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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 via digits on a receiver's remote control. A "virtual channel" was first used for DigiCipher 2 in North America and then later used and referred to as a logical channel number (LCN) for private European Digital Video Broadcasting extensions widely used by the NDS Group and NorDig in other markets. Pay television operators were the first to use either of these systems as a method of channel reassignment and/or rearrangement that suited their need to group multiple channels by their content or origin as well as to a lesser extent to localize advertising to a particular market. Free-to-air ATSC uses the DigiCipher 2 method to maintain the same radio frequency channel allocation that the NTSC channel was using when both were simulcasting so the same number could bring up either service. Free-to-air DVB network operators such as Freeview in the UK and New Zealand use the NorDig method and follow the same practice as pay TV operators. The exception is FreeTV Australia, which also use the NorDig method and partially 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. ==Implementation== The DigiCipher 2 method uses a privately defined virtual channel table (VCT) to set the channels major and minor numbers that appear on-screen separated by a decimal point. The major number for ATSC represents the original analog or non-simulcast channel frequency while the minor is a sequentially assigned number for the selected channel with zero reserved for the analog channel. The channel may also be marked as hidden from the viewer. The DVB extensions use privately defined descriptors within the ''Bouquet Association Table'' for DVB-S or the ''Network Information Table'' for DVB-T. The (NorDig ) version allows for marking a channel as hidden, while the NDS Group version simply omits the channel entry. The DVB system doesn't promote nor mention either system due to the simple fact that the already defined H.222 ''Program number'' and ''Transport Stream ID'' can achieve the same purpose and also hide a channel by simply omitting it from the ''Program Association table.'' All these methods share the same principle of not allowing any kind of viewer reordering as could be achieved under analog and generic digital systems. This ''locked-down'' ordering is one of the main criticisms of using either method. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Virtual channel」の詳細全文を読む スポンサード リンク
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