  We investigate the charge density wave transport in a quasi-one-dimensional
conductor, orthorhombic tantalum trisulfide ($o$-TaS$_3$), by applying a
radio-frequency ac voltage. We find a new ac-dc interference spectrum in the
differential conductance, which appear on both sides of the zero-bias peak. The
frequency and amplitude dependences of the new spectrum do not correspond to
those of any usual ac-dc interference spectrum (Shapiro steps). The results
suggest that CDW phase dynamics has a hidden degree of freedom. We propose a
model in which $2\pi$ phase solitons behave as liquid. The origin of the new
spectrum is that the solitons are depinned from impurity potentials assisted by
an ac field when small dc field is applied. Our results provide a new insight
as regards our understanding of an elementary process in CDW dynamics.
