Introduction - Java Software Solutions: Foundations of Program Design - page 8

system. Computers have been created that use other number systems to store and
move information, but they aren't as convenient.
Some computer memory devices, such as hard drives, are magnetic in nature.
Magnetic material can be polarized easily to one extreme or the other, but intermedi-
ate levels are difficult to distinguish. Therefore, magnetic devices can be used to rep-
resent binary values quite effectively—a magnetized area represents a binary 1 and a
demagnetized area represents a binary 0. Other computer memory devices are made
up of tiny electrical circuits. These devices are easier to create and are less likely to fail
if they have to switch between only two states. We're better off reproducing millions
of these simple devices than creating fewer, more complicated ones.
Binary values and digital electronic signals go hand in hand. They improve our
ability to transmit information reliably along a wire. As we've seen, an analog signal
has continuously varying voltage with infinitely many states, but a digital signal is
discrete, which means the voltage changes dramatically between one extreme (such
as +5 volts) and the other (such as -5 volts). At any point, the voltage of a digital
signal is considered to be either “high,” which represents a binary 1, or “low,”
which represents a binary 0. Figure 1.6 compares these two types of signals.
As a signal moves down a wire, it gets weaker and degrades due to environ-
mental conditions. That is, the voltage levels of the original signal change slightly.
The trouble with an analog signal is that as it fluctuates, it loses its original infor-
mation. Since the information is directly analogous to the signal, any change in
the signal changes the information. The changes in an analog signal cannot be
recovered because the degraded signal is just as valid as the original. A digital
signal degrades just as an analog signal does, but because the digital signal is
originally at one of two extremes, it can be reinforced before any information is
lost. The voltage may change slightly from its original value, but it still can be
interpreted correctly as either high or low.
The number of bits we use in any given situation determines the number of
unique items we can represent. A single bit has two possible values, 0 and 1, and

Analog signal Digital signal

FIGURE 1.6 An analog signal vs. a digital signal