Chapter 1: Introduction: Analog
vs. Digital
We often hear that we live in a digital age. This refers to the massive adoption of computer systems
within every aspect of our lives from smart phones to automobiles to household appliances. This
statement also refers to the transformation that has occurred to our telecommunications infrastructure
that now transmits voice, video, and data using 1’s and 0’s. There are a variety of reasons that digital
systems have become so prevalent in our lives. In order to understand these reasons, it is good to start
with an understanding of what a digital system is and how it compares to its counterpart, the analog
system. The goal of this chapter is to provide an understanding of the basic principles of analog and
digital systems.
Learning Outcomes—After completing this chapter, you will be able to:
1.1 Describe the fundamental differences between analog and digital systems.
1.2 Describe the advantages of digital systems compared to analog systems.
1.1 Differences Between Analog and Digital Systems
Let’s begin by looking at signaling. In electrical systems, signals represent information that is
transmitted between devices using an electrical quantity (voltage or current). An analog signal is defined
as a continuous, time-varying quantity that corresponds directly to the information it represents. An
example of this would be a barometric pressure sensor that outputs an electrical voltage corresponding
to the pressure being measured. As the pressure goes up, so does the voltage. While the range of the
input (pressure) and output (voltage) will have different spans, there is a direct mapping between the
pressure and voltage. Another example would be sound striking a traditional analog microphone. Sound
is a pressure wave that travels through a medium such as air. As the pressure wave strikes the
diaphragm in the microphone, the diaphragm moves back and forth. Through the process of inductive
coupling, this movement is converted to an electric current. The characteristics of the current signal
produced (e.g., frequency and magnitude) correspond directly to the characteristics of the incoming
sound wave. The current can travel down a wire and go through another system that works in the
opposite manner by inductively coupling the current onto another diaphragm, which in turn moves back
and forth forming a pressure wave and thus sound (i.e., a speaker). In both of these examples, the
electrical signal represents the actual information that is being transmitted and is considered analog.
Analog signals can be represented mathematically as a function with respect to time.
In digital signaling the electrical signal itself is not directly the information it represents; instead, the
information is encoded. The most common type of encoding is binary (1’s and 0’s). The 1’s and 0’s are
represented by the electrical signal. The simplest form of digital signaling is to define a threshold voltage
directly in the middle of the range of the electrical signal. If the signal is above this threshold, the signal is
representing a 1. If the signal is below this threshold, the signal is representing a 0. This type of signaling
is not considered continuous as in analog signaling; instead, it is considered to be discrete because the
information is transmitted as a series of distinct values. The signal transitions between a 1 to 0 or 0 to
1 are assumed to occur instantaneously. While this is obviously impossible, for the purposes of
information transmission, the values can be interpreted as a series of discrete values. This is a digital
signal and is not the actual information, but rather the binary encoded representation of the original
information. Digital signals are not represented using traditional mathematical functions; instead, the
digital values are typically held in tables of 1’s and 0’s.
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Springer International Publishing Switzerland 2017
B.J. LaMeres, Introduction to Logic Circuits & Logic Design with VHDL,
DOI 10.1007/978-3-319-34195-8_1
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