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Analog television: Wikis


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Analog (or analogue) television encodes television picture and sound information and transmits it as an analog signal: one in which the message conveyed by the broadcast signal is a function of deliberate variations in the amplitude and/or frequency of the signal. All systems preceding digital television, such as NTSC, PAL or SECAM are analog television systems.

Broadcasters using analog television systems encode their signal using NTSC, PAL or SECAM analog encoding and then modulate this signal onto a VHF or UHF carrier. An analog television picture is "drawn" on the screen an entire frame each time, in the manner of a motion picture (cinematograph) film, irrespective of the picture content.



In black and white television based on a cathode ray tube (CRT), a single electron beam scans a phosphor screen from left to right and then returns to the top. The electron beam is brightness-modulated to create intensity changes which cause the different shades of grey. Analog television equipment has been manufactured using alternative forms of display, such as LCD, but the picture display is still updated a frame at a time in the same manner as the flying-spot CRT.

To support color signals contained in the broadcast, a color synchronization signal called a "color burst" is added to the basic black and white information. When color television was introduced, engineers ensured that black and white televisions would still be able to display signals that were broadcast in color. To do this, the original monochrome information is still transmitted in the color signal, and then the color difference information is added on top.

Analog broadcast television systems come in a variety of frame rates and resolutions. Further differences exist in the frequency and modulation of the audio carrier. The monochrome combinations still existing in the 1950s are standardized by the ITU as capital letters A through N. When color television was introduced, the hue and saturation information was added to the monochrome signals in a way that black & white televisions ignore. This way backwards compatibility was achieved. That concept is true for all analog television standards.

However there are three standards for the way the additional color information can be encoded and transmitted. The first was the American NTSC (National Television Systems Committee) color television system. The European/Australian PAL (Phase Alternation Line rate) and the French-Former Soviet Union SECAM (Séquentiel Couleur Avec Mémoire) standard were developed later and attempt to cure certain defects of the NTSC system. PAL's color encoding is similar to the NTSC systems. SECAM, though, uses a different modulation approach than PAL or NTSC.

In principle all three color encoding systems can be combined with any of the scan line/frame rate combinations. Therefore, in order to describe a given signal completely, it's necessary to quote the color system and the broadcast standard as capital letter. For example the United States uses NTSC-M, the UK uses PAL-I, France uses SECAM-L, much of Western Europe uses PAL-B/G, most of Eastern Europe uses PAL-D/K or SECAM-D/K and so on.

However not all of these possible combinations actually exist. NTSC is currently only used with system M, even though there were experiments with NTSC-A (405 line) and NTSC-I (625 line) in the UK. PAL is used with a variety of 625-line standards (B,G,D,K,I) but also with the North American 525-line standard, accordingly named PAL-M. Likewise, SECAM is used with a variety of 625-line standards.

For this reason many people refer to any 625/25 type signal as "PAL" and to any 525/30 signal as "NTSC", even when referring to digital signals, e.g., on DVD-Video which don't contain any analog color encoding, thus no PAL or NTSC signals at all. Even though this usage is common, it is misleading as that is not the original meaning of the terms PAL/SECAM/NTSC.

Shutdown and transition to digital

As of late 2009, ten countries had completed the process of turning off analog terrestrial broadcasting. Many other countries had plans to do so or were in the process of a staged conversion. The first country to make a wholesale switch to digital over-the-air (terrestrial) broadcasting was Luxembourg in 2006, followed later in 2006 by the Netherlands; in 2007 by Finland, Andorra, Sweden, Norway and Switzerland; in 2008 by Belgium (Flanders) and Germany; in 2009 by the United States (high power only), Isle of Man and Denmark.

In the United States, high-power over-the-air broadcasts are solely in the ATSC digital format since June 12, 2009, the date that the Federal Communications Commission (FCC) set for the end of all high-power analog TV transmissions. As a result, almost two million households could no longer watch TV because they were not prepared for the transition. The switchover was originally scheduled for February 17, 2009 until the US Congress passed the DTV Delay Act.[1] By special dispensation, some analog TV signals ceased on the original date.[2]

In Japan, the switch to digital is scheduled to happen July 24, 2011. In Canada, it is scheduled to happen August 31, 2011. China is scheduled to switch in 2015. In the United Kingdom, the digital switchover has different times for each part of the country; however, the whole of the UK will be digital by 2012. Brazil switched to digital on December 2, 2007 in major cities and it is estimated it will take seven years for complete signal expansion over all of the Brazilian territory. Australia will turn off analog signals between 2010 and 2013, region by region.[3]

In Malaysia, the Malaysian Communications & Multimedia Commission (MCMC) will call for tender bids in the third quarter of 2009 for the UHF 470–742 megahertz spectrum which will pave the way for the country to move into the digital television era. The awarding of the spectrum will see the winner having to build a single digital terrestrial transmission/TV broadcast (DTTB) infrastructure for all broadcasters to ride on to transmit their TV programs. The winner will be announced at the end of 2009 or early 2010 and has to commence digital roll-out soon after the award where the analog switch-off is planned for 2015.

While the majority of the viewers of over-the-air broadcasting in the USA watch full-power stations (which number about 1800), there are three other categories of TV stations in the US: low-power stations, Class A stations, and TV translator stations. There is presently no deadline for these stations, about 7100 in number, to convert to digital broadcasting.

Common analog television systems

See also




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