One of the fundamental and central problems of radio engineering and communication theory was and remains the scientific problem of finding ways to potentially reduce the frequency band occupied by the signal, as well as reducing the power of transmitting devices, which provide the necessary speed and reliability of information transfer, that is, the problem of increasing the specific bandwidth. One of the key forms of solving this problem is the transition to the reception of information messages in conditions of strong intersymbol interference. Sufficiently encouraging results in this direction are shown by the theory of resolution time presented in the papers [1-5] for APSK-N- and PSK-n- signals. At the same time, as a mathematical apparatus in these works, it is used as a mathematical apparatus in the field of optimization and analysis of transient processes. At the same time, as shown in these papers, the correct choice of the mathematical apparatus for the analysis of transient processes is of paramount importance. At present, due to the constant growth of the volume of transmitted information, more and more attention is paid to the issues of the possibility of increasing the transmission speed due to the use of the transmission mode "above the Nyquist rate". According to the theory of resolution time, which has been developing quite rapidly recently, it is required to analyze the transient processes in linear selective systems (LSS). In this case, LSS makes it possible to implement frequency selective properties of real communication channels in the channel model. At the same time, the question of choosing a method for analyzing the transient process is quite acute, since it primarily determines the complexity of estimating capacity procedure. This paper presents a retrospective, analytical review of methods for analyzing transient processes in LSS and substantiates the need to apply the method of slowly varying amplitudes S.I. Evtyanov and its developing. On its basis, the simplest and most convenient method for analyzing transient processes was chosen as applied to phase radio engineering systems for transmitting information.