A method of synchronizing the phase of a local image synchronization signal generator of a local video data processor in communication with an asynchronous switched packet network to the phase of a reference image synchronization signal generator of a reference video data processor also coupled to the network, the local and reference processors having respective clocks, the reference and local image synchronization signal generators generating periodic image synchronization signals in synchronism with the reference and local clocks respectively including: frequency synchronizing the local and reference clocks; sending an image timing packet providing reference image synchronization data indicating the difference in timing, measured with respect to the reference processor's clock, between the time at which the image timing packet is launched onto the network and the time of production of a reference image synchronization signal; and controlling the timing of the production of the local image synchronization signal.