There are three primary color transmission standards in use today:
• NTSC (National Television Systems Committee): Used in the United States, Canada, Central America, most of South America, and Japan. In addition, NTSC is used in various countries or possessions heavily influenced by the United States.
• PAL (Phase Alternation each Line): Used in England, most countries and possessions influenced by the British Commonwealth, many western European countries and China. Variation exists in PAL systems.
• SECAM (Sequential Color with [Avec] Memory): Used in France, countries and possessions influenced by France, the USSR (generally the former Soviet Bloc nations), and other areas influenced by Russia. The three standards are incompatible for a variety of reasons (see Benson and Whitaker, 1991). Television transmitters in the United States operate in three frequency bands:
• Low-band VHF (very high frequency), channels 2 through 6
• High-band VHF, channels 7 through 13
• UHF (ultra-high frequency), channels 14 through 83 (UHF channels 70 through 83 currently are assigned to mobile radio services)
Maximum power output limits are specified by the FCC for each type of service. The maximum effective radiated power (ERP) for low-band VHF is 100 kW; for high-band VHF it is 316 kW; and for UHF it is 5 MW.
The ERP of a station is a function of transmitter power output (TPO) and antenna gain. ERP is determined by multiplying these two quantities together and subtracting transmission line loss.
The second major factor that affects the coverage area of a TV station is antenna height, known in the broadcast industry as height above average terrain (HAAT). HAAT takes into consideration the effects of the geography in the vicinity of the transmitting tower.
The maximum HAAT permitted by the FCC for a low- or high-band VHF station is 1000 ft (305 m) east of the Mississippi River and 2000 ft (610 m) west of the Mississippi. UHF stations are permitted to operate with a maximum HAAT of 2000 ft (610 m) anywhere in the United States (including Alaska and Hawaii).
The ratio of visual output power to aural output power can vary from one installation to another; however, the aural is typically operated at between 10 and 20% of the visual power. This difference is the result of the reception characteristics of the two signals.
Much greater signal strength is required at the consumer’s receiver to recover the visual portion of the transmission than the aural portion. The aural power output is intended to be sufficient for good reception at the fringe of the station’s coverage area but not beyond. It is of no use for a consumer to be able to receive a TV station’s audio signal but not the video.
In addition to high power stations, two classifications of low-power TV stations have been established by the FCC to meet certain community needs: They are:
• Translator: A low-power system that rebroadcasts the signal of another station on a different channel. Translators are designed to provide “fill-in” coverage for a station that cannot reach a particular community because of the local terrain. Translators operating in the VHF band are limited to 100 W power output (ERP), and UHF translators are limited to 1 kW.
• Low-Power Television (LPTV): A service established by the FCC designed to meet the special needs of particular communities. LPTV stations operating on VHF frequencies are limited to 100 W ERP, and UHF stations are limited to 1 kW. LPTV stations originate their own programming and can be assigned by the FCC to any channel, as long as sufficient protection against interference to a full-power station is afforded.