Page 303 - Satellite Communications, Fourth Edition

Chapter
10

Digital Signals

10.1 Introduction
As already mentioned in connection with analog signals, baseband sig-
nals are those signals which occupy the lowest, or base, frequency band
in the frequency spectrum used by the telecommunications network. A
baseband signal may consist of one or more information signals.
For example, a number of telephony signals in digital form may be
combined into one baseband signal by the process known as time-
division multiplexing.
Analog signals may be converted into digital signals for transmis-
sion. Digital signals also originate in the form of computer and other
data. In general, a digital signal is a coded version of the original data
or analog signal. In this chapter, the characteristics of the more common
types of digital baseband signals are described, along with representative
methods of digital modulation.

10.2 Digital Baseband Signals
Digital signals are coded representations of information. Keyboard
characters, for example, are usually encoded in binary digital code. A
binary code has two symbols, usually denoted as 0 and 1, and these
are combined to form binary words to represent the characters. For
example, a teleprinter code may use the combination 11000 to repre-
sent the letter A.
Analog signals such as speech and video may be converted to a digi-
tal form through an analog-to-digital (A/D) converter. A particular form
of A/D conversion is employed, known as pulse-code modulation, which
will be described in detail later. Some of these sources are illustrated
diagrammatically in Fig. 10.1.
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