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.
No comments:
Post a Comment