3 Virtual channel


Virtual channel

In telecommunications, a logical channel number (LCN), also known as virtual channel, is a channel designation which differs from that of the actual radio channel (or range of frequencies) on which the signal travels.

The term is most often applied to television, where digital television (DTV) channels may operate on different physical channels from the displayed channels. A station branded as Channel 8, for example, might actually use channel 32 for its ATSC, DVB or ISDB transmission, but a virtual channel map or virtual channel table (VCT) allows viewers to tune in the station on channel 8, displayed as 8.1 on a digital set. DTV tuners have a menu item that scans all channels for this information and stores it in memory for instant access later on.

The most common reason for a television station using a virtual channel is to minimize viewer confusion when a digital transmission is airing on a different channel from the one the station used in analog mode. The virtual channel thus enables viewers to tune in the station by choosing the same channel number as they would have previously.

Contents

Digital TV subchannels

Because DTV can carry multiple programs simultaneously, virtual channels also map out subchannels.

North America

Continuing the "Channel 8" example, if the station were in the United States and Canada, typically its main programming would air on channel 8 (the "major channel") or 8.1 (the first "subchannel"). Other programs could be found on channels 8.2, 8.3, and so forth, up to 8.99, with additional data channels ranging from 8.100 to 8.999. The subchannels can also be represented using a hyphen (8-1, 8-2, and so forth). There is no practical difference between the dot and hyphen forms, although the dot form is more familiar to FM radio listeners, and avoids confusion with ranges of values (for example, 2-4 may be misinterpreted as the range 2 to 4 instead of the multiplex 4 of channel 2). A dot is also easier to display on LEDs in most set-top boxes, as it need not occupy a full digit position.

Traditionally, most stations in the United States have adopted either their former analog channel for the virtual channel, or their actual digital channel position if they signed on as a digital station without having an analog station beforehand. There are a few rare exceptions; for instance, New York City's WNYZ-LP, which broadcasts on channel 6 in digital, uses the virtual channel 1.1, instead of 6.

The assignment of virtual channels in the United States is defined by ATSC document "A/65", Annex B.[1][2] Rules for assignment of major channel numbers are as follows:

  • Existing analog stations were assigned a major channel number matching their existing analog number
  • New digital stations assigned to a channel whose matching major channel number is not in use must 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 guarantee that no overlapping will occur. Additionally, stations may apply for a license to broadcast some of their subchannels under a secondary major channel in the 70-99 range; these numbers are certain to be unused, as 69 was the highest assigned channel prior to the conversion to digital broadcasting. The document does not address the use of certain other major channel numbers:

  • Numbers below 70 that were never used in NTSC (0,1, and 37)
  • The real numbers of stations that are using virtual channels from 53-69 (these stations are not covered by the reciprocity rule, as real station numbers are not assigned above 52)
  • Numbers in the range of 53-69 that are not being used by a former analog station

Additionally, broadcasters owning more than one station that overlap in coverage area may set all of the channels to use the major channel of just one of the stations, so long as different minor channel numbers are used to avoid overlap.

Australia

In Australia, allocation of logical channel numbers is governed by guidelines set by the commercial broadcasters' association, Free TV Australia.[3]

LCNs in Australia may have one, two or three digits. Each network is allocated LCNs starting with a certain prefix - for instance, all metropolitan Nine Network services use LCNs beginning with the digit '9'. Generally, but not always, the single digit LCN is allocated to the primary SD service (Network Ten's HD sub-channel One being the main exception). LCNs need not be contiguous, and a channel may be identified by more than one LCN. For instance, ABC Television's primary ABC1 service is allocated LCNs 2 and 21; the latter allows it to be easily accessed amongst other ABC services which lie in the 21-24 range.

Regional affiliates of the three metropolitan networks are provided with a different LCN prefix. For instance, channels owned by affiliates of the Nine Network (in this case, WIN Television and NBN Television) are prefixed with the digit '8' rather than '9'. This allows areas that are part of both a metropolitan market and a regional market, such as the Gold Coast, Sunshine Coast and Central Coast, to receive all local commercial services. The ABC and SBS use the same prefix in all areas.

Prefixes for remote-area services are intended to be overlaid over this model. When digital transmission starts in these areas, services licensed for the Remote Central and Eastern Australia licence area (Imparja and Southern Cross Central) have been reserved the "metropolitan" prefixes corresponding to their affiliation; those in Remote Western Australia (GWN and WIN WA) the "regional" prefixes.

A number of LCNs are reserved for various reasons:

  • LCN 4 was originally intended for a free-to-air video program guide. In practice, the LCN 4 prefix has for most of its life been largely unused, except in Sydney (where it was used by the Digital Forty Four trial datacasting service from 2004-2010). As of 2010, capital city community television stations (or "Channel 31" stations, after their typical analogue channel position) have begun to air in digital using LCN 44; one of which (West TV in Perth) airs exclusively via digital transmission.
  • The LCN range 350-399 is intended to be allocated by receivers to channels which either duplicate a stronger signal's LCN, or are transmitted without an LCN. For instance, if two broadcasts of LCN 2 were found, one signal (generally the stronger) will be allocated to LCN 2, and the weaker should be allocated to, say, LCN 350.
  • The LCN range 450-499 is intended for use by trial services by non-broadcasters.

Europe, Africa and the Middle East

In Europe, Africa and the Middle East, there is no special numbering system for subchannels; two related "channels" (that is, programme streams) may have completely unrelated numbers (for example, in the UK, ITV is channel 3 and ITV2 channel 6, on Freeview).

In continental and eastern Europe, virtual channels are not used, since television sets and receivers there allow users to freely assign arbitrary "programme numbers" or "programme letters" to channels.

Stations still market themselves as "first", "second", or "third" channel (and so on), or "channel A", "channel B" or "channel C", etc., but this reflects the programme number at which the channel should be stored, not the RF channel used for transmitting the signals.

Virtual channels are also used on direct broadcast satellites, such as Dish Network, DirecTV, and Astra. Rather than a few dozen channels with a few subchannels each, these services map to a range of hundreds of individually numbered channels. This is true of digital cable and satellite radio services, as well.

Japan

In Japan, digital terrestrial TV broadcasters in each region are allocated a "remote control key ID" (or, "remocon key ID"), currently numbered from 1 to 12.[4] Remote control ID allocations for broadcasters outside the Kanto region generally follow their Tokyo-based network flagships, however stations in some regions deviate from this. Current technical standards allow for expansion to a maximum of 16 broadcasters per region.

Each underlying channel is then assigned a three-digit number, which is based on their assigned remote control ID, followed by the sub-channel number. For example, NHK Educational TV is assigned remote control ID 2 (nationwide). Their primary channel is therefore assigned virtual channel 021. If the broadcaster multichannels (of which the ISDB-T standard allows up to three standard definition streams), the additional streams would be assigned virtual channels 022 and 023, respectively. Current standards allow for a maximum of eight virtual channels per broadcaster (in this example 021-028).

Additional datacasting services use virtual channels in the 200-799 range - in this example, the network could use the 22x, 42x and 62x ranges.

Philippines

As the Philippines started its transition to digital terrestrial television back in 2008, virtual channels have been tentatively assigned to TV networks who are now in operation. In June 2010, the National Telecommunications Commission finally adopted ISDB-T as the sole digital terrestrial television standard in the country.

LCN used in ISDB-T in the Philippines was pre-assigned to the currently operating networks in digital TV. Small-player GEM HD on DZCE-TV was the first ever Philippine TV network to go ISDB-T, being assigned to LCN 2.11 which is using the analog channel 49. Government-owned National Broadcasting Network or NBN was assigned to 1.1 using its analog channel 48 because of its status as government-owned. High definition channels are being assigned with the decimal with "11", while a multiple-SD channel uses decimal with "1, 2, 3... and so on" as its subchannel.

In the first quarter of 2011, the NTC convened to form the TWG-IRR that will draft the implementing rules and regulations on digital TV. Aside from that, it will cover the frequency planning for the upcoming TV networks that will go digital.

Digital radio

Digital radio also uses channels and subchannels in the DAB format. iBiquity's HD Radio uses HD1, HD2, ..., HD7 channels. HD1-3 are available in FM hybrid mode, while all seven HD channels are available in the pure digital mode.

IBOC system (Digital Radio Mondiale) stations do not currently use any virtual channels because of the limited bandwidth available in analog sidebands.

References

  1. ^ "ATSC Standard: Program and System Information Protocol for Terrestrial Broadcast and Cable(PSIP)". 23 December 1997. http://www.atsc.org/cms/standards/a_65-2009.pdf. Retrieved 29 November 2010. 
  2. ^ Eyer, Mark; Mark C. Whitaker (2002). PSIP: program and system information protocol; naming, numbering, and navigation for digital television. pp. 105–107. http://books.google.com/books?id=0LktvPgIGuAC. 
  3. ^ Free TV Australia (July 2005). Free TV Australia Operational Practice OP-41: Logical Channel Descriptor (Issue 3). Retrieved on 2009-09-01.
  4. ^ Association of Radio Industries and Businesses (2006). ARIB Technical Report TR-B14 version 2.8: Operational Guidelines for Digital Terrestrial Television Broadcasting (official English translation, Fascicle 3). Volume 7: Provisions for Carrier Operations. Accessed on 2009-09-01.

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