FIGURE 1 

(A) A linear time-invariant (LTI) system takes an input signal and converts it to an output signal. The system is characterized by a response function. (B) Input signal, output signal and response function can be written as a function of time. (C) An LTI system is linear, that is, an output to a set of inputs can be formulated as the sum of outputs to each input. (D) An LTI system is time-invariant, that is, the output that an input generates is independent of the time that the input is applied. (E) Input–output relation is mediated through the response function. However, it is harder to decode response function from this relation. (F) All the functions can be transformed to the frequency domain using Fourier transforms. (G) In the frequency domain, input–output relation becomes a multiplication rule, which makes it easier to decode the response function.