 When film cameras, which run at 24 frames per second, film directly  off the screen of US televisions, which run at 30 fps, the result is  a dark or light bar across the image, rolling from top to bottom. To  avoid this effect in the scene when Sammy and Orlok are watching  "The Criminal Code" (1931), in most shots a film picture was matted  in over the TV screen, giving a steady picture with no bar. However,  according to Bogdanovichs commentary, they couldnt afford to use a  matte for the establishing shot for the scene, which pans across the  TVs screen, so the bar appears in that shot and only that shot (In  Europe, where the TV frame rate is 25 fps, often they simply run the  film camera at 25 fps also, for shots with a TV picture in them.)