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Concept
Canal virtuel
Résumé
In most telecommunications organizations, a virtual channel is a method of remapping the program number as used in H.222 Program Association Tables and Program Mapping Tables to a channel number that can be entered as digits on a receiver's remote control.
Often, virtual channels are implemented in digital television to help users to go to channels easily and in general to ease the transition from analogue to digital broadcasting. Assigning virtual channels is most common in parts of the world where TV stations were colloquially named after the RF channel they were transmitting on ("Channel 6 Springfield"), as was common in North America during the analogue TV era. In other parts of the world, such as Europe, virtual channels are rarely used or needed, because TV stations there identify themselves by name, not by RF channel or callsign.
A "virtual channel" was first used for DigiCipher 2 in North America. It was later called a logical channel number (LCN) and used for private European Digital Video Broadcasting extensions widely used by the NDS Group and by NorDig in other markets.
Pay television operators were the first to use these systems for channel reassignment and rearrangement to meet their need to group channels by their content or origin and, to a lesser extent, to localize advertising.
Free-to-air stations using Advanced Television Systems Committee standards (ATSC) used the same television frequency channel allocation that the NTSC channel was using when both were simulcasting. They achieved this by the DigiCipher 2 method. Viewers could then use the same number to bring up either service.
Free-to-air DVB network operators, such as DTV Services Ltd. (d.b.a. Freeview) and Freeview New Zealand Ltd., use the NorDig method and follow the same practice as pay-TV operators. The exception is Freeview Australia Ltd., which also use the NorDig method and partly follow the ATSC practice of using the same VHF radio-frequency channel allocation that the PAL channel is simulcasting on from the metropolitan station's main transmission point (i2, 7, 9, and 10) with the major and minor format emulated by multiplying by ten.
Source officielle
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