MODULATION/DEMODULATION— Modul-
ation is the modifying of a signal to carry intelligent
data over the communications channel. Several types
of modulation are available, depending on the system
requirement and equipment. The most frequently
used types of modulation are amplitude modulation,
frequency modulation,
and phase modulation.
Demodulation is the act of returning modulated data
signals to their original form.
AMPLITUDE MODULATION— Amplitude
modulation refers to modifying the amplitude of a
sine wave to store data.
FREQUENCY MODULATION— Frequency
modulation refers to changing the frequency of a
signal to indicate a logic 1 or a logic 0.
One
frequency indicates a logic 1, and the other frequency
indicates a logic 0.
PHASE MODULATION— Phase modulation is
more complex than amplitude modulation or
frequency modulation.
Phase modulation uses a
signal frequency sine wave and performs phase shifts
of the sine wave to store data. A modification of
phase modulation involves the use of several discrete
phase shifts to indicate the state of two or more data
bits.
M O D E M S — A modem is a device that
MOdulates and Demodulates data in a digital
communications system. Modems are available in a
variety of types, with various speeds and capabilities.
A modem consists of two functionally separate
areas—the transmitter section and the receiver
section.
The transmitter section prepares, or
modulates, the data for transmission. The receiver
section demodulates, or returns, incoming data to its
original form.
MULTIPLEXING— Multiplexing refers to
processes used in digital communications systems to
make the most efficient use of system time.
Multiplexing can involve time-sharing of the
communications channel by several users or assigning
several frequencies for the parallel transmission of
data.
1-11
