Last month we began covering the components of video systems — too much to cover in one article. This month we continue with components; next month we'll start to cover design.
It is not necessary that you understand ever minute detail of these components, but it is important that you understand what these items are, how they operate and how they interact with all the other components.
Communications media is an electronics term that refers to the method of bringing a signal (in this case a video signal) from one place to another; that is, the wiring method. Coaxial cable has always been the industry standard, although it is now being replaced by optical fiber. Microwave transmission is used only where required, as it costs significantly more than the other methods.
When using coaxial cable, high-quality cable is essential. For installations less than 1,000 ft, RG59U cable is fine. But for distances of 1,000 ft to 2,000 ft, RG11U should be used. Installations of more than 2,000 ft long require amplifiers to keep the signals at usable levels, or the use of optical fiber as the communication media.
Virtually all cameras require appropriate power sources and power cords. All camera locations need 120V AC power. You'll need to correctly match components. A few specialty cords and cameras bring operating power to the cameras, but they are not used frequently.
Especially for large installations, with so many cameras to monitor, sequential switching of cameras shown on the monitors is essential. If these switchers were not used, the people watching the monitors would be overwhelmed.
The many sizes of switchers and sequencers offer varied capabilities. They range from systems that monitor a few cameras to micro-processor-controlled models that control many cameras through dozens of monitors. Switchers and synchronizers become more necessary with larger CCTV systems.
Switchers simply switch the monitor (usually manually) from one camera's signal to another's. These are needed when there are fewer monitors than there are cameras.
Sequencers automatically switch monitors from one camera to another in a preset sequence. These units come with any number of features. Useful switchers and synchronizers include:
Character display generators. A character generator displays on screen the identification of the camera whose view is being seen, as well as date and time. This can be useful when monitoring several cameras.
Bridging. This feature allows the operator to choose any of the cameras in the sequence for continuous monitoring on a separate monitor. This can be done without interrupting the sequence.
Synchronizing. This feature allows for switching between cameras without a vertical roll, which obviously is undesirable. The synchronizing of all cameras to the power-line frequency is accomplished with the use electronic circuits, which sense the zero crossing point of the AC line voltage. This eliminates the vertical roll. This is somewhat more difficult to do with three-phase systems than with single-phase systems. For three-phase systems, a phase adjustable line lock is a must. This feature allows individual cameras to be synchronized to any of the three electrical phases in a three-phase system.
For the most part, CCTV monitors used for security work have black-and-white picture tubes, and are of a relatively small size. The most common sizes are 9-in., 12-in., 15-in. and 19-in. They are also fitted with hardware for rack mounting.
For monitoring in residences the signal is generally sent to an unused channel on a standard television.
VCRs provide security records that are very important in analyzing an intrusion or other event. Two types of VCRs are commonly 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 standard videotape, but up to 600 hours of recording can be squeezed onto a standard T-120 cassette. Most are equipped with automatic speed switching mechanisms, which increase the recording speed to standard when an alarm is triggered.
Event alarm recorders. These devices have many of the same features as time-lapse recorders, but don't turn on unless an alarm is triggered. After the alarm, they record for a specified period of time and then shut off until the next alarm condition. Both types of recorders should have built-in character generators that identify which camera is displaying the area where the alarm condition is sensed and a date and time.
Next month we will cover more video components and design.
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:
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.