Television

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Television

Television is a telecommunication device for sending (broadcasting) and receiving video and audio signals. The name television is derived from the Greek root tele for far sight and the Latin root visio meaning sight, or combined together to mean distance seeing. Broadly speaking, television, or TV, is, thus, the overall technology used to transmit pictures with sound using radio frequency and microwave signals or closed-circuit connections.

Television programming was regularly broadcast in such countries as the United States, England, Germany, France, and the Soviet Union before World War II (1939–1945). However, television in the U.S., for instance, did not become common in homes until the middle 1950s. By the early 2000s, over 250 million television sets were in use in the U.S., nearly one TV set per person.

History

german engineer and inventor Paul Julius Gottlieb Nipkow (1860–1940) designed the mechanism in 1884 that provided for television. Nipkow placed a spiral pattern of holes onto a scanning disk (later called a Nipkow disk.) He turned the scanning disk while it was in front of a brightly lit picture so that each part of the picture was eventually exposed. This technology was later used inside cameras and receivers to produce the first television images.

The invention of the cathode ray tube in 1897 by German inventor and physicist Karl Ferdinand Braun

(1850–1918) quickly made possible the technology that today is called television. Indeed, by 1907, the cathode ray tube was capable of supplying images for early incarnations of the television.

English inventor Alan Archibald Campbell-Swinton (1863–1930) invented electronic scanning in 1908. He used an electron gun to neutralize charges on an electrified screen. Later, he wrote a scientific paper describing the electronic theory behind the concept of television. Russian-born American physicist Vladimir Kosma Zworykin (1889–1982) added to Campbell-Swinton’s idea when he developed an iconoscope camera tube later in the 1920s.

American inventor Charles Francis Jenkins (1867–1934) and English inventor John Logie Baird (1888–1946) used the Nipkow scanning disc in the early 1920s for their developmental work with television. Baird used his working television model to show a 1926 audience. By 1928, he had set up an experimental broadcast system. In addition, Baird demonstrated a color transmission of television. Then, Philo Traylor Farnsworth (1906–1971), an American inventor and engineer, invented a television camera that could convert elements of an image into an electrical signal. Farnsworth demonstrated the first completely electronic television system in 1934, which he eventually patented.

Within 50 years, television had become a dominant form of entertainment and an important way to acquire information. This remains true in the mid-2000s, as the average U.S. citizen spends between two and five hours each day watching television.

Television operates on two principles that underlie how the human brain perceives the visual world. First, if an image is divided into a group of very small colored dots (called pixels), the brain is able to reassemble the individual dots to produce a meaningful image. Second, if a moving image is divided into a series of pictures, with each picture displaying a successive part of the overall sequence, the brain can put all the images together to form a single flowing image. The technology of the television (as well as computers) utilizes these two features of the brain to present images. The dominant basis of the technology is still the cathode ray tube.

Operation of the cathode ray tube

A cathode ray tube contains a positively charged region (the anode) and a negatively charged region (the cathode). The cathode is located at the back of the tube. As electrons exit the cathode, they are attracted to the anode. The electrons are also focused electronically into a tight beam, which passes into the central area of the television screen. The central region is almost free of air, so that there are few air molecules to deflect the electrons from their path. The electrons travel to the far end of the tube where they encounter a flat screen. The screen is coated with a molecule called phosphor. When an electron hits a phosphor, the phosphor glows. The electron beam can be focused in a coordinated way on different part of the phosphor screen, effectively painting the screen (a raster pattern). This process occurs very quickly—about 30 times each second—producing multiple images each second. The resulting pattern of glowing and dark phosphors is what is interpreted by the brain as a moving image.

Black and white television was the first to be developed, as it utilized the simplest technology. In this technology, the phosphor is white. Color television followed, as the medium became more popular, and demands for a more realistic image increased. In a color television, three electron beams are present. They are called the red, green, and blue beams. Additionally, the phosphor coating is not just white. Rather, the screen is coated with red, green, and blue phosphors that are arranged in stripes. Depending on which electron beam is firing and which color phosphor dots are being hit, a spectrum of colors is produced. As with the black and white television, the brain reassembles the information to produce a recognizable image.

High definition television

High definition television (HDTV) is a format that uses digital video compression, transmission, and presentation to produce a much crisper and lifelike image than is possible using the cathode ray tube technology. This is because more information can be packed into the area of the television screen. Conventional cathode ray tube screens typically have 525 or 625 horizontal lines of dots on the screen. Each line contains approximately from 500 to 600 dots (or pixels). Put another way, the information possible is 525× 500 pixels. In contrast, HDTV contains from 720 to 1080× 500 pixels. The added level of detail produces a visually-richer image.

Televisions of the 1950s and 1960s utilized an analog signal. The signals were beamed out into the air from the television station, to be collected by an antenna positioned on a building or directly on the television (commonly called rabbit ears). Today, the signal is digitized. This allows the electronic pulses to be sent through cable wire to the television, or to a satellite, which then beams the signal to a receiving dish in a format known as MPEG-2 (exactly like the video files that can be loaded on to a computer).

The digital signal is less subject to deterioration that is the analog signal. Thus, a better quality image reaches the television.

Cable television

Television signals are transmitted on frequencies that are limited in range. Only persons residing within a few dozen miles of a TV transmitter can usually receive clear and interference-free reception. Community antenna television systems, often referred to as CATV, or simply cable, developed to provide a few television signals for subscribers far beyond the service area of big-city transmitters. As time passed cable moved to the big cities. Cable’s appeal, even to subscribers able to receive local TV signals without an outdoor antenna, is based on the tremendous variety of programs offered. Some systems provide subscribers with a choice of hundreds of channels.

Cable systems prevent viewers from watching programs they have not contracted to buy by scrambling or by placing special traps in the subscriber’s service drop that remove selected channels. The special tuner box that descrambles the signals can often be programmed by a digital code sent from the cable system office, adding or subtracting channels as desired by the subscriber.

Wired cable systems generally send their programming from a central site called a head end. TV signals are combined at the head end and, then, sent down one or more coaxial-cable trunk lines. Signals for various neighborhoods along the trunk split away to serve individual neighborhoods from shorter branches called spurs.

Coaxial cable, even the special type used for CATV trunk lines, is made of material that dissipates the electrochemicals passing through. Signals must be boosted in power periodically along the trunk line, usually every time the signal level has fallen by approximately 20 decibels, the equivalent of the signal having fallen to one-hundredth (1/100th) of its original power. The line amplifiers used must be very sophisticated to handle the wide bandwidth required for many programs without degrading the pictures or adding noise. The amplifiers must adjust for changes in the coaxial cable due primarily to temperature changes. The amplifiers used are very much improved over those used by the first primitive community antenna systems, but even today trunk lines are limited in length to about a dozen miles. Not much more than about one hundred line amplifiers can be used along a trunk line before problems become unmanageable.

Cable’s program offerings are entirely confined within the shielded system. The signals provided to subscribers must not interfere with over-the-air radio and television transmissions using the same frequencies. Because the cable system’s offerings are confined within the shielded system, pay-per-view programs can be offered on cable.

Cable is potentially able to import TV signals from a great distance using satellite or terrestrial-microwave relays. Cable systems are required to comply with a rule called Syndex, for syndication exclusivity, where an over-the-air broadcaster can require that imported signals be blocked when the imported stations carry programs they have paid to broadcast.

The current step in CATV technology is the replacing of wire-based coaxial systems with fiber optic service. Fiber optics is the technology where electrical signals are converted to light signals by solid-state laser diodes. The light waves are transmitted through very-fine glass fibers so transparent that a beam of light will travel through this glass fiber for miles.

Cable’s conversion to fiber optics results in an enormous increase in system bandwidth. Virtually the entire radio and TV spectrum is duplicated in a fiber optic system. As an example, every radio and TV station transmitting over the airwaves can be carried in a single thread of fiberglass; 500 separate television channels on a single cable system is easily handled. A fiber optic CATV system can be used for two-way communication more easily than can a wire-cable plant with electronic amplifiers. Fiber optic cable service greatly increases the support needed for interactive television services.

Latest television

Plasma television

Plasma television has been available commercially since the late 1990s, and are becoming popular in the early 2000s. Plasma televisions do not have a cathode ray tube. Thus, the screen can be very thin. Typically, plasma televisions screens are about 6 in (15 cm) thick but can reach down to 1 in (2.5 cm) thick. This allows the screen to be hung from a wall. Along with plasma television, flat panel LCD television is also available. Both are considered flat panel televisions. Some models are used as computer monitors.

In a plasma television, fluorescent lights are present instead of phosphors. Red, green, and blue fluorescent lights enable a spectrum of colors to be produced, in much the same way as with conventional television. Each fluorescent light contains a gas called plasma. Plasma consists of electrically charged atoms (ions) and electrons (negative in charge). When an electrical signal encounters plasma, the added energy starts a process where the particles bump into one another. This bumping releases a form of energy called a photon. The release of ultraviolet photons causes a reaction with phosphor material, which then glows.

Rear projection

Rear projection television send the video signal and its images to a projection screen with the use of a lens system. They were introduced in the 1970s but declined in the 1990s as better alternatives were made available. Many used large screens, some over 100 in (254 cm). These projection systems are divided into three groups: CRT-based (cathode ray tube-based), LCD-based (liquid crystal display-based), and DLP-based (digital light processing-based). Their quality since the 1970s have improved drastically so that in the 2000s they are still a viable type of television.

KEY TERMS

Chrominance —Color information added to a video signal.

Coaxial cable —A concentric cable, in which the inner conductor is shielded from the outer conductor; used to carry complex signals.

Compact disc —Digital recording with extraordinary fidelity.

Field —Half a TV frame, a top to bottom sweep of alternate lines.

Frame —Full TV frame composed of two interlaced fields.

Parallax —Shift in apparent alignment of objects at different distances.

Phosphor —Chemical that gives off colored light when struck by electron.

ATV

ATV stands for advanced television, a name created by the U.S. Federal Communications Commission (FCC) for digital TV (or DTV). It is the television system replacing the current analog system in the United States. Television technology is rapidly moving toward the ATV digital system planned to replace the aging analog process. ATV is a digital-television system, where the aspects that produce the image are processed as computer-like data. Digitally processed TV offers several tremendous advantages over analog TV methods. In addition to sharper pictures with less noise, a digital system can be much more frugal in the use of spectrum space. ATV includes high-definition television (HDTV), which is a format for digital video compression, transmission, and presentation. For instance, high-definition television (HDTV), which was developed in the 1980s, uses 1,080 lines and a wide-screen digital format to provide a very clear picture when compared to the traditional 525- and 625-line televisions.

Most TV frames are filled with information that has not changed from the previous frame. A digital TV system can update only the information that has changed since the last frame. The resulting picture looks to be as normal as the pictures seen for years, but many more images can be transmitted within the same band of frequencies.

TV audiences have been viewing images processed digitally in this way for years, but the final product has been converted to a wasteful analog signal before it leaves the television transmitter. Satellite relays have relayed TV as digitally compressed signals to maximize the utilization of the expensive transponder equipment in orbit about the Earth. The small satellite dishes offered for home reception receive digitally-encoded television signals.

ATV will not be compatible with current analog receivers, but it will be phased in gradually in a carefully-considered plan that will allow older analog receivers to retire gracefully over time. Television in the 2000s includes such features as DVD (digital video recorders) players, computers, VCRs (video cassette recorders), computer-like hard drives (to store programming), Internet-access, video game consoles, pay-per-view broadcasts, and a variety of other advanced add-on features.

See also Digital recording; Electronics.