Signals typically represent physical variables such as displacement, velocity, pressure, energy, ...
In most cases, we are concerned with variables that are time-dependent.
The discrete-time or digital time index is generally specified by , where is an integer and is the sampling interval or period.
It is common to drop the explicit reference to (or assume ) and index discrete-time signals by the letter n.
A discrete-time signal that is only dependent on time can be represented by for
We typically assume our systems are causal or do not depend on future inputs (they should have zero “activity” before a non-zero input is applied). This can also be stated as
for .