  We theoretically propose a synthetic frequency dimension scheme to control
the spectrum of a light beam propagating through an array of evanescently
coupled waveguides modulated in time by a propagating sound wave via the
acousto-optical effect. Configurations are identified where the emerging
two-dimensional synthetic space-frequency lattice displays a non-trivial
topological band structure. The corresponding chiral edge states can be
exploited to manipulate the frequency spectrum of an incident beam in a robust
way. In contrast to previous works, our proposal is not based on discrete
high-Q cavity modes, which paves the way to the manipulation of broadband
signals.
