Understanding the individual components of a closed circuit television system is key to maintaining a successful video monitoring security plan.
In Lesson 3, last month, we explained some of the essential components used in a Closed-Circuit Television (CCTV) system, including communication media, switchers and synchronizers, monitors, video cassette recorders, and video motion detectors. In this lesson, we'll continue our discussion of CCTV systems by taking a closer look at how cameras, lenses, and mountings operate as well as how to apply them to a building's overall security system. We'll also go over Code requirements and the downside of video monitoring.
Cameras. The first thing you should remember is cameras do not "see" things the same way the human eye sees. Our eyes adjusts to various focal lengths and light conditions automatically (usually faster than you can notice). Video cameras can't. They remain limited to fixed fields of view, focal lengths, and light sensitivities. When designing a system, it's best to go to the job site with a camera in hand, to verify it picks up the desired images and fields of view you desire in the design.
Cameras come in several varieties. However, we will restrict our coverage to CCD cameras -- by far the most commonly used type. CCD stands for Charged-Coupled Device, which is simply an arrangement of special semiconductors. Why are these solid-state cameras the most popular on the market today? Their prices have dropped dramatically in the past few years, they have excellent operating characteristics, and they exhibit no geometric distortion, lag, or image retention. In fact, they are so good, TV studios even use them.
An array of photodiodes (solid-state photosensitive elements, each serving as one pixel) arranged in a grid (rows and columns) performs the sensing function of the CCD. You mount this grid on a silicon substrate. The individual photosensitive elements store up electrical charges proportional to the amount of light that strikes them. If a large amount of light strikes an element between discharges, it accumulates a high charge; if it receives little or no light between discharges, the electrical charge in the element will be small. The CCD generates a television signal by discharging the individual elements in the proper order (the same order as the tube scanning process). You can handle the discharging process either directly, or through shift registers. However, the final result is the same -- the conversion of photoelectric charges into a television signal.
Lenses. Video lenses come in half-in., 2/3-in., and 1-in. sizes to match video cameras. However, it's possible to use a lens larger than the format of the camera. In other words, you can use a 1-in. lens on a 2/3-in. camera. You cannot, however, use a 2/3-in. lens on a 1-in. camera.
The most important factor in choosing lenses is you must match them to the area you want to monitor. You must coordinate the proper focal length (the distance at which the camera properly focuses) and field of view to get the desired results.
We use two formulas to calculate these values. (These formulas are for 2/3-in. formats.)
For calculating the required lens size (measured in millimeters) for monitoring a certain width, you use the equation: (8.8 x distance)/width = focal length
For calculating the required lens size for an area of a certain height, you use the equation: (6.6 x distance)/height = focal length
Keep in mind lenses are not available in all sizes, and you'll almost always need to round the actual size up or down.
We can change these two formulas around to give us the field of view for any size lens. The formula for the field of view is: (8.8 x distance)/focal length = field width
The formula for the height of the field of view is: (6.6 x distance)/focal length = field height
For half-in. formats, the formula uses 5.9 instead of 8.8 for the width constant, and 4.4 instead of 6.6 for the height constants.
All lenses should come with an automatic iris, except those on vidicon cameras (which you use in indoor locations with a more or less constant light level). The iris controls the amount of light entering the camera. This keeps the light level reaching the sensor within acceptable limits.
Mountings. You need to mount surveillance cameras firmly. In addition to standard wall-mounting brackets, you may also mount cameras in vandal-resistant cases for trouble-prone areas.
Mountings are also available with built-in panning and tilting mechanisms. Obviously, these will cost a bit more, but will provide an additional benefit. However, you can't use them in all installations, particularly with video motion detector devices.
Surge suppressors. While not required by the National Electrical Code (NEC), surge suppressors are a practical necessity for virtually all outdoor coaxial cable runs. Since the pieces of equipment connected to these cables are very sensitive, voltage spikes can easily damage them. The most commonly used type of surge suppressors for communications circuits is metal oxide varistors (MOVs). They are usually called protectors in the communications industry. These devices are made of sintered zinc oxide particles pressed into a wafer and equipped with connecting leads or terminals.
This device has a more gradual clamping action than either spark-gap arresters or gas tubes. As the surge voltages increase, these devices conduct more heavily and provide clamping action. And unlike spark-gap arresters and gas tubes, these devices absorb energy during surge conditions. They also tend to wear out over time.
Code requirements. Art. 820 of the NEC covers cable television and security monitoring circuits. As you'll notice, Art. 820 has a broad title: "Community Antenna Television." It covers all radio frequency signals sent through coaxial cables.
While the NEC does not define the term radio frequency (often abbreviated as RF), it would generally include every frequency from several kilohertz to hundreds of megahertz. This would include all types of radio signals, television signals, and computer network signals. Art. 820 does not, however, cover television cabling that is not coaxial (see fine print note to 820-1); it applies to coaxial cable only.
The primary safety requirements of Art. 820 are that the voltage applied to coaxial cables cannot exceed 60V, and the power source must be energy-limited. (Sec. 725-31 defines energy limitation.)
The downside of video monitoring. From a technical standpoint, video monitoring offers immense benefits. But our coverage of video surveillance would not be complete unless we address the human difficulties associated with its use.
Imagine sitting at your desk working with a video camera pointing at you all day. It observes your every motion, every action. You probably wouldn't like that feeling. It is intimidating and intrusive. It makes many of us feel vulnerable and mistrusted.
As an owner of a surveillance system, there are several things you can do to minimize the human costs of surveillance.
• Make sure to explain your decision to use video surveillance to every person affected. Your people should understand you are spying on areas, not on people.
• Explain to personnel how the new security measures will enhance safety. Give them details on the system's design, how it operates, and how it will keep them much safer. This approach has a dual purpose: First, it makes the employees feel better about the situation. Secondly, your people will talk to their friends, who will talk to still more people. In a short time, most of your community will know your facility is well protected. Remember our discussions in earlier lessons about the potential attacker perceiving your facility as a difficult target? This is one of the ways this gets done.
• When explaining the system to employees, talk about the threats you face, and make distinctions between the bad individuals who are threats, and the good people working in the facility.
• Develop an explicit policy on how you will use the videotapes. For example, only one or two security people should see them; you should review them for security purposes only (no spying to see who takes an overly long coffee break); and destroy them after 30 days, unless there is a security-based reason to preserve them.
• Stick to your policy; no cheating. If you break policy and use the tapes for other than the stated reasons, people will eventually find out, and the results will be purely negative.
• Keep track of how your people feel about the video monitoring. Do they feel like it keeps them safer? Or do they feel like slaves under the watchful eye of big brother? Remember: You have a lot to lose from video monitoring if you offend your employees.
Next month, we'll focus on electronic systems, which are designed to keep unauthorized people out of your facility.
Sidebar: Concerns of CCTV Design
Lighting. You must have at least two footcandles (fc) for black and white, and five for color work (except for special low-light cameras).
Contrast. Too much contrast in the scene creates glaring reflections.
Connections. Bad connections or wrong types of wire cause an image with very low contrast. A common cause of this problem is mixing RG-58U and RG-59U cables, which look similar but have different levels of impedance.
Power voltage drops. For cameras located in remote locations, long runs of power wiring are required, and voltage drops may result. If they do, you lose contrast.
Multiple ground points. If the image circuit is grounded at more than one point, ground loops can form and cause video hum (furrows in the image).
Lens selection. Automatic-iris lenses are necessary where changes in lighting occur. In these cases, a fixed-iris lens yields poor results.
Scene selection. The focal length and camera position must be coordinated to serve the camera's intended purpose.