  We derive a general expression that quantifies the total entanglement
production rate in continuous variable systems, where a source emits two
entangled Gaussian beams with arbitrary correlators.This expression is
especially useful for situations where the source emits an arbitrary frequency
spectrum,e.g. when cavities are involved. To exemplify its meaning and
potential, we apply it to a four-mode optomechanical setup that enables the
simultaneous up- and down-conversion of photons from a drive laser into
entangled photon pairs. This setup is efficient in that both the drive and the
optomechanical up- and down-conversion can be fully resonant.
