive experimentation in the 1920s and 1930s through the advent of commercial television in the late 1940s, to 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 Huxley's Brave New World, television has become the most influential medium of the second half of the twentieth century. While the medium continues to evolve and change, its importance, influence and pervasiveness appear to continue unabated. How will new technology 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 has been with us, with only minor changes and improvements, for well over 50 years.

-Steve Runyon

FURTHER READING

Benson, K. Blair. Television Engineering Handbook. New York: McGraw-Hill, 1986.

Hartwig, Robert. Basic TV Technology: A Media Manual. Boston: Focal Press, 1990; 2nd edition, 1995.

Inglis, A.F. Video Engineering. White Plains, New York: Knowledge Industry Publications, 1992.

Mazda, Fraidoon, editor. Telecommunications Engineer's Reference Book. Stoneham, Massachusetts: Butterworth-Heinemann, 1993.

Shrader, Robert L. Electronic Communication. New York: McGraw-Hill, 1959; 6th edition, 1990.

Townsend, Boris, and Kenneth Jackson, editors. TV & Video Engineer's Reference Book. Stoneham, Massachusetts: Butterworth-Heinemann, 1991.

 

See also All Channel Legislation; Color Television; High-Definition Television; Low Power Television; Microwave; Steadicam; Translator; United States: Cable Television; United States: Networks; Video Editing; Videotape