14

Signal Processing

14.1 Signals and Spectral Analysis

14.2 Modulation, Sampling, and Multiplexing

14.3 Interference and Noise

14.4 Learning Objectives

14.5 Practical Application: A Case Study—Antinoise Systems—Noise Cancellation

Problems

The essential feature of communication, control, computation, and instrumentation systems is

the processing ofinformation. Because of the relative ease and flexibility of processing and

transmitting electrical quantities, usually the information obtained from a nonelectrical source is

converted into electrical form. An electricsignalis a voltage or current waveform whose time or

frequency variations correspond to the desired information. The information-bearing signals are

processed either for purposes of measurement in an instrumentation system, or for transmitting

over long distance in a communication system. All such systems, regardless of their particular

details, share certain basic concepts and common problems.

Continuous signals(shown in Figure 6.0.1) are described by time functions which are defined

for all values oft(a continuous variable). Commercial broadcast systems, analog computers,

and various control and instrumentation systems process continuous signals. The information

processed inanalog systemsis contained in the time function which defines the signal. Analog

systems are often thought of as performing signal processing in thefrequency domain.

Discrete signals(shown in Figure 6.0.2), on the other hand, exist only at specific instances

of time, and as such, their functional description is valid only for discrete-time intervals. Dis-

crete signals are invariably a sequence of pulses in which the information is contained in the

pulse characteristics and the relation amidst the pulses in the sequence during a specified time

interval. Digital computers, pulsed-communication systems (modern telephone and radar), and

microprocessor-based control systems utilize discrete signals.Digital systemsprocess digits, i.e.,

pulse trains,in which the information is carried in the pulse sequence rather than the amplitude–

time characterization of the pulses. Digital systems are often thought of as performing signal

processing in thetime domain.Because of the advantages of economy in time, low power

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