Abstract: Apparatus for combining video signals from a video source, such as a video disc player, with computer-generated graphics/text output on a single display, for overlaying the two. The computer-generated video is provided in RGB format; the other video is converted to RGB format if not already in that form and the two sets of RGB signals are provided to a switch. The switch (i.e., multiplexer) selects which one of the two RGB signal sets to display; this selection is made separately for each pixel. In one embodiment, the color of the computer-generated signals controls the switch's selection of source. A master-slave synchronization system maintains registration between the two sets of RGB signals. When the video source is unstable (as, for example, with a video disc player), a master sync generator provides a house (coarse) synchronization signal to the video disc player.