Television

Television or TV is a telecommunication system for sending and receiving moving pictures and sound. The receiving device is also called television or TV, or full television set.

Television

In the 20th century, television has grown into a large mass medium, reaching billions of people. Initially, the images sent were only in black and white, but later they were also broadcast in color. This required a new kind of television: the color television. After analogue television came digital television. The latest innovation is the 3D television, which provides a three-dimensional image.

History

The development of television initially followed two distinct lines: one based on both mechanical and electronic principles and a second based on purely electronic principles. From the latter line, which originated in the United States, all modern televisions have been derived, but these would not have been possible without discoveries and insights into mechanical systems.

Mechanical line: the Nipkow disk

The electromechanical television that Paul Nipkow developed and patented in 1884 was the basis for this, the so-called Nipkow disc. Small holes were formed in a spiral pattern in this rapidly rotating flat disc. The light and dark areas were converted into an electrical signal with the help of a photocell (selenium cells). This activated a neon lamp that projected the image onto a screen in the same sequence of light and dark via a second synchronized rotating disk. However, it wasn’t until 1907 that advances in reinforcement tube technology made the design practical.

Another electromechanical system based on a mirror drum in both the camera and the monitor was developed in the Soviet Union around 1925 by Léon Theremin. Starting with 16 picture lines, the number was increased to 100 lines in two years, then surpassed in 1931 by the RCA’s 120-line cathode ray tube (CRT).

In the period 1907-1910, Boris Rosing and his student Vladimir Zworykin showed a television system to the outside world with a mechanical mirror-drum scanner and a cathode-ray tube in the receiver. This cathode ray tube, invented by Karl Ferdinand Braun in 1897, is a glass vacuum tube where the image is projected by means of an electron beam on the fluorescent end. Rosing disappeared during the 1917 Revolution, but Zworykin later went to work for the RCA to build a real electronic television.

A semi-mechanical analog television system was first shown in London in February 1924 by John Logie Baird with an image of Felix the Cat and a moving image by Baird on October 30, 1925. The company (Baird Television Development Company) realized the first trans in 1928 -Atlantic television signal, between London and New York.

In 1932 Baird introduced the ultra-short wave television. Baird’s system was approved by the BBC, who discontinued this use in 1937 in favor of purely electronic television.

Electronic television

Although the discoveries of Nipkow, Rosing, Baird and others were extraordinary, little of their technology is still used in modern television: by 1934 all electromechanical television systems were obsolete.

Already in 1908, the Briton Alan Archibald Campbell Swinton had described the concept of an electronic television system using the aforementioned cathode ray tube. Braun suggested using an electron beam in both the camera and the receiver, but his system was never built.

A fully electronic system was first shown by Philo Farnsworth in the fall of 1927. Farnsworth, a Mormon farm boy from Rigby, Idaho, made his first system at the age of 14. He discussed the idea with his teacher, who could think of no reasons why the system wouldn’t work (Farnsworth would later credit this teacher, Justin Tolman, as providing very important insight into his invention). He continued to pursue the idea at Brigham Young Academy (now Brigham Young University). At the age of 21, he showed a working system at his own laboratory in San Francisco.

His breakthrough freed television from its reliance on spinning discs and other mechanical parts. All modern picture tube televisions are derived directly from its design. Russian Vladimir Zworykin is sometimes cited as the father of electronic television because of his invention of the iconoscope in 1923 and his invention of kinescope in 1929; his design was one of the first to develop a television system with all the features of modern picture tubes.

The controversy about who first (Farnsworth or Zworykin) invented modern television is still debated to this day.

TV

Technic

There are three common types of display technologies: the LCD television, the plasma television and the CRT or video tube television. LCD television is relatively new, plasma television slightly older (introduction on the consumer market around 1995) and CRT television has been around for some time (introduction in the Netherlands around 1951). This page explains the difference in technology between the LCD television and the CRT television.

Video tube television

A television picture is divided into a large number of pixels in the colors red, green and blue. A CRT television has a CRT composed of phosphorescent pixels on the front display and three electron guns mounted in the back of the CRT. There are separate cannons for the color red, blue and green. By shooting electrons at an enormous speed against the pixels, these points light up, so that they become visible on the outside of the screen. Depending on the image to be formed, one point is shot less hard than the other, causing a difference in brightness in the image. However, electrons don’t fly to the front of the tube by themselves, you have to accelerate them. By applying a high voltage of an average of 30,000 V (at 1-3 mA) to the picture tube, the electrons are pulled from the back of the guns and shot towards the screen, in the direction of the pixels. In order to hit all points of the screen, the electron beam must be deflected horizontally and vertically. For both the horizontal and vertical deflection there are electromagnets that deflect the beams from the cannons. Depending on which position the bundle should be, it will be more or less deflected.

By adding a shadow mask that lies just in front of the pixels, it is ensured that the bundle with red image information can only land on the pixels that light up red. The sieving effect of this mask then prevents the beams with blue and green information from landing on red pixels.

To obtain a random color, the colors red, green and blue are mixed by letting the colored pixels light up in a certain proportion in a certain place. The human eye is unable to distinguish these individual points at a greater distance and will perceive it as a particular color. In this way a whole palette of colors can be generated.

This process repeats 50 times per second in Europe (using the frequency of the mains), but due to interlacing, only half an image is shown on each repetition – alternating the even and odd lines. Because film works at 24 frames per second, they are sometimes played a little too fast on television. In the past, film actors had an unnaturally high voice on television as a result.

LCD television

An LCD screen itself does not emit light, but manipulates the ambient light or the light that can radiate from the back. Each pixel (see text above) in the screen consists of two groups of liquid crystals (hence the name). These crystals have the property of rotating in polarization depending on whether or not applied voltage, incident or transmitted light. If there is no voltage on one of the layers, nothing will happen and the light can just pass through. However, if voltage is applied, the light can no longer or worse (depending on the amount of voltage) pass through.

Plasma television

In a plasma television, the pixels are formed by small gas discharge lamps, somewhat comparable to the principle of, for example, a fluorescent lamp. Due to the correct choice of material, the different colors are emitted per pixel. In a plasma display, electrical energy is added to a gas mixture. Plasma is unstable and gives off the absorbed energy in the form of heat and an ultraviolet light beam (photon). A phosphorescent layer converts the ultraviolet light into visible light. The color of this visible light is determined by the phosphor. In order to be able to make more than 17 million different colors, red, green and blue colors are generated. These colors blend into the desired color.

Receiving television programs

Part of television technology are the frequencies for reception and transmission.
Antennas on the roof for television reception. With the introduction of cable television and satellite television, the rake antennas virtually disappeared.

For television, the airwaves frequencies between 470 and 860 MHz are used in the United Kingdom (channels 21 to 69), while channels 2 to 12 are also used in the rest of Europe. For use on closed cable networks, other frequencies are also in use, which lie between channels 12 and 21, the so-called S and H band. In the airwaves the trend is to switch from analog signal to DVB-T, for which only UHF channels are used. In the Netherlands, all analog airwaves (VHF and UHF) have already expired since December 11, 2006. The best known of these were the Lopik channels 4, 27 and 30.

As a result of the introduction of DVB-C and internet via cable, there is also a trend in cable television to leave the VHF, S and H channels.

In television, a monochrome black-and-white image is applied to the carrier wave via amplitude modulation (AM). The associated color information is sent with a subcarrier and quadrature amplitude modulation (QAM). As a result, when a color signal is introduced, the signal can also be properly received on a black-and-white television, which only processes the first signal.

To keep the bandwidth of the entire signal limited to about 6 MHz, residual sideband modulation is used, a special form of AM.

The associated sound is sent on a subcarrier some distance from the image signal. Frequency modulation (FM) is usually used for this.

Since AM is much more sensitive to pulse signals than FM, lightning in the vicinity will distort the picture slightly, but the sound will usually remain unaffected. Ignition sources such as mopeds can also have this effect.

Resources:
Wikipedia