This paper introduces signal-controlled time-series modeling based on arbitrarily chosen buliding blocks. Such modeling is used in the design of a nested-form transver-sal structure based on Almost-Symmetrical ARMA (AS-ARMA) building blocks. This structure can operate in the transient mode, in contrast to the commonly used linear line-enhancers based on an conventional ARMA, leading to practically sound processing of short-duration signals. It is shown that the proposed time-series modeling can be effectively applied towards the separation of superimposed signals of heavily overlapping spectra.