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. (For stable sources, this is unnecessary) The slave synchronization generator locks the video switch, display and computer video generator to the timing of the video image source (such as video disc player). Thus, the rest of the system tracks the jitter of the video source. When the video disc player is scanning or is being spun up or down, the slave sync generator locks onto the house sync signal of the master sync generator, instead of the video disk player's output, to avoid rolling and tearing of the display.