1.
Analog television
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Analog television or analogue television is the original television technology that uses analog signals to transmit video and audio. In an analog television broadcast, the brightness, colors and sound are represented by variations of either the amplitude. Analog signals vary over a range of possible values which means that electronic noise. So with analog, a weak signal becomes snowy and subject to interference. In contrast, a moderately weak signal and a very strong digital signal transmit equal picture quality. Analog television may be wireless or can be distributed over a network using cable converters. All broadcast television systems preceding digital transmission of digital television used analog signals, analog television around the world has been in the process of shutting down since the late 2000s. The earliest systems were mechanical systems which used spinning disks with patterns of holes punched into the disc to scan an image. A similar disk reconstructed the image at the receiver, synchronization of the receiver disc rotation was handled through sync pulses broadcast with the image information. However these mechanical systems were slow, the images were dim and flickered severely, camera systems used similar spinning discs and required intensely bright illumination of the subject for the light detector to work. Analog television did not really begin as an industry until the development of the cathode-ray tube, the electron beam could be swept across the screen much faster than any mechanical disc system, allowing for more closely spaced scan lines and much higher image resolution. Also far less maintenance was required of an all-electronic system compared to a spinning disc system, all-electronic systems became popular with households after the Second World War. Broadcasters using analog television systems encode their signal using different systems, the official systems of transmission are named, A, B, C, D, E, F, G, H, I, K, K1, L, M and N. These systems determine the number of lines, channel width, vision bandwidth, vision-sound separation, each frame of a television image is composed of lines drawn on the screen. The lines are of varying brightness, the set of lines is drawn quickly enough that the human eye perceives it as one image. The next sequential frame is displayed, allowing the depiction of motion, the analog television signal contains timing and synchronization information, so that the receiver can reconstruct a two-dimensional moving image from a one-dimensional time-varying signal. The first commercial systems were black-and-white, the beginning of color television was in the 1950s. A practical television system needs to take luminance, chrominance, synchronization, and audio signals, the transmission system must include a means of television channel selection
2.
NTSC
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The first NTSC standard was developed in 1941 and had no provision for color. In 1953 a second NTSC standard was adopted, which allowed for television broadcasting which was compatible with the existing stock of black-and-white receivers. NTSC was the first widely adopted broadcast color system and remained dominant until 1997, North America, parts of Central America, and South Korea are adopting or have adopted the ATSC standards, while other countries are adopting or have adopted other standards instead of ATSC. After nearly 70 years, the majority of over-the-air NTSC transmissions in the United States ceased on January 1,2010, the majority of NTSC transmissions ended in Japan on July 24,2011, with the Japanese prefectures of Iwate, Miyagi, and Fukushima ending the next year. In March 1941, the committee issued a standard for black-and-white television that built upon a 1936 recommendation made by the Radio Manufacturers Association. Technical advancements of the side band technique allowed for the opportunity to increase the image resolution. The NTSC selected 525 scan lines as a compromise between RCAs 441-scan line standard and Philcos and DuMonts desire to increase the number of lines to between 605 and 800. The standard recommended a frame rate of 30 frames per second, other standards in the final recommendation were an aspect ratio of 4,3, and frequency modulation for the sound signal. In January 1950, the committee was reconstituted to standardize color television, in December 1953, it unanimously approved what is now called the NTSC color television standard. The compatible color standard retained full backward compatibility with existing black-and-white television sets, Color information was added to the black-and-white image by introducing a color subcarrier of precisely 315/88 MHz. These changes amounted to 0.1 percent and were tolerated by existing television receivers. The FCC had briefly approved a different color standard, starting in October 1950. However, this standard was incompatible with black-and-white broadcasts and it used a rotating color wheel, reduced the number of scan lines from 525 to 405, and increased the field rate from 60 to 144, but had an effective frame rate of only 24 frames per second. CBS rescinded its system in March 1953, and the FCC replaced it on December 17,1953, with the NTSC color standard, later that year, the improved TK-41 became the standard camera used throughout much of the 1960s. The NTSC standard has been adopted by countries, including most of the Americas. With the advent of television, analog broadcasts are being phased out. Most US NTSC broadcasters were required by the FCC to shut down their analog transmitters in 2009, low-power stations, Class A stations and translators were required to shut down by 2015. NTSC color encoding is used with the System M television signal, each frame is composed of two fields, each consisting of 262.5 scan lines, for a total of 525 scan lines
3.
PAL
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Phase Alternating Line is a colour encoding system for analogue television used in broadcast television systems in most countries broadcasting at 625-line /50 field per second. Other common colour encoding systems are NTSC and SECAM, all the countries using PAL are currently in process of conversion or have already converted standards to DVB, ISDB or DTMB. This page primarily discusses the PAL colour encoding system, the articles on broadcast television systems and analogue television further describe frame rates, image resolution and audio modulation. To overcome NTSCs shortcomings, alternative standards were devised, resulting in the development of the PAL, the goal was to provide a colour TV standard for the European picture frequency of 50 fields per second, and finding a way to eliminate the problems with NTSC. PAL was developed by Walter Bruch at Telefunken in Hannover, Germany, with important input from Dr. Kruse, the format was patented by Telefunken in 1962, citing Bruch as inventor, and unveiled to members of the European Broadcasting Union on 3 January 1963. When asked, why the system was named PAL and not Bruch the inventor answered that a Bruch system would not have sold very well. The first broadcasts began in the United Kingdom in June 1967, the one BBC channel initially using the broadcast standard was BBC2, which had been the first UK TV service to introduce 625-lines in 1964. Telefunken PALcolor 708T was the first PAL commercial TV set and it was followed by Loewe-Farbfernseher S920 & F900. Telefunken was later bought by the French electronics manufacturer Thomson, Thomson also bought the Compagnie Générale de Télévision where Henri de France developed SECAM, the first European Standard for colour television. The term PAL was often used informally and somewhat imprecisely to refer to the 625-line/50 Hz television system in general, accordingly, DVDs were labelled as PAL or NTSC even though technically the discs do not carry either PAL or NTSC composite signal. CCIR 625/50 and EIA 525/60 are the names for these standards, PAL. Both the PAL and the NTSC system use a quadrature amplitude modulated subcarrier carrying the chrominance information added to the video signal to form a composite video baseband signal. The frequency of this subcarrier is 4.43361875 MHz for PAL and NTSC4.43, the SECAM system, on the other hand, uses a frequency modulation scheme on its two line alternate colour subcarriers 4.25000 and 4.40625 MHz. Early PAL receivers relied on the eye to do that cancelling, however. The effect is that phase errors result in changes, which are less objectionable than the equivalent hue changes of NTSC. In any case, NTSC, PAL, and SECAM all have chrominance bandwidth reduced greatly compared to the luminance signal. The 4.43361875 MHz frequency of the carrier is a result of 283.75 colour clock cycles per line plus a 25 Hz offset to avoid interferences. Since the line frequency is 15625 Hz, the carrier frequency calculates as follows,4.43361875 MHz =283.75 ×15625 Hz +25 Hz
4.
SECAM
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SECAM, also written SÉCAM, is an analogue colour television system first used in France. It was one of three major television standards, the others being the European PAL and North American NTSC. Development of SECAM began in 1956 by a led by Henri de France working at Compagnie Française de Télévision. The first SECAM broadcast was made in France in 1967, making it the first such standard to go live in Europe, the system was also selected as the standard for colour in the Soviet Union, who began broadcasts shortly after the French. The standard spread from these two countries to many client states and former colonies, SECAM remained a major standard into the 2000s. It is in the process of being phased out and replaced by DVB, work on SECAM began in 1956. The technology was ready by the end of the 1950s, a version of SECAM for the French 819-line television standard was devised and tested, but not introduced. The first proposed system was called SECAM I in 1961, followed by studies to improve compatibility. These improvements were called SECAM II and SECAM III, with the latter being presented at the 1965 CCIR General Assembly in Vienna, further improvements were SECAM III A followed by SECAM III B, the adopted system for general use in 1967. Soviet technicians were involved in the development of the standard, and created their own incompatible variant called NIR or SECAM IV, the team was working in Moscows Telecentrum under the direction of Professor Shmakov. The NIR designation comes from the name of the Nautchno-Issledovatelskiy Institut Radio, two standards were developed, Non-linear NIR, in which a process analogous to gamma correction is used, and Linear NIR or SECAM IV that omits this process. SECAM was inaugurated in France on 1 October 1967, on la deuxième chaîne, a group of four suited men—a presenter and three contributors to the systems development—were shown standing in a studio. Following a count from 10, at 2,15 pm the black-and-white image switched to color, in 1967, CLT of Lebanon became the third television station in the world, after the Soviet Union and France, to broadcast in color utilizing the French SECAM technology. The first color television sets cost 5000 Francs, color TV was not very popular initially, only about 1500 people watched the inaugural program in color. A year later, only 200,000 sets had been sold of an expected million and this pattern was similar to the earlier slow build-up of color television popularity in the US. SECAM was later adopted by former French and Belgian colonies, Greece, the Soviet Union and Eastern bloc countries, and Middle Eastern countries. However, with the fall of communism, and following a period when multi-standard TV sets became a commodity, other countries, notably the United Kingdom and Italy, briefly experimented with SECAM before opting for PAL. Since late 2000s, SECAM is in the process of being phased out, some have argued that the primary motivation for the development of SECAM in France was to protect French television equipment manufacturers
5.
Digital television
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Digital television is the transmission of audio and video by digitally processed and multiplexed signal, in contrast to the totally analog and channel separated signals used by analog television. Digital TV can support more than one program in the channel bandwidth. It is a service that represents the first significant evolution in television technology since color television in the 1950s. Several regions of the world are in different stages of adaptation and are implementing different broadcasting standards and this standard has been adopted in Europe, Singapore, Australia and New Zealand. Advanced Television System Committee uses eight-level vestigial sideband for terrestrial broadcasting and this standard has been adopted by six countries, United States, Canada, Mexico, South Korea, Dominican Republic and Honduras. Integrated Services Digital Broadcasting is a designed to provide good reception to fixed receivers. It utilizes OFDM and two-dimensional interleaving and it supports hierarchical transmission of up to three layers and uses MPEG-2 video and Advanced Audio Coding. This standard has adopted in Japan and the Philippines. ISDB-T International is an adaptation of this standard using H. 264/MPEG-4 AVC that been adopted in most of South America and is also being embraced by Portuguese-speaking African countries. Digital Terrestrial Multimedia Broadcasting adopts time-domain synchronous OFDM technology with a signal frame to serve as the guard interval of the OFDM block. The DTMB standard has adopted in the Peoples Republic of China, including Hong Kong. Digital TVs roots have been tied very closely to the availability of inexpensive and it wasnt until the 1990s that digital TV became a real possibility. S. Until June 1990, the Japanese MUSE standard—based on an analog system—was the front-runner among the more than 23 different technical concepts under consideration, then, an American company, General Instrument, demonstrated the feasibility of a digital television signal. This breakthrough was of significance that the FCC was persuaded to delay its decision on an ATV standard until a digitally based standard could be developed. In March 1990, when it became clear that a standard was feasible. The new ATV standard also allowed the new DTV signal to be based on new design principles. Although incompatible with the existing NTSC standard, the new DTV standard would be able to incorporate many improvements, the final standard adopted by the FCC did not require a single standard for scanning formats, aspect ratios, or lines of resolution. This outcome resulted from a dispute between the electronics industry and the computer industry over which of the two scanning processes—interlaced or progressive—is superior
6.
ATSC standards
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Advanced Television Systems Committee standards are a set of standards for digital television transmission over terrestrial, cable, and satellite networks. It is largely a replacement for the analog NTSC standard, and like that standard, used mostly in the United States, other former users of NTSC, like Japan, have not used ATSC during their digital television transition. The standard is now administered by the Advanced Television Systems Committee, the standard includes a number of patented elements, and licensing is required for devices that use these parts of the standard. Key among these is the 8VSB modulation system used for over-the-air broadcasts, ATSC includes two primary high definition video formats, 1080i and 720p. It also includes standard-definition formats, although initially only HDTV services were launched in the digital format, the high-definition television standards defined by the ATSC produce wide screen 16,9 images up to 1920×1080 pixels in size – more than six times the display resolution of the earlier standard. However, many different image sizes are also supported, the reduced bandwidth requirements of lower-resolution images allow up to six standard-definition subchannels to be broadcast on a single 6 MHz TV channel. ATSC standards are marked A/x and can be downloaded for free from the ATSCs website at ATSC. org, ATSC Standard A/72 was approved in 2008 and introduces H. 264/AVC video coding to the ATSC system. ATSC supports 5. 1-channel surround sound using the Dolby Digitals AC-3 format, numerous auxiliary datacasting services can also be provided. Many aspects of ATSC are patented, including elements of the MPEG video coding, the AC-3 audio coding, the cost of patent licensing, estimated at up to $50 per digital TV receiver, has prompted complaints by manufacturers. As with other systems, ATSC depends on numerous interwoven standards, broadcasters who used ATSC and wanted to retain an analog signal were temporarily forced to broadcast on two separate channels, as the ATSC system requires the use of an entire separate channel. Channel numbers in ATSC do not correspond to RF frequency ranges, there is also a standard for distributed transmission systems, a form of single-frequency network which allows for the synchronised operation of multiple on-channel booster stations. Dolby Digital AC-3 is used as the audio codec, though it was standardized as A/52 by the ATSC and it allows the transport of up to five channels of sound with a sixth channel for low-frequency effects. In contrast, Japanese ISDB HDTV broadcasts use MPEGs Advanced Audio Coding as the audio codec, MPEG-2 audio was a contender for the ATSC standard during the DTV Grand Alliance shootout, but lost out to Dolby AC-3. The Grand Alliance issued a statement finding the MPEG-2 system to be equivalent to Dolby. Later, a story emerged that MIT had entered into an agreement with Dolby whereupon the university would be awarded a sum of money if the MPEG-2 system was rejected. Dolby also offered an incentive for Zenith to switch their vote, however, the ATSC system supports a number of different display resolutions, aspect ratios, and frame rates. The formats are listed here by resolution, form of scanning, for transport, ATSC uses the MPEG systems specification, known as an MPEG transport stream, to encapsulate data, subject to certain constraints. ATSC uses 188-byte MPEG transport stream packets to carry data, before decoding of audio and video takes place, the receiver must demodulate and apply error correction to the signal
7.
Digital Video Broadcasting
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Digital Video Broadcasting is a set of internationally open standards for digital television. The interaction of the DVB sub-standards is described in the DVB Cookbook, many aspects of DVB are patented, including elements of the MPEG video coding and audio coding. Devices interact with the physical layer via a parallel interface, synchronous serial interface. All data is transmitted in MPEG transport streams with additional constraints. A standard for temporally-compressed distribution to mobile devices was published in November 2004 and these distribution systems differ mainly in the modulation schemes used and error correcting codes used, due to the different technical constraints. DVB-S uses QPSK, 8-PSK or 16-QAM, DVB-S2 uses QPSK, 8-PSK, 16-APSK or 32-APSK, at the broadcasters decision. QPSK and 8-PSK are the only regularly used. DVB-C uses QAM, 16-QAM, 32-QAM, 64-QAM, 128-QAM or 256-QAM, lastly, DVB-T uses 16-QAM or 64-QAM in combination with OFDM and can support hierarchical modulation. The DVB-T2 specification was approved by the DVB Steering Board in June 2008, ETSI adopted the standard on 9 September 2009. DVB has established a 3D TV group to identify what kind of 3D-TV solution does the market want and need, the CM-3DTV group held a DVB 3D-TV Kick-off Workshop in Geneva on 25 January 2010, followed by the first CM-3DTV meeting the next day. DVB now defines a new standard for 3D video broadcast, DVB 3D-TV, older technologies such as teletext and vertical blanking interval data are also supported by the standards to ease conversion. However, for many more advanced alternatives like DVB-SUB for subtitling are available. The conditional access system defines a Common Scrambling Algorithm and a physical Common Interface for accessing scrambled content, dVB-CA providers develop their wholly proprietary conditional access systems with reference to these specifications. Multiple simultaneous CA systems can be assigned to a scrambled DVB program stream providing operational and commercial flexibility for the service provider, dVB-CPCM has been the source of much controversy in the popular press and It is said that CPCM is the DVBs answer to the failed American Broadcast Flag. By comparison, the rival DigiCipher 2 based ATSC system will not have this issue until 2048 due in part to 32 bits being used, recently, DVB has adopted a profile of the metadata defined by the TV-Anytime Forum. This is an XML Schema based technology and the DVB profile is tailored for enhanced Personal Digital Recorders, DVB lately also started an activity to develop a service for IPTV which also includes metadata definitions for a broadband content guide. The DVB Multimedia Home Platform defines a Java-based platform for the development of consumer video system applications, in addition to providing abstractions for many DVB and MPEG-2 concepts, it provides interfaces for other features like network card control, application download, and layered graphics. DVB has standardised a number of channels that work together with DVB to create bi-directional communication
8.
ISDB
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The Integrated Services Digital Broadcasting is a Japanese standard for digital television and digital radio used by the countrys radio and television networks. ISDB replaced NTSC-J analog television system and the previously used MUSE Hi-vision analogue HDTV system in Japan, Digital Terrestrial Television Broadcasting services using ISDB-T started in Japan in December 2003 and in Brazil in December 2007 as a trial. Since many countries have adopted ISDB over other digital broadcasting standards, the standards can be obtained for free at the Japanese organization DiBEG website and at ARIB. The core standards of ISDB are ISDB-S, ISDB-T, ISDB-C and 2, ISDB-T and ISDB-Tsb are for mobile reception in TV bands. 1seg is the name of an ISDB-T service for reception on cell phones, laptop computers, the concept was named for its similarity to ISDN, because both allow multiple channels of data to be transmitted together. This is also much like another digital radio system, Eureka 147, which each group of stations on a transmitter an ensemble. ISDB-T operates on unused TV channels, an approach taken by countries for TV. The various flavors of ISDB differ mainly in the modulations used, the 12 GHz band ISDB-S uses PSK modulation,2.6 GHz band digital sound broadcasting uses CDM, and ISDB-T uses COFDM with PSK/QAM. Besides audio and video transmission, ISDB also defines data connections with the internet as a channel over several media. This is used, for example, for interactive interfaces like data broadcasting, the ISDB specification describes a lot of interfaces, but most importantly the Common Interface for Conditional Access System. While ISDB has examples of implementing various kinds of CASes, in Japan CAS called B-CAS is used, defines the Common Scrambling Algorithm system called MULTI2 required for scrambling television. The ISDB CAS system in Japan is operated by a company named B-CAS, the Japanese ISDB signal is always encrypted by the B-CAS system even if it is a free television program. That is why it is commonly called Pay per view system without charge, an interface for mobile reception is under consideration. Since all digital systems carry digital data content, a DVD or high-definition recorder could easily copy content losslessly. Hollywood requested copy protection, this was the reason for RMP being mandated. The content has three modes, “copy once”, “copy free” and “copy never”, “Copy never” programming may only be timeshifted and cannot be permanently stored. There are two types of ISDB receiver, Television and set-top box, the aspect ratio of an ISDB-receiving television set is 16,9, televisions fulfilling these specs are called Hi-Vision TV. There are three TV types, Cathode ray tube, plasma display panel and liquid crystal display, with LCD being the most popular Hi-Vision TV on the Japanese market nowadays
9.
Digital Terrestrial Multimedia Broadcast
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DTMB is the TV standard for mobile and fixed terminals used in the Peoples Republic of China, Cuba, Hong Kong, and Macau. Previously known as DMB-T/H, the DTMB is a merger of the standards ADTB-T, DMB-T and TiMi, the DTMB was created in 2004 and finally became an official DTT standard in 2006. DTMB system is compatible with fixed reception and mobile digital terrestrial television, mobile reception, is compatible with digital broadcasting TV in standard definition, digital audio broadcasting, multimedia broadcasting and data broadcasting service. Fixed reception, in addition to the services, also supports high definition digital broadcasting. This system gives flexibility to the services offered to support the combination of single-frequency networks, the different modes and parameters can be chosen depending on the type of service and networks environment. The sequence of pattern is defined in time domain and the information of the Discrete Fourier transform is defined in the frequency domain. The two frames are multiplexed in the domain, resulting in Time domain synchronization. This transmission system makes the conversion of the signal to the output data of terrestrial TV signal. The data passes through the encoder, the error protection process FEC, through the constellation mapping process, the data block and the TPS information are multiplexed, and pass through the data processor to form the body structure. It combines information from the body and the head to form the frame, finally the signal is modulated to put it in the corresponding frequency band. Bit-rate, from 4.813 Mbit/s to 32, mobile TV, DVB-H, DMB, 3G Systems and Rich Media Applications. Focal Press Media Technology Professional Series, http, //en. dtvstatus. net/ http, //www. altera. com/support/refdesigns/sys-sol/broadcast/ref-dmb-t. html IEEE Spectrum - Does China Have the Best Digital Television Standard on the Planet
10.
Black and white
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Black and white, often abbreviated B/W or B&W, and hyphenated black-and-white when used as an adjective, is any of several monochrome forms in visual arts. Black-and-white images are not usually starkly contrasted black and white and they combine black and white in a continuum producing a range of shades of gray. Even when most studios had the capability to make color films they were not heavily utilized as using the Technicolor process was expensive, for the years 1940–1966, a separate Academy Award for Best Art Direction was given for black-and-white movies along with one for color. Television, Television program was first transmitted in black-and-white, scottish inventor John Logie Baird demonstrated the worlds first color television transmission on July 3,1928 using a mechanical process. Some color broadcasts in the US began in the 1950s, with color becoming common in industrialized nations during the late 1960s. In the United States, the Federal Communications Commission settled on a color NTSC standard in 1953, color television became more widespread in the U. S. between 1963 and 1967, when the CBS and ABC networks joined NBC in broadcasting full color schedules. Canada began airing color television in 1966 while the United Kingdom began to use a different color system from July 1967 known as PAL. The Republic Of Ireland followed in 1970, in China, black and white television sets were the norm until as late as the 1990s, color TVs not outselling them until about 1989. While seldom used now, many consumer camcorders have the ability to record in black-and-white. Photography, Photographs were either black-and-white or shades of sepia, color photography was originally rare and expensive and again often containing inaccurate hues. Color photography became more common from the mid-20th century, Today, black-and-white is a niche market for photographers who use the medium for artistic purposes. This can take the form of film or digital conversion to grayscale. For amateur use certain companies such as Kodak manufactured black-and-white disposable cameras until 2009, also, certain films are produced today which give black-and-white images using the ubiquitous C41 color process. Printing press, Most American newspapers were black-and-white until the early 1980s, The New York Times, some claim that USA Today was the major impetus for the change to color. In the UK, color was only introduced from the mid-1980s. Even today, many newspapers restrict color photographs to the front, similarly, daily comic strips in newspapers were traditionally black-and-white with color reserved for Sunday strips. Sometimes color is reserved for the cover, magazines such as Jet magazine were either all or mostly black-and-white until the end of the 2000s when it became all-color. Manga are typically published in black-and-white although now it is part of its image, many school yearbooks are still entirely or mostly in black-and-white
11.
Color television
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Color television is a television transmission technology that includes information on the color of the picture, so the video image can be displayed in color on the television set. It is an improvement on the earliest television technology, monochrome or black and white television, Television broadcasting stations and networks in most parts of the world upgraded from black and white to color transmission in the 1960s and 1970s. The invention of color television standards is an important part of the history of television, in its most basic form, a color broadcast can be created by broadcasting three monochrome images, one each in the three colors of red, green, and blue. When displayed together or in succession, these images will blend together to produce a full color image as seen by the viewer. The higher resolution black-and-white and lower resolution color images combine in the eye to produce a seemingly high-resolution color image, the NTSC standard represented a major technical achievement. Although all-electronic color was introduced in the U. S. in 1953, high prices, the first national color broadcast occurred on January 1,1954, but during the next ten years most network broadcasts, and nearly all local programming, continued to be in black-and-white. The first all-color prime-time season came just one year later, early color sets were either floor-standing console models or tabletop versions nearly as bulky and heavy, so in practice they remained firmly anchored in one place. The introduction of GEs relatively compact and lightweight Porta-Color set in the spring of 1966 made watching color television a more flexible, in 1972, sales of color sets finally surpassed sales of black-and-white sets. Also in 1972, the last holdout among daytime network programs converted to color, Color broadcasting in Europe was also not standardized on the PAL format until the 1960s. By the late 1980s even these areas switched to color sets, the human eyes detection system in the retina consists primarily of two types of light detectors, rod cells that capture light, dark, and shapes/figures, and the cone cells that detect color. A typical retina contains 120 million rods and 4.5 million to 6 million cones and this means that the eye has far more resolution in brightness, or luminance, than in color. The eye has limited bandwidth to the rest of the visual system and this illusion starts to work at about 16 frame/s, and common motion pictures use 24 frame/s. Experiments in television systems using radio broadcasts date to the 19th century, a key problem was the need to convert a 2D image into a 1D radio signal, some form of image scanning was needed to make this work. Early systems generally used a known as a Nipkow disk. A single photodetector behind the disk captured the image brightness at any given spot, a similar disk was used at the receiver side, with a light source behind the disk instead of a detector. A number of systems were being used experimentally in the 1920s. The best-known was John Logie Bairds, which was used for regular public broadcasting in Britain for several years. Indeed, Bairds system was demonstrated to members of the Royal Society in London in 1926 in what is recognized as the first demonstration of a true
12.
Monochrome
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Monochrome describes paintings, drawings, design, or photographs in one color or values of one color. A monochromatic object or image reflects colors in shades of limited colors or hues, images using only shades of grey are called grayscale or black-and-white. However, scientifically speaking, monochromatic light refers to light of a narrow band of wavelengths. In computing, monochrome has two meanings, it may mean having only one color which is either on or off, a monochrome computer display is able to display only a single color, often green, amber, red or white, and often also shades of that color. In film photography, monochrome is typically the use of black-and-white film, originally, all photography was done in monochrome. Although color photography was possible even in the late 19th century, easily used color films, if the red channel is eliminated and the green and blue combined then the effect will be similar to that of orthochromatic film or the use of a cyan filter on panchromatic film. The selection of weighting thus allows a range of artistic expression in the final monochromatic image. For production of an image the original color stereogram source may first be reduced to monochrome in order to simplify the rendering of the image. This is sometimes required in cases where an image would render in a confusing manner given the colors and patterns present in the source image. In physics, monochromatic light is electromagnetic radiation of a single frequency, even very controlled sources such as lasers operate in a range of frequencies. In practice, filtered light, diffraction grating separated light and laser light are all referred to as monochromatic. Often light sources can be compared and one be labeled as “more monochromatic”
13.
Chrominance
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Chrominance is the signal used in video systems to convey the color information of the picture, separately from the accompanying luma signal. Chrominance is usually represented as two color-difference components, U = B′ − Y′ and V = R′ − Y′, each of these difference components may have scale factors and offsets applied to it, as specified by the applicable video standard. In digital-video and still-image color spaces such as Y′CbCr, the luma, separating RGB color signals into luma and chrominance allows the bandwidth of each to be determined separately. Typically, the bandwidth is reduced in analog composite video by reducing the bandwidth of a modulated color subcarrier. The idea of transmitting a color signal with distinct luma and chrominance components originated with Georges Valensi. Previous schemes for color television systems, which were incompatible with existing monochrome receivers, in analog television, chrominance is encoded into a video signal using a subcarrier frequency. Depending on the standard, the chrominance subcarrier may be either quadrature-amplitude-modulated or frequency-modulated. In the PAL system, the subcarrier is 4.43 MHz above the video carrier. The NTSC and PAL standards are the most commonly used, although there are other standards that employ different subcarrier frequencies. For example, PAL-M uses a 3.58 MHz subcarrier, the presence of chrominance in a video signal is indicated by a color burst signal transmitted on the back porch, just after horizontal synchronization and before each line of video starts. If the color burst signal were visible on a television screen, in NTSC and PAL, hue is represented by a phase shift of the chrominance signal relative to the color burst, while saturation is determined by the amplitude of the subcarrier. In SECAM and signals are transmitted alternately and phase does not matter, chrominance is represented by the U-V color plane in PAL and SECAM video signals, and by the I-Q color plane in NTSC. Digital video and digital photography systems sometimes use a luma/chroma decomposition for improved compression. On decompression, the Y′CbCr space is rotated back to RGB
14.
Bandwidth (signal processing)
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Bandwidth is the difference between the upper and lower frequencies in a continuous set of frequencies. It is typically measured in hertz, and may refer to passband bandwidth, sometimes to baseband bandwidth. Passband bandwidth is the difference between the upper and lower frequencies of, for example, a band-pass filter, a communication channel. In the case of a filter or baseband signal, the bandwidth is equal to its upper cutoff frequency. A key characteristic of bandwidth is that any band of a given width can carry the amount of information. For example, a 3 kHz band can carry a telephone conversation whether that band is at baseband or modulated to some higher frequency, Bandwidth is a key concept in many telecommunications applications. In radio communications, for example, bandwidth is the range occupied by a modulated carrier signal. An FM radio receivers tuner spans a range of frequencies. A government agency may apportion the regionally available bandwidth to broadcast license holders so that their signals do not mutually interfere, each transmitter owns a slice of bandwidth. For different applications there are different precise definitions, which are different for signals than for systems. One definition of bandwidth, for a system, could be the range of frequencies over which the system produces a level of performance. A less strict and more practically useful definition will refer to the frequencies beyond which frequency response is small, small could mean less than 3 dB below the maximum value, or more rarely 10 dB below, or it could mean below a certain absolute value. As with any definition of the width of a function, many definitions are suitable for different purposes, in some contexts, the signal bandwidth in hertz refers to the frequency range in which the signals spectral density is nonzero or above a small threshold value. That definition is used in calculations of the lowest sampling rate that will satisfy the sampling theorem, the threshold value is often defined relative to the maximum value, and is most commonly the 3dB point, that is the point where the spectral density is half its maximum value. The word bandwidth applies to signals as described above, but it could apply to systems. To say that a system has a certain bandwidth means that the system can process signals of that bandwidth, or that the system reduces the bandwidth of a white noise input to that bandwidth. If the maximum gain is 0 dB, the 3 dB bandwidth is the range where the gain is more than −3 dB. This is also the range of frequencies where the gain is above 70. 7% of the maximum amplitude gain
15.
Interlaced video
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Interlaced video is a technique for doubling the perceived frame rate of a video display without consuming extra bandwidth. The interlaced signal contains two fields of a video frame captured at two different times and this enhances motion perception to the viewer, and reduces flicker by taking advantage of the phi phenomenon. This effectively doubles the resolution as compared to non-interlaced footage. Interlaced signals require a display that is capable of showing the individual fields in a sequential order. CRT displays and ALiS plasma displays are made for displaying interlaced signals, Interlaced scan refers to one of two common methods for painting a video image on an electronic display screen by scanning or displaying each line or row of pixels. This technique uses two fields to create a frame, one field contains all odd-numbered lines in the image, the other contains all even-numbered lines. A Phase Alternating Line -based television set display, for example, the two sets of 25 fields work together to create a full frame every 1/25 of a second, but with interlacing create a new half frame every 1/50 of a second. To display interlaced video on progressive displays, playback applies deinterlacing to the video signal. The European Broadcasting Union has argued against interlaced video in production and they recommend 720p 50 fps for the current production format—and are working with the industry to introduce 1080p 50 as a future-proof production standard. 1080p 50 offers higher vertical resolution, better quality at lower bitrates, despite arguments against it, television standards organizations continue to support interlacing. It is still included in digital video formats such as DV, DVB. Progressive scan captures, transmits, and displays an image in a similar to text on a page—line by line. The interlaced scan pattern in a CRT display also completes such a scan, the first pass displays the first and all odd numbered lines, from the top left corner to the bottom right corner. The second pass displays the second and all even numbered lines and this scan of alternate lines is called interlacing. A field is an image that contains only half of the lines needed to make a complete picture, persistence of vision makes the eye perceive the two fields as a continuous image. In the days of CRT displays, the afterglow of the displays phosphor aided this effect, interlacing provides full vertical detail with the same bandwidth that would be required for a full progressive scan of twice the perceived frame rate and refresh rate. To prevent flicker, all analog broadcast television systems used interlacing, format identifiers like 576i 50 and 720p 50 specify the frame rate for progressive scan formats, but for interlaced formats they typically specify the field rate. This can lead to confusion, because industry-standard SMPTE timecode formats always deal with frame rate, not field rate
16.
Utility frequency
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The utility frequency, line frequency or mains frequency is the nominal frequency of the oscillations of alternating current in an electric power grid transmitted from a power station to the end-user. In large parts of the world this is 50 Hz, although in the Americas, current usage by country or region is given in the list of mains power around the world. During the development of electric power systems in the late 19th and early 20th centuries. Large investment in equipment at one frequency made standardization a slow process, however, as of the turn of the 21st century, places that now use the 50 Hz frequency tend to use 220–240 V, and those that now use 60 Hz tend to use 100–127 V. Both frequencies coexist today with no technical reason to prefer one over the other. Unless specified by the manufacturer to operate on both 50 and 60 Hz, appliances may not operate efficiently or even if used on anything other than the intended frequency. In practice, the frequency of the grid varies around the nominal frequency, reducing when the grid is heavily loaded. However, most utilities will adjust the frequency of the grid over the course of the day to ensure a constant number of cycles occur and this is used by some clocks to accurately maintain their time. Several factors influence the choice of frequency in an AC system, lighting, motors, transformers, generators and transmission lines all have characteristics which depend on the power frequency. All of these factors interact and make selection of a power frequency a matter of considerable importance, the best frequency is a compromise between contradictory requirements. When large central generating stations became practical, the choice of frequency was made based on the nature of the intended load, eventually improvements in machine design allowed a single frequency to be used both for lighting and motor loads. A unified system improved the economics of electricity production, since system load was more uniform during the course of a day, the first applications of commercial electric power were incandescent lighting and commutator-type electric motors. Both devices operate well on DC, but DC could not be changed in voltage. Commutator-type motors do not operate well on high-frequency AC, because the changes of current are opposed by the inductance of the motor field. Though commutator-type universal motors are common in AC household appliances and power tools, they are small motors, less than 1 kW. The induction motor was found to work well on frequencies around 50 to 60 Hz, once AC electric motors became common, it was important to standardize frequency for compatibility with the customers equipment. Generators operated by slow-speed reciprocating engines will produce lower frequencies, for a number of poles, than those operated by, for example. For very slow prime mover speeds, it would be costly to build a generator with enough poles to provide a high AC frequency, as well, synchronizing two generators to the same speed was found to be easier at lower speeds
17.
Electric power distribution
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Electric power distribution is the final stage in the delivery of electric power, it carries electricity from the transmission system to individual consumers. Distribution substations connect to the system and lower the transmission voltage to medium voltage ranging between 2 kV and 35 kV with the use of transformers. Primary distribution lines carry this medium voltage power to distribution transformers located near the customers premises, Distribution transformers again lower the voltage to the utilization voltage of household appliances and typically feed several customers through secondary distribution lines at this voltage. Commercial and residential customers are connected to the distribution lines through service drops. Customers demanding a larger amount of power may be connected directly to the primary distribution level or the subtransmission level. Electric power distribution only became necessary in the 1880s when electricity started being generated at power stations, before that electricity was usually generated where it was used. Both were supplanting gas lighting systems, with arc lighting taking over large area/street lighting, the Edison DC system needed thick copper conductor cables, and the generating plants needed to be within about 1.5 miles of the farthest customer to avoid excessively large and expensive conductors. With much cheaper transmission costs and the economies of scale of having large generating plants supply whole cities and regions. Edisons propaganda campaign was short lived with his company switching over to AC in 1892, in the first half of the 20th century, the electric power industry was vertically integrated, meaning that one company did generation, transmission, distribution, metering and billing. Starting in the 1970s and 1980s, nations began the process of deregulation and privatisation, the distribution system would remain regulated, but generation, retail, and sometimes transmission systems were transformed into competitive markets. Electric power begins at a station, where the potential difference can be as high as 13,800 volts. However, High-voltage DC can be advantageous for isolating alternating-current systems or controlling the quantity of electricity transmitted, for example, Hydro-Québec has a direct-current line which goes from the James Bay region to Boston. From the generating station it goes to the generating station’s switchyard where a step-up transformer increases the voltage to a suitable for transmission. Once in the system, electricity from each generating station is combined with electricity produced elsewhere. Electricity is consumed as soon as it is produced and it is transmitted at a very high speed, close to the speed of light. Transformers step down transmission voltages, 35kV or more, down to primary distribution voltages and these are medium voltage circuits, usually 600-35,000 V. From the transformer, power goes to the busbar that can split the power off in multiple directions. The bus distributes power to distribution lines, which fan out to customers, urban distribution is mainly underground, sometimes in common utility ducts
18.
Beat (acoustics)
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In acoustics, a beat is an interference pattern between two sounds of slightly different frequencies, perceived as a periodic variation in volume whose rate is the difference of the two frequencies. When tuning instruments that can produce sustained tones, beats can readily be recognized, tuning two tones to a unison will present a peculiar effect, when the two tones are close in pitch but not identical, the difference in frequency generates the beating. The volume varies like in a tremolo as the sounds alternately interfere constructively and destructively, as the two tones gradually approach unison, the beating slows down and may become so slow as to be imperceptible. It can be proven that the envelope of the maxima and minima form a wave frequency is half the difference between the frequencies of the two original waves. Instead, it is perceived as a variation in the amplitude of the first term in the expression above. It can be said that the lower frequency cosine term is an envelope for the higher frequency one, the frequency of the modulation is f1 + f2/2, that is, the average of the two frequencies. It can be noted that every second burst in the pattern is inverted. Each peak is replaced by a trough and vice versa, however, because the human ear is not sensitive to the phase of a sound, only its amplitude or intensity, only the magnitude of the envelope is heard. A physical interpretation is that when cos =1 the two waves are in phase and they interfere constructively, when it is zero, they are out of phase and interfere destructively. Beats occur also in more complex sounds, or in sounds of different volumes, beating can also be heard between notes that are near to, but not exactly, a harmonic interval, due to some harmonic of the first note beating with a harmonic of the second note. For example, in the case of perfect fifth, the harmonic of the bass note beats with the second harmonic of the other note. Musicians commonly use interference beats to objectively check tuning at the unison, perfect fifth, piano and organ tuners even use a method involving counting beats, aiming at a particular number for a specific interval. The composer Alvin Lucier has written many pieces that feature interference beats as their main focus, composer Phill Niblocks music is entirely based on beating caused by microtonal differences. Binaural beats are heard when the right ear listens to a different tone than the left ear. Here, the tones do not interfere physically, but are summed by the brain in the olivary nucleus and this effect is related to the brains ability to locate sounds in three dimensions. Combination tone Gamelan tuning Heterodyne Consonance and dissonance Moiré pattern, a form of interference that generates new frequencies
19.
Deinterlacing
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Deinterlacing is the process of converting interlaced video, such as common analog television signals or 1080i format HDTV signals, into a non-interlaced form. An interlaced video frame consists of two sub-fields taken in sequence, each sequentially scanned at odd, and then even, lines of the image sensor. Analog television employed this technique because it allowed for less transmission bandwidth, cRT-based displays were able to display interlaced video correctly due to their complete analogue nature. Newer displays are inherently digital, in that the display comprises discrete pixels, consequently, the two fields need to be combined into a single frame, which leads to various visual defects. The deinterlacing process should try to minimize these, deinterlacing has been researched for decades and employs complex processing algorithms, however, consistent results have been very hard to achieve. Both video and photographic film capture a series of frames in succession, however. In analog television, each frame is divided into two fields, one containing all even lines, another with the odd lines. The fields are captured in succession at a rate twice that of the frame rate. For instance, PAL and SECAM systems have a rate of 25 frames/s or 50 fields/s and this process of dividing frames into half-resolution fields at double the frame rate is known as interlacing. This results in a doubling of time resolution as compared with non-interlaced footage. Most modern displays, such as LCD, DLP and plasma displays, are not able to work in interlaced mode, because they are fixed-resolution displays and only support progressive scanning. In order to display interlaced signal on such displays, the two interlaced fields must be converted to one frame with a process known as de-interlacing. A good deinterlacing algorithm should try to avoid interlacing artifacts as much as possible and not sacrifice image quality in the process, deinterlacing techniques require complex processing and thus can introduce a delay into the video feed. While not generally noticeable, this can result in the display of video games lagging behind controller input. Many TVs thus have a mode in which minimal processing is done in order to maximize speed at the expense of image quality. Deinterlacing is only responsible for such lag, scaling also involves complex algorithms that take milliseconds to run. Interlaced video can carry progressive scan signal, and deinterlacing process should consider this as well, typical movie material is shot on 24 frames/s film, when converting film to interlaced video using telecine, each film frame can be presented by two progressive segmented frames. This format does not require complex deinterlacing algorithm because each contains a part of the very same progressive frame
20.
Flat panel display
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They are far lighter and thinner than traditional cathode ray tube television sets and video displays and are usually less than 10 centimetres thick. Flat-panel displays can be divided into two display device categories, volatile and static, volatile displays require that pixels be periodically electronically refreshed to retain their state. A volatile display only shows an image when it has battery or AC mains power, static flat-panel displays rely on materials whose color states are bistable, and as such, flat-panel displays retain the text or images on the screen even when the power is off. As of 2016, flat-panel displays have almost completely replaced old CRT displays, most 2010s-era flat-panel displays use LCD and/or LED technologies. Most LCD screens are back-lit to make them easier to read or view in bright environments, flat-panel displays are thin and lightweight and provide better linearity and they are capable of higher resolution than typical consumer-grade TVs from earlier eras. The highest resolution for consumer-grade CRT TVs was 1080i, in contrast, many touchscreen-enabled devices can display a virtual QWERTY or numeric keyboard on the screen, to enable the user to type words or numbers. In many instances, an MFM also includes a TV tuner, the first engineering proposal for a flat-panel TV was by General Electric as a result of its work on radar monitors. Their publication of their findings gave all the basics of future flat-panel TVs, but GE did not continue with the R&D required and never built a working flat panel at that time. The first production flat-panel display was the Aiken tube, developed in the early 1950s and this saw some use in military systems as a heads up display, but conventional technologies overtook its development. Attempts to commercialize the system for television use ran into continued problems. The Philco Predicta featured a relatively flat cathode ray tube setup and would be the first commercially released flat panel upon its launch in 1958, the plasma display panel was invented in 1964 at the University of Illinois, according to The History of Plasma Display Panels. The first active-matrix addressed display was made by T Peter Brodys Thin-Film Devices department at Westinghouse Electric Corporation in 1968, in 1977, James P Mitchell prototyped and later demonstrated what was perhaps the earliest monochromatic flat panel LED television display LED Display. As of 2012, 50% of global market share in flat-panel display production is by Taiwanese manufacturers such as AU Optronics, Liquid crystal displays are lightweight, compact, portable, cheap, more reliable, and easier on the eyes than cathode ray tube screens. LCD screens use a thin layer of crystal, a liquid that exhibits crystalline properties. It is sandwiched between two conducting plates. The top plate has transparent electrodes deposited on it, and the plate is illuminated so that the viewer can see the images on the screen. By applying controlled electrical signals across the plates, various segments of the crystal can be activated. These segments can either transmit or block light, an image is produced by passing light through selected segments of the liquid crystal to the viewer
21.
Plasma display
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A plasma display panel is a type of flat panel display common to large TV displays 30 inches or larger. They are called plasma displays because they use small cells containing electrically charged ionized gases, Plasma displays are bright, have a wide color gamut, and can be produced in fairly large sizes—up to 3.8 metres diagonally. They had a very low-luminance dark-room black level compared with the grey of the unilluminated parts of an LCD screen at least in the early history of the competing technologies. LED-backlit LCD televisions have been developed to reduce this distinction, the display panel itself is about 6 cm thick, generally allowing the devices total thickness to be less than 10 cm. The plasma that illuminates the screen can reach a temperature of at least 1200 °C, typical power consumption is 400 watts for a 127 cm screen. Most screens are set to shop mode by default, which draws at least twice the power of a setting of less extreme brightness. The lifetime of the latest generation of displays is estimated at 100,000 hours of actual display time. This is the time over which maximum picture brightness degrades to half the original value. Plasma screens are made out of glass. This may causes glare from reflected objects in the viewing area, currently, plasma panels cannot be economically manufactured in screen sizes smaller than 82 centimetres. Although a few companies have been able to make plasma enhanced-definition televisions this small, with the trend toward large-screen television technology, the 32 inch screen size is rapidly disappearing. Competing display technologies include cathode ray tube, organic light-emitting diode, AMLCD, Digital Light Processing DLP, SED-tv, LED display, field emission display, and quantum dot display. Capable of producing deeper blacks allowing for superior contrast ratio Wider viewing angles than those of LCD, images do not suffer from degradation at less than straight ahead angles like LCDs. LCDs using IPS technology have the widest angles, but they do not equal the range of plasma primarily due to IPS glow, LCD panel backlights nearly always produce uneven brightness levels, although this is not always noticeable. High-end computer monitors have technologies to try to compensate for the uniformity problem, unaffected by clouding from the polishing process. Some LCD panel types, like IPS, require a process that can introduce a haze usually referred to as clouding. Less expensive for the buyer per square inch than LCD, particularly when equivalent performance is considered, earlier generation displays were more susceptible to screen burn-in and image retention. Recent models have a pixel orbiter that moves the entire picture slower than is noticeable to the human eye, earlier generation displays had phosphors that lost luminosity over time, resulting in gradual decline of absolute image brightness. Newer models have advertised lifespans exceeding 100000 hours, far longer than older CRTs Uses more electrical power, on average, older CCFL backlights for LCD panels used quite a bit more power, and older plasma TVs used quite a bit more power than recent models
22.
Film
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A film, also called a movie, motion picture, theatrical film or photoplay, is a series of still images which, when shown on a screen, creates the illusion of moving images due to the phi phenomenon. This optical illusion causes the audience to perceive continuous motion between separate objects viewed rapidly in succession, the process of filmmaking is both an art and an industry. The word cinema, short for cinematography, is used to refer to the industry of films. Films were originally recorded onto plastic film through a photochemical process, the adoption of CGI-based special effects led to the use of digital intermediates. Most contemporary films are now fully digital through the process of production, distribution. Films recorded in a form traditionally included an analogous optical soundtrack. It runs along a portion of the film exclusively reserved for it and is not projected, Films are cultural artifacts created by specific cultures. They reflect those cultures, and, in turn, affect them, Film is considered to be an important art form, a source of popular entertainment, and a powerful medium for educating—or indoctrinating—citizens. The visual basis of film gives it a power of communication. Some films have become popular worldwide attractions by using dubbing or subtitles to translate the dialog into the language of the viewer, some have criticized the film industrys glorification of violence and its potentially negative treatment of women. The individual images that make up a film are called frames, the perception of motion is due to a psychological effect called phi phenomenon. The name film originates from the fact that film has historically been the medium for recording and displaying motion pictures. Many other terms exist for a motion picture, including picture, picture show, moving picture, photoplay. The most common term in the United States is movie, while in Europe film is preferred. Terms for the field, in general, include the big screen, the screen, the movies, and cinema. In early years, the sheet was sometimes used instead of screen. Preceding film in origin by thousands of years, early plays and dances had elements common to film, scripts, sets, costumes, production, direction, actors, audiences, storyboards, much terminology later used in film theory and criticism apply, such as mise en scène. Owing to the lack of any technology for doing so, the moving images, the magic lantern, probably created by Christiaan Huygens in the 1650s, could be used to project animation, which was achieved by various types of mechanical slides
23.
Telecine
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Telecine is the process of transferring motion picture film into video and is performed in a color suite. The term is used to refer to the equipment used in the post-production process. Telecine enables a motion picture, captured originally on film stock, to be viewed with standard equipment, such as television sets, video cassette recorders, DVD. Within the film industry, it is referred to as a TK. With the advent of popular broadcast television, producers realized they needed more than live television programming, however, the difference in frame rates between film and television meant that simply playing a film into a television camera would result in flickering. Originally the kinescope was used to record the image from a display to film. This could then be re-played directly into a camera for re-display. Non-live programming could also be filmed using the cameras, edited mechanically as normal. As the film was run at the speed as the television. Color was supported by using a video camera, prisms. However, this still left film shot at cinema frame rates as a problem, the obvious solution is to simply speed up the film to match the television frame rates, but this, at least in the case of NTSC, is rather obvious to the eye and ear. This problem is not difficult to fix, however, the solution being to play a selected frame twice. For NTSC, the difference in rates can be corrected by showing every fourth frame of film twice. A more convincing technique is to use 2,3 pulldown, discussed below, PAL uses a similar system,2,2 pulldown. In recent decades, telecine has primarily been a film-to-videotape process, changes since the 1950s have primarily been in terms of equipment and physical formats, the basic concept remains the same. Home movies are video tapes of films that used this technique, the most complex part of telecine is the synchronization of the mechanical film motion and the electronic video signal. Every time the part of the telecine samples the light electronically. This is relatively easy when the film is photographed at the frame rate as the video camera will sample, but when this is not true
24.
576i
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576i is a standard-definition video mode originally used for broadcast television in most countries of the world where the utility frequency for electric power distribution is 50 Hz. Because of its association with the colour encoding system, it is often referred to as simply PAL, PAL/SECAM or SECAM when compared to its 60 Hz NTSC-colour-encoded counterpart. The 576 identifies a vertical resolution of 576 lines, and the i identifies it as an interlaced resolution and its basic parameters common to both analogue and digital implementations are,576 scan lines or vertical pixels of picture content,25 frames per second. Digital information not to be displayed as part of the image can be transmitted in the lines, teletext and other services. Analogue television signals have no pixels, they are rastered in scan lines, in digital applications, the number of pixels per line is an arbitrary choice as long as it fulfils the sampling theorem. Values above about 500 pixels per line are enough for conventional broadcast television, DVB-T, DVD and DV allow better values such as 704 or 720. The video format can be transported by major digital television formats, ATSC, DVB and ISDB, and on DVD, when 576i video is transmitted via baseband, most of the differences between the one-letter systems are no longer significant, other than vertical resolution and frame rate. Digital video uses its own separate space, so even the minor colour space differences between PAL and SECAM become moot in the digital domain. When 576i is used to transmit content that was composed of 25 full progressive frames per second. This is the opposite of NTSC, motion pictures are typically shot on film at 24 frames per second. When telecined and played back at PALs standard of 25 frames per second and this also applies to most TV series that are shot on film or digital 24p. Depending on the system in use, it also slightly increases the pitch of the soundtrack by 70.67 cents. More recently, digital conversion methods have used algorithms which preserve the pitch of the soundtrack. Conversion methods exist that can convert 24 frames per second video to 25 frames per second with no speed increase, however image quality suffers when conversions of this type are used. This method is most commonly employed through conversions done digitally, and is employed in situations where the importance of preserving the speed of the video outweighs the need for image quality. Many movie enthusiasts prefer PAL over NTSC despite the formers speed-up, because the results in telecine judder. States the majority of authorities on the subject favour PAL over NTSC for DVD playback quality, also DVD reviewers often make mention of this cause. For example, in his PAL vs. NTSC article, the founder of MichaelDVD says, Personally, I find all but intolerable and find it very hard to watch a movie on an NTSC DVD because of it
25.
Cathode ray tube
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The cathode ray tube is a vacuum tube that contains one or more electron guns and a phosphorescent screen, and is used to display images. It modulates, accelerates, and deflects electron beam onto the screen to create the images, the images may represent electrical waveforms, pictures, radar targets, or others. CRTs have also used as memory devices, in which case the visible light emitted from the fluorescent material is not intended to have significant meaning to a visual observer. In television sets and computer monitors, the front area of the tube is scanned repetitively and systematically in a fixed pattern called a raster. An image is produced by controlling the intensity of each of the three beams, one for each additive primary color with a video signal as a reference. A CRT is constructed from an envelope which is large, deep, fairly heavy. The interior of a CRT is evacuated to approximately 0.01 Pa to 133 nPa. evacuation being necessary to facilitate the flight of electrons from the gun to the tubes face. That it is evacuated makes handling an intact CRT potentially dangerous due to the risk of breaking the tube and causing a violent implosion that can hurl shards of glass at great velocity. As a matter of safety, the face is made of thick lead glass so as to be highly shatter-resistant and to block most X-ray emissions. Flat panel displays can also be made in large sizes, whereas 38 to 40 was about the largest size of a CRT television, flat panels are available in 60. Cathode rays were discovered by Johann Hittorf in 1869 in primitive Crookes tubes and he observed that some unknown rays were emitted from the cathode which could cast shadows on the glowing wall of the tube, indicating the rays were traveling in straight lines. In 1890, Arthur Schuster demonstrated cathode rays could be deflected by electric fields, the earliest version of the CRT was known as the Braun tube, invented by the German physicist Ferdinand Braun in 1897. It was a diode, a modification of the Crookes tube with a phosphor-coated screen. In 1907, Russian scientist Boris Rosing used a CRT in the end of an experimental video signal to form a picture. He managed to display simple geometric shapes onto the screen, which marked the first time that CRT technology was used for what is now known as television. The first cathode ray tube to use a hot cathode was developed by John B. Johnson and Harry Weiner Weinhart of Western Electric and it was named by inventor Vladimir K. Zworykin in 1929. RCA was granted a trademark for the term in 1932, it released the term to the public domain in 1950. The first commercially made electronic television sets with cathode ray tubes were manufactured by Telefunken in Germany in 1934, in oscilloscope CRTs, electrostatic deflection is used, rather than the magnetic deflection commonly used with television and other large CRTs
26.
Phosphor
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A phosphor, most generally, is a substance that exhibits the phenomenon of luminescence. Somewhat confusingly, this includes both phosphorescent materials, which show a slow decay in brightness, and fluorescent materials, where the emission decay takes place over tens of nanoseconds, Phosphors are often transition metal compounds or rare earth compounds of various types. The most common uses of phosphors are in CRT displays and fluorescent lights, CRT phosphors were standardized beginning around World War II and designated by the letter P followed by a number. Phosphorus, the element named for its light-emitting behavior, emits light due to chemiluminescence. A material can emit light either through incandescence, where all atoms radiate, or by luminescence, in inorganic phosphors, these inhomogeneities in the crystal structure are created usually by addition of a trace amount of dopants, impurities called activators. The wavelength emitted by the center is dependent on the atom itself. The scintillation process in inorganic materials is due to the band structure found in the crystals. An incoming particle can excite an electron from the band to either the conduction band or the exciton band. This leaves a hole behind, in the valence band. Impurities create electronic levels in the forbidden gap, the excitons are loosely bound electron-hole pairs that wander through the crystal lattice until they are captured as a whole by impurity centers. The latter then rapidly de-excite by emitting scintillation light, in case of inorganic scintillators, the activator impurities are typically chosen so that the emitted light is in the visible range or near-UV where photomultipliers are effective. The holes associated with electrons in the band are independent from the latter. Those holes and electrons are captured successively by impurity centers exciting certain metastable states not accessible to the excitons, the delayed de-excitation of those metastable impurity states, slowed down by reliance on the low-probability forbidden mechanism, again results in light emission. Many phosphors tend to lose efficiency gradually by several mechanisms, the degradation of electroluminescent devices depends on frequency of driving current, the luminance level, and temperature, moisture impairs phosphor lifetime very noticeably as well. Harder, high-melting, water-insoluble materials display lower tendency to lose luminescence under operation, examples, BaMgAl10O17, Eu2+, a plasma display phosphor, undergoes oxidation of the dopant during baking. Thin coating of aluminium phosphate or lanthanum phosphate is effective in creation a barrier layer blocking access of oxygen to the BAM phosphor, for the cost of reduction of phosphor efficiency. Addition of hydrogen, acting as an agent, to argon in the plasma displays significantly extends the lifetime of BAM, Eu2+ phosphor. Y2O3, Eu phosphors under electron bombardment in presence of oxygen form a non-phosphorescent layer on the surface, the traps also damage the crystal lattice
27.
Electromagnet
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An electromagnet is a type of magnet in which the magnetic field is produced by an electric current. The magnetic field disappears when the current is turned off, electromagnets usually consist of insulated wire wound into a coil. A current through the wire creates a field which is concentrated in the hole in the center of the coil. The main advantage of an electromagnet over a permanent magnet is that the field can be quickly changed by controlling the amount of electric current in the winding. However, unlike a permanent magnet that needs no power, an electromagnet requires a supply of current to maintain the magnetic field. Electromagnets are also employed in industry for picking up and moving heavy objects such as scrap iron. Danish scientist Hans Christian Ørsted discovered in 1820 that electric currents create magnetic fields, british scientist William Sturgeon invented the electromagnet in 1824. His first electromagnet was a piece of iron that was wrapped with about 18 turns of bare copper wire. The iron was varnished to insulate it from the windings, when a current was passed through the coil, the iron became magnetized and attracted other pieces of iron, when the current was stopped, it lost magnetization. Sturgeon displayed its power by showing that although it only weighed seven ounces, however, Sturgeons magnets were weak because the uninsulated wire he used could only be wrapped in a single spaced out layer around the core, limiting the number of turns. Beginning in 1830, US scientist Joseph Henry systematically improved and popularized the electromagnet, the first major use for electromagnets was in telegraph sounders. A portative electromagnet is one designed to just hold material in place, a tractive electromagnet applies a force and moves something. The solenoid is a coil of wire, and the plunger is made of a such as soft iron. Applying a current to the solenoid applies a force to the plunger, the plunger stops moving when the forces upon it are balanced. For example, the forces are balanced when the plunger is centered in the solenoid, the maximum uniform pull happens when one end of the plunger is at the middle of the solenoid. For units using inches, pounds force, and amperes with long, slender, solenoids, for example, a 12-inch long coil with a long plunger of 1-square inch cross section and 11,200 ampere-turns had a maximum pull of 8.75 pounds. The maximum pull is increased when a stop is inserted into the solenoid. The stop becomes a magnet that will attract the plunger, it adds little to the pull when the plunger is far away
28.
Inductance
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According to Lenzs law, a changing electric current through a circuit that contains inductance induces a proportional voltage, which opposes the change in current. The varying field in this circuit may also induce an EMF in neighbouring circuits, the term inductance was coined by Oliver Heaviside in 1886. It is customary to use the symbol L for inductance, in honour of the physicist Heinrich Lenz. In the SI system, the measurement unit for inductance is the henry, with the unit symbol H, named in honor of Joseph Henry, who discovered inductance independently of, an electronic component that is intended to add inductance to a circuit is called an inductor. Inductors are typically manufactured from coils of wire and this design delivers two desired properties, a concentration of the magnetic field into a small physical space and a linking of the magnetic field into the circuit multiple times. The relationship between the self-inductance, L, of a circuit, the voltage, v. A voltage is induced across an inductor, that is equal to the product of the inductors inductance, all circuits have, in practice, some inductance, which may have beneficial or detrimental effects. For a tuned circuit, inductance is used to provide a frequency-selective circuit, practical inductors may be used to provide filtering, or energy storage, in a given network. The inductance of long AC power transmission lines affects the power capacity of the line, sensitive circuits, such as microphone and computer network cables, may utilize special cabling construction, limiting the inductive coupling between circuits. The generalization to the case of K electrical circuits with currents, here, inductance L is a symmetric matrix. The diagonal coefficients Lm, m are called coefficients of self-inductance, the coefficients of inductance are constant, as long as no magnetizable material with nonlinear characteristics is involved. This is a consequence of the linearity of Maxwells equations in the fields. The coefficients of inductance become functions of the currents in the nonlinear case, the inductance equations above are a consequence of Maxwells equations. There is a derivation in the important case of electrical circuits consisting of thin wires. Here Nm denotes the number of turns in loop m, Φm, the flux through loop m. This equation follows from Amperes law - magnetic fields and fluxes are linear functions of the currents and this agrees with the definition of inductance above if the coefficients Lm, n are identified with the coefficients of inductance. Because the total currents Nnin contribute to Φm it also follows that Lm, ∑ n =1 K ∂ W ∂ i n d i n. This must agree with the change of the field energy, W
29.
Spectral density
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The power spectrum S x x of a time series x describes the distribution of power into frequency components composing that signal. According to Fourier analysis any physical signal can be decomposed into a number of discrete frequencies, the statistical average of a certain signal or sort of signal as analyzed in terms of its frequency content, is called its spectrum. When the energy of the signal is concentrated around a time interval, especially if its total energy is finite. More commonly used is the spectral density, which applies to signals existing over all time. The power spectral density then refers to the energy distribution that would be found per unit time. Summation or integration of the spectral components yields the total power or variance, identical to what would be obtained by integrating x 2 over the time domain, the spectrum of a physical process x often contains essential information about the nature of x. For instance, the pitch and timbre of an instrument are immediately determined from a spectral analysis. The color of a source is determined by the spectrum of the electromagnetic waves electric field E as it fluctuates at an extremely high frequency. Obtaining a spectrum from time series such as these involves the Fourier transform, however this article concentrates on situations in which the time series is known or directly measured. The power spectrum is important in signal processing and in the statistical study of stochastic processes, as well as in many other branches of physics. Typically the process is a function of time but one can similarly discuss data in the domain being decomposed in terms of spatial frequency. Any signal that can be represented as an amplitude that varies in time has a frequency spectrum. This includes familiar entities such as light, musical notes, radio/TV. When these signals are viewed in the form of a frequency spectrum, in some cases the frequency spectrum may include a distinct peak corresponding to a sine wave component. And additionally there may be corresponding to harmonics of a fundamental peak. In physics, the signal might be a wave, such as an electromagnetic wave, the power spectral density of the signal describes the power present in the signal as a function of frequency, per unit frequency. Power spectral density is expressed in watts per hertz. When a signal is defined in terms only of a voltage, for instance, in this case power is simply reckoned in terms of the square of the signal, as this would always be proportional to the actual power delivered by that signal into a given impedance
30.
Teletext
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Teletext is a television information retrieval service created in the United Kingdom in the early 1970s by the Philips Lead Designer for VDUs, John Adams. It offers a range of text-based information, typically including news, weather, paged subtitle information is also transmitted within the television signal. It is closely linked to the PAL broadcast system used in Europe, other teletext systems have been developed to work with the SECAM and NTSC systems, but teletext failed to gain widespread acceptance in North America and other areas where NTSC is used. In contrast, teletext is nearly ubiquitous across Europe as well as other regions. Common teletext services include TV schedules, regularly updated current affairs and sport news, Teletext is broadcast in numbered pages. For example, a list of news headlines appear on page 110. The broadcaster constantly sends out pages in sequence, there will typically be a delay of a few seconds from requesting the page and it being broadcast and displayed, the time being entirely dependent in the number of pages being broadcast. More sophisticated receivers use a memory to store some or all of the teletext pages as they are broadcast. For this reason, some pages are broadcast more than once in each cycle, Teletext is also used for carrying special packets interpreted by TVs and video recorders, containing information about channels, programming, etc. Transmitting and displaying subtitles was relatively easy and it requires limited bandwidth, at a rate of perhaps a few words per second. However, it was found that by combining even a slow data rate with a memory, whole pages of information could be sent. In the early 1970s work was in progress in Britain to develop such a system, the goal was to provide UK rural homes with electronic hardware that could download pages of up-to-date news, reports, facts and figures targeting U. K. agriculture. The original idea was the brainchild of Philips Laboratories in 1970, in 1971, CAL engineer John Adams created a design and proposal for UK broadcasters. A major objective for Adams during the development stage was to make Teletext affordable to the home user. In reality, there was no scope to make an economic Teletext system with 1971 technology, however, as low cost was essential to the projects long term success, this obstacle had to be overcome. Meanwhile, the General Post Office, whose telecommunications division later became British Telecom, had been researching a similar concept since the late 1960s, unlike Teledata which was a one-way service carried in the existing TV signal, Viewdata was a two-way system using telephones. Since the Post Office owned the telephones, this was considered to be an excellent way to more customers to use the phones. In 1972 the BBC demonstrated their system, now known as Ceefax, the Independent Television Authority announced their own service in 1973, known as ORACLE
31.
Closed captioning
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Closed captioning and subtitling are both processes of displaying text on a television, video screen, or other visual display to provide additional or interpretive information. Both are typically used as a transcription of the portion of a program as it occurs. Other uses have been to provide a textual alternative language translation of a presentations primary audio language that is usually burned-in to the video, hTML5 defines subtitles as a transcription or translation of the dialogue. When sound is available but not understood by the viewer and captions as a transcription or translation of the dialogue, sound effects, relevant musical cues, when sound is unavailable or not clearly audible. The term closed indicates that the captions are not visible until activated by the viewer, on the other hand, open, burned-in, baked on, or hard-coded captions are visible to all viewers. Most of the world does not distinguish captions from subtitles, in the United States and Canada, however, these terms do have different meanings. Subtitles assume the viewer can hear but cannot understand the language or accent, or the speech is not entirely clear, so they transcribe only dialogue and some on-screen text. Also, the closed caption has come to be used to also refer to the North American EIA-608 encoding that is used with NTSC-compatible video. The United Kingdom, Ireland, and most other countries do not distinguish between subtitles and closed captions and use subtitles as the general term, the equivalent of captioning is usually referred to as subtitles for the hard of hearing. In New Zealand, broadcasters superimpose an ear logo with a line through it that represents subtitles for the hard of hearing, even though they are currently referred to as captions. In the UK, modern digital television services have subtitles for the majority of programs, so it is no longer necessary to highlight which have captioning and which do not. Remote control handsets for TVs, DVDs, and similar devices in most European markets often use SUB or SUBTITLE on the used to control the display of subtitles/captions. Regular open-captioned broadcasts began on PBSs The French Chef in 1972, WGBH began open captioning of the programs Zoom, ABC World News Tonight, and Once Upon a Classic shortly thereafter. Closed captioning was first demonstrated at the First National Conference on Television for the Hearing Impaired in Nashville, the closed captioning system was successfully encoded and broadcast in 1973 with the cooperation of PBS station WETA. As a result of tests, the FCC in 1976 set aside line 21 for the transmission of closed captions. PBS engineers then developed the caption editing consoles that would be used to caption prerecorded programs, real-time captioning, a process for captioning live broadcasts, was developed by the National Captioning Institute in 1982. In real-time captioning, court reporters trained to write at speeds of over 225 words per minute give viewers instantaneous access to news, sports. As a result, the sees the captions within two to three seconds of the words being spoken, explaining spelling and grammatical errors and garbled characters
32.
Widescreen
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Widescreen images are a variety of aspect ratios used in film, television and computer screens. In film, a film is any film image with a width-to-height aspect ratio greater than the standard 1.37,1 Academy aspect ratio provided by 35mm film. For television, the screen ratio for broadcasts was 4,3. Between the 1990s and early 2000s,16,9 TV displays came into wide use and they are typically used in conjunction with high-definition television receivers, or Standard-Definition DVD players and other digital television sources. With computer displays, aspect ratios wider than 4,3 are also called widescreen, widescreen computer displays were previously of 16,10 aspect ratio, but now are usually 16,9. Widescreen was first used in the film of the Corbett-Fitzsimmons Fight in 1897 and this was not only the longest film that had been released to date at 100 minutes, it was also the first widescreen film being shot on 63mm Eastman stock with five perforations per frame. Widescreen was first widely used in the late 1920s in some films and newsreels. Claude Autant-Lara released a film Pour construire un feu in the early Henri Chretien widescreen process, in 1927, The American aka The Flag Maker was released. In 1926, Spoor and Berggren had released a Natural Vision film of Niagara Falls, the Natural Vision widescreen process used 63. 5mm film and had a 2,1 aspect ratio. On May 26,1929, Fox Film Corporation released Fox Grandeur News, which premiered at Graumans Chinese Theatre in Hollywood on October 2,1930, all of which were also made in the 70mm Fox Grandeur process. On November 13,1930, United Artists released The Bat Whispers directed by Roland West in a 70mm widescreen process known as Magnafilm, warner Brothers released Song of the Flame and Kismet in a widescreen process they called Vitascope. In 1930, after experimenting with the system called Fanthom Screen for The Trail of 98, MGM filmed The Great Meadow in Realife—however, its unclear if it was ever released in that widescreen process due to declining interest of the movie-going public. S. However, a few producers and directors, among them Alfred Hitchcock, have been reluctant to use the anamorphic widescreen size featured in such formats as Cinemascope. Hitchcock alternatively used VistaVision, a widescreen process developed by Paramount Pictures. Masked widescreen was introduced in April 1953, a detriment is that the film grain size is thus increased because only part of the image is being expanded to full height. Films are designed to be shown in cinemas in masked widescreen format, in such an instance, a photographer will compose for widescreen, but protect the full image from things such as microphones and other filming equipment. Standardized flat wide screen ratios are 1.66,1,1.75,1,1.85,1,1.85,1 has become the predominant aspect ratio for the format. 35mm anamorphic – This type of widescreen is used for CinemaScope, Panavision, the film is essentially shot squeezed, so that the actors appear vertically elongated on the actual film
33.
Overscan
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Overscan is the situation in which not all of a televised image is present on a viewing screen. It exists because cathode-ray tube television sets from the 1930s through to the early 2000s were highly variable in how the image was positioned within the borders of the screen. The solution was to have the show less than the full image. Early analogue televisions varied in the image because of manufacturing tolerance problems. There were also effects from the design limitations of power supplies. Because of this, TV producers could not be certain where the edges of the image would be. In order to compensate, they defined three areas, Title safe, An area visible by all reasonably maintained sets, where text was not to be cut off. Action safe, A larger area that represented where a set would cut the image off. Underscan, The full image area to the edge of the signal. Studio monitors and camera viewfinders were set to show this area, when used, this mode is called underscan. Todays TV sets are based on newer fixed-pixel technology like liquid crystal displays, as overscan reduces picture quality, it is undesirable for 1080i and 1080p sets, therefore,1,1 pixel mapping is preferred. On LCDs driven from a signal, no adjustment is necessary because all pixels are in fixed positions. Thus all modern computers can safely assume that all pixels are visible to the viewer, analog video signals such as VGA, however, are subject to timing variations and even when using an LCD panel do not have this exactness. When video or animation content is designed to be viewed on computers, CRTs made for computer display are set to underscan with an adjustable border, usually colored black. The border will change size and shape if required to allow for the tolerance of low precision, as such, computer CRTs use less physical screen area than TVs, to allow all information to be shown at all times. Computer CRT monitors usually have a black border —these can be seen in the video card timings, video game systems have been designed to keep important game action in the title safe area. Older systems did this with borders for example, the Super Nintendo Entertainment System windowboxed the image with a border, visible on some NTSC television sets. Newer systems frame content much as live action does, with the area filled with extraneous details
34.
Modulation
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Most radio systems in the 20th century used frequency modulation or amplitude modulation to make the carrier carry the radio broadcast. Modulation of a sine waveform transforms a narrow frequency range baseband message signal into a passband signal, a modulator is a device that performs modulation. A demodulator is a device that performs demodulation, the inverse of modulation, a modem can perform both operations. The aim of digital modulation is to transfer a digital bit stream over an analog bandpass channel, in music synthesizers, modulation may be used to synthesise waveforms with an extensive overtone spectrum using a small number of oscillators. In this case the carrier frequency is typically in the order or much lower than the modulating waveform. In analog modulation, the modulation is applied continuously in response to the information signal. Where an AM carrier also carries a variable phase f. TM is f In digital modulation, a carrier signal is modulated by a discrete signal. Digital modulation methods can be considered as digital-to-analog conversion, and the corresponding demodulation or detection as analog-to-digital conversion, the changes in the carrier signal are chosen from a finite number of M alternative symbols. A simple example, A telephone line is designed for transferring audible sounds, for example tones, computers may however communicate over a telephone line by means of modems, which are representing the digital bits by tones, called symbols. If there are four symbols, the first symbol may represent the bit sequence 00, the second 01, the third 10. If the modem plays a melody consisting of 1000 tones per second, since each tone represents a message consisting of two digital bits in this example, the bit rate is twice the symbol rate, i. e.2000 bits per second. This is similar to the used by dialup modems as opposed to DSL modems. According to one definition of digital signal, the signal is a digital signal. According to another definition, the modulation is a form of digital-to-analog conversion, most textbooks would consider digital modulation schemes as a form of digital transmission, synonymous to data transmission, very few would consider it as analog transmission. The most fundamental digital modulation techniques are based on keying, PSK, FSK, a finite number of frequencies are used. ASK, a number of amplitudes are used. QAM, a number of at least two phases and at least two amplitudes are used
35.
Phase-locked loop
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A phase-locked loop or phase lock loop abbreviated as PLL is a control system that generates an output signal whose phase is related to the phase of an input signal. While there are several differing types, it is easy to visualize as an electronic circuit consisting of a variable frequency oscillator. The oscillator generates a signal, and the phase detector compares the phase of that signal with the phase of the input periodic signal. Bringing the output signal back toward the signal for comparison is called a feedback loop since the output is fed back toward the input forming a loop. Keeping the input and output phase in lock step also implies keeping the input and output frequencies the same, consequently, in addition to synchronizing signals, a phase-locked loop can track an input frequency, or it can generate a frequency that is a multiple of the input frequency. These properties are used for clock synchronization, demodulation. Phase-locked loops are widely employed in radio, telecommunications, computers, for a practical idea of what is going on, consider an auto race. There are many cars, and the driver of each of them wants to go around the track as fast as possible, each lap corresponds to a complete cycle, and each car will complete dozens of laps per hour. The number of laps per hour corresponds to an angular velocity, but the number of laps corresponds to a phase. During most of the race, each car is on its own and the driver of the car is trying to beat the driver of other car on the course. However, if there is an accident, a car comes out to set a safe speed. None of the cars are permitted to pass the pace car. While it is on the track, the car is a reference. Each driver will measure the difference between him and the pace car. If the driver is far away, he will increase his speed to close the gap. If hes too close to the car, he will slow down. The result is all the race cars lock on to the phase of the pace car, the cars travel around the track in a tight group that is a small fraction of a lap. Phase can be proportional to time, so a difference can be a time difference
36.
Automatic gain control
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The signal to be gain controlled goes to a diode & capacitor, which produce a peak-following DC voltage. This is fed to the RF gain blocks to alter their bias, traditionally all the gain-controlled stages came before the signal detection, but it is also possible to improve gain control by adding a gain-controlled stage after signal detection. In 1925, Harold Alden Wheeler invented automatic volume control and obtained a patent, karl Küpfmüller published an analysis of AGC systems in 1928. By the early 1930s most new commercial broadcast receivers included automatic volume control, AGC is afrom linearity in AM radio receivers. If the circuit were not fairly linear, the signal could not be recovered with reasonable fidelity. However, the strength of the signal received will vary widely, depending on the power and distance of the transmitter, for a very weak signal, the AGC operates the receiver at maximum gain, as the signal increases, the AGC reduces the gain. Since the AM detector diode produces a DC voltage proportional to signal strength, design of the AVC system has a great effect on the usability of the receiver, tuning characteristics, audio fidelity, and behavior on overload and strong signals. FM receivers, even though they incorporate limiter stages and detectors that are insensitive to amplitude variations. A related application of AGC is in systems, as a method of overcoming unwanted clutter echoes. This method relies on the fact that clutter returns far outnumber echoes from targets of interest, the receivers gain is automatically adjusted to maintain a constant level of overall visible clutter. While this does not help detect targets masked by stronger surrounding clutter, in the past, radar AGC was electronically controlled and affected the gain of the entire radar receiver. As radars evolved, AGC became computer-software controlled, and affected the gain with greater granularity, an audio tape generates a certain amount of noise. If the level of the signal on the tape is low, to produce the least noisy recording, the recording level should be set as high as possible without being so high as to clip or distort the signal. In professional high-fidelity recording the level is set using a peak-reading meter. When high fidelity is not a requirement, a recording level can be set by an AGC circuit which reduces the gain as the average signal level increases. This allows a usable recording to be made even for some distance from the microphone of an audio recorder. Some reel-to-reel tape recorders and cassette decks have AGC circuits and those used for high-fidelity generally dont. Most VCR circuits use the amplitude of the vertical blanking pulse to operate the AGC, a voice-operated gain-adjusting device or volume-operated gain-adjusting device is a type of AGC or compressor for microphone amplification
