Although many people don't know what vertical interval switching is, they see its effect every time they watch the evening news. Vertical interval switching allows for seamless switching between the various studio cameras and the studio video sources. Without vertical interval switching there would be a seam or noticeable glitch each time the studio switches to a different camera or video source.
To properly understand vertical interval switching it is first necessary to understand genlock, a technique which is often confused with vertical interval switching. Genlock is a major factor in vertical interval switching; thus vertical interval switching alone doesn't assure seamless/glitch free switching.
Genlock
Genlock is a technique that synchronizes the scanning cycles of multiple video sources. Figure 1 shows a system which uses genlock. The figure shows a synchronization signal from a time base generator being fed to all the cameras. Circuitry within each camera will lock that camera's scanning to the reference signal from the time base generator so that all the cameras will scan in sync.
While genlock can be used to synchronize scanning, it does not control when a switcher will switch. This is critical for seamless switching. If a switch is made when the scanning cycle is in process of drawing an image, there will a noticeable seam when the display switches to scanning the new image. This is because the top part of the field has one camera image while the lower part has an image from another camera.
Vertical Interval Switching
During the scanning cycle the electron beam reaches the bottom of the CRT and then goes into the vertical interval stage. This is the point where the beam is blanked (shut off) as it swings back to the upper left corner of the display (see figure 2) to start the next field. This is the best time to switch between sources since the beam is turned off and any switching won't be noticed.
Where genlock synchronizes the scanning, vertical interval switching controls when in the scanning cycle the switch takes place. This is what produces glitch free switching. Since the switcher is receiving the genlock signal (see figure 1) it can detect the start of the vertical interval, and switch at that point. Since the switch to the new source is made during the vertical blanking time, the switch won't be seen, resulting in a seamless switch.
Vertical Switching Interval Switching Without Genlock
There are certain switchers that have a built in time base corrector. These switchers use the time base corrector to re-sync the input signals. This way it is possible for the switchers to provide glitch free switching. These switchers are quite expensive and are typically found only in the broadcast industry.
Claims have been made that standard switchers can do vertical interval switching without genlock. These switchers operate by waiting until the source that is being switched to is at its vertical interval point. However, since there is no genlock, it's unlikely that both sources will be at the vertical interval point. Without both sources being at the vertical interval there is bound to be a noticeable seam or glitch, because the display will have to resynchronize to the new source signal. The only way to be assured of glitch free seamless switching is to use genlock and switch at the vertical interval.