CCTV - Glossary of terms.

2:1 INTERLACE - The precise combination of two fields of 312½ lines to create a single frame of 625 lines (CCIR).

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.

ALARM ACTIVATED VCR - From selecting 'record', a normal VCR would take from 15 to 21 seconds before it actually starts recording useable pictures. With an alarm-activated recorder it can be set so that the tape is spooled up and ready to commence recording in about one second. The signal to go into recording can be triggered from an alarm sensor or any other input.

ALC (AUTOMATIC LEVEL CONTROL) - This is a manual adjustment you will find on Auto Iris lenses, and is also known as peak/coverage control. Adjusting this control allows the auto iris circuitry to either take bright spots into greater consideration (peak), bringing out the detail in bright areas, or to take them into less consideration (average) bringing out detail in shadows.

AMBIENT LIGHT LEVEL - The background or general light level of a given area.

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

ANGLE OF VIEW - Angle of view is the angle covered by a picture for a particular lens and camera combination. The higher the number, the wider the area that can be covered by the camera-lens combination. However, the angle will decrease with the format size, of either the camera or the lens. For example with lenses: - 1/3" 2.6mm lens on a 1/3" camera will give a 75 deg angle of view - 1/2" 2.6mm lens on a 1/2" camera will give a 90 deg angle of view

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.

ASPECT RATIO - The ratio of the vertical to the horizontal dimension of an image. This is normally 3:4.

ASPHERICAL LENS - These types of lenses are specially shaped to better focus the light rays entering the lens. This allows approximately 25% more light onto the sensor. This will in turn improve the overall lux level, enabling the camera to perform better at lower light levels.

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

AUTOMATIC IRIS - An electronic circuit fitted to the iris of a lens to help compensate for large changes in light levels. This type of lens is recommended for use where there are low light or changing light conditions, for example external use. An Auto Iris lens adjusts the aperture automatically dependent on the light level, very much like the human eye.

AUTO-TERMINATING - Feature whereby the equipment automatically selects the correct termination depending on whether a BNC video output is connected.

AUTO WHITE BALANCE - Feature on colour cameras whereby the camera constantly monitors the light and adjusts its colour to maintain white areas.

BACKBONE - A high-capacity network that links together other networks of lower capacity. A local backbone network would typically be an FDDI network acting as an in-building backbone to link together multiple LANs.

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 be transferred on to a twisted pair cable.

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.

10Base2 - A form of Ethernet and IEEE 802.3 network cabling using thin coaxial. It refers to 10Mbit/s speed Baseband transmission over 200 meters maximum length (in practice 185m) and is commonly known as Cheapernet.

Base5 - A form of Ethernet and IEEE 802.3 network cabling using thick coax. It refers to 10Mbit/s speed Baseband transmission and 500m maximum length.

10BaseT - A form of Ethernet and IEEE 802.3 network cabling using twisted pair cabling. It refers to 10Mbit/s speed baseband transmission twisted pair cable with a maximum segment length of 100m.

100BaseT - IEEE standard from proposals by the Fast Ethernet Alliance (including 3Com and Syn-Optics). It will support Category 3, 4 & 5 UTP cabling.

BIFURCATOR - An adaptor with which a loose tube containing two optical fibres can be split into two single fibre cables. (See loose tube).

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.

BLACK LEVEL - The dark parts of a video signal corresponding to approximately 0.3 volts.

BLANKING PERIOD - The period of the composite video signal at black level and below when the retrace occurs, making it invisible on the screen.

BLC (BACK LIGHT COMPENSATION) - A feature of modern CCD cameras, which electronically compensates for high background lighting, to give details that would normally be silhouetted.

BNC - Video connector used in CCTV installations.

BRIDGE - Device connecting two separate networks. Once bridging is accomplished, the bridge makes interconnected LANs look like a single LAN, passing data between the networks and filtering local traffic.

BUFFER - The material surrounding the fibre to protect it from physical damage.

CABLE EQUALISER - An amplifier to increase a video signal to the optimum value. This is usually to compensate for cable losses.

CATEGORY 5 - Cabling standard specified by EIA/TIA 568 for use with speeds up to 100Mbit/s including FDDI (TP PMD), 100BaseT and 100BaseVG-AnyLan, and potentially ATM at 155Mbit/s.

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.

CHROMA BURST - The reference signal included in the video signal after the horizontal sync pulse. This enables a colour monitor to lock on to a colour composite video signal.

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

CLADDING - The outermost region of an optical cable, less dense than the central core. Acts as an optical barrier to prevent transmitted light leaking away from the core.

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).

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 a C-mount configuration. These lenses are more compact and cheaper than the C-mount equivalents.

CS TO C MOUNT ADAPTOR - This is also known as a spacer ring, and is used when you want to fit a C-mount lens on a CS-mount camera.

CO-AXIAL CABLE - Any cable that has a conductor and shield sharing the same axis. Also known as RG59.

COMPOSITE SYNC - A signal comprising of both Line and Field sync pulses but without any video information.

COMPOSITE VIDEO - A complete video signal, comprising of sync and video information. The sync pulse should be 0.3 volts and the video signal should be 0.7 volts.

CORE - The central region of an optical fibre through which a signal carrying infrared is transmitted. Manufactured from high-density silica glass.

CROSSTALK - Unwanted interference from another adjacent communications channel..

DB (DECIBEL) - a logarithmic ratio between two signals. It is used as the unit of noise or signal strength.

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.

DESKTOP 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 microprocessor.

DIRECT DRIVE LENSES - This is an Auto-Iris lens, which has a motor or galvanometer coil moving the iris vanes in the lens, driven directly by the camera. There are no active electronics in the lens, hence they are simpler, smaller and cheaper than the equivalent auto-iris lenses.

DUPLEX - Used in multiplexing terminology, whereby two single (simplex) units are assembled in one unit enabling the system to record all camera images onto the VCR and display them on the monitor simultaneously.

DWELL TIME - The length of time a switcher will hold a camera before moving onto the next camera in the sequence.

EI (Electronic Iris) - Automatically varies a CCD camera's shutter to mimic Auto Iris control. Allowing fixed or manual iris lenses to be used in a wider range of areas.

EIA - The American 525 line standard for the video signal.

EIA/TIA - The US Electronics Industries Association and Telecommunications Industries Association, which have merged. The EIA is a US trade organisation for the electronics industry that concentrates on hardware interface standards.

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

ETHERNET - The most widely used LAN transmission network. Based on a bus network topology, it runs at a maximum speed over 100 meters of 10Mbit/s. It operates over conventional 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.

FAST ETHERNET - Proposed 100Mbit/s technology for workstation LANs.

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.

FC - Foot candles, used in some USA specifications to define sensitivity. 10fc is approx. 1 lux.

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. This is more effective than copper wire over long distances.

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 312½ lines. 50 fields are created every second.

FLANGE BACK LENGTH - The distance from the back flange of a lens to the sensor face. This is 17.526mm for C-mount and 12.5mm for CS-mount lenses.

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

FOCAL POINT - The point on the axis of a lens to which parallel rays of light will meet.

FRAME - One complete TV picture made up of approximately 625 lines. 25 frames are transmitted every second.

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. This method helps to eliminate the vertical streaking caused by, for example, car headlights.

FULL PICTURE UPDATE - This technology enables a transmission system to update a full, high quality picture at very fast speeds.

GALVANOMETRIC - This is one method used on Auto Iris and Direct Drive lenses to move the iris vanes, open and closed using a coil operation.

GAMMA CORRECTION - An electronic correction carried out by 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.

GRADED INDEX (GRADED INDEX PROFILE) - A measurement, shown in the form of a diagram, which illustrates how the quality of glass used in optical fibre alters gradually, from densest at the core to the optically less dense cladding.

GROUND LOOP - An alternating current (AC) 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 of the signal path. This results in interference of the video pictures in the form of a black shadow bar across the screen or as a tearing effect in the top comer of a picture.

GROUND LOOP TRANSFORMER - An isolation transformer. There is no direct connection between input and output.

HARDWIRED - Direct cables connected from one electronic product to another. Can be 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 (e.g. picture frames, alternating of the current, etc).

HUB - The centre of a star topology network or cabling system. A multi-node network topology that has a central multiplexer with many nodes feeding into and through the multiplexer or hub. The other nodes do not usually directly interconnect. LAN hubs are increasingly popular with the growth of structured cabling and the need for LAN management.

IDENT - Characters generated onto a video signal, for example, titles, time and date.

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

IMAGE SENSOR FORMAT - Size of the CCD element in the camera.

IMPEDANCE - A measure of the total opposition to current flow in an alternating current circuit, measured in Ohms (O).

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.

INTERLACE - The accurate combining of two fields, one odd one even, into a single frame. For standard video picture make up, you would have 50 fields per second interlaced, which equates to 25 frames per second.

INTERLEAVING - Method used with alarms or activity detection which allows extra frames of video, from alarmed cameras, to be added to a time multiplexed sequence whilst a state of alarm exists.

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 within the camera, which is typically used 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.

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IP RATING - This is used as an index of environmental protection. The number combination defines the protection from outside influences afforded by an enclosure. The minimum rating for most external housings for cameras is IP65.

IRIS - The mechanism within a lens that is adjusted to vary the amount of light falling upon the imaging device. If this is an Auto Iris lens for example, then the mechanism would close when it is very bright, and open as it becomes darker, to give a good usable picture.

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

ISDN (INTEGRATED SERVICES DIGITAL NETWORK) - A higher transmission speed for telephone circuits, which allows video, signals to be transmitted using fastscan at speeds of 128Kb/second: used with terminal adapters.

ISIT (INTENSIFIED SILICON INTENSIFIED TARGET) - A camera used in exceptionally low light conditions. This type of camera is susceptible to damage and requires great care when used in normal lighting conditions. Some cameras have an internal sensor that automatically cuts out the first level of intensification.

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.

J. SYNC - Pulses generated, normally by the camera, to tell other pieces of equipment that a picture, or one line of a picture, is about to start.

LAN (Local Area Network) - A communications system that links computers into a network, usually via a wiring based cabling scheme. LANs 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.

LASER - A source of exceptionally pure light that can consist of a single wavelength concentrated into a straight beam. This is used to transmit infrared light along an optical fibre. A laser light source is used when extra high data rate performance is required.

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 AMPLIFIER - A device to make good the loss of signal strength and quality due to long cable runs.

LINE FEED CAMERA - A camera that receives its power along the same cable that is used to send its picture back to the monitor. Makes for ease of installation, but cabling distances can be restricted and connections to other equipment can cause problems.

LINE LOCKED - The sync pulses of cameras that 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.

LOOP FRAME STORE - There are a variety of proprietary names for this latest advance in video storage technology. The principle is that a series of video frames is compressed and stored in digital form, in what is likened to a continuous loop. This records, for instance, 200 frames and then records over the top again and carries on doing this until an alarm signal is received. When this happens, the equipment carries on recording for (say) 150 frames and then stops. This means that 50 frames BEFORE the event causing the alarm and 150 frames after the event are captured. It eliminates the tedious searching through hours of dubious quality video recording and concentrates the reviewing on the period of real activity. There are currently systems that can record four or eight cameras simultaneously using this technique.

LOOSE TUBE - This refers to a type of cable in which one or more optical fibres are laid loosely within a tube.

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.

MAN (METROPOLITAN AREA NETWORK) - A high-speed network designed to link together sites in a metropolitan or campus area.

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.

MECHANICAL FOCUS - The mechanical aligning of the imaging device with the focal point of the lens, most often on zoom lenses, to ensure the image stays in focus throughout the zoom range that the system has been set to.

MICROWAVE TRANSMISSION - Another method of transmitting video signals and telemetry signals over free space. This is less affected by weather conditions and transmits over longer distances. Most countries require a licence to use this method.

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.

MODE - (In fibre optics) The path taken by a light ray as it travels along a fibre.

MONOCHROME - A black and white picture. Recommended for use with external cameras, as they have better picture resolution at night.

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.

MULTIMODE FIBRE - An optical fibre that allows the signal carrying light to travel along more than one path.

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 pre-programmed 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. However, this requires a multiplexer at each end of the line.

ND FILTER - A glass filter that attenuates light evenly over the visible spectrum. It would be used to reduce the amount of light entering a lens and thus force the iris open to its maximum. This is to simulate conditions of low light in order to set the focus accurately on the desired part of the scene, for low light operation.

ND SPOT FILTER - A graduated filter at the very centre of a lens element adjacent to the iris plane. This restricts the amount of light that can be passed when the iris is fully closed. This is because the iris does not close to a zero aperture and exceptionally bright light could damage the camera. It has no effect once the iris has opened slightly.

NETWORK ADAPTER CARD - A printed circuit board, installed in a computer, enabling the computer to be joined to the LAN.

NODES - Devices on a network where transmission paths are connected. The node term is often used instead of workstation.

NOISE - This 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.

PEAK-TO-PEAK - The measurement of a video signal from the base of the sync pulse to the top of the white level. For a full video signal this should be one volt.

PHASE ADJUSTABLE - The ability to delay the line locking process so as to align cameras fed from AC voltages of different phases.

PHOTOCELL - A device that automatically switches on the infra-red lights when light levels fall to a pre-set level.

PHOTO DETECTOR - A device at the receiving end of an optical fibre link that converts light to electrical power.

PIC IN PIC - An electronic device that superimposes the view from one camera over that of another.

PORT - A computer interface that can be used to attach another device, such as a modem, for communicating with a remote terminal or, if the port is within a hub, to a workstation.

PRESETS - A function contained within a telemetry system that, on receipt of a signal, causes a particular camera to pan (left to right), tilt (up and down) and zoom (lens) to a predetermined field of view. Most systems can accommodate up to 16 pre-set positions for each camera. This is an especially useful feature on larger systems with alarmed areas.

PRIMARY COATING - A thin plastic coating applied to the outer cladding of an optical fibre. This is essential to protect the fibre from contamination and abrasion.

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P.S.T.N. (PUBLIC SWITCHED TELEPHONE NETWORK) - The telephone network provided by both BT and Mercury through which video, data or audio signals can be sent.

QUAD SPLITTER - A product that can display the views from 4 cameras simultaneously on one monitor. It is also possible to select any individual camera for full-screen display on real time monitoring, dependent on model.

RANDOM INTERLACE - A method of combining two fields to make one frame where strict timing is not a requirement.

REFLECTANCE - The ratio of light returned from a surface expressed as a percentage.

REFLECTED LIGHT - Scene illumination multiplied by reflectance. This is the amount of light returned to the camera and determines the quality of picture.

REFRACTED INDEX PROFILE - A description shown in the form of a diagram illustrating how the optical density of an optical fibre alters across its diameter.

REGENERATORS - Devices placed at regular intervals along a transmission line to detect weak signals and re-transmit them. These are seldom required in fibre optic systems. (Often incorrectly referred to as 'repeaters').

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 desktop 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 - Router Network interconnector device. A router can be used to link LANs together locally or remotely as part of a WAN. A network built using routers is often termed an internetwork.

S/N (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.

SCANNER - A device that can be remotely controlled to provide horizontal movement only to a camera.

SCART - A standard European 20 pin connector, used for carrying both audio and video signals in domestic TV appliances. The majority of modern televisions and VCRs have a scart connector on them.

SCENE ILLUMINATION - The density of light in LUX falling on the area to be viewed. For best results the ratio of the lightest to the darkest areas should not exceed a factor of two.

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 into another.

SENSITIVITY - Of a camera, usually specified in LUX, should be the amount of light falling upon an imaging device to produce a video signal of 1-volt peak to peak. Confusingly, it is often referred to as the amount of light at the lens, which is not technically accurate and can be misleading unless the lens specification is provided in detail, including its light transmission value.

SHEATH - The outer finish of a cable. Usually this consists of an extruded layer of PVC or polyethylene.

SILICA GLASS - The exceptionally pure glass used to make an optical fibre.

SIMPLEX - This is a type of multiplexer. There is only one PCB inside a Simplex unit, so it can only do one of the three functions at a time. It can either record all cameras simultaneously, playback all cameras or display all cameras onto a monitor at the same time. It is mainly used on multiple camera systems, but because of its limitations, it tends only to be used on an un-named installation.

SINGLE MODE FIBRE - An optical fibre constructed in such a way that light travelling along the core can only follow one path. (Also called mono-mode).

SIT (SILICONE INTENSIFIED TARGET) - a camera for use in very low light levels.

SLOW SCAN - A term usually applied to a method of transmitting video signals through the public telephone network. The speed of transmission is dependent on the type of network between the transmitter and receiver. The quality of picture is proportional to the speed of transmission. Modern systems, using digital compression techniques, can transmit good quality pictures in a few seconds.

SPECTRAL RESPONSE - The sensitivity of the imaging device to different frequencies of light. This is usually in the range of 300-1000 nanometres. Visible light is up to 730nm, dull red infrared lighting from 700-850nm, invisible infrared is over 850nm.

STEP INDEX - (Step index profile). A measurement shown in the form of a diagram which illustrates how the quality of glass used in optical fibre graduates from the highest density to the lowest. The shift from one density to another causes the light to bounce as it travels.

STRAIN MEMBER - The part of an optical fibre cable that removes any strain on the fibres. Commonly used materials are steel and synthetic yarns.

S.V.H.S. (Super Video Home System) - New format high-resolution VHS video recorders, capable of giving greatly improved picture if all features are used. VHS compatible.

SWITCHER - A video switcher to which the cables from the cameras are connected and which contains the switching electronics. This allows the user to switch between the pictures given by the different cameras, either manually or automatically after a preset time. Many switchers also have the facility that, if an alarm is activated, the switcher will switch to the camera covering the area where the alarm has been activated.

SWIVEL MOUNT - Used on a tower or bracket to enable the housing to be pointed at any angle, vertically or horizontally.

TELEMETRY - The system by which a signal is transmitted to a remote location in order to control the operation of equipment. In CCTV systems this may include controlling pan, tilt and zoom functions, switch on lights, move to pre-set 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.

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.

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.

TIGHT BUFFERED - A type of cable in which the optical fibres are tightly bound.

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 is now nearing the end of its life in industrial security systems.

TOUCH SCREEN CONTROL - A system that allows all the camera controls to be displayed on the screen of a special monitor. Controlling any function simply requires the screen to be touched at the appropriate symbol 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.

TRIPLEX - Multiplexer feature that gives you the ability to simultaneously view both playback and live cameras within the same multi-screen, while still recording.

T.V.L. - Television Lines. The maximum number of changes between light and dark on a picture, across 3/4 of the width, dictates the resolution of a CCTV product, measured in TVL.

TWISTED PAIR - A cable, often screened, that consists of two conductors twisted together along their length.

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

UNTERMINATED - Video input of apiece of equipment, wired so as to allow the video signal to be fed to further equipment. Does not necessarily include extra sockets for the extra cables.

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.

VARIFOCAL LENS - A type of lens that enables a manual selection between two focal lengths, to give the desired picture view. A good option for customers, who are unsure of angle or distance, as it saves having to try multiple lenses.

VEXT - multiplexer feature that uses a pulse generated by the VCR to automatically adjust to the VCR time lapse speed.

V.H.S. (Video Home System) - Name given to ½" tapes, used with domestic video recorders.

VIDEO LAUNCH AMPLIFIER - An interface between the video source and distribution cable. This is used where the video signal requires to be processed before being launched over coaxial or balanced lines.

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 (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 that covers a larger geographical area than a LAN and where telecommunications links are implemented, normally leased from the appropriate PTO(s). Examples of WANs include packet switched networks, public data networks and Value Added Networks.

WAVELET - Compression that 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).

WORKSTATION - Term used freely to mean a PC, node, terminal or high-end desktop processor (for CAD/CAM and similar intensive applications) - in short, a device that has data input and output and operated by a user.

Y/C - A colour camera producing separate luminance (Y) and chrominance (C) signals to provide greatly improved picture quality from video recorders. Can only be used with a restricted range of equipment.

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