CCTV Surveillance System
CCTV, Closed Circuit TeleVision, is a TV system that does not broadcast TV signals to public but transmits them over to limited monitor(s). CCTV system usually utilizes CCD video cameras (to produce the video), cable or wireless transmitters/receivers or Internet (to transmit the video), and monitors (to see the video). CCTV system is not only applied to the security and surveillance purpose but also to the other fields like laboratory in schools or in private companies, even to production lines in factories. As the application of CCTV system increased, the CCTV industry has developed variety of CCTV equipment like Time Lapse VCRs, combiners, infrared illuminators, Pan/Tilt, etc.
CCD and CMOS cameras
The very first and important element in a camera is the sensor that captures the image. As in photographic and film cameras the images are captured on film, in CCTV cameras the images are converted into electrical signals with a help of an image sensor which can be a CCD(Changed Coupled Device) or CMOS(Complementary MOSFET) . CCD cameras are more sensitive and produces higher resolution compared to the newly developed CMOS cameras. Ideally for CCTV application CCD cameras are only to be used. CMOS sensors have been long used for imaging in photocopy machines and other scanners. Now same have been experimented for CCTV cameras. However the resolution offered by CMOS cameras is very low and has a shorter life compared to CCD cameras. The only advantage is that they are cheaply priced.
About CCD Chip
A CCD (charge coupled device) consists of several hundred thousand individual picture elements (pixels) on a tiny 1/2″, 1/3″, or 1/4″ chip. Each pixel responds to light falling on it hy storing a tiny charge of electricity. The pixels are arranged on a precise grid, with vertical and horizontal transfer registers carrying the signals to the camera’s video processing circuitry. This transfer of signals occurs sixty times per second.
The 1/3″ CCD chip is the most widely used sensor format these days; its size is 5.5mm (diagonal), 4.4mm (horizontal) and 3.3mm (vertical). The 1/4″ sensor format, recently being used in color cameras, is 4mm (diagonal), 3.2mm (horizontal) and 2.4mm (vertical).
For 1/3″ CCD chip camera, the following formula is a simple way to determine the focal length of a lens needed.
For example, if you want to watch a subject of 5.5ft width at 10ft distance with a 1/3″ CCD camera, you will need a 8.0mm FL lens. That is, 10ft / 5.5ft x 4.5mm = 8.18mm. A 8.0mm FL lens mounted on 1/3″ CCD camera will cover the subject you want to watch.
Reasolution is usally meant by Vertical resoluiton and it is determind by the number of vertical TV lines e.g. 380 TVL or 550 TVL. Horizontal resolution is seldom quoted is a function of frequency. Overall resolution is a function of vertical & horizontal resolutions. Overall resolution may be quantified by number of pixels or picture elements. For B/W camera normal resolution is is 350 – 400 TVLines and high resolution means 550 – 600 TVLines, whereas for a colour camera normal resolution means 250 – 300 TVLines and 450 – 480 TVLines means high resolution.
Note that the resolution of monitor is also important because afterall the resolution of the camera has to be produced on the monitor. So it is meaningless to have a monitor with a lesser resolution than that of the camera.
F-number (Optical Speed)
Optical Speed is about how fast a lens collects lights and is defined by the f-number like f/1.2, f/2.0, etc. This speed is determined by the Focal Length(FL) and the Diameter(D) of a lens; f-number = FL/D.
Larger FL, if the Diameter is same, makes higher f-number (like f/4 or f/8) lens that collects less lights to the camera sensor and results a slower lens. Lower f-number (like f/1.2 or f/1.4) lenses pass more lights to the camera sensor. Larger Diameter, if the FL is given, will make a lower f-number lens which can operate in lower light level.
The amount of lights is defined by LUX (Lumens per Square Meter). One LUX is a candle light volume at one-meter distance. Followings are some examples of natural lights expressed in LUX.
Full daylight; 10,000 LUX
Very dark day: 100 LUX
Twilight: 10 LUX
Deep twilight: 1 LUX
Full moon: 0.1 LUX
Quarter moon; 0.01 LUX
A good B/W camera can see in full moon condition. But, a color camera will need an additional artificial light in full moon.
Electronic Shutter, one of the major features of CCD camera, is not really a moving shutter, but a clever piece of signal processing. Under low light conditions, the CCD is allowed to gather signal at the electronic shutter speed of full 1/60th of a second. Under brighter lighting conditions, the video processing chip automatically responds by reading the CCD and then immediately “early purging” it, resulting in precise control over the video level. Even at a shutter speed of 1/100,000th of a second, the CCD camera is still delivering 60 images per second, but each image is gathered over a much shorter period of time. It doesn’t end with the CCD…the image is constantly monitored and optimized by advanced on board signal processing circuitry. The end result is a fantastic picture, with no fiddling, no adjustments, and best of all, unbelievable reliability.
Economically commercialized infrared cameras, some also water-resistant to be installed outdoor, can see up to 40 to 80ft distance in total darkness. You may also use infrared illuminators, which can illuminate up to 100ft (Color cameras, unless their IR filter is removed, do not work with infrared illuminators.) Infrared cameras work as ordinary video camera during daytime. Color infrared cameras work as B/W ones at night.
Quad Splitter is a combiner to combine up to 4 cameras and to show them on a monitor screen split into 4 at the same time. Usually, it also has built-in switcher which can display the pictures one by one. Some models have BNC jacks for video inputs and others RCA jacks. You may choose one according to your camera’s video connection type. But, if the video input jack is different with your camera’s connection, you can get a BNC/RCA converter easily at any electronic parts shop like Radio Shack or Best Buy, etc. B/W splitter works with B/W cameras and color splitter with color cameras. A quad splitter is to be connected to a CCTV monitor, TV or VCR.
DVR (Digital Video Recorder)
DVR, Digital Video Recorder, records video pictures digitally on a hard disk drive(HDD). This HDD, usually built-in, has capacity of 250 Gb, 320 Gb or 1.5TB to store the records. You can program the picture resolution and recording speed (how many frames per second) according to the application; real-time or time lapse recording also available. Overwriting the oldest pictures is programmable.
Event alarm recording which records only when a movement is captured within the image frame is easier to program and more reliable than the Time Lapse VCR’s alarm recording function. You just assign dots over the screen where you want to detect the movement. As DVR records digitally, the image quality remains the same regardless of how many times the images are stored or rerecorded. And, you can select images quickly by using time/date or alarm search, or just browsing through.
Cabling of CCTV
Co-axial cables were designed specifically to assist with the reduction of many forms of interface found in TV network. Co-axial are the more common, most well understood form of signal transmission and should always be a first consideration. Co-axial cable are graded using RG numbers which define the attenuation loss in decibels per meter. eg RG-59, RG-6, RG-11. They are also classified by their standardized per unit impedences. For CCTV application coaxial cable of 75 Ohm with single inner conductor and braided outter conductor is specified. CCTV signals may also be transmitted via following medium / link depending on distance and system used.
Power Source for Cameras
Most board cameras, mini cameras and about one third of the professional cameras work with 12VDC, 100mA to 200mA for B/W cameras and 150mA to 300mA for color ones. These cameras usually have DC jacks to accept DC power plugs. You should be careful about the polarity (positive and negative) for this power source. 12VDC power can be supplied by AC Adapter or battery pack. You may even use a “power cord” plugged to the cigarette lighter in a car.
About two thirds of the professional cameras work with 24VAC, 20VA to 40VA. The cameras usually have screw type connections and you don’t need to worry about the polarity. This power is usually supplied by AC Adapter and you need to prepare separate power cable for the connection. Unlike 12VDC, this power can be transmitted to a long distance up to 450ft and is proper for the cameras that are to be installed outdoor or when you don’t have the power outlet near the camera.
B/W Monitor and Color Monitor
In the past, 9 to 12 inch (diagonal) B/W monitors were widely used in the field. These days, many people are looking for 14 inches for color monitor and 17 inches for B/W. As the color monitor needs 3 different color dots to produce one pixel of information on the monitor, it usually has lower resolution than B/W monitor.