Virtual channel

For the SDRAM technology, see VC-RAM. For the computer networking term, see virtual circuit.

In telecommunications, a logical channel number (LCN), also known as virtual channel in North America, is a channel designation which differs from the actual radio channel or frequency on which the signal travels.

The term is most often applied to television, where digital television (DTV) channels are in-band adjacent to analog ones. A station branded as Channel 8, for example, might actually use channel 32 for its ATSC or DVB transmission, but a virtual channel map or virtual channel table (VCT) allows viewers to tune in the station on channel 8 on a digital set. DTV tuners typically scan all channels for this information while they are turned "off", and store 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 than 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.

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 99, with additional data channels ranging from 100 to 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.

Australia

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

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 20-23 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'. 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 is currently used only in Sydney, by the Digital Forty Four trial datacasting service.
• 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, as two related channels 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. 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.

Digital radio

Digital radio also uses channels and subchannels, but only in the DAB format. IBOC systems (iBiquity and Digital Radio Mondiale) do not currently use any virtual channels because of the limited bandwidth available in analog sidebands. As IBOC progresses and bandwidth improves due to new technology and the discontinuation of stereo subcarriers (and later analog altogether), virtual channels may take hold there as well.

External links

• Logical Channel Number technical specifications