Wednesday, October 21, 2009

CCTV Glossary of Common Terms and Definitions




Closed-Circuit Television (CCTV)

A collection of cameras, tv/monitors, and recording devices directly connected together via cables or other direct means (such as wireless transmitters/receivers).

Alarm Input
Some cameras and DVRs have the ability to accept alarm inputs. These are inputs from standard sensor devices such as, Passive Infra-red (PIR) detectors, door contacts, active beams etc. or relay outputs from intruder alarm panels, or  access control panels. Alarm input circuits can come in a number of forms, but most are commonly self powered, so will operate with a passive switch style circuit. However, in some cases, they expect to be provided with power (normally 12V) as the signal of open or closed.


Alarming
Ability of CCTV equipment to respond to an input signal, normally a simple contact closure. The response will vary depending on your equipment type.

Ambient Light Level
The amount of background light in any given situation.

Analog Signal
Most CCTV cameras, although internally working with digital components, produce a standard analog signal. This is where the image is represented by a variable voltage level and frequency timings. In the USA, the standard used is called NTSC.
In the UK and Europe, the standard for the analog signal output from CCTV cameras is called PAL.

Annunciator
An audible and/or visual signaling device.

Aperture
The aperture of a lens, is the area that allows light to pass into the lens. Lenses with large apertures allow more light in than lenses with small apertures.


Aperture Correction 
Compensation for the loss in sharpness of detail because of the finite dimensions of the image elements or the dot-pitch of the monitor.
Aspect Ratio
The ratio between the the height and width of an image.


Auto Iris
Cameras with an Auto Iris feature have the ability to compensate for large variations in light levels. This is particularly useful for cameras that need to compensate for changes from bright sunlight to dark shadows (ex. near the front door of a building with the door opening and closing a lot). The auto iris circuitry is normally linked to a motorized iris drive that physically opens and shuts the iris on the lens.





Automatic Gain Control (AGC)
This is a feature of many CCTV cameras to help keep the strength of the output signal constant, even with varying light levels. In other words, it boosts signal strength at low light levels, and caps it at higher levels.


Automatic Level Control (ALC)
Allows the camera to either bring out detail in bright areas of the image, or bring out more detail in the darker areas, depending on how it is set up. This is often useful where the view has a window in the background, where the sunlight is causing the image of the room to be much darker than required.

Automatic Terminating (Auto-terminating)
Video signals are normally transmitted along co-axial cable, which require a terminating resistor at either end. If the signal is looped through a piece of equipment that has an input and an output, then it should be terminated if it is the final piece of equipment, but not terminated if it is in between other equipment. Some appliances have a manual switch for this setting, while others automatically detect if termination is required and applies the additional resistor as necessary.

Automatic White Balance
Automatically adjusts the color settings to maintain the quality of the white areas of an image.


Back Focus
Alignment of the rear of the lens to the imaging device.

Back Light Compensation
Automatically adjusts the image to compensate for bright sunlight or bright lights, to give more detail on the darker areas of the image. (ex- Can focus on a persons face, even if there is a bright light coming from behind them).



Balanced Signal
This is a method of transmitting audio or video signals over a pair of wires, (ex- twisted pair cable) by sending two equal, but opposite signals. This system minimizes external interference, and maintains signal quality.


Balun
Short for Balanced - Unbalanced. It is a device used to interface between balanced lines and unbalanced lines.Typically used for Cat5e to Coax cable.
 
 
Bandwidth 
A measure of the carrying capacity of information over a network. Video, for example, takes up more bandwidth to transmit over a network than text would.
 
Bar Test Pattern 
This is a special test pattern for adjusting color TV receivers or color encoders. The upper portion consists of vertical bars of saturated colors and white. The power horizontal bars have black and white areas and I and Q signals.
 
BNC Connector 
BNC is a bayonet style connector for coaxial cable that is most commonly used for CCTV installations. The 3 types of connector types are Compression, Crimp and Twist.
Burn-In (burn)
An image which persists in a fixed position in the output signal of a camera tube after the camera has been turned to a different scene or, on a monitor screen. This is why it's a good idea to use CCTV specific monitors, vs a standard monitor.

Charge Coupled Device (CCD)
One of the two main types of image sensing device used in cameras. It operates by converting light energy into electrical charge.





Chromaticity 
The color quality of light which is defined by the wavelength (hue) and saturation. Chromaticity defines all the qualities of color except its brightness.

Chrominance
This refers to the part of the video signal that contains the color information.


C Mount Lens / CS Mount Lens
C Mount lens has a flange back distance of 17.5mm while the CS Mount lens has a flange back distance of 12.5mm. C mount lenses therefore have a longer focal distance.


Co-Axial Cable (Coax)
This refers to cable that has a central conductor, surrounded by a shield sharing the same axis. The shield can be made from a variety of materials including, braided copper, or lapped foil.There are various standards for specific types of coax cable. RG59 is the most common used cable for CCTV application. You can also get a siamese coax, which includes 2 wires for power.

Composite Video
A full video signal that combines picture signal and synchronisation pulses.

Compression
Digital video pictures can be compressed with a number of techniques. These include: JPEG and JPEG-2000 (for still images), M-JPEG and MPEG (for moving pictures).


Depth of Field
Depth of field or depth of focus refers to the range of distance from the camera that is in good focus. The smaller the aperture of the lens, the longer the depth of field. However, less light coming into the lens, means that the image will be dimmer, so the camera needs to compensate by increasing the gain. For example, the best (longest) depth of field would be obtained on a bright, sunny day, with a very small aperture on the camera. In this case the imaging device still gets a reasonable amount of light, even with a small aperture. The worst (shortest) depth of field would be in a dark room, with a large aperture. To improve depth of field, the best approach is to increase the amount of light on the subject, or use a more sensitive camera, like an IR Camera



Digital Versatile Disk (DVD)
This is an optical disk,  the same size as a CD, used for storing data of various formats. Including video, audio and computer data.



Digital Video Recorder (DVR)
This is a device that records video signal digitally. Normally this is on a large hard drive in the machine. 


Direct Drive (DD)
Some auto iris lenses require a DC signal from the camera. These are known as direct drive lenses.


Electronic Iris (EI)
This is an electronic implementation of an auto iris. It uses electronics to simulate the effect of opening and closing the iris, by increasing or decreasing the effective shutter time of the camera.

External Sync
External sync allows a piece of equipment to take its video synchronization from another unit, so that it can align itself with the system as a whole.


Focal Length
The distance between the center of a lens, or its secondary principal point and the imaging sensor. Lower lengths give a greater field of view and less magnification. Longer lengths give a narrower field of view and greater magnification. The table below gives an approximate value for the angle of the field of view for lenses of various focal lengths. 30˚ is considered to be a normal view, telephoto (longer) lenses have lower angles. Most CCTV cameras have one of the 3 sizes of imaging devices listed below, 1/4", 1/3" or 1/2".

Frame
Refers to a full TV picture. The PAL signal transmits 25 full frames per second.


Gamma Correction
This is an adjustment to ensure that variations in light input, produce a corresponding output in light intensity.

Ghost
This refers to poor image quality, where the primary signal is weak, allowing a secondary signal to interfere with the picture.


Hertz (Hz)
Cycles per second.


Impedance
The total opposition offered by a device to the flow of an alternating current (measured in Ohms).

Infra Red (IR)
Low frequency light below the visible spectrum. This is often used for covert or semi-covert surveillance to provide a light source for cameras to record images in dark or zero light conditions.
 


Interlacing
PAL video signals transmit odd and even lines alternately. This is a 2:1 interlace. The two sets of lines are combined to form each single frame.

Internal Sync
Devices with internal sync, have an internal crystal to provide sync pulses, without needing reference from any external device.

IP Waterproof Rating (IP66 - IP68)
IP ratings are a BSi standard measurement for how waterproof something is. Many cameras or camera housings are designed for outdoor use, and therefore need to be waterproof to some degree. The details of the tests are defined in BS EN 60529 : 1992. The IP number has two digits, and optional letters after them.

Iris
A mechanical device that adjusts to vary the amount of light passing through the lens of a camera. Think of it like the iris of your eye. When it's dark outside, the iris opens bigger to let more light in.



JPEG
JPEG is a standard for coding/compression of still pictures. It is used in the CCTV systems to compress and store individual frames of video.


Line Amplifier
This device takes a weak video signal as its input and outputs a new stronger version of the same signal. These are useful/necessary for transmitting video over very long cable runs. This is sometimes known as a video line corrector.

Line Fed Camera
This refers to the use of equipment that can provide power to the camera and take the video signal along the same cable. It normally involves placing an "encoder" unit at one end and a "decoder" unit at the other. These units effectively combine and then split the power from the video signal at either end of the cable run.

Liquid Crystal Display (LCD)
This is a technology used for flat screen displays. Aside from being smaller and lighter, it also has the advantage of using less power than traditional cathode ray tube screens.



Looping
This refers to connecting an additional device in parallel with an existing video cable. For example, when driving a video recorder as well as a monitor from the same video signal, simply by T'ing off the cable.

Luminance
This refers to the part of a video signal that carries the monochrome information. i.e. brightness information.



Lux
This is a measure of the amount of light striking a surface. i.e. the luminus flux density at a surface. One lux is one lumen per square metre. Cameras for use in good lighting conditions, or in daylight would normally be rated at 2 Lux or more. Cameras with a Lux rating of 0.2 Lux or less would be considered low-light cameras. It is not possible to get good colour definition in low light levels, so in general low light cameras are always monochrome. However, day/night cameras use electronics to switch from colour during the daytime, to monochrome during night or low light conditions. Many low light cameras are also infra-red sensitive, so that infra-red illumination can be used. Particularly useful in zero light conditions.


Matrix Switcher
This is a device that allows any of its camera inputs to be switched to one or more of its monitor outputs. The outputs can also be video recorders.

Monitor
A device used to view video pictures. These devices don't normally have television RF frequency receivers. They normally have composite, or component video inputs.

Monochrome
Refers to a black and white image rather than color.

MPEG
MPEG is a standard used for coding and compression of moving images. It is now used widely for the compression of video images. However MPEG isn't just one standard. They have developed several standards for different uses. For example MPEG-2 is used for DVD's and set top boxes. MPEG-4 was developed for multi-media applications for fixed and mobile web applications.

Multiplexer
A device that takes inputs from 2 or more video channels and combines them into one signal. This is often done by using time division multiplexing, which interleaves frames from each channel in such a way that they can be split out again. Frequency division multiplexing uses different frequencies to achieve the separation of the signals.


Network Camera (IP Camera)
This refers to a camera that is designed to record pictures and transmit them directly over a computer network or dialup internet connection. Network cameras normally do not have any analog video outputs. The images are encoded directly in one of the standard compression techniques, such as JPEG or MPEG.

NTSC
The standard for TV signals developed by the National Television Standards Committee in the USA. The UK and Europe, use a similar, but different standard known as PAL.


PAL
This is the standard for TV signals used in the UK. It stands for Phase Alternating Line.

Pinhole Lens
This is a type of lens with a very small aperture. Normally used for covert applications, where it can easily hide behind or within another object.

Pixel
A pixel refers to an individual area on the surface of the imaging device, normally a CCD. It is made from photosensitive material which converts light into electrical energy. In the context of a display monitor, a pixel is also referred to as an individual area on the surface of the screen which converts electrical energy to visible light.


Roll
This is a fault condition on a video signal which refers to loss of vertical synchronization. It causes the picture to continuously move up or down.


SECAM
The system used for TV signals in France.

Shutter
The shutter in a camera is the device that controls the time period for which light is gathered by the collecting device. Old cameras used mechanical shutters. However modern cameras use electronic circuits to perform the same function with no moving parts.


Television Lines (TVL)
This is a measure of the resolution of a video device. Higher number is higher resolution. 380 TVL is considered medium resolution. 480 TVL or greater is considered high resolution.


Varifocal (Zoom)
This refers to a type of lens that has the facility to change the focal length. This allows adjustment of the magnification and field of view of a camera.

Video Motion Detection
This is a feature that detects motion within a video signal. Normally this is used to trigger recording of images. Advanced video motion detection systems have the facility to adjust the sensitivity and object size that will trigger the system. They also allow the image to be blocked out, such that only certain areas of the image are taken into account when scanning for motion.


Wide Area Network (WAN)
 Multiple LANs connected together, usually over a great distance, to share information.

Wavelets
This is a compression technique used to give high compression without degrading the image quality.


Zoom Lens
A lens that may be effectively used as a wide angle, standard or telephoto lens by varying the focal length of the lens.

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