0195136047.pdf

626 SIGNAL PROCESSING

consumption, accuracy, and reliability, digital communication systems are increasingly used for

transmitting information.

Foremost among signal concepts isspectral analysis(representation of signals in terms of

their frequency components), a concept that serves as a unifying thread in signal processing and

communication systems. Signals and spectral analysis are considered first in Section 14.1. Then

in Section 14.2, processing techniques such as equalization, filtering, sampling, modulation, and

multiplexing are presented, while topics on interference and noise are exposed in Section 14.3. The

circuit functions required for time-domain processing parallel those needed for frequency-domain

processing.

14.1 SIGNALS AND SPECTRAL ANALYSIS

Figure 14.1.1 shows the functional block diagram of a signal-processing system. The information

source may be a speech (voice), an image (picture), or plain text in some language. The output

of a source that generates information may be described in probabilistic terms by arandom

variable, when the random orstochasticsignal is defined by a probability density function. The

output of a source may not bedeterministic, given by a real or complex number at any instant of

time. However, in view of the scope of this text, random signals and random processes are not

discussed here.

A transducer is usually required to convert the output of a source into an electrical signal

that is suitable for transmission. Typical examples include a microphone converting an acoustic

speech or a video camera converting an image into electric signals. A similar transducer is needed

at the destination to convert the received electric signals into a form (such as voice, image, etc.)

that is suitable for the user.

The heart of any communication system consists of three basic elements:transmitter, trans-

mission mediumorchannel,andreceiver.The transmitter (input processor) converts the electric

signal into a form that is suitable for transmission through the physical channel or transmis-

sion medium. For example, in radio and TV broadcasts, since the FCC (Federal Communi-

cations Commission) specifies the frequency range for each transmitting station, the trans-

mitter must translate the information signal to be transmitted into the appropriate frequency

range that matches the frequency allocation assigned to the transmitter. This process is called

modulation, which usually involves the use of the information signal to vary systematically

the amplitude, frequency, or phase of a sinusoidal carrier. Thus, in general, carrier modula-

tion such asamplitude modulation(AM),frequency modulation(FM), orphase modulation

Information

source

Input

transducer

Input processor

(transmitter)

Output signal

at destination

Output

transducer

Output processor

(receiver)

Transmission medium

Attenuation, distortion, interference, noise (channel)

Figure 14.1.1Functional block diagram of a signal-processing system.