The principle of television and its development. Television is the transmission of sound and images over a distance. The arrival of television in Russia
Television - the field of science, technology and culture associated with the transmission of visual information (moving images) over a distance by radio electronic means; actually the method of such transmission. Along with radio broadcasting, television is one of the most widespread means of disseminating information and one of the main means of communication used for scientific, organizational, technical, and other applied purposes. The final link of a television transmission is the human eye, so television systems are built taking into account the peculiarities of vision. The real world is perceived by a person visually in colors, objects - in relief, located in the volume of some space, and events in dynamics, movement: therefore, an ideal television system should provide the ability to reproduce these properties of the material world. In modern television, the tasks of transmitting motion and color have been successfully solved. Television systems capable of reproducing the relief of objects and the depth of space are at the testing stage.
TV reception with a kinescope The TV has a cathode-beam with magnetic control, called a kinescope. In a kinescope, an electron gun creates an electron beam that is focused on a screen covered in crystals that can glow when struck by fast-moving electrons. On their way to the screen, the electrons fly through the magnetic fields of two pairs of coils located outside the tube. The transmission of television signals to any point in our country is provided with the help of relaying artificial Earth satellites in the Orbita system.
The antenna of the television receiver receives ultrashort waves emitted by the antenna of the television transmitter, modulated by the signals of the transmitted image. To obtain stronger signals in the receiver and reduce various interferences, as a rule, a special reception room is made TV antenna. In the simplest case, it is a so-called half-wave vibrator, or dipole, that is, a metal rod with a length slightly less than half the wavelength, located horizontally at right angles to the direction of the television center. The received signals are amplified, detected and amplified again in a manner similar to conventional audio receivers. A feature of a television receiver, which can be direct amplification or superheterodyne type, is that it is designed to receive ultrashort waves. The voltage and current of the image signals obtained as a result of amplification after the detector repeat all changes in the current that produced the modulation on the television transmitter. In other words, the image signal at the receiver accurately represents the 25 times per second serial transmission of the individual elements of the transmitted object. The image signals act on the television receiver, which is the main part of the television. How is television reception?
The use of a cathode ray tube for receiving television images was proposed by the professor of the St. Petersburg Institute of Technology B. L. Rosing as early as 1907 and ensured the further development of high-quality television. It was Boris Lvovich Rosing who laid the foundations of modern television with his work.
Kinescope A kinescope is a cathode-beam device that converts electrical signals into light signals. Main parts: 1) an electron gun, designed to form an electron beam, in color kinescopes and multibeam oscilloscope tubes are combined into an electron-optical projector; 2) a screen covered with a phosphor substance that glows when an electron beam hits it; 3) a deflecting system controls the beam in such a way that it forms the required image.
Historically, television has evolved from transmitting only the brightness characteristics of each image element. In a black and white TV, the luminance signal at the output of the transmitting tube is amplified and converted. The communication channel is a radio channel or a cable channel. In the receiving device, the received signals are converted in a single-beam kinescope, the screen of which is covered with a white phosphor.
1) Electron guns 2) Electron beams 3) Focusing coil 4) Deflecting coils 5) Anode 6) Mask, due to which the red beam hits the red phosphor, etc. 7) Red, green and blue grains of the phosphor 8) Mask and phosphor grains (enlarged). Color kinescope device
Red Blue Green The transmission and reception of color images require the use of more sophisticated television systems. Instead of one falling tube, it is required to use three tubes transmitting signals of three single-color images - red, blue and green. red green blue blue red green The screen of a color TV kinescope is covered with three types of phosphor crystals. These crystals are located in separate cells on the screen in a strict order. On a color TV screen, three beams produce simultaneously three images of red, green, and blue. The superimposition of these images, consisting of small luminous areas, is perceived by the human eye as a multi-color image with all shades of colors. At the same time, the glow of crystals in one place in blue, red and green is perceived by the eye as white, so black and white images can also be obtained on a color TV screen.
(TK-1) The first TV set for individual use KVN-49 Teleradiol "Belarus-5" Color TV sets "Minsk" and "Rainbow"
Conclusion In conclusion, I would like to say that a fairly large amount of popular science literature, as well as encyclopedias and reference books, has been studied. The principle of radio communication, the processes of amplitude modulation and detection were studied in detail. Based on what has been studied, the following conclusions can be drawn: Radio played a huge role in the life of mankind in the 20th century. It occupies an important place in the economy of any country. Thanks to the invention of radio in the 20th century, various means of communication were greatly developed. Scientists all over the world, including Russian and Soviet scientists, continue to improve modern means of communication. And without the invention of radio, this would hardly have been possible. Already by 2014, our country will introduce the transfer of information using digital communications.
References 1. I.V. Brenev "The invention of radio by A.S. Popov" MOSCOW "Soviet radio" B.B. Bukhovtsev, G.Ya. 3. V.S. Virginsky, V.F. Khoteenkov "Essays on the history and science of technology" MOSCOW "Enlightenment" F.M. Diaghilev "From the history of physics and the life of its creators" MOSCOW "Enlightenment" O.F. Kabardin, A.A. Pinsky "Physics grade 11. Textbook for general education institutions and schools with in-depth study of physics" Moscow "Enlightenment" e edition 6. V.P. Orekhov "Oscillations and waves in the course of high school physics" Moscow "Enlightenment" 1977. 7. Popov V.I. Basics cellular communication GSM standard ("Engineering Encyclopedia of the Fuel and Energy Complex"). M., "Eco-Trends", 2005
In the 80s of the XIX century. - 30s of XX century. mechanical television systems were developed, which for the first time implemented the basic principle of modern TV - the sequential transmission of image elements. This principle was put forward at the end of the 19th century. the Portuguese scientist A. di Paiva and, independently of him, the Russian scientist P.I. Bakhmetiev. In 1884 ᴦ. German engineer P. Nipkov received a patent in Germany for an optical-mechanical television set.
The 30-80s were the period of development of electronic television systems. At the heart of modern television are the principles of decomposing an image of an object into many elements (formation of a raster), converting the light flux from each element into electrical video signals, transmitting them on the air and inversely converting video signals into an image of an object. The process is carried out using cathode ray tubes (CRT) with magnetic beam focusing. The cathode-ray tube created in 1907 ᴦ served as a prototype. Professor of Petersburg University B.L. Rosing. The tube located in the transmitting chamber is commonly called an iconoscope, in the receiver - a kinescope.
Rice. 2. The principle of operation of the transmitting tube
Under the action of light, parts of the screen acquire positive charges. An electron beam is directed to the screen inside the transmitting tube, moving periodically from left to right along 625 horizontal lines - lines. During the run of the beam along the line, the neutralization of electric charges occurs in certain parts of the screen and in electrical circuit, connecting the electron gun and the screen, a current pulse flows. An electron beam with a diameter of only 0.02 mm falls on each individual element of the screen. This makes it possible to read 820 elements per line. Changes in the current strength in the pulse correspond to changes in the illumination of the screen along the path of the electron beam. high frequency electromagnetic oscillations in a television transmitter, they are modulated by the signal received at the output of the transmitting tube and fed to the transmitter antenna. An antenna emits electromagnetic waves. In a television receiver - a television set there is an electrovacuum tube called a kinescope. In a kinescope, an electron gun creates an electron beam (Fig. 3). Electrons under the action of an electric field move inside the tube to a screen covered with phosphor crystals that can glow under the impact of fast moving electrons. On their way to the screen, the electrons fly through the magnetic fields of two pairs of coils located outside the tube. The magnetic field of one pair of coils causes the electron beam to deviate horizontally, the second - vertically. Periodic changes in the current strength in the coils cause changes in the magnetic fields, as a result of which the electron beam runs 625 times across the screen from left to right and once from top to bottom in 1/25 of a second. During the movement of the beam along the first line, the current in the electron beam is controlled by the signal received by the receiver from the transmitter during the movement of the beam in the transmitting tube along the first line; when the beam moves along the second line, the current in the beam is controlled by the signal from the second line, and so on. As a result, in 1/25 s, the beam “draws” the same image on the TV screen that was built by the lens on the screen of the transmitting tube. Frames succeed each other at a frequency of 25 frames per second, the sequence of successive high frequency frames are perceived by the human eye as continuous movement. Sound accompaniment is transmitted via a separate frequency-modulated channel.
To transmit a color image, color signals are added to the complete TV signal. To do this, the color image of the object is decomposed into three single-color images (red, green and blue), which are transmitted by three CRTs. Accordingly, the TV receiver has three electronic searchlights, the rays of which, passing through the holes in the mask, cause the glow of red, green and blue phosphors. The mask is a thin metal sheet with 550,000 holes 0.25 mm in diameter. The phosphor of a color kinescope contains 1.5 million grains of phosphors of red, green and blue glow, located exactly opposite the holes in groups of three grains of each color. Three beams from three CRTs, reduced to one point, fall at each individual moment of time on one group of phosphors, while each beam causes the glow of one phosphor grain of its “own” color. When sweeping, the beams move to the next hole in the mask, which allows the signals of three single-color images to be combined on the screen.
Regular transmissions of black and white TV began in our country in 1938 ᴦ., color - in 1967 ᴦ. There are three color TV systems in the world today. The NTSC system operates in the USA, Canada, Japan and a number of countries in Central and South America. The PAL system operates in Germany, Great Britain and other countries of Western Europe. The SECAM system operates in France, Russia, the republics of the former USSR and a number of countries in Eastern Europe. The systems differ in the features of the formation of color channels, but they can be united by the unified digital video recording standard currently being developed.
A television. A set of devices whose action is aimed at transmitting a moving image and sound over a distance is television. In everyday life, this refers to organizations that are engaged in production, as well as distribute television programs. Television and radio broadcasting are the most popular means for disseminating various kinds of information, as well as the main means of communication.
Basic principles of television
Television is based on the principle of transmitting an image using a radio signal or wires. The TV chain includes several devices:
- a transmitting television camera that converts the image (received through the lens) into a television video signal.
- a telecine projector that converts the "picture" and sound on film into a television signal, as well as for showing films on television.
- a video recorder is necessary for recording and reproducing, as necessary, a video signal generated by a television camera or telecine projector.
- video switcher allows switching among several image sources (camera, VCR, etc.)
- a transmitter that transmits by radio or by wire, a high-frequency signal simulated by a television signal
- receiving device - TV. Sync pulses from the video signal help to recreate the television image on the screen.
To make a TV show, they use an audio track, which is similar to a radio show track. Audio is transmitted on a separate frequency, usually by frequency modulation, using technology that is similar to FM radio. Digital television operates with multi-channel audio, which is transmitted simultaneously with the "picture", in a single data stream.
Television standards and systems
For television broadcasting, its own standard has been adopted. This is the name of the sum of the number of lines into which the frame is decomposed, the frequency of its change, or the fields with the scan type. There are three standards used worldwide for analogue and digital television, standard definition. The digital television standard uses digital image anamorphism that is adapted to today's 16:9 screen aspect ratio.
High definition television (HDTV) is replacing traditional standards. HDTV has two main decomposition standards.
A television system is a technique that allows a message to be encoded about a color. For television with standard definition, three color systems are characteristic: NTSC, PAL, SECAM.
Terrestrial television is a system for transmitting a television signal to the consumer using television towers and transmitters (range 47-862 MHz). An indoor or outdoor antenna is used to receive the signal.
The method of transmitting a television signal from the transmitting center to the consumer, which uses for this artificial satellites, located within the geostationary orbit of the Earth in space, above the equator - is called satellite television. They have transceiver equipment. This system ensures the transmission of a high-quality TV signal to areas that the traditional method cannot cover.
To transmit analog television via satellite, an encoded or encrypted form is used, in the NTSC, PAL or SECAM standard.
Digital TV signal modulation is carried out using QPSK or 8SPK. Digital television, in particular transmitted via satellite, is based on MPEG, DVB-S, DVB-S2.
Analog television is a system that uses an analog electrical signal to receive, output and transmit images and sound. Before the advent of digital television, analog signals were used, which were transmitted via cable or radio. Currently being translated into digital television. By 2015, Russia and China plan to completely switch to this television.
Digital TV has one major advantage. It has better picture and sound quality than analog TV. The range of radio waves will also be released, which will allow you to create a new wireless network.
However, digital television also has its downsides. It has a sharp limitation on the area that the signal covers. Reception takes place within it. However, it is still greater than that of analog television, with the same transmitter power. Another disadvantage of digital television is the stop or unfolding of the “picture” into “squares” when the incoming signal is low.
The main standards are DVB (European standard), ATSC (American standard), ISDB (Japanese standard).
History of the invention of television
Television was not invented by anyone alone. The base is the discovery of the photoelectric effect in selenium, which was made in 1873 by Willoughby Smith. Then the invention of the scanning disk (invented by Nipkow), which led to the development of mechanical television. This type of television was popular before World War II.
For the first time in the world, a moving image was transmitted in 1923 by Charles Jenkins, who used a mechanical scan to transmit. The display he was transmitting was in silhouette, containing no midtones. The system by which halftone images were transmitted was invented in 1926.
At that time, there were several mechanical television systems, but none of them could compete with cheap and more reliable electronic systems.
The patent for electronic television technology, which is used to this day, was obtained by Professor Boris Rosing. He was able to transmit a still image over a distance. This experiment was staged in 1911. He needed a cathode ray tube to reproduce the image, for transmission he used a mechanical scan.
The world's first transmission of an image that moved was shown in 1928 by B.P. Grabovsky and I.F. Belyansky. Although the image was crude and unclear (as the act that recorded the result claims), it is this experience that is considered the birth of today's electronic television. The television receiver that participated in the experiment was called "telephoto".
The invention of the "iconoscope", in 1923 by V. Zvorykin, brought clarity to the image and decided the fate of electronic television. This handset made it possible to organize electronic television broadcasting. The first transmissions took place with a decomposition of 240 lines. The signal was received at a distance of up to 100 km on TVs, the release of which was carried out by RCA.
Beginning of regular television broadcasting
The first WCFL television station appeared in 1928 in Chicago. A single range of radio waves was used to transmit image and sound.
In the Soviet Union, the mechanical television standard (30 line resolution and 12.5 frames per second) has existed since 1931. There was no sound transmission. Regular broadcasting began on November 15, 1934 - 12 times a month for 1 hour. In 1938, regular electronic television broadcasting began. The first models of VRK TVs came out.
In Moscow, in 1939, the television center on Shabolovka began broadcasting. Initially, the programs were 4 times a week for 2 hours. The first electronic TV "KVN-49" appeared in the USSR in 1949. In 1950 - the remote control was invented remote control that has been connected to the TV with a cable.
In 1953, color television began in the United States.
Recording TV programs on television
Regular commercial television broadcasting began to develop. There was a need to save TV shows in order to broadcast and distribute them later. The first TV stations with a VHF band had a short range. By the mid-1950s, radio-relay TV signal transmission lines began to appear, which made it possible to reach a large audience. In the beginning, film recording technology was used for recording. After the creation of the first VCR (1956), it became easier to store TV programs. Today's television broadcasting uses digital technologies for video recording and video editing. Today they are an integral part of television.
The popularity of the query "television" in the Yandex search engine
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Media and news agencies mentioned Yandex.News 13,938 times.
It is precisely on television that the politicians of Ukraine, Russia, Belarus, Azerbaijan, Uzbekistan and other CIS countries, the European Union, the USA, etc. enter with an appeal. Baskov, etc.) and real-show stars (Dom-2, Vacation in Mexico, Battle of Psychics, etc.).
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Today we will find out the history of the invention of television. But first, let's look at how television works.
How television works
In the television center, the transmitted image is projected by the lens onto a photosensitive plate in a special cathode ray tube. This plate consists of small photocells (photocells) isolated from each other, in which electric charges of various strengths arise at different illumination levels. The electron beam emerging from the tail of the cathode ray tube runs around all the photocells at high speed and in a certain sequence and removes the charges arising there, turning them into pulses of various strengths. These impulses, amplified and processed accordingly, are transmitted as video signals and received in our television sets. The most important part of the TV is the receiving TV tube (now it is not used in new TV models), which has a fluorescent screen coated with a special substance - a phosphor. The electron beam in the tube, acting synchronously with the transmitting station, runs around the screen at a certain speed. The speed of the electron beam on the screen of the latest televisions reaches almost 30 thousand km/h. The different strength of the received signals causes at each point of the receiving screen a different intensity of the luminescence of the composition. This is what gives the image that is projected onto the screen during a quick change of frames. Television broadcasting can be considered as the highest form of radio broadcasting (see), combining the simultaneous transmission of sound and images by radio.
The first experiments on the transmission of a television signal
The idea of transmitting still images over the wires of an electric communication line was put forward as early as the middle of the 19th century, shortly after the invention of the electromagnetic telegraph. In 1875, in the USA, a system was proposed for the simultaneous transmission of an image over individual points by electrical signals over telegraph wires, based on the use of selenium photocells. However, such devices were extremely cumbersome due to the large number of connecting wires and therefore did not receive practical application.
The first improvement in the field of image transmission over a distance was developed in 1878 by the French scientist De Paiva. He proposed the alternate transmission of electrical impulses along one pair of wires, the intensity of which corresponds to the degree of illumination of a separate photocell of the "tube". However, the technical means of that time did not allow this idea to be put into practice.
In the history of the development of television, the invention of the so-called "electric telescope", proposed in 1884 by the German inventor P. II, was especially important. Nipkov. Nipkov's invention marked the beginning of the development of the principle of mechanical decomposition (sweep) of an image into elements. At Nipkow, an opaque rotating disk with spirally arranged holes of small diameter was used for reaming.
At the receiving station, electrical impulses were converted into light signals using a flat neon lamp, due to the rapid change in the brightness of its glow (depending on the change in the signal voltage supplied to the electrodes of the lamp). And finally, with the help of a similar disk rotating synchronously, an image (of the order of 2×3 cm) was reproduced. The correct image was obtained only when the disks of the transmitter and receiver rotated strictly synchronously, which was very difficult to achieve at that time. Establishing the need to synchronize the transmitting and receiving devices was the next stage in the development of television systems. This made it possible to subsequently manage with one communication channel between the transmitter and receiver.
The advent of the cathode ray tube
In 1907, the Russian scientist B. L. Rosing suggested using a cathode ray tube to reproduce television images.
He used a mechanical scanning system for image transmission, and an electronic one for receiving, and used alkaline photovoltaic cells with an external photoelectric effect instead of selenium ones. The signals from the photocell were applied to the capacitor plates, between which an electron beam passed, which, together with a special diaphragm with a hole, made it possible to control the brightness of the screen glow.
Having created a working model of a TV set with a single lamp - a receiving cathode ray tube, B.L. Rosing in 1911 obtained the simplest image in the form of 3-4 parallel lines, having carried out, in his terminology, "cathode telescoping".
Modern cathode ray tubes are the result of the work of many inventors (read ""). First - in the 10-20s of the XX century. - these tubes had serious shortcomings, for example, there were no devices for amplifying pulses, due to the imperfection of vacuum tubes. In the early 1920s, ideas were expressed about using radio to transmit images, and the first experimental television broadcasts were conducted over long distances.
By the 1930s, thanks to advances in the development of tube amplifiers, photocells, and vacuum technology, the situation had changed. Attention to cathode ray tubes has increased, and they have replaced mechanical systems with a Nipkow disk.
Kinescope
Developing the principle of operation of the receiving tube, V.K. Zworykin in 1929 in the USA created a receiving tube with electrostatic focusing, which he called a kinescope. Similar studies were carried out in the Soviet Union. By the end of the 1930s, receiving tubes with magnetic focusing and magnetic deflection were created in the USSR. Modern kinescopes usually use an electromagnetic beam steering system.
In modern television transmission tubes, elements of the optical image are converted by the photoelectric effect into electrical signals. The first transmitting television tube based on these principles was proposed in 1923 by V.K. Zworykin. Image transmission in the tube was based on the decomposition of an image projected onto a multi-element (mosaic) photocathode by an electron beam. However, this tube was not used. In 1928, the image dissector transmission tube was created in the United States, which also had a number of major drawbacks.
Television tubes that used the effect of charge accumulation (in particular, the iconoscope) were more advanced. It was the development of a workable design of a tube with the accumulation of electric charges that was a turning point in the development of television.
Iconoscope - mechanical television
The idea of a tube with capacitive charge storage was proposed in 1930 by the Soviet physicist A.P. Konstantinov and V.K. Zvorykin, who lived in the USA. In 1931–1932 S.I. Kitaev developed a device for transmitting a cathode ray tube with a mosaic photocathode and electronic image transfer by fast electrons. At the same time, V.K. Zworykin created such a tube in the USA, where it was called an iconoscope. The principle of operation and design of the mosaic photocathodes of the Kitaev and Zworykin tubes were similar. In 1933, engineer A. V. Moskvin created the first iconoscope in the Soviet Union.
Iconoscope - a television transmission tube with a one-sided mosaic and a secondary electronic emission. The main part of the iconoscope is a mosaic, a mosaic photocathode is a mica plate coated on one side large quantity(several million) grains of silver (elementary miniature cathodes) with cesium deposited on them, and on the other hand - a layer of metal. Impulses of different strength ( alternating current, which is an image signal and flows through a closed circuit in the mosaic-load-mosaic section) are amplified and transmitted to the receiving station.
The appearance of the iconoscope opened a new, modern stage in the development of television. In 1933, Soviet scientists P. V. Timofeev and P. V. Shmakov created a new design for the transmitting tube. This tube, called the image-transfer iconoscope, or supericonoscope, had two electrodes, a photocathode and a mosaic target. The optical image was projected not onto a mosaic, but onto a solid semitransparent photocathode, with the subsequent transfer of the image onto the mosaic, which was scanned by an electron beam. Due to the secondary emission, an enhancement of the electronic image was achieved, which improved the image quality.
In 1931, regular transmissions of mechanical television began in several cities of the USSR. In 1932, the first transmission of a moving image was made. Regular TV broadcasts began in 1936 almost simultaneously in Germany and Great Britain. In the USSR, regular television programs began in Moscow and Leningrad in 1939, in the USA - in 1941.
In 1939, American engineers A. Rose and H. Yamsem created an orticon in which the electrons in the sweeping beam have a low speed, which basically eliminated the secondary emission of electrons from the mosaic.
In 1943, A. Rose, P. Wenmer and H. Lowe created the super-orthicon, which combined the positive aspects of the super-iconoscope and the orticon. In this design, a double-sided capacitive target (mosaic) was used. In the superorthicone, separation of the charge and discharge regions of the mosaic, transfer of the electronic image, and amplification of the image signal by an electron multiplier are achieved. This tube is the most sensitive television transmitting tube in the world today. The sensitivity of the superorthicon is far superior to that of the orthicon. The advent of the superorticon made it possible to transmit a good image not only from specially equipped illuminated studios, but also from theaters and sports grounds.
The birth of color television
In the 1920s, work began on the transfer of color images. In 1925, the Soviet engineer I. A. Adamyan proposed the principle of sequential transmission of the three primary colors of an image. However, at that time there were no conditions for its implementation.
In the process of improving television technology, color television systems were implemented in two main versions.
The first option is the sequential transmission of color images at a sufficiently high speed. Decomposition of colors into three main components and their reproduction during reception is carried out using a rotating three-color disk filter. It was installed between the object under consideration and the photocathode of the transmitting tube, on the one hand, and in front of the screen of the receiving tube, on the other. Each color frame has its own impulse, which is amplified and transmitted sequentially, as in black and white television. Due to the fact that the number of pulses here is tripled, instead of 25 frames per second, 75 frames must be transmitted - three times each frame - through the red, green and blue filters.
For the first time, the experience of color television according to this principle was carried out on the small screen by J. Byrd in England in 1928.
Although color television with a frame change of colors using a rotating disk was the simplest, it had several disadvantages: during transmission, colors changed due to the overlap of one color with another, and when an object moved quickly, color halos appeared.
The second option was based on the simultaneous transmission of colors. Here, too, it is necessary to decompose the entire gamut of colors into three primary colors, but their transmission and reception are carried out simultaneously using three transmitting and three receiving tubes with their own communication channels. It also had its drawbacks: a triple set of video devices was needed both in the transmitter and in the receiver and an extended (triple) video bandwidth compared to the black-and-white television standard. Difficulties arose in the regulation of reception. For a long time it was not possible to achieve optical alignment of three images on a common screen. Both of these systems were incompatible with black and white television.
In 1953, a color television system was developed with the simultaneous transmission of colors without increasing the bandwidth of the video signals. It is compatible with black-and-white television and makes it possible to watch a color broadcast on a black-and-white TV, while a black-and-white picture of a regular program can be viewed on a color TV. The main part of the television receiver of such a system is a special receiving tube with a three-beam round three-color screen. In the transmitting chamber there are three tubes with light filters.
In the 50s, a flat kinescope was designed in the form of a rectangular parallelepiped, which simplified the scheme of a color TV. In it, the inner surface of the front wall of the flask had a colored mosaic fluorescent layer. Directly behind the screen was the so-called shadow mask - a protective plate opaque to the electron beam with a huge number of small holes, and the tube itself had a three-beam electron gun (cathode) and a complex scanning system. To obtain the desired color at the desired point, the electron beam passed through the mask so that each of the three beams excited a color mosaic of the screen at the corresponding point, creating a spot of a certain color. The mask produces a separation of color components. A large number of closely spaced individual dots of different colors on such a screen merged when viewed into a common color picture.
The era of modern television
The transmission of a television signal is limited to line of sight, so the antennas of broadcasting stations were installed on high objects or special towers were built. Later, they began to build small relay stations located at a line-of-sight distance. After the start of space flights, special communications satellites began to be launched. Several such satellites are enough to relay signals to any point on Earth.
We invite you to familiarize yourself with a brief history the invention of the television.
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In a cathode-ray video camera mosaic screen 1 is formed by several million isolated from each other silver grains coated with cesium. They are located on a mica plate 2 glued to a metal plate 3. The light 5 incident on the grains is able to “knock out” electrons from them, which “flow down” along the collector 4.
Depending on the brightness of the light, each grain acquires a greater or lesser positive charge. The charges of all grains of the mosaic "describe" the image. The elements at the bottom left of the camcorder create scanning electron beam. Consistently falling on the grains, the beam gives up its electrons to the place of those knocked out by light. There is a "recharge" - the grains change charges from "+" to "-". Note that the grains together with the metal plate 3 form a set of microscopic capacitors. When they are sequentially recharged in the external circuit between the metal plate 3 and the collector 4, a changing current arises - a video signal.
In a cathode ray video monitor
an electron beam is also used to convert a video signal into an image. Its intensity (the flow of flying electrons) varies according to the video signal. Getting on the mosaic screen, consisting of grains of the phosphor substance, electrons cause them to glow. It lasts for some time, while the beam "runs around" other grains on the screen, which we perceive as a video image.
In these devices electron beams scan the screens synchronously at a frequency of 25 Hz, that is, they run through them simultaneously 25 times per second (line by line, like reading a book). This allows you to transmit and receive rapidly changing images.
In a semiconductor video camera
a mosaic screen (matrix) is formed by several million "electronic pockets" in a p-type silicon wafer, above which control electrodes are located. If a positive charge is applied to them, then a pocket “opens” in the silicon plate under the electrode, and electrons released under the action of light accumulate in it. Correspondingly, holes formed at the sites of electron release are pushed aside by the electric field into the thickness of the plate. The number of electrons accumulated in the pocket depends on the brightness of the image fragment falling on it. The charges of all pockets collectively "describe" the image.
Under the action of control signals of a special microprocessor, a sequential "reading" of the charge of the pockets is carried out. As shown in the figure, only the first electrode is charged at the time of image capture. This charge is then switched to the next electrode and the electrons move to the adjacent pocket. And so on, up to the edge of the screen, where additional electrodes are located, to which the video signal “flows”.
On a semiconductor video monitor
to convert a video signal into a light image, a layer " liquid crystals". It is enclosed between special translucent films with a mosaic grid of control electrodes. The microprocessor distributes the video signal to all mosaic elements one by one. The electric fields that arise between the electrodes cause the crystals of each piece of the mosaic to rotate differently in the liquid layer. Depending on this, the amount of light transmitted by each element of the mosaic changes. As a result, we see an image that is made up of individual dots - pixels.
By the end of the 20th century, black and white television was replaced by color television. His the basic principles remain the same:
mosaic screen in the transmitter and receiver, sequential scanning of mosaic elements by an electron beam or microprocessor to form a video signal or a light image, transmission of a video signal by radio waves. Only the mosaic of screens became more complicated: each of its elements was replaced by a red-green-blue triad of elements capable of transmitting all shades of colors.
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