Biomedical Engineering Reference
In-Depth Information
There are some useful approximations of a signal that can be used to quantify
the signal such that the signal is represented in some mathematically precise format.
The most general representation of a signal, which is a function of time, is the abstract
symbol
x
(
t
)or
y
(
t
) . These abstract symbols can be used with linear systems because
the system characteristics are independent of the explicit forms of the signals. But also
note that the abstract representation,
x
(
t
), is
NOT
a quantitative representation, since
the abstract notation does not specify the magnitude of the signal at any instant in
time.
To specify the magnitude at any instant of time, the signal must be specified
more explicitly by a function of time defined for all instants of time. Realistically, any
quantitative representation is only an approximation. So that leaves the engineer with
the next problem, which is that of deciding how good the approximation is in order to
obtain meaningful results from its use.
3.1 HOW ARE SIGNALS CLASSIFIED
Let us begin to examine the various ways that signals are classified. In this section, only
the most used classifications are presented. Engineers should ask the following questions
(see Table 3.1).
TABLE 3.1:
Signal Classification
1.
Is the signal periodic or nonperiodic?
2.
Is the signal random or nonrandom (deterministic)?
3.
Is the signal an energy signal or a power signal?
4.
What is the bandwidth of the signal?
5.
What is the time duration of the signal?
6.
What is time-bandwidth product?
7.
What is the dimensionality of the signal?
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