Digital communication systems offer much more efficiency, better performance, and much greater flexibility.
Analog communication systems, amplitude
modulation (AM) radio being a typifying example, caninexpensively communicate a bandlimited analog signal from one
location to another (point-to-point communication) or from onepoint to many (broadcast). Although it is not shown here, the
coherent
receiver provides the largest possible signal-to-noise
ratio for the demodulated message.
An analysis of this receiver thus indicates
that some residual error will
always be
present in an analog system's output.
Although analog systems are less expensive in
many cases than digital ones for the same application,digital systems offer much
more efficiency, better performance, and much greaterflexibility.
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Efficiency : The Source Coding
Theorem allows quantification of just how complex a givenmessage source is and allows us to exploit that complexity
by source coding (compression). In analog communication,the only parameters of interest are message bandwidth and
amplitude. We cannot exploit signal structure to achieve amore efficient communication system.
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Performance : Because of the Noisy Channel
Coding Theorem, we have a specific criterion by which toformulate error-correcting codes that can bring us as close
to error-free transmission as we might want. Even though wemay send information by way of a noisy channel, digital
schemes are capable of error-free transmission while analogones cannot overcome channel disturbances; see
this
problem for a comparison.
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Flexibility : Digital communication systems
can transmit real-valued discrete-time signals, which couldbe analog ones obtained by analog-to-digital conversion,
and symbolic-valued ones (computer
data, for example). Any signal that can be transmitted byanalog means can be sent by digital means, with the only
issue being the number of bits used in A/D conversion (howaccurately do we need to represent signal amplitude).
Images can be sent by analog means (commercial television),but better communication performance occurs when we use
digital systems (HDTV). In addition to digitalcommunication's ability to transmit a wider variety of
signals than analog systems, point-to-point digital systemscan be organized into global (and beyond as well) systems
that provide efficient and flexible informationtransmission.
Computer networks ,
explored in the next section, are what we call such systemstoday. Even analog-based networks, such as the telephone
system, employ modern computer networking ideas rather thanthe purely analog systems of the past.
Consequently, with the increased speed of digital computers, thedevelopment of increasingly efficient algorithms, and the
ability to interconnect computers to form a communicationsinfrastructure, digital communication is now the best choice for
many situations.
