When one video signal replaces another, the switching process causes a random interruption in the first video signal (in the midst of a frame) and a random entrance into the second video signal (also in the middle of a frame). The result is a visible jump in the edited or viewed image. This situation is amplified when the tape is copied, and the disturbance on playback is much more serious. To avoid this phenomenon, switching is performed at a very specific point during the vertical blanking retrace period which is also known as the vertical interval. This allows very smooth replacement of a whole frame by a second whole frame provided that the video sources are Genlocked to each other. VIS is used also in the presentation and graphics worlds, where sources are switched during the black period of the vertical interval, thus allowing glitch-free transitions.
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