Glossary
Automatic Gain Control (AGC)
This is a feature of many CCTV cameras that helps keep the strength of the output signal constant, even when the light level changes. In other words, it boosts the signal strength at low light levels, and limits it at higher levels.
Analogue Signal
Most CCTV cameras, although internally working with digital components, produce a standard analogue signal. This is where the image is represented by a variable voltage level and frequency timings. In the UK and Europe, the standard for the analogue signal output from CCTV cameras is called PAL. In the USA, the standard used is called NTSC.
Aperture
The aperture of a lens is the area that allows the light to pass into the lens. Lenses with large apertures allow more light in than lenses with small apertures.
Aspect Ratio
This is the ratio between the height and width of an image.
Automatic Level Control (ALC)
This is a feature of some cameras that 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 White Balance
This is a feature of some cameras that automatically adjusts the colour settings to maintain the quality of the white areas of the image.
Back Light Compensation
This is a feature of cameras that automatically adjusts the image to compensate for bright sunlight or bright lights, to give more detail on the darker areas of the image. For example to focus on the detail of a face of a person that has the sunlight shining from behind.
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.
Chrominance
This refers to the part of the video signal that contains the colour information.
Composite Video
A full video signal that combines picture signal and synchronisation pulses.
Compression
Compression is the method by which the amount of digital data required to represent an image may be minimised. This generally involves eliminating redundant information, and/or removing information that the human eye does not notice. Compression techniques can broadly be split into two categories:
- Lossless, where the original image may be reproduced exactly from the compressed data (such as PNG and GIF), and
- Lossy, where the original image cannot be reproduced exactly, but the reduction in the picture quality is tolerated in order to increase the compression ratio.
Common types of compression for digital video pictures include: JPEG and JPEG-2000 (for still images), M-JPEG and MPEG (for moving pictures).
Digital Video Recorder (DVR)
This is a device that records video signal digitally.
Depth of Field
Depth of field (also known as 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.
Decibel (dB)
The decibel is a logarithmic scale. In the context of CCTV it refers to voltage or signal strength. A 6dB increase is equivalent to doubling the signal strength.
External Sync
An external sync allows a piece of equipment to take its video synchronisation from another unit, so that it can align itself with the system as a whole.
Focal Length
The distance between the centre 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. 30˚ is considered to be a normal view, telephoto (longer) lenses have lower angles.
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.
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.
IP Waterproof Rating (Ingress Protection)
IP ratings are a standard measurement for the level of dust and water protection.
Iris
This is a mechanical device that adjusts to vary the amount of light passing through the lens of a camera.
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.
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.
MPEG
MPEG is a standard used for coding and compression of moving images. It is now used widely for the compression of video images. There are 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
This is 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
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 analogue video outputs. The images are encoded directly in one of the standard compression techniques, such as JPEG or MPEG.
NTSC
This is 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.
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.
SECAM
This is the system used for TV signals in France.
SCART
This is a standard used in Europe for a 20 pin connector used to carry video and audio signals. It is most widely used in domestic TV appliances.
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.
Signal to Noise Ratio (S/N Ratio)
This is the ratio between the signal strength and the noise levels on an audio or video signal.