A system for combining two or more video signal inputs using a chrome key like system. The keying is not done to one color, such as saturated blue, but to any color within a range. One of the video signal inputs (1) is frozen and appears as a still picture on a display (6). Graphic means (16) are used to define an area of the picture and the maximum and minimum values of the color components of the signals in that area are detected. Whenever a signal with components falling within the detected range is present in a switch (14) it operates to select a channel for output. This works to build up a composite picture.