Vertical blanking interval
Vertical Blanking Interval
The vertical blanking interval (VBI) is a portion of the television signal that occurs between the end of one frame and the start of the next. During this interval, the electron beam in a cathode ray tube (CRT) television returns from the bottom of the screen to the top to begin drawing the next frame. This period is used to prevent the beam from drawing on the screen while it is moving back to the top.
Technical Details
The VBI is a critical component of analog television systems, such as NTSC, PAL, and SECAM. It typically lasts for several lines of video, during which no image is displayed. In NTSC, for example, the VBI consists of 21 lines, while in PAL, it consists of 25 lines.
During the VBI, additional data can be transmitted without interfering with the visible image. This data can include closed captions, teletext, and other information. The VBI is also used for vertical synchronization, ensuring that the display device starts drawing each new frame at the correct time.
Historical Use
In the early days of television, the VBI was simply a necessary part of the signal to allow for the retrace of the electron beam. However, as technology advanced, engineers found ways to utilize this otherwise unused portion of the signal to transmit additional data. This led to the development of services like closed captioning and teletext, which provided viewers with additional information and accessibility options.
Modern Applications
With the advent of digital television, the traditional VBI is no longer necessary, as digital signals do not rely on electron beam scanning. However, the concept of a blanking interval still exists in digital video, where it is used to manage the timing of data transmission and synchronization.
Related Pages
References
- "Vertical Blanking Interval." Wikipedia, The Free Encyclopedia. https://en.wikipedia.org/wiki/Vertical_blanking_interval
- "Television Technology." Encyclopedia of Television. https://www.encyclopedia.com/television-technology
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