vertical blanking interval [VBI]

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(n.) In a video signal, the period during which picture information is suppressed and in which the scanning beam returns from the bottom of the picture to the top. Technically, the VBI consists of lines 1 through 21 of video field 1 and lines 263 through 284 of field 2 in the NTSC format. Frame numbers, flags, and closed captions are placed in this interval to maintain image stability and to provide access to a frame. A cable modem may use this space to send the equivalent of a 9600-bps signal during blanking interval. See also horizontal blanking interval.