- 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
*t*_{n}=*n T*, where*n*is an integer and*T*is the sampling interval or period. - It is common to drop the explicit reference to
*T*(or assume*T*= 1) and index discrete-time signals by the letter n. - A discrete-time signal that is only dependent on time can be represented by
*x*[*n*] for*n*= 0, 1, 2, ... - 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
*n*< 0.

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