This "Opto" box allows you to hook up a VHS & DVD to the same preview booth. Basically the 2 players are stacked on top of each other, the audio and video wires from both players are "Y"ed together. Then scan and play wires plug into the Opto box. Basically it isolates the trigger voltages from each player, so you don't "smoke" the outputs to your scan and play functions on the players.