Security cameras often store video in a multiplexed video format. Instead of an individual video recorder for each camera, the security system rotates through the cameras and records an individual image from each camera to a single video recorder. The reordering process goes something like this (keep in mind there are 30 frames per second available on NTSC video):
Camera 1/Frame 1 - Camera 2/Frame 2 - Camera 3/Frame 3
Once the rotation is complete, the recording goes back to Camera 1 and starts the whole process over again. dPlex, from Ocean Systems, is a universal video demultiplexing tool that allows you to isolate individual cameras from multiplexed video. The results are amazing.
