ANALOG DATA,ANALOG SIGNALS
Modulation has been defined as the process of combining an input signal met) and a
carrier at frequency fe to produce a signal set) whose bandwidth is (usually) centered
on fe. For digital data, the motivation for modulation should be clear: When only
analog transmission facilities are available, modulation is required to convert the
digital data to analog form. The motivation when the data are already analog is less
clear. After all, voice signals are transmitted over telephone lines a their original
spectrum (referred to as baseband transmission). There are two principal reasons for
analog modulation of analog signals:
• A higher frequency may be needed for effective transmission. For unguided
transmission, it is virtually impossible to transmit baseband signals; the
required antennas would be many kilometers in diameter.
• Modulation permits frequency division multiplexing, an important technique
that was discussed in Chapter 2.
In this section we look at the principal techniques for modulation using analog
data: amplitude modulation (AM), frequency modulation (FM), and phase modulation
(PM). As before, the three basic characteristics of a signal are used for modulation.
Modulation has been defined as the process of combining an input signal met) and a
carrier at frequency fe to produce a signal set) whose bandwidth is (usually) centered
on fe. For digital data, the motivation for modulation should be clear: When only
analog transmission facilities are available, modulation is required to convert the
digital data to analog form. The motivation when the data are already analog is less
clear. After all, voice signals are transmitted over telephone lines a their original
spectrum (referred to as baseband transmission). There are two principal reasons for
analog modulation of analog signals:
• A higher frequency may be needed for effective transmission. For unguided
transmission, it is virtually impossible to transmit baseband signals; the
required antennas would be many kilometers in diameter.
• Modulation permits frequency division multiplexing, an important technique
that was discussed in Chapter 2.
In this section we look at the principal techniques for modulation using analog
data: amplitude modulation (AM), frequency modulation (FM), and phase modulation
(PM). As before, the three basic characteristics of a signal are used for modulation.
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