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Basic CCTV Technology before you select CCTV system
Here I want to post my first article related to basic CCTV technology.
CCTV-surveillance Basics are explained first, to let you guys what needs to be understood for making informed choices.
CCTV Meaning
Closed Circuit Television simply means that the signals picked up by the cameras are not broadcast over the air, or to a public cable net, but are fed directly to monitors. This may not be fully correct for wireless outfits, but then the range of effective broadcast is minimal, and there is only one intentional receiver.
We should now learn a few basic concepts in order to make sense of the language of those proposing us their CCTV-surveillance ware. Otherwise how will we decide, if unable to get what they are talking about?
Camera performance depends largely upon the reflected light at the scene to be surveyed and upon the quality of the imager or sensitive element.
A few fundamental CCTV-surveillance related concepts play an important role when coming to select a CCTV Surveillance system. It is therefore necessary to explain them in brief to understand what they mean and how they apply in practice to the features of different components. These terms are listed hereafter:
- Video signal
- Illumination
- Lighting
- Sensitivity
- Resolution
Video signal
A Video signal capable of reproducing a Video picture is generated in an imager or camera sensor.
A two dimensional picture can be represented as a two dimensional array (like a checkered page) of picture elements (pixels), each one containing data on color and brightness.
The Video signal of CCTV-surveillance presents the camera output as a continuous waveform which is obtained by streaming the data of each pixel, one by one, when scanned from the first to the last of any frame or picture.
The waveform provides a one dimensional sequence of analog data that are fed to the monitor device. To achieve a correct reproduction of the image, timing signals are added according to specific protocols, to indicate the start of a line, the serial number of the line, and, after the last pixel, a signal is included to perform the vertical retrace and prepare to scan the next field.
A composite color signal consists of the standard monochrome signal with color information added.
In North America the video signal is a standard, called NTSC (National Television Standards Committee), while in Europe the standard is PAL or SECAM in different places. Monitor and camera must operate according to the same standard.
Sound and video are completely separate in analog TV. A color TV signal starts off looking just like a black-and-white signal, but an extra chrominance signal is added to provide color data.
A monochrome (black and white) TV filters out and ignores the chrominance signal. A color TV picks it out of the signal and decodes it, along with the normal intensity signal, to determine how to modulate the three color beams (RGB or Red, Green and Blue).
The radio frequency (RF) waves used as a carrier to transmit signals over the air, are modulated, or modified, to include the information from video and audio signals. The TV extracts the composite video signal and the sound signal from the modulated radio waves collected by the antenna.
In a CCTV-surveillance camera signal there are no radio frequency waves to deal with, therefore the monitor has no tuner to pick up the right channel. The Video signal is fed directly to the screen according to the proper scanning protocol.
To be fed to a computer screen, instead of an analog monitor, the Video signal of a CCTV-surveillance camera has to be digitized by the grabber card. When so transformed the digital image can be compressed according to usual computer protocols (JPEG, MPEG etc.) and is no more dependent on synchronization signals for correct display.
Illumination.
A scene presents different colors, surfaces and materials that reflect varying amounts of light. Before selecting equipment, one should determine the minimum light level that will arrive from the scene to the camera lens.
The target area can be lighted by natural or artificial light sources.
The illumination of a scene describes the quantity of light reflected by the visible objects to the camera lens. This is a quantity expressed in Lux, measurable with an instrument called photometer. Even without such an instrument one can see from available tables that there is a huge difference between the light available in a moonless night, and that of full sunlight.
Under a given lighting condition, black absorbs most of the light available and reflects only a minimum part. So do also rugged materials and rough surfaces. On the contrary flat and bright colored surfaces reflect most of the light falling on them.
The amount of reflected light collected from any setting depends on the amount of lighting available and on the scene reflective characteristics. The human eye can readily adjust to widely varying illumination levels. On the contrary, optical viewing and recording systems have much reduced latitude of adaptation to conditions suitable for the display of interpretable pictures.
Lighting.
In any case the light reflected from a scene depends strongly on the amount of light projected on the place by lamps or by a bright day light. For best CCTV-surveillance, lighting should be adequate.
If a given image from a certain CCTV-surveillance camera is not satisfactory because of insufficient illumination, one can either increase the lighting, by adding more powerful lamps, or change the camera to a more sensitive one of lower Lux rating.
We should remark however that at low illumination levels most color images look like black and white on monitors.
Sensitivity...
... ratings are generally given as the minimum Lux levels (a measure of the intensity of light) at which the camera will still produce a useable image. The lower the Lux number, the more light sensitive is the camera, the lower the illumination levels at which the CCTV-surveillance camera will still produce an acceptable image.
This is known as the camera Lux rating or minimum illumination rating. The closer to zero the Lux rating the better the camera will perform in a low light application.
Lux level ratings (and the amount of light entering the camera lens) are inversely proportional to the aperture of the camera iris and so are stated at a specific F/Stop (Focal Length divided by Aperture).
In general, for CCTV-surveillance, the better the light, the better the picture. The lower the lux number the more sensitive is the camera to marginally illuminated areas. However the number of lux appearing in the camera data sheet is not a guarantee that a picture taken at that illumination level will be meaningful. A sensitive camera will show even dimly lit images. Contrast and details though may be weak.
There is no optimum lux sensitivity for a given CCTV camera. It depends on the light available when picture taking is needed. For viewing in full darkness a CCTV-Surveillance camera needs Infrared Red (IR) Illuminators, that project light invisible to the human eye but not to the camera sensors.
Resolution...
... is the ability to resolve or see distinctly small details in an image. Resolution for CCTV cameras (as well as for TV monitors and recorders) is usually specified in terms of lines of resolution. The number of TV lines in the video produced by the CCTV-surveillance camera is a measure of picture resolution (sharpness) also described as quality of definition and clarity of a picture.
If a sequence of pictures, containing a progressive larger number of finer distinct horizontal or vertical lines, alternately black and white, are captured by a camera and displayed by a monitor, there will be a limit to the number of separate lines that can theoretically be seen clearly.
A word of caution is here in place. As reported elsewhere while dealing with Camera monitor, this subject is open somewhat to manipulation depending on exactly how and with which other accessories the test is performed.
That number is reported as the resolution of the particular device, camera or monitor. The more the lines, the higher the resolution and the better the picture quality. CCTV cameras range from 200 to more than 1,000 lines of resolution.
The larger the number of TV lines, the better the resolution and the overall picture quality. Black and White cameras have usually higher resolution than Color ones. Resolution is directly proportional to the number of picture elements (pixels) in the solid state chip sensor (imager).
Due to the somewhat confusing usage of resolution lines reported by certain manufacturers, readers are advised that actually two different types of resolution are to be considered.
Vertical resolution refers to the number of horizontal lines resolvable. Vertical resolution is constant for a given scanning system (NTSC, PAL or SECAM), between 350 and 400 lines.
Horizontal resolution refers to the number of vertical lines resolvable. The color cameras used in CCTV-surveillance systems generally offer horizontal resolution between 330 and 480 lines. Over 600 lines is considered high resolution.
If different elements of a CCTV-surveillance system are capable of different numbers of resolution lines, the overall resolution of the system will be that of the least line number element.
It should be noted that the number of lines reported above for defining resolution has nothing to do with the number of the horizontal scanning lines specified by the regulation authorities establishing the uniform protocol for capturing and displaying TV and video pictures. In the USA this number is 525 per NTSC, in Europe and elsewhere this number is 625 according to PAL or SECAM. Therefore one should always care that the hardware considered be suitable to that in use in the region.
The concept of contrast should be added to this presentation although it is not specific to CCTV and depends on color and other factors. The visual perception of details is a property related to the contrast, or the difference in aspect, that makes one object distinguishable from another or from the background. Increasing ambient light reduces contrast across surfaces.
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