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The North American broadcast television frequencies are on designated television channels numbered 2 through 69, approximately between 54 and 806 MHz. Traditionally, the frequencies are divided into two sections, the very high frequency (VHF) band and the ultra high frequency (UHF) band. The VHF band is further subdivided into two more sections, VHF-Lo (band I) and VHF-Hi (band III). In between lies the FM broadcast band (band II) used for frequency-modulated radio transmissions and a VHF radio band typically used by civil service agencies, amateur radio and aircraft (often called the airband).
On many FM radios, the audio for channel 6 can often be picked-up by turning the tuner dial below the lower FM band edge, at 87.75. The volume is low as the frequency deviation for TV audio is only ±25 kHz, versus ±75 kHz for FM broadcasting. WRGB attempted to keep this even after putting its digital signal on 6, but it caused destructive interference with the ATSC DTV signal, despite having opposite radio antenna polarizations. The lower sideband of HD Radio signals on 88.1 also overlap slightly from 87.9 to 88.0. The FM audio carrier is always 4.5 MHz above the VSB video carrier, and the total channel bandwidth is 6 MHz. The video carrier is nominally 1.25 MHz above the lower channel edge. In some cases, analog TV stations are assigned carrier frequency "offsets" of +10 or −10 kHz to minimize interference with distant stations on the same channel (see NTSC for more details). Analog stations must be separated by at least one unused channel except for non-adjacent channel pairs 4 and 5, 6 and 7, and 13 and 14. Wireless microphones and medical telemetry devices already share some of the TV bands, but transmit at a very low power. [edit] Changes and variationsChannel 1 was removed early on as a community television reservation and given to amateur radio and other uses. Channels 70 through 83 were removed from the bandplan in the 1980s to make way for AMPS mobile phone service. These channels were mainly used in the U.S., and mainly for broadcast translators, some of which continued in operation if their frequencies were not used by cellular. Channel 37 is allocated to radio astronomy and may not be used by any station. In the U.S.A., channels 52 through 69 are being reallocated for other purposes as the transition to digital television broadcasting is completed 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 instead, because frequency-use rights could be auctioned for a greater price. Channels 14 through 19 are used for two-way radio in major cities on a non-interference basis, although the transition to digital television (DTV) has caused problems in certain instances when a previously unused channel has begun to be used for DTV broadcasts. The same holds true for wireless microphones and medical telemetry devices in that band. In most regions, new digital television stations are placed on UHF (14 to 51, except 37 and sometimes 14 and 20) or high-VHF channels (7-13), although others are used in some of the more crowded media markets. 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. Subchannel 0 (e.g., 4-0) designates the analog broadcast. In the U.S., all channels from 7-36 and 38-51 are frequently used for digital TV broadcasts. However, VHF 2 - 6 are rarely used by DTV broadcasters due to ongoing problems with impulse noise, and channel 6 being an adjacent channel to the FM broadcast band. The VHF bandplan was modified several times before 1948. The last change was the transfer of channel 1, originally intended as a low-power (less than 1,000 watts) LPTV community channel, to two-way land-mobile radio and the six-meter amateur radio band. [1] Amateur television (ATV) is used on four channels in the 420-450 MHz (70-centimeter) amateur band; UHF TV channel 14 starts at 470 MHz. These ATV channels are popular for repeater output and direct communications. ATV is also used on the other amateur bands above 450 MHz. Note that some channels operate with a frequency offset of 10kHz on the analog video carrier wave, which helps avoid TV interference. Positive-offset station will therefore end in .26, while negative-offset stations will end in .24, and are usually denoted with a plus or minus sign immediately after the number (8+, 37-). While offsets are rare in digital TV, positive-offset stations end in .3380556 while non-offset stations end in .30944056 (rounded to .31).
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