  The dynamics of self-oscillatory extended systems, resonantly forced at a
frequency close to that of the natural oscillations (1:1 resonance), is shown
to be universally described by a complex Ginzburg-Landau equation containing an
inhomogeneous term. The case of amplitude modulated forcing is considered,
which generalizes previous studies. Application to the two-frequency forcing in
a 1:1 resonance is considered as an example.
