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 select channels easily and, in general, to ease the transition from analogue to digital broadcasting. Assigning virtual channels is most common 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 allow them to group channels by 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 used when both were simulcasting. They achieved this by the DigiCipher 2 method. Viewers could then use one 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 (ie. 2, 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 channel's major and minor numbers that appear separated by a decimal point or dash on-screen. 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 be hidden from the viewer. 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 neither promotes nor mentions either system due to the 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 principle of not allowing any viewer reordering as could be achieved under analog and generic digital systems. This locked-down ordering is one of the main criticisms of these methods.
Digital television multiple channels Because DTV can carry any number of streams (referred to as multiplexing), program numbers can be used to group them into more than one channel which can then be reassigned by virtual or logical channel numbers.
United States and Canada
An example of the ATSC major and minor numbers would be to have main programming airing on say channel 8 (the "major channel") with analog on 8.0 and digital on 8.1 (the first two "minor channels") with other entertainment channels below 8.99 on channels 8.2, 8.3, and up and informational data channels ranging from 8.100 to 8.999. The channels can be displayed using a hyphen (such as 8-1) or a space; however, on a common seven-segment display, a decimal point would save one segment. The decimal point is more familiar to FM radio listeners who tune by frequency rather than channel, and avoids confusion with ranges of values (for example, 2-4 may be misinterpreted as the range 2 to 4 instead of the fourth sub-channel of channel 2). Most US stations follow ATSC numbering guidelines; however, low-power stations such as New York City's WNYZ-LD are exceptions. It was temporarily broadcasting on VHF channel 6 in digital, but used the virtual channel 1.1, instead of 6. This persisted for approximately one year, after which WNYZ-LD reverted to low-power analog. The assignment of virtual channels in the US is defined within the stream via terrestrial or cable versions of a "Virtual Channel Table" as outlined by ATSC document "A/65", Annex B. Rules for assignment of major channel numbers are:
Existing analog stations were assigned a major channel number matching their analog number. New digital stations assigned to a channel whose matching major channel number is not active use that number. New digital stations assigned to a channel whose matching major channel number is in use (by a former analog station) must reciprocate, using the major channel number that matches the actual channel of the station in question. These rules generally prevent overlapping, although in the case of stations where large numbers of stations in adjacent markets are in close proximity, it is possible that overlaps can happen (see, for example, the cases of WJLP and WDPN-TV). Additionally, stations may broadcast some of their subchannels under major channel(s) in the 70–99 range, so long as multiple stations do not attempt to use the same major channel(s). These numbers are certain to be unused, as 69 was the highest assigned channel prior to digital broadcasting. The document does not address the use of certain other major channel numbers:
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