686 COMMUNICATION SYSTEMS
15 kHz. Distortion at most stations is below 1%. Besides broadcast applications, FM is also used
in satellite links, aircraft altimetry, radars, amateur radio, and various two-way radio applications.
Because of the larger bandwidth (up to 20 times more than in AM), an FM system has more
freedom from interference and better performance in noise than any AM station.
Commercial television broadcasting is allocated frequencies that fall in the VHF and UHF
bands. Table 15.2.1 lists the television channel allocations in the United States, with the channel
bandwidth of 6 MHz. In contrast to radio broadcasting, television signal-transmission standards
vary from country to country. The U.S. standard is set by the National Television Systems Com-
mittee (NTSC). Commercial television broadcasting began as black-and-white (monochrome)
picture transmission in London in 1936 by the British Broadcasting Corporation (BBC). Although
color television was demonstrated a few years later, due to the high cost of color television
receivers, color television signal transmission was slow in its development. With the advent of
the transistor and microelectronic components, the cost of color television receivers decreased
significantly, and by the middle 1960s, color television broadcasting was widely used by the
industry. The NTSC color system is compatible with monochrome receivers, so that the older
monochrome receivers still function receiving black-and-white images out of the transmitted
color signal.Amplitude Modulation (AM)
In AM the message signal is impressed on the amplitude of the carrier signal. There are several
different ways of amplitude modulating the carrier signal by the message signal, each of which
results in different spectral characteristics for the transmitted signal. Four methods are:- Conventional (standard) double-sideband AM
- Double-sideband suppressed-carrier AM
- Single-sideband AM
- Vestigial-sideband AM.
AM, as its name implies, carries the modulating signalx(t)inatime-varying amplitudeof
the form
a(t)=Ac[1+mAx(t)] (15.2.1)
where the constantAcstands for the unmodulated carrier amplitude, andmAis themodulation
index.The resulting modulated wave is
xc(t)=a(t)cos( 2 πfct+φc)
=Accos( 2 πfct+φc)+AcmAx(t)cos( 2 πfct+φc) (15.2.2)TABLE 15.2.1VHF and UHF Allocations for
Commercial TV in the United States
Frequency Band
Channel (6-MHz bandwidth/station)VHF 2–4 54–72 MHz
VHF 5–6 76–88 MHz
VHF 7–13 174–216 MHz
UHF 14–69 470–896 MHz