A computer based video/graphics display system includes a computer system (22) having a graphics generator (24). A video signal from a video source (28) is fed through an input stage (26) to an asynchronous converter (30). The converter (30) synchronises the video signal to the graphics generator (24). The output from the asynchronous converter (30) and the output from the graphics generator (24) are fed to a fading/mixing matrix (34). The combined signal output from the fading/mixing matrix is fed to a computer display monitor (14). The system allows the mixing or windowing of computer graphics and a video image on a common display.