  Oscillatory behavior is ubiquitous in out-of-equilibrium systems showing
spatio-temporal pattern formation. Starting from a linear large-scale
oscillatory instability -- a conserved-Hopf instability -- that naturally
occurs in many active systems with two conservation laws, we derive a
corresponding amplitude equation. It belongs to a hierarchy of such universal
equations for the eight types of instabilities in homogeneous isotropic systems
resulting from the combination of three features: large-scale vs.\ small-scale
instability, stationary vs.\ oscillatory instability, and instability without
and with conservation law(s). The derived universal equation generalizes a
phenomenological model of considerable recent interest, namely, the
nonreciprocal Cahn-Hilliard model, and may be of a similar relevance for the
classification of pattern forming systems as the complex Ginzburg-Landau
equation.
