6.31 Comparison of analog and digital communication

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.

• 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.
• 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.
• 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.