Computers and special processors are widely used today for filtering, control, and regulation in conjunction with continuous processes. The digital elements can only influence the process at discrete points in time, while the process is continuously disturbed, and the effectiveness of the entire system must also be evaluated in continuous (real) time. For such a mixed system, both treating it as a time-invariant continuous system and transitioning to a purely time-discrete description are generally fraught with fundamental errors. The book develops an exact description of mixed systems using the parametric transfer function, which also allows for measurement acquisition as frequency response. This approach in the frequency domain is novel, aligns with the engineer's way of thinking, and yields extensive results.