Abstract: A system is described for testing the video output of a computer of the type in which a video output connector is provided with a data channel, such as a DDC channel, permitting data to be transmitted from a monitor connected to the video output connector to the computer. The system comprises test apparatus having a connector for connection to the video output connector of a computer under test, the test apparatus comprising means to measure parameters of test video signals received through the connector and means to generate a result signal from the measured parameters for transmission to the computer under test over the data channel. Software executing on the computer under test to generate the test video signals and to receive and process the result signal received over the data channel.