Linear, time-invariant (LTI) systems are the primary signal-processing tool for modeling the action of a physical phenomenon on a signal, such as propagation and measurement. LTI systems also are a very important tool for processing signals. For example, filters are almost always LTI systems. In this lesson you will learn the definition of a system and the important system properties of linearity, time-invariance, and causality.
There are four different common methods for relating the input of an LTI system to the output. You will also gain an introductory understanding of these methods for describing the action of systems. Different methods for describing the system behavior are used because they reveal different system attributes. Future lessons will study these four descriptions in more depth.