A LTI system is also stable if every bounded input function f(t) produces a bounded output y(t).Alternatively, a physically realizable and stable LTI system can be described by the FRF H(3). The convolution integral of (1.6) reduces to a simple multiplication when it is expressed in terms of the FRF and the Fourier transforms of the input— F(3)—and the output一Y(3)—(see also Chap. 2):