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An analog TV signal is made up of a video signal broadcast on AM radio waves, and an audio signal broadcast on FM waves. Analog technology is currently being replaced by digital technology throughout the world.
In the U.S., black-and-white analog TV transmissions were standardized in 1941 by the National Television System Committee (NTSC), later followed by an updated color standard in 1953. NTSC was adopted by North America, Central America, parts of South America, Japan and other nations. Other parts of the world developed Phase Altering Line (PAL) and Sèquentiel Couleur Avec Mèmoire (SECAM) analog standards. Less popular standards were also developed.
Analog broadcasts have an aspect ratio of 4:3, or nearly square in configuration. An NTSC signal has 525 scan lines, though only 486 make up the visible raster. The remaining lines carry synchronization and vertical retrace information. The lines are painted in two passes across an analog screen, each pass painting every other line, interlacing the passes to create a flicker-free image. Frame rate is 30-frames per second, resulting in an actual frame rate of 29.97 frames per second.
A PAL analog TV signal comes in many flavors, including B/G/H/I/D/M and PAL Nc. Most consist of 625 scan lines, interlaced, at 25 frames per second, though audio carrier frequencies differ between standards. PAL M, like NTCS, uses 525 scan lines and 29.97 frames per second. Brazil uses PAL M, while other flavors of PAL are used in most of South America, Australia, China and other territories, noting again that digital broadcasting is replacing this technology at various rates, regionally.
The SECAM standard was developed in France and has also evolved over the years into different flavors. It also uses 625 scan lines, except for its M version, which like PAL M and NTSC has 525 scan lines. SECAM was used in France, Africa, Russia and other parts of the world, though many territories migrated to PAL throughout the 1990s.
An analog TV signal is subject to interference that can cause undesired effects like ghosting and snow. Distance from the transmitter and intervening topography factor into signal clarity.
An analog television is quite heavy for its size due to the lead-encased, vacuumed chamber that houses the scanning mechanism known as a cathode ray tube (CRT). This mechanism converts the broadcast signal into a moving picture by shooting electrons against the back of the phosphorous television screen many times per second to re-create each frame of information. The analog TV has a deep footprint, taking up a large amount of space, and emits off a fair amount of radiation compared to digital TVs.
Analog TV broadcasting is legacy technology, though it continues to be used in territories that have not yet switched to digital standards. Digital technologies use less bandwidth to deliver more information on the signal, resulting in the ability for higher resolutions, non-interlaced signals, and a 16:9 aspect ratio (resembling a movie screen in configuration). Other advantages of digital TV include the ability to broadcast at lower resolutions that are still higher than analog TV, creating room for multiple channel broadcasting within the same allotted frequency band.