Analog communication systems, amplitude
modulation (AM) radio being a typifying example, can
inexpensively communicate a bandlimited analog signal from one
location to another (point-to-point communication) or from one
point 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 greater
flexibility.
- Efficiency: The Source Coding
Theorem allows quantification of just how complex a given
message 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 a
more efficient communication system.
- Performance: Because of the Noisy Channel
Coding Theorem, we have a specific criterion by which to
formulate error-correcting codes that can bring us as close
to error-free transmission as we might want. Even though we
may send information by way of a noisy channel, digital
schemes are capable of error-free transmission while analog
ones cannot overcome channel disturbances; see this
problem for a comparison.
- Flexibility: Digital communication systems
can transmit real-valued discrete-time signals, which could
be analog ones obtained by analog-to-digital conversion,
and symbolic-valued ones (computer
data, for example). Any signal that can be transmitted by
analog means can be sent by digital means, with the only
issue being the number of bits used in A/D conversion (how
accurately 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 digital
communication's ability to transmit a wider variety of
signals than analog systems, point-to-point digital systems
can be organized into global (and beyond as well) systems
that provide efficient and flexible information
transmission. Computer networks,
explored in the next section, are what we call such systems
today. Even analog-based networks, such as the telephone
system, employ modern computer networking ideas rather than
the purely analog systems of the past.
Consequently, with the increased speed of digital computers, the
development of increasingly efficient algorithms, and the
ability to interconnect computers to form a communications
infrastructure, digital communication is now the best choice for
many situations.
"Electrical Engineering Digital Processing Systems in Braille."