the counter 13 to zero the interval stops on receipt of a signal from the timing packet generator that the count has been loaded into the packet thus the count u03b4t f at that time represents the time interval between the time of launch of the timing packet onto the network and the most recent preceding sync pulse in the example of fig1 the video data is transmitted across the network 6 as packets in conventional manner time stamp and frame timing data packets hereinafter referred to as timing packets an example of which is shown in fig3 are also produced separately from the video and also transmitted across the network 6 the receiver 4 comprises a network interface 17 corresponding to the interface 15 and an interface 18 corresponding to the interface 16 which feeds video packets to a video processor 22 and timing packets to a time packet selector 20 the selector 20 extracts the timing data from the timing packet and also supplies a sampling signal indicating the time at which the packet was received by the selector 20 the timing data and sampling signal are supplied to a frequency locked loop fll which includes and controls the local clock 30 the fll is a sample data control system details.