The screen of a circuit television system shows an image of whatever is "caught" in range of the camera. In modern systems, image processing software continuously compares the current image to the previous one. When a change is detected, the image is stored on a memory device. This uses less computer memory than older systems which stored 24 full hours of images.
If a change in the image is detected, it usually leads to a change on the observer's screen, such as a shift to a full-screen image. Sometimes a buzzer also sounds to alert the guard.
But what happens on Shabbat? The Zomet Institute has experience in helping public institutions and private individuals modify their equipment to meet halachic requirements. The continuous formation of an image or the action of the image processing system do not present any problem, since this is defined from the halachic point of view as simply changing the level of the current, which is not prohibited.
Other aspects of this type of system must be studied in detail, and as noted above Zomet has developed an expertise in solving the halachic problems involved.
The halachic basis for using closed-circuit television under these circumstances on Shabbat can be found in the section on halachic-technical instructions for closed-circuit tv.