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Communications
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incorporating solid-state technology (which is itself a digital medium), the
use of digital signal processing has increased as well. Some familiar
examples are CDs (compact disks), on which music is stored digitally and
reconverted into analog sounds by your player, and telephone links (fiber-
optic digital telecommunications). Even radio and television stations are
beginning to digitize their signals. In this section, we will discuss how a
baseband signal (information) is digitized, what the characteristics of such
signals are, and how the resulting signal is modulated and demodulated.

Basic Binary

Digital signals are merely representations of the original information
using a simple (binary) coding system. The invention of the transistor in
the 1950s began the interest in this field. With the transistor, it was possi-
ble to represent a situation (“true” or “false,” for instance) by indicating
either “true” by allowing current flow through the device (device “on”),
or “false” with no current flow (device “off’). Since only two situations
are possible, a transistor is a binary (2-ary) device, and in binary language,
“0n” could represent a digital “1” and “off’ a digital “0.”
A single transistor can only represent two situations, or states, at any
one time (either 1 or 0 could be indicated). If two transistors are combined
simultaneously, four different states could be represented at any one time
(0 and 0,O and 1, 1 and 0, or 1 and 1, respectively, on the two transistors).
Three transistors allow eight different states to be represented, four tran-
sistors 16 states, and so on. Table 5-1 shows the possible states combina-
tions of three and four transistors. (Note: Each 1 or 0 that represents a pos-
sible state for a transistor is known as a bit.)
Notice that, in going down the table, to get from one state to the next on
the list you need only add (in binary) a “one” to the preceding value. There
is an entire branch of mathematics concerned with the manipulation of bina-
ry numbers, but for our purposes we need only the simple relationships
which relate the number of states that a certain number of bits can represent:

2” = k (5- 12)

and

n = 1.44271n (k) (5-13)