Difference between revisions of "North American Television Frequency Allocation"

Revision as of 19:19, 30 January 2012

With both digital and analog TV, there are two television bands on the broadcast frequency spectrum: the VHF (very high frequency) band and UHF (ultra high frequency) band. VHF radio frequency channels run from 2 to 13, occupying most of the 54 to 216 megahertz range. During analog TV broadcasting, UHF radio frequency channels ran from 14 to 69, occupying the 470 to 806 megahertz range.

In the television radio spectrum, each channel spans a range of six megahertz frequencies per channel. For example, radio frequency (RF) channel number 3 is assigned to the 60 to 66 megahertz frequency range. RF channel 4 goes from 66 to 72 megahertz.

A television broadcast station's "radio frequency channel" is not the same thing as the "virtual channel" you tune into on your television set when you watch TV. 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. With DTV, a TV station could be assigned to RF channel 19, for instance, but you might know it as channels 2-1 and 2-2 on your television set. DTV channels operate on different physical channels from the displayed channels. A virtual channel map or virtual channel table (VCT) allows viewers to tune in the station on channel 2, displayed as 2.1 on a digital set. Because DTV can carry multiple programs simultaneously, virtual channels also map out subchannels.

With the digital conversion, the channel system has not changed. Now that the transition to digital TV is complete, the UHF television band has been reduced. The UHF band now runs from channels 14 to 51 (470 to 698 megahertz). The 52 to 69 RF channels have been reassigned for other broadcast uses.

The reallocated channels above 51 are now used for low-power TV stations. All full-power analog broadcasts ended in June 2009, and all temporary full-power digital broadcasts moved back down to the lower channels. While most other countries abandoned VHF due to its poor suitability for TV broadcasting and allocation for Digital Audio Broadcasting, the FCC chose to cut high-UHF channels.

New digital television stations are placed on UHF (14 to 51, except 37 and sometimes 14 and 20). In some areas digital television is broadcast on VHF frequencies channels (7-13). All UHF channels from 7-36 and 38-51 are frequently used for digital TV broadcasts and in some areas VHF is still being used but that is proving to be problematic.

With virtual channel numbering, many digital televisions group digital channels with their corresponding analog broadcasts. For example, the first digital TV stream of a station that broadcasts analog TV on channel 4 will usually appear as 4-1 or 4.1 on a DTV receiver, even though the digital transmissions may be on channel 38. Several digital subchannels can be multiplexed together, so 4-1 through 4-5 might be used by one station, while 4-0 is reserved to designate the analog broadcast.

Assignment of Virtual Channels

• 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

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.