Abstract: In order to synchronise video and audio signals, a video test signal and an audio test signal are generated at the transmitting end of a transmission link, and transmitted over the link. The video test signal has first and second active picture periods of contrasting states. The audio test signal has first and second periods of contrasting states. As generated, the video and audio test signals have a predetermined timing relationship—for example, their changes of respective states may be coincident in time. At the receiving end of the link, the video and audio test signals as received are detected, and any difference of timing between the video and audio test signals is derived from their changes of respective states, measured and displayed, including an indication of whether the video signal arrived before the audio signal or vice-versa.