 On the VHS, the drum turns a full revolution per frame, and carries two picture heads, each of which sweeps the tape surface once for every revolution.If the device were made to record progressive scanned video, the switchover of the heads would fall in the middle of the picture and appear as a horizontal band.Interlacing allows the switchovers to occur at the top and bottom of the picture, areas which in a standard TV set are invisible to the viewer.