**Block Diagrams / Lohkokaavio**

A causal LTI system can be written with a differential equation

There are three basic functions in LTI systems.

FIR (finite impulse response) systems have an impulse response *h*[*n*]of finite length. For example
is zero when *n*<0 or *n*>1. FIRs are therefore always BIBO-stable.
Transfer function *H*(*z*) consists only of numerator polynomial.

IIR (infinite impulse response) systems have an impulse response *h*[*n*]of infinite length. For example
*h*[*n*] = 0.5^{n} *u*[*n*]is nonzero when .
IIRs contain feedback loops.
Transfer function *H*(*z*) consists of numerator and denominator
polynomials

Stability can be derived from positions of poles (roots of denominator). If poles are strictly inside unit circle, then system is stable.