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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 caps it at higher levels.

Auto Iris (AI)
Cameras with an Auto Iris feature, have the ability to compensate for large variations in light levels. Particularly useful for cameras that need to compensate for changes from bright sunlight to dark shadows. The auto iris circuitry is normally linked to a motorised iris drive that physically opens and shuts the iris on the lens. Closing a physical iris is a much better way to protect a camera from being damaged by bright sunlight then simply using electronics to reduce the signal strength.

Alarm Input
Some cameras and video servers 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. Most commonly they are 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. In a few cases manufactures only supply TTL (very low voltage) level inputs. In that case, they can only be safely used with normal alarm sensors in conjunction with some additional isolation circuitry. In this case, it is best to contact the manufacturer for advice.

Ambient Light Level
This refers to the amount of background light in any given situation.

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.

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 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 Focus
This is the alignment of the rear of the lens to the imaging device.

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.

Balanced Signal
This is a method of transmitting audio or video signals over a pair of wires, often twisted pair cable. By sending two equal, but opposite signals, this system minimises external interference, and maintains signal quality.

Balun stands for Balanced - Unbalanced. It is a device used to interface between balanced lines and unbalanced lines. For example, twisted pair to co-axial.

BNC Connector
BNC is a bayonet style connector for coaxial cable that is most commonly used for CCTV installations.

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.


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

C Mount Lens & CS Mount Lens
There are two main types of lens used in CCTV cameras. The C mount lens has a flange back distance of 17.5mm. The CS mount lens has a flange back distance of 12.5mm. C mount lenses therefore have a longer focal distance. CS mount became widely used, because it its more practical for many of today's more compact cameras. Lenses are often supplied with a 5mm spacer ring (sometimes called a C ring) that allows a C mount lens to be used on a CS camera. Most modern cameras are CS.

Co-Axial Cable
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 co-axial cable. The cable used for normal CCTV installations is called RG59.

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

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. Normally this is on a large hard drive in the machine. For this reason it is sometimes known as a hard disk recorder (HDR).

Direct Drive (DD)

Some auto iris lenses require a DC signal from the camera. These are known as direct drive lenses.

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.

Distribution Amplifier
This is a device that accepts one video input and sends it out to 2 or more devices. This is often used in a situation where the signal needs to be split between a number of video monitors or recording devices. 2 or 3 devices can often (but not always) be driven just by splitting the signal with a "T" connector, but more than that, and invariably a distribution amplifier is required to give an acceptable signal quality.

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

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
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. 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". As you can see, this makes a big impact when choosing lenses.


This 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.


This refers to a poor image, where the primary signal is weak, allowing a secondary signal to interfere with the picture.
Hertz (Hz)


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.

This is a mechanical device that adjusts to vary the amount of light passing through the lens of a camera.


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. JPEG was developed by the Joint Photographic Experts Group.


JPEG-2000 is a successor to the JPEG compression standard, that uses wavelet-based compression as opposed to the Discrete Cosine Transformation (DCT). It provides a higher compression ratio than the JPEG standard, for the same image quality.

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.

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.


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.


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.

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

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 of course also be video recorders.

Mechanical Focus

This refers to the process of manually setting the focus on a lens.

The device used to view video pictures. These devices do not normally have television RF frequency receivers. They normally have composite, or component video inputs.

This refers to a black and white image rather than a colour one.

MPEG is a standard used for coding and compression of moving images. It was developed by the Moving Pictures Experts Group. 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.


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.

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.

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.


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.


This is the system used for TV signals in France.


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.

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.

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 the 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.

Video Server (Network Video Server)

This is a device that accepts inputs from CCTV cameras using the PAL, composite video signal. It digitises the signal and then transmits images via a network, dialup or GSM connection. Advanced video servers have built-in video motion detection, and can buffer images while they are being uploaded to a remote server, where they are recorded.
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