Fundamentals of Signals
What is a Signal?
A signal is a quantitative description of a physical phenomenon, event or process.
Some common examples include:
1. Electrical current or voltage in a circuit.
2. Daily closing value of a share of stock last week.
3. Audio signal: continuous-time in its original form, or discrete-time when stored
on a CD.
More precisely, a signal is a function, usually of one variable in time. However, in
general, signals can be functions of more than one variable, e.g., image signals.
In this class we are interested in two types of signals:
1. Continuous-time signal x(t), where t is a real-valued variable denoting time,
i.e., t ∈ R. We use parenthesis (·) to denote a continuous-time signal.
2. Discrete-time signal x[n], where n is an integer-valued variable denoting the
discrete samples of time, i.e., n ∈ Z. We use square brackets [·] to denote a
discrete-time signal. Under the definition of a discrete-time signal, x[1.5] is not
defined, for example.