Understanding the individual components of a closed circuit television system is key to maintaining a successful video monitoring security plan.
Video monitoring (also known as Closed-Circuit Television or CCTV) is an essential technology for the security field. It not only allows you to see things developing at a distance so you can immediately diffuse a threatening or undesirable situation, but you can also use it as a motion detection system to permanently record a sequence of events for future use.
In this article, and in Lesson 4 next month, we will cover all of the essential design factors for CCTV systems, explain the common components and how they operate, and how you apply CCTV to a building's overall security system.
While not an intrusion detection system in itself, CCTV is probably most useful in physical security operations. You place cameras at critical locations to provide direct visual monitoring anywhere you wish from a single vantage point. Probably the easiest way to explain this basic TV technology is to use the fax machine as an example. We refer to fax machines as slow-scan television. The system works by sending television picture frames (in black and white) over telephone lines. We are all familiar with the process: the machine scans one line at a time, interprets that area as black or white space, translates the information into electronic pulses, and then transfers the information to a receiver.
Black and white television uses the same technology as the fax machine, except at 60 frames per second; using a camera rather than a scanner. Each line of the image is scanned and translated into a complex video signal. A color process system is the same, except it uses an even more complex signal.
The principal elements of a CCTV system consist of communication media, switchers, synchronizers, monitors, video cassette recorders, video motion detectors, cameras, lenses, and mountings. In addition to these components, CCTV systems also use the following devices:
• Amplifiers, which simply increase the strength of the video 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 serve to keep power surges, spikes, and noise off the cables connecting the system.
Let's look at these elements in more detail.
Communication media. Communications media is the electronics term referring to the method of bringing a signal (in this case a video signal) from one place to another (i.e. wiring). Coaxial cable has always been the industry standard, although fiber optics is now replacing it. We use microwave transmission only where required, as it's much more costly. Using a high-quality coaxial cable is essential. For installations of less than 1000 ft, RG59U cable is fine. However, for distances of 1000 ft to 2000 ft, use RG11U. Installations of more than 2000 ft in length require the use of amplifiers to keep the signals at usable levels or the use of optical fiber. Most CCTV installations use BNC connectors, a bayonet mount connector, or occasionally an "F" connector, like those used in cable television systems.
Installing video cabling is relatively simple, however, you must be careful not to pull the cable beyond its tension limits, and bend it sharply. In addition, the cables must remain safe from physical damage and environmental hazards.
Take care when strapping or (especially) stapling cables to structural surfaces (walls, ceilings, etc.). If you cinch the staple or strap too tightly, it will deform the cable and alter its transmission characteristics.
Switchers and synchronizers. Especially for large installations with many cameras to monitor, sequential switching of cameras shown on the monitors is essential. If you don't use these switchers, you may overwhelm security personnel watching the monitors.
There are many sizes of switchers and sequencers -- each with varied capabilities. They range from systems that monitor a few cameras to microprocessor-controlled models that control many cameras through dozens of monitors. The larger the CCTV system, the more necessary switchers and sequencers become. Switchers simply switch the monitor (usually manually) from one camera's signal to another's.
Sequencers automatically switch monitors from one camera to another in a preset sequence. These units come with any number of features. Some of the more useful are:
• Character display generators. A character generator displays on screen the identification of the camera you're viewing. This can be useful when monitoring a large number of cameras.
• Bridging. This feature allows the operator to choose any camera in the sequence for continuous monitoring on a separate monitor. You can do this without interrupting the pre-programmed sequence.
• Synchronizing. This feature allows switching between cameras without a vertical roll, which obviously is undesirable. By synchronizing all of the cameras to the power-line frequency by means of electronic circuits (which sense the zero crossing point of the AC line voltage they synchronize), you eliminate the vertical roll.
This is somewhat more difficult with 3-phase systems than with single-phase units. For 3-phase systems, you must use a phase-adjustable line lock. This feature allows you to synchronize the camera to either of the three electrical phases in a 3-phase system.
Videocassette recorders (VCRs). VCRs provide security records that are important when you're trying to analyze an intrusion or other event. There are two types of VCRs used for security surveillance systems.
Time-lapse video recorders use microprocessor-based time-lapse techniques to compress long periods of recording time into a short length of tape. The quality is not as good as a standard videotape, but you can squeeze up to 600 hr of recording onto a standard T-120 cassette. Most units come with automatic speed switching mechanisms, which increase the recording speed to standard when an alarm triggers.
Event alarm recorders. These devices have many of the same features as time-lapse recorders, but do not turn on unless an alarm triggers. After the alarm, they record for a specified period of time and then shut off until the next alarm condition occurs.
Both types of recorders should have built-in character generators that identify which camera is displaying the area where the alarm condition exists.
Video motion detectors (VMDs). This device can detect irregular motion in the camera's field of view and set off an alarm. Not only will this alert guards to the intrusion, but it may also switch the scene onto a monitor for continuous observation. It may also automatically switch on a VCR and capture the action on film.
VMDs, however, are not without their problems; they are susceptible to false alarms, and can sometimes be more trouble than they're worth. Even the move ments of the automatic iris of the camera can cause trouble. Some VMDs avoid this difficulty by allowing you to set a certain viewing window (area of view). Only irregular activities within that window will set off the alarm. This reduces the possibility of false alarms.
For instance, by setting the window on the top of an unused door, employees walking past the door will not set off the alarm. However, an intruder opening the door will. Analog VMDs typically offer one window per camera, but digital VMDs may offer up to seven (at a higher cost, of course).
In addition to using windows to cut out false alarms, many VMDs have sensitivity settings. By using shorter focal lengths, these devices reduce the sensitivity.
Another option, called a retention control, specifies how long the irregular action must go on before the alarm triggers. The use of high-resolution cameras with low-noise characteristics also helps reduce the number of false alarms.
When using VMDs, there can be no camera motion such as panning or zooming, because such motion would set off the alarm.
In Lesson 4 next month, we'll continue our discussion of CCTV systems by taking a closer look at cameras, lenses, and mountings. We'll also cover Code requirements as well as the downside of video monitoring.
