  We study transport properties of an inertial Brownian particle moving in
viscous symmetric periodic structures and driven by an oscillating signal of
two harmonic components. We analyze the influence of symmetric, antisymmetric
and asymmetric signals on directed transport and reveal the shift symmetry of
the stationary averaged velocity of the Brownian particle with respect to the
relative phase of two components of the signal. The shift symmetry holds true
in all regimes.
