AGC: Automatic gain control – electronic circuitry to increase the video signal in low light conditions. This usually introduces ‘ noise’ in the picture giving a grainy appearance. Camera specifications must always be considered with AGC off.

ANALOGUE SIGNAL: In video, the representation of a camera scene by varying voltages in the video signal, the voltage being directly proportional to the light level.

APERTURE: The light gathering area of a lens. The iris controls the size of the aperture.

ARMOUR: Extra protection for a cable that improves resistance to cutting and crushing. The most common material used is steel.

ATTENUATION: A term that refers to signal loss in a transmission system.

AUTOMATIC IRIS: A lens that automatically adjusts to allow the correct amount of light to fall on the imaging device. There is a tiny motor and amplifier built in which generally receives a control signal from the camera to maintain a constant one volt peak to peak (pp) video level. There are two manual controls on the lens to allow compensation for varying conditions of ‘peak’ and ‘average’ light.

BACK FOCUS: A mechanical adjustment in a camera that moves the imaging device relative to the lens to compensate for different back focal lengths of lenses. An important adjustment when a zoom lens is fitted.

BALANCED SIGNAL: A video signal converted to a balanced signal, usually to enable it to be transmitted along a ‘twisted pair’ cable. Used in situations where the cabling distance is too great and which would produce unacceptable losses in a coaxial cable.

BALUN: A transformer that levels out impedance differences so that a signal generated on to a coaxial cable can transfer on to a twisted pair.

BANDWIDTH: The range of frequencies a transmission line or channel can carry: the greater the bandwidth, the greater the information-carrying capacity of a channel. For a digital channel this is defined in bit/s.

BITS PER SECOND: The rate at which individual bits are transmitted across a communications link or circuit; written bit/s. One thousand bit/s is 1 Kbit/s, and one million bit/s is 1 Mbit/s.

Cat5 / Network cable: Cabling standard specified by EIA/TIA 568 for use with speeds up to 100Mbit/s 100BaseT

C-MOUNT: The standard screw mounting for 2/3” and 1” camera lenses. The distance from the flange surface to the focal point is 17.526mm. A C-mount lens can be used on a camera with a CS-mount by adding an adaptor ring to reduce this distance to 12.5mm (See CS mount).

CCD: Charge coupled device, a flat thin wafer that is light sensitive and forms the imaging device of most modern cameras. Size is measured diagonally and can be 1/3”-1/2” or 2/3”. There are two types, frame transfer and interline transfer.

CCIR: The European 625 line standard for the video signal.

CHROMINANCE: The part of a colour video signal that carries the colour information.

COMPOSITE VIDEO: The complete video signal comprising the sync and video information. The sync pulse should be 0.3 volts and the video signal should be 0.7 volts.

CROSSTALK: Unwanted interference from another adjacent communications channel. The signal from the channel, adjacent channel is inserted into the original communications.

CS MOUNT: A new generation of lenses designed for 2/3”, 1/2” and 1/3” cameras incorporating CS-mounts. The distance from the flange surface to the focal point is 12.5mm. CS-mount lenses cannot be used on cameras with C-mount configuration. These lenses are more compact and cheaper than the C-mount equivalents.

dB: Decibel, a logarithmic ratio between two signals.

DEPTH OF FIELD: The proportion of the field of view that is in correct focus. The depth of field in focus DECREASES when: the focal length is longer, the f number is smaller, or the object distance is shorter.

SWITCHER: A device for switching the video signal from several cameras to one or more monitors. The cables from the cameras are connected to the back of the unit.

DIGITAL SIGNAL: An analogue signal that has been converted to a digital form so that it can be processed by a micro processor.

EIA/TIA 568: The EIA/TIA’s most famous standard specifying a maximum horizontal cable run of 90 metres, allowing 10 metres for hub and device attachment resulting in a total cable length.

ETHERNET: The most widely LAN transmission network. Based on a bus network topology, it runs at a of 100 metres, maximum 10Mbit. Techniques operating over convention co-axial cable, thin wire co-axial cable and unshielded twisted pair cabling. This has several implementations – 10Base5 for use over conventional co-axial cable, 10BaseF for use over optic fibre, and 10BaseT for use over Unshielded Twisted Pair (UTP) cabling.

F STOP: This is the ratio of the focal length to the effective diameter of the lens (f/A). It is not a measure of the efficiency or the transmission value of the lens. The smaller the f number the more light is passed.

FIBRE OPTIC: A very efficient method of transmitting video and telemetry signals over very long distances using fibre optic cable. Signals can be multiplexed and sent along a single fibre.

FIELD OF VIEW: The relationship between the angle of view and the distance of the object from the lens.

FIELD: One half of a frame consisting of 312 1/2 lines. 50 fields are created every second.

FOCAL LENGTH: The distance between the secondary principal point in the lens and the plane of the imaging device. The longer the focal length, the narrower is the angle of view.

FRAME STORE: An electronic method of capturing and storing a single frame of video. All slow scan transmitters include a frame store that holds the picture at the moment of alarm, while the control is being dialled up. When the link is confirmed, the picture is transmitted.

FRAME TRANSFER: A type of CCD imaging device in which the entire matrix of pixels is read into storage before being processed by the electronics of the camera.

FRAME: The combination of two interlaced fields, 25 frames are created every second.

GAMMA CORRECTION: An electronic correction carried out in the camera circuitry to balance the brightness seen by the camera to that of the monitor.

GATEWAY: A node that allows connection to another network using another protocol.

GEN LOCK: Also called external sync. A separate coaxial cable is run to each camera and carries sync pulse information to ensure that all cameras are producing fields at exactly the same time. This eliminates picture bounce during switching and can improve quality and update time in multiplexers.

GROUND LOOP TRANSFORMER: An isolation transformer, no direct connection between input and output.

GROUND LOOP: An AC current that can be produced in a cable. This is usually caused by parts of the system being fed from different electrical sources resulting in different earth potentials at each end. The result is interference on the signal.

HARDWIRED: Controlling remote equipment by direct voltage transmitted along a multicore cable from the main controller. This is very labour intensive to install and is only used in simple systems with short cable runs.

H COMPRESSION: H261, H263, H321 & H324 offering a high frame rate, low image quality, these compression techniques are popular for video conferencing applications. The low image quality is particularly acute when the image contains moving images.

HERTZ (Hz): The number of variations per second.

ILLUMINANCE: The measurement of light in lumens per square metre, the unit of which is the lux.

IMPEDANCE: A measure of the total opposition to current flow in an alternating current circuit, measured in Ohms.

INFRA RED LIGHT: The wavelength of light produced above the visible part of the spectrum.

INFRA RED TRANSMISSION: A method of transmitting video and telemetry signals across free space along an infrared beam. This opens possibilities for using CCTV where it had been previously impossible to run cables. Distance can be limited and the signal can be degraded in adverse weather conditions.

INTERLINE TRANSFER: Another type of CCD imaging device in which the rows of charge are stepped down one at a time and processed straight away.

INTERNAL SYNC: The internal generation of sync pulses in a camera without reference to external sources. This uses a crystal controlled oscillator and is needed on non-mains powered cameras. IP (Internet Protocol): The TCP/IP standard protocol that defines the IP datagram as the unit of information passed across an Internet and provides the basis for connectionless packet delivery service.

IP Address: The 32-bit address assigned to hosts that want to participate in a TCP/IP Internet. IP datagram – The basic unit of information passed across a TCP/IP Internet.

IP RATING: Index of protection, a number combination that defines the protection from outside influences afforded by an enclosure.

IR SHIFT: The difference in the field of view in focus between daylight and infrared light.

IRIS: The mechanism that can be adjusted to vary the amount of light falling on the imaging device.

JPEG: A popular compression standard for still images. Native to most web browsers this is the preferred image standard for many Network cameras.

JPEG2000: A new compression medium but as yet uncommon standard, based on Wavelet technology.

LAN (Local Area Network): A communications system that links computers into a network, usually via a wiring-based cabling scheme. LAN's connect PCs, workstations and servers together to allow users to communicate and share resources like hard disk storage and printers. Devices linked by a LAN may be on the same floor or within a building or campus. It is user owned and does not run over leased lines, though a LAN may have gateways to the PSTN or other, private, networks.

LED: (Light Emitting Diode). An electronic component that produces light when stimulated by electricity. This is the cheapest and most common type of transmitter for infrared light along an optical fibre.

LENS MOUNT: See C-mount and CS-mount.

LINE LOCKED: The sync pulses of cameras are locked to the AC mains frequency.

LINE POWERED: A camera in which the power is supplied along the same coaxial cable that carries the video signal. Makes for ease of installation, but cabling distances can be restricted and connections to other equipment can cause problems.

LUMINANCE: The brightness element of a video signal.

LUX: Density of light measured in lumens/sq metre. This is used< to define the sensitivity of camera imaging devices.

MATRIX SWITCHING: This is an advanced method of switching video signals where there can be any number of inputs directed to any number of outputs. These are generally associated with larger systems where it is necessary to have several control positions, each of which can be programmed to control different groups of cameras.

MJPEG: This compression standard generally refers to JPEG images shown at high frame rate, generally 30 frames per second. It gives high quality video images, but the comparatively large file sizes of each individual image does put demands on the transmission bandwidth.

MICROWAVE TRANSMISSION: Another method of transmitting video and telemetry signals over free space. This is less affected by weather conditions and transmits over longer distances. The need for a licence should be checked for the system being considered.

MPEG1: The video standard that typically delivers 30/25 (NTSC/PAL) frames per second. With many variations, this format provides low-resolution images but places low demand on the transmission medium.

MPEG2: Compression offers higher resolution images and same frame rates as MPEG1. Only modern computers can decode this format, as it generally demands high computing capacity.

MPEG4: A video compression standard that offers high performance video with good resolution and moderate demand on transmission bandwidth. Most MPEG4 solutions today however are focused on high-end applications, since they are very costly.

MULTIPLEXER: An electronic system that can accept a number of camera inputs and record them virtually simultaneously. Most also provide many other features such as multi screen displays with four, nine, sixteen etc. cameras on the screen at once. Most have alarm inputs that trigger several preprogrammed functions. Multiplexers can be used to transmit up to sixteen pictures down a single video line whether it is a coaxial cable, microwave, infrared link etc. This requires a multiplexer at each end of the line.

NOISE: Is inherent in all electronic circuits. In a video signal this appears as snow or graininess in the picture.

PAN TILT: A device that can be remotely controlled to provide both vertical and horizontal movement for a camera.

PIP: An electronic device to superimpose the view from one camera over that of another.

PORT: A computer interface capable of attachment to another device, such as a modem for communicating with a remote terminal, or, if the port is within a hub, to a workstation.

QUAD SPLITTER: A device that can display the views from four cameras simultaneously on one monitor. It is also possible to select any camera as a full screen view.

REMOTE SWITCHER: A video switcher to which the cables from the cameras are connected and which contains the switching electronics. This unit may be remotely located and connected to a desk top controller by a single cable for each monitor.

RJ11: Popular name for the standard four-wire modular connector for phone connections.

RJ45: Popular name for the eight-pin modular connector in the 10BaseT standard for UTP connections to workstations or smart wiring hubs.

ROUTER: A router can be used to link Networks, PC’s, cameras together locally or remotely as part of a WAN.

S/N RATIO: Signal to noise ratio, a measurement of the noise level in a signal expressed in dB (decibels). In a video signal values from 45dB to 60dB produce an acceptable picture. Less than 40dB is likely to produce a ‘noisy’ picture.

SCREEN SPLITTER: A term usually used for a device that can combine the views from two cameras on a single screen. The split can be arranged horizontally, vertically or one picture inserted in another.

SENSITIVITY: Of a camera – usually specified in LUX and should be the amount of light falling on the imaging device to produce a video signal of 1 volt pp (peak to peak). However, it is more often given as the amount of light at the lens. This can be confusing unless the lens specification is provided in detail, including its light transmission value. SIGNAL TO NOISE: The ratio, expressed in decibels, of the signal voltage to the noise voltage in an electronic circuit.

TELEMETRY TRANSMITTER: The unit that is at the control position of a CCTV system and contains the keys, joysticks etc. for the remote control of pan/tilt/zoom cameras.

TELEMETRY: The system by which a signal is transmitted to a remote location in order to control operation of equipment. In CCTV systems to control pan and tilt and zoom functions, switch on lights, move to preset positions, etc. The controller at the operating position is the transmitter and there is a receiver at the remote location. The signal can be transmitted along a simple ‘twisted pair’ cable or along the same coaxial cable that carries the video signal.

TERMINATION: The video cable requires an impedance of 75 ohms at normal video signal bandwidth. This is often called ‘low Z’. There is a switch on the back of the monitors to select either 75 ohm or ‘high Z’ (sometimes ‘high/low’). If a signal is looped through more than one monitor all should be set to ‘high’ except at last, which should be to ‘low’ or 75 ohm.

TIME LAPSE VCR: A type of industrial video recorder that can be set to record continuously over long periods. Typically, this can be from three hours to 480 hours, achieved by the tape mechanism moving in steps and recording one frame at a time. This means that if set to record over long periods much information can be lost. For instance in the 72-hour mode only 3 frames/second will be recorded instead of 25 frames/second in the real time mode. On receipt of an alarm signal these machines can be automatically switched to real time mode. With rapid advances in digital storage and retrieval techniques the mechanical video recorder must now be nearing the end of its life in industrial security systems.

TOUCH SCREEN CONTROL: A system by which all the camera controls are displayed on the screen of a special monitor. To control any function simply requires the screen to be touched at the appropriate symbol which can be to select a camera or pan, tilt and zoom. The system is computer driven and can include maps, diagrams, etc. that are automatically displayed according to the alarm received.

UNBALANCED SIGNAL: A composite video signal transmitted along a coaxial cable is an example of an unbalanced signal. (See balanced signal).

UPS (Uninterruptible Power Supply): A battery, attached to a piece of hardware, for example a server, that provides back up power for conducting an orderly shutdown if the server’s normal power supply fails.

UTP (Unshielded Twisted Pair): The standard cabling used for telephone lines. The standard IEEE 802.3, 10BaseT, defines use of Ethernet over UTP for rates up to 10Mbit/s. The general LAN medium of choice for the 1990s.

VIDEO SWITCHER: A device to enable different cameras to be selected to a monitor. The switching is normally arranged to take place during the blanking period of the video signal to prevent a picture bouncing when a new one is selected.

VMD: Short for VIDEO MOTION DETECTION, a method of detecting movement in the view of the camera by electronic analysis of the change in picture contrast.

WAN (Wide Area Network): A network which covers a larger geographical area than a LAN and where telecommunications links are implemented, normally leased from the appropriate PTO(s). Examples of WAN's include packet switched networks, public data networks and Value Added Networks..

WAVELET: Compression is optimised for images containing low amounts of data. The relatively inferior image quality is offset against the low bandwidth demands on transmission mediums.

WHITE LEVEL: The brightest part of a video signal corresponding to approximately 1.0 volt (0.7 volts above the black level).