This article will conclude our coverage of video-system components and then begin our discussion of CCTV design considerations. Last month we left off describing VCRs.
VIDEO MOTION DETECTORS
One of the more popular accessories to security-monitoring systems is a device called a video motion detector (VMD). This device, when properly set up, is able to detect irregular motion in the camera's field of view and set off an alarm. Not only will this alert the intrusion, but it may also switch the scene onto a monitor, so it may be constantly monitored. It may also automatically switch on a videocassette recorder and capture all the action on film.
VMDs, however, are not without problems; they are susceptible to false alarms and can be more trouble than they are worth. Even the movements of the automatic iris can cause trouble. Some VMDs avoid this difficulty by allowing a certain viewing window (area of view) to be set. Only irregular activities within that window will set off the alarm. This greatly reduces the possibility of false alarms. For instance, by setting the window on the top of an unused door, the alarm will not be set off by employees walking past that door. However, an intruder opening the door will trigger the alarm. Analog VMDs typically offer one window per camera, but digital VMDs may offer up to seven windows (at a higher cost).
In addition to using windows to cut out false alarms, many VMDs have various sensitivity settings. By using shorter focal lengths, the sensitivity is reduced. Also, many models have separate sensitivity settings. Another option, called a retention control, specifies how long the irregular action must go on before the alarm is triggered. The reduction of false alarms is also helped by the use of high-resolution cameras with low-noise characteristics.
When using VMDs, there can be no camera motion (such as panning or zooming), as such motion would set off the alarm immediately.
In addition to the specialized components we have covered in this chapter, CCTV systems use all the standard little black boxes used in the cable TV trade. Most common are:
Amplifiers, which simply increase the strength of the signal.
Splitters, which operate as a “Y” connection, splitting the signal and sending it several different ways.
Screen splitters, which put two or more images on the same screen.
Surge suppressors, which keep power surges, spikes and noise off the cables connecting the system.
These devices are usually easy to connect. Simply connect the coaxial cables to a properly specified device as directed by the manufacturer.
The eight primary concerns of CCTV design are the following:
- Scene lighting. You must have at least 2 footcandles for black/white and 5 footcandles for color work. (Except for special low-light cameras, which require less than 1 footcandle.)
- 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. When cameras are located in remote locations, long runs of power wiring are required, and voltage drops may result, causing a loss in contrast. If such a voltage drop becomes a problem, two solutions are available. The first is to increase the size of the power wires, and the second would be to install a small buck-boost transformer at the end of the long power run.
- Multiple ground points. If the image circuit is grounded at more than one point, ground loops can form and cause video hum (dark and light alternating furrows in the image).
- Lens selection. Automatic-iris lenses are necessary where changes in lighting will occur. In this case, a fixed-iris lens will yield poor results.
- Scene selection. The focal length and camera position must be coordinated to serve the camera's intended purpose. An image must show what was intended.
- Security system and UPS systems coupling.
The camera should be able to operate under any marginal light conditions and provide adequate security surveillance of low light-level type.
The installation should provide continuous operation, which is advantageous in that it eliminates delay time required for the camera to warm up and be properly adjusted.
Adjustment of the camera may be important, depending on the camera installation.
To detect intruders in 360-deg field, the camera should be able to:
Pan — turn horizontally, left to right or right to left.
Tilt — aim the camera up or down.
Zoom — change the camera's field of view.
Scan automatically (optional).
CCTV operation capability should include the following:
The ability to operate by remote control by security personnel at guard headquarters.
Routine and continuous monitoring of activity.
Preplanned monitor and automatic zoom of activity at specified time intervals.
CONTINUING EDUCATION FROM IOWA STATE
This year, Iowa State University has completely overhauled its electrical courses to meet state requirements. Eleven courses are now being offered. A Code Changes course is also planned. Usually, one or two of these will meet the requirements in your state. The courses are:
- National Electrical Code
- Code Refresher Course
- Electrical Controls
- Data Networking
- Fiber Optics
- Closed-Circuit TV Monitoring
Of course, you can take any of these courses at any time for your own improvement; but to meet your renewal requirements, you will have to take the courses that are required by your state.
Since the requirements vary for each state, a public Web site has been set up at www.ElectricRenewal.com. All registration information, course descriptions and state requirements can be found there. You can also call the University office at (800)262-0015 or fax the office at (515)294-6223.