The line of push button switches on the control panel which select and indicate the preview output, and represents the next video that will appear when the DSK fader is pulled. It can also select and indicate key sources to other keyers due to the fact that it is a “split” bus. That is, reentries can be selected for the next video as well as bus inputs for a key source, both at the same time. This type of bus is exclusive to 4100 series switchers.