Digital Switchover
See also: Digital television transition in the United States, Digital television in Canada, Television in Mexico, and Television in Latin America#El SalvadorATSC replaced much of the analog NTSC television system in the United States on June 12, 2009, replaced NTSC on August 31, 2011 in Canada, and will replace NTSC by December 31, 2015 in Mexico, January 1, 2019 in El Salvador., and by end of 2012 in South Korea.
Broadcasters who use ATSC and want to retain an analog signal must broadcast on two separate channels, as the ATSC system requires the use of an entire channel. Virtual channels allow channel numbers to be remapped from their physical RF channel to any other number 1 to 99, so that ATSC stations can either be associated with the related NTSC channel numbers, or all stations on a network can use the same number. There is also a standard for distributed transmission systems (DTx), a form of single-frequency network which allows for the synchronised operation of multiple on-channel booster stations.
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