Nonlinear signal spectra transformations

Echo formation occurs in media consisting of many individual oscillators which are able to interact with the external exciting field and have slightly different frequencies. Each this oscillator is considered as a quadripole transforming the input signals. We show that the delay operation with some conversion of the signals will be carried out if each quadripole transforming, has a nonlinearity, in which the spectrum of its output is connected to the spectrum of the input signal through a nonlinear transformation.