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After a camera converts an image into an electrical signal, the video signal is transported to a remote point for viewing and possibly recording. This is common in applications where one or more cameras are monitored at a security kiosk or an administrative office. Here the video signal is then converted from its electronic form back to light, via a CCTV monitor, so security guards, office personnel, or others can perform surveillance duties.
CCTV monitors are designed to convert the electrical video signals transmitted over coaxials, fiber-optic cables, or a radio- based media into light energy for the eye to see. Although these monitors share some similarities to conventional television sets, in reality there are many differences between the two that set them a world apart.
Televisions, for example, are designed to receive commercial video and audio signals that are broadcasted over the UHF and VHF frequency bands. CCTV monitors, on the other hand, are designed to receive composite video signals direct over a coaxial cable, whether they arrived over the same coaxial cable, a fiber-optic link or a microwave/radiated radio-frequency link.
There are other differences between conventional TV sets and CCTV monitors that also should be noted. The most significant is durability and life expectancy.
Where a typical conventional television set is designed to operate up to 5 years at eight-hour-a-day intervals of operation, for example, a commercial-grade CCTV monitor will operate up to 5 years when operated on a 24-hour-a- day basis. This is not to say that a conventional television set cannot be used as a CCTV monitor. With the use of a video-to-RF modulator, composite video signals can be converted to operate over the VHF television frequency band. However, conventional television sets will not operate for extended periods of time as CCTV monitors will.
Viewing Distance and Resolution
Commercial CCTV monitors come in a number of sizes. As a general rule, the farther away security personnel are from the monitor, the larger the monitor should be.
One accepted method of determining monitor distance and size is based on the mathematical formula:
Monitor (inches) - 4 = Viewing Distance (+/- 25%)
For example, to find out how far away a 12-in. monitor should be used, plug the values into the equation and do the math:
12 - 4 = 8 ft.
To establish the upper and lower viewing limits, multiply the value of 8 ft. by 1.25 and .75, or:
8 x 1.25 = 10 ft.
8 x .75 = 6 ft.
Thus, a 12-in. monitor should be viewed effectively at a distance of 6 to 10 ft.
The same formula can be used to calculate the approximate viewing distance of a common 9-in. monitor:
9 - 4 = 5 (+/- 25%)
The viewing range is then calculated in the same manner:
5 x 1.25 = 6.25 ft.
5 x .75 = 3.75 ft.
In this case, a 9-in. monitor can be viewed effectively from a distance of 3.75 to 6.25 in.
Another concern when purchasing a monitor is its resolution. The number lines of resolution is usually determined by the resolution of the camera(s) used.
For example, if a monitor with a 300 line resolution is used with an 800- to 1000-line camera, the result will not be as good as if a 700-line monitor were used. On the other hand, when a 700- line monitor is used with a 300-line camera, the result will usually be just as good. This is because the 700-line monitor will reproduce the 300 lines of available video by dividing them over the 700-line screen. This will produce video images that may not as good as the originals produced by the 300-line camera.
The last category is that of CCTV Peripherals, which consists of camera lenses, monitors, switchers and splitters, as well as event recorders, time-lapse tape recorders and pan & tilt mechanisms. In this discussion we will discuss lenses and camera control devices, which include camera switchers of various types, microprocessor-based matrix control systems, computer-driven camera control systems, and over-the-phone camera control systems.
Color LCD monitors
These monitors became a standard in new CCTV installations. Their biggest advantage is small depth, they need little space on operator's desk, or can even be mounted on a wall in the monitoring center. The popularity of LCD panels in CCTV applications is the result of large scale manufacturing and low price - they are mainly used as computer monitors. However, the typical PC monitors can only be used with devices equipped with VGA output.
The manufacturers of digital video recorders began to equip their products with such interface, or they offer additional composite video/VGA converters, like the M8543 for CP-CAM DVRs.
The group of LCD monitors dedicated for CCTV applications are professional devices with CVBS inputs. The monitor has standard BNC inputs for connecting any CCTV video devices using PAL standard, like DVR, CCTV camera, video splitter etc.
Another group of displays that are often used in CCTV systems are universal PC/TV LCD implementations, usually available at very competitive price. Aside from built-in TV tuner, the panels have been equipped with video inputs suitable for connecting CCTV devices. Such models are manufactured e.g. by Samsung and LG.
LCD televisions are also used when one needs bigger screens, over 22". When using analog CCTV cameras, the LCD monitors should have 4:3 aspect ratio. Most of such cameras provide images in this format and they would be deformed on the displays with 16:9 aspect ratio. The latter are used in IP-CCTV systems. The role of DVR is performed by PC and special software that can adjust the images from the cameras to screen parameters of the monitor used.
Small thickness of LCD monitors allows to use them in places where CRT solutions were practically not possible. LCD monitors in metal vandal-proof casings are mounted in vehicles. Larger panels, up to 21", mounted e.g. in buses (public transport) work as information/ advertising/ entertainment screens. The smaller ones, mounted close to the driver, allow him to monitor the inside of the bus.
The main parameters of LCD monitors include maximum contrast and brightness, pixel response time, horizontal and vertical viewing angles. Additional differences concern number of audio and video inputs/outputs, built-in speakers, the way of mounting.
Choosing LCD monitor it is important to know the terms of warranty concerning possible degradation of the screen (falling-out pixels). This phenomenon is not so irritating as in the case of a PC monitor, but also lowers quality of images. |
