The vertical blanking interval (VBI) is the transition between the visible picture in one video frame or field and the visible picture in the next. In analog television, it allowed the scan to return to the top of the image and provided time for signals and data to be carried outside the normal displayed picture.
What does “vertical blanking interval” mean?
Video displayed as a raster is drawn line by line. The VBI is the time between the last line of one frame or field and the beginning of the first line of the next. It is also called the vertical interval or VBLANK. The International Telecommunication Union (ITU) defines it in those terms in its television accessibility glossary.
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“Blanking” does not mean that the interval necessarily contains no signal. It means the interval is not intended to appear as ordinary visible picture. Synchronization signals and data could occupy it.
Why did analog television have a VBI?
In analog television, the picture was scanned from top to bottom. After the last visible line, the signal needed a transition period before the next frame or field began. The VBI accommodated that return and offered a place to carry information without putting it into the normal picture area.
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Depending on the format and service, the interval could carry closed captions, teletext, time codes, test signals, copy-control indicators, or other data. These are possible uses, not a list of services present in every broadcast. The ITU glossary describes these examples.
How many lines are in the VBI?
There is no single line count that applies to every video format. For NTSC, the Advanced Television Systems Committee (ATSC) glossary describes the first 21 horizontal lines in each field as VBI lines not intended for direct display by a receiver. That figure is specific to the NTSC description; it should not be treated as a universal count for all television systems.
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Is NTSC caption line 21 part of the VBI?
It depends on the convention being used. CEA-608 closed-caption data is commonly associated with NTSC line 21, but that line sits at the boundary between the VBI and active picture. The Federal Agencies Digitization Guidelines Initiative glossary notes that some authorities classify it as part of the VBI and others as the first active-picture line. On underscan displays and broadcast measurement equipment, caption data can appear above the visible picture as lines or dashes.
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Does digital video have a VBI?
Digital video does not have the analog VBI as an unchanged interval in its picture timing. ATSC’s glossary makes that distinction. However, digital transport standards can carry legacy VBI information so that compatible equipment can process it or insert it into an output waveform.
For example, ANSI/SCTE 127 2019 (reaffirmed 2024) specifies carriage of analog VBI information in compressed digital television bitstreams for output waveform insertion or direct receiver handling. Its scope excludes CEA-608 Line 21 and CEA-708 caption carriage. Separately, ANSI/SCTE 20 2020 specifies carriage for CEA-608 captions and certain other VBI services in MPEG-2-compliant streams. These are examples of transporting information associated with legacy services, not evidence that digital video itself contains the same analog blanking interval.
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