Video Security Basics

Vertical Interval Switching

With the increased use of time lapse video tape recorders, it has become important to eliminate frame loss due to video sequential switching (switching on a random basis). The significance of the loss of even a single frame may possibly have an effect on information required in utilization of the video tape recorder. Vertical interval switching in sequential switchers eliminates information loss, which greatly enhances the video tape recorders value in CCTV security applications.

Vertical interval switching is employed in all the Pelco sequential switchers in order to eliminate visual flashes, jitter, or roll on the monitor or VTR.

With vertical interval switching, the person watching the monitor may do so with ease. (Viewing a rolling picture over a period of time tends to be an annoying or fatiguing situation.)

What is Vertical Interval Switching?
Vertical interval switching is a method of switching video during that portion of the scan between video displays. The electronics inside the Pelco sequential switchers look for the vertical sync pulse of each of the input cameras and then switches the video during this vertical pulse – eliminating the vertical roll or jitter on the monitor.

For Proper Vertical Interval Switching
For roll-free switching conditions from a vertical interval sequential switcher, certain requirements must first be met. First, all the cameras should have their vertical sync pulse occurring during the same time period. To do this you must select cameras that have a line-locked circuitry. This circuitry causes the vertical sync pulse to occur at the same time in multiple camera installations, that is, when the cameras are powered by the same AC power phase. This is one of the reasons why 24 VAC power cameras are used a great deal in larger CCTV systems because power to the camera can be easily wired to the same AC power source.

But, what if cameras in a larger system get their AC power from different power phases and 24 VAC cameras cannot be used? Do you now have to live with the vertical roll on the monitor? NO. There are a few more options in the marketplace.

1. With cameras that are equipped with external sync or genlock capability you can supply external sync to all of the cameras to produce a vertical lock. However, you must use more cable and a master sync generator to do this, which could get quite costly.

2. The next option available in some cameras is adjustable phase control. This control lets the installer or dealer adjust for the differences in the vertical sync timing which occurs between different AC power phases. This feature again would give you a nice, roll-free switching action.