Sampling Theory and Analog-to-Digital Conversion - Signal Processing of Random Physiological Signals - page 37

CHAPTER 6

Sampling Theory and
Analog-to-Digital
Conversion

6.1 BASIC CONCEPTS
Use of a digital computer for processing an analog signal implies that the signal must be
adequately represented in digital format (a sequence of numbers). It is therefore necessary
to convert the analog signal into a suitable digital format.
By definition, an analog signal is a continuous physical phenomenon or indepen-
dent variable that varies with time. Transducers convert physical phenomena to electrical
voltages or currents that follow the variations (changes) of the physical phenomena with
time. One might say that an electrical signal is an Analog Signal if it is a continuous
voltage or current that represents of a physical variable. Analog signals are represented
as a deterministic signal, x ( t ), for use in a differential equation. Or, one may simply state
that an analog signal is any signal that is not a digital signal, but then you must define a
digital signal. There are three classes of analog signals.

a) Continuous in both time and amplitude (Fig. 6.1).

b) Continuous in time (Fig. 6.2) but taking on only discrete amplitude values, that

is, analog signals used to approximate digital bit streams in serial communication,
which are used primarily for time codes or markers.