Page 273 - Satellite Communications, Fourth Edition

Chapter
9

Analog Signals

9.1 Introduction
Analog signals are electrical replicas of the original signals such as
audio and video. Baseband signals are those signals which occupy the
lowest, or base, band of frequencies, 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 analog teleph-
ony signals may be combined into one baseband signal by the process
known as frequency-division multiplexing (FDM). Other common types
of baseband signals are the multiplexed video and audio signals which
originate in the TV studio. In forming the multiplexed baseband signals,
the information signals are modulated onto subcarriers. This modula-
tion step must be distinguished from the modulation process, which
places the multiplexed signal onto the microwave carrier for transmis-
sion to the satellite.
In this chapter, the characteristics of the more common types of analog
baseband signals are described, along with representative methods of
analog modulation.

9.2 The Telephone Channel
Natural speech, including that of female and male voices, covers a fre-
quency range of about 80 to 8000 Hz. The somewhat unnatural quality
associated with telephone speech results from the fact that a consider-
ably smaller band of frequencies is used for normal telephone trans-
mission. The range of 300 to 3400 Hz is accepted internationally as the
standard for “telephone quality” speech, and this is termed the speech
baseband. In practice, some variations occur in the basebands used by
different telephone companies. The telephone channel is often referred
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