A) That part of the video signal where the voltage level is at 0 IRE and the electron beam sweeps back from the bottom to the top of the screen. b) A period during which the electron beam in a display is blanked out while it travels from the bottom of the screen to the top. It is the black bar that becomes visible when the vertical hold on a television set is not correctly adjusted. The VBI is usually measured in scanning lines. When the VBI is subtracted from the total number of scanning lines, the result is the number of active scanning lines. In NTSC, the VBI has a duration of 20.5 or 21 lines (depending on the field), of which nine lines are devoted to the vertical synchronising signal that lets television sets know when a field has been completed. The remaining lines have long been used to carry auxiliary information, such as test and reference signals, time code, and encoded text, such as captions for the hearing impaired. Some ATV schemes propose expanding the VBI to accommodate widescreen images by the letterbox technique; some propose using it as a sub-channel for additional picture information. See also Blanking.