Technical Page

AC Alternating Current.

ACTIVITY DETECTION A simple form of video motion detection, activity detection merely produces an alarm indication when light levels change. Often used with video multiplexers to increase the update of cameras where activity is occurring.

ADDRESS A sequence of bits, a character or a group of characters that identifies a network station.

AFC Automatic Frequency Control - the automatic tracking of an oscillator to a specific narrow range of frequencies.

AGC Automatic Gain Control - an electronic circuit normally found in cameras which will increase the video signal in low light conditions. In normal operation it will maintain a constant level.

AI Automatic Iris - a function of a lens that automatically adjusts to varying light levels to allow the correct amount of light to fall upon the image sensor.

ALARM ACTIVATED VCR A VCR that is triggered to start recording from the idle position. This can take as long as 20 seconds to spool the tape and start recording for a useable picture. If the VCR is in time lapse mode an alarm trigger can take considerably less for a useable picture.

ALARMING Ability for CCTV equipment to respond to an external alarm input, providing numerous functions such as to switch the relevant camera signal and alarm start a VCR.

ALC Automatic Level Control - on AI lenses a manual adjustment also known as Peak/Average, a facility to make the lens respond more to the highlights (Peak), or more to the darker scenes (Average) , providing a user adjustment of information seen in the highlight.

ALGORITHM A rule or procedure for problem solving, a commonly used term in mathematics.

AM Amplitude Modulation.

AMBIENT LIGHT The general light level of any given scene.

ANALOGUE An electrical voltage that is directly proportional to a given input, ie. giving a correspondingly variable output.

ANGLE OF VIEW The limits of the viewing area of a camera/lens.

APD Avalanche Photo Diode.

APERTURE The measure of the light gathering capability of a lens. The maximum aperture is the minimum 'f' stop of a lens, generally the lower the 'f' stop the better.

APERTURE CORRECTION Switchable correction circuit which electronically sharpens the image of a picture.

ARMOUR Protection given to a cable to provide resistance to damage that may be incurred from being crushed or cut.

ASPECT RATIO Ratio between the vertical and horizontal image size of a camera sensor, the ratio is 4:3.

ASYNCHRONOUS DATA Most common form of data, where data is passed without any clocks or timing information. Uses start and stop bits to synchronise.

AT COMMANDS A protocol used between a modem and terminal equipment for autodialling and configuration of the modem.

ATTENUATION Term used to describe the amount of loss seen in a video signal when transmitted from one point to another.

AUX Auxiliary.

BACK FOCUS A mechanical adjustment for focusing a camera. It moves the image sensor in relationship to the lens to accommodate for different back focal lengths of lenses.

BALANCED SIGNAL Term generally associated with the transmission of video signals over twisted pair cables. A circuit of two conductors equal to each other in voltage but opposite in polarity.

BANDWITH A value expressing the difference between the upper and lower limits through a range of allowable frequencies.

BAUD The speed at which data is transmitted, ie. 1 baud = 1 Bit per second.
BIT A binary digit, the smallest element of information in a binary system.

BLACK LEVEL A measure of picture signal level, that matches to a specified maximum limit for black peaks.

BLANKING PERIOD The period of the composite video signal at black level (0.3V) and below when the retrace occurs.

BLC Back Light Compensation - an electronic feature of modern CCD cameras to allow viewing of highlighted scenes that would normally be silhouetted, thus providing more detail.

BNC The most common form of video connector used in CCTV.

BPS Bits per second.

BROADBAND Communications transmission where the signal is impressed on a higher frequency carrier.

BRIDGING The connection of more than 1 electrical circuit or component in parallel.

BURN An image that has become permanently frozen onto the sensitive region of a camera or monitor tube.

BUS NETWORK A topology network where all terminals are attached to a transmission medium serving as a bus.

BYTE A unit of 8 Bits.

CABLE EQUALISER Increases the video signal level by amplification, used on long cable runs to compensate for the loss in video signal level.

CAD Computer Aided Design.

CAMEO Term seen in video multiplexer terminology, represents one sixteenth of the area of a full screen.

CATV Communal Aerial Television.

CCD Charge Coupled Device - modern form of camera imaging device.

CCIR Committee Consultative International Radiocommunications. Monochrome Video Signal for most of Europe, Australia and the Middle East - 625 lines, 50Hz.

CCTV Closed Circuit Television - A television system used for private purposes and not for public or general broadcast.

CHROMA BURST A reference signal (4.43Mhz) included in the video signal after the horizontal synchronisation line pulse.

CHROMINANCE The part of a video signal that contains all of the colour information.

CLADDING In Fibre Optics the outermost region of an optical cable, less dense than the central core, it acts as an optical barrier to prevent transmitted light leaking away from the core.

CLOCK A name commonly used for any of the sources of timing signals used in synchronous data transmission.

CMOS Complimentary Metal Oxide Semiconductor - Solid state switching device, also used as a camera pick up device - MOS chip.

C-MOUNT Industry standard thread mounting portion on the rear of a lens. Note a C mount lens can be used on a CS mount camera with a 5mm adaptor ring, however a CS mount lens cannot be used on a C mount format camera.

COAXIAL CABLE A shielded cable containing a central conductor.

COMET TAILS A condition that appears on a VDU that is caused by near burn combined with image movement.

COMPOSITE SYNC A signal containing Line and Field pulses, but one which has no video information.

COMPOSITE VIDEO The combination of all electronic information required to produce a video signal. Comprising 0.7 volts video and 0.3 volts sync., hence the term one volt peak to peak.

COMPRESSED PICTURE A compressed picture is a full size picture that has been reduced in size while still displaying all of the original screen information.

COMPRESSION Taking standard video signals and reducing them into compact digital data.

CONDITIONAL REFRESHMENT A technique used by some video transmission systems. Once the first image has been constructed only the part of the image that changes is subsequently transmitted, allowing high speed updates when little movement is seen. However, the speed of update decreases with more movement.

CORE Central region of a cable.

CPU Central Processing Unit - the main device that contains the processing logic.

CROSS TALK Electrical interference caused by electromagnetic or electrostatic coupling by nearby conductors or external sources. Interference between two or more signals in close proximity within a bandpass.

CRT Cathode Ray Tube a device that is used to display information on a TV or Monitor.

CS to C MOUNT ADAPTOR A spacer ring of 5mm that allows the use of C mount lenses on CS mount cameras.

CS-MOUNT Uses the same thread as C mount but requires the lens to be 5mm closer to the image sensor, new industry standard for lens mounting.

20mA CURRENT LOOP Electronic signalling method for CCTV, used for controlling remote camera functions. Data is sent via a pair of wires, offers greater distance capability and is less susceptible to interference than similar 2 wire data transmission systems.

DARK CURRENT The thermally induced current that exists in a photo diode in the absence of incident optical power.

DAT Digital Audio Tape, a technology for storing massive amounts of digital information in a small package.

DB Decibel - a logarithmic ratio between two signals.
DC Direct Current.

DD Direct Drive - a new form of auto iris drive for lenses, whereby the lenses requires a DC reference from the camera opposed to a video level required by AI lenses.

DECODE Seen in multiplexer terminology referring to the playback of images from tape.

DECOMPRESSION Taking digitally compressed DVST information and restoring this to normal video images.

DEFINITION The ability of a camera or monitor to resolve fine detail, measured in lines.

DEPTH OF FIELD Area of view that is in focus, the depth of field decreases as the aperture increases.

DESKTOP SWITCHER A device to select incoming signals from a number of cameras for display onto one or more monitors.

DIGITAL SIGNAL A signal that comprises binary information ie. 1 or 0, to allow processing by micro processor based equipment.

DISTRIBUTION AMPLIFIER A device that separates and amplifies input signals for multiple outputs.

DROPOUT The loss of video signal from a magetic tape playback head or worn or damaged tapes.

DTMF Dial Tone Multi Frequency - used by some manufacturers for telemetry signalling over twisted pair cables.

DUPLEX Seen in multiplexer terminology where two simplex units have been assembled as one unit, allowing the equipment to perform two functions simultaneously, ie. record pictures to tape whilst displaying multiscreen images of cameras at the same time.

DVST Digital Video Storage & Transmission - name given to equipment that can compress video pictures to a fraction of their former size for transmission over communication networks or for digital storage. Note, this equipment transmits the total image every time.

DWELL TIME The time duration a video switcher will stay on a camera before moving onto the next camera.

EI Electronic Iris - a feature of modern CCD cameras where the camera electronics automatically varies the shutter to mimic an AI lens, allowing the use of fixed or manual iris lenses in a wider range of areas.

EIA Electrical Industries Association - Monochrome video signal for North America and Japan TV standard - 525 lines 60Hz,

EMI Electro Magnetic Interference - electrical or electromagnetic energy that may cause unwanted responses, degradation or failure in electronic equipment.

ENCODE Seen in multiplexer terminology as the recording of images to tape.

ERROR CORRECTION Method employed by modems to ensure that data is transmitted or received error free.

EXTERNAL SYNC The ability of electronic equipment (normally seen in cameras) to accept a synchronisation signal from an external source and synchronise itself to it.

'f' STOP The ratio of the focal length to the effective diameter of the lens. The smaller the 'f' stop, the more light is passed. The 'f' stop directly affects the depth of field.

FC Foot Candle - American measurement of light, 10fc equals 1 lux.

FI Fixed Iris.

FIBRE OPTIC More efficient method of sending signals than the more conventional method of copper cable.

FIELD One half of a frame comprising 312.5 lines, there are 50 fields per second.

FIELD OF VIEW The image area produced by any camera/lens combination.

FLANGE LENGTHS Referring to lenses - C mount = 15.5mm, CS mount = 12.5mm.

FM Frequency Modulation.

FOCAL LENGTH The focal length of a lens measured in millimetres, has a direct relationship to the angle of view that is obtained. A short focal length provides a wide angle of view and a large focal length provides a narrower angle of view.

FOCAL POINT The point at which light passing through a lens is concentrated.

FRAME One complete TV picture, one frame is made up of two fields, TV pictures are made up of 25 frames per second.

FRAME STORE Digital storage medium capable of holding video information in memory.
FRAME TRANSFER Type of CCD imager used by some cameras.

FREQUENCY RESPONSE The capability of a device to transmit or receive a given range of frequencies.

FRONT PORCH A portion of a composite video signal that is located between the start of the horizontal blanking pulse and the start of the corresponding sync. pulse.

FSK Frequency Shifted Keying - a form of signalling employed by some coaxial born telemetry equipment.

FULL PICTURE UPDATE Used to describe video transmission products that send the total image on each update.

GALVANOMETRIC A method of converting the minute electric currents produced by AI circuits, used by both AI & DD lenses.

GAMMA CORRECTION Balances the light seen by the camera to that of the monitor.

GEN LOCK As per external sync, allows cameras to synchronise to a common signal, providing bounce free switching of images.

GHOSTING Mutliple images caused by video signal transmission echoes, not to be confused with burn in.

GIGA Order of magnitude 10 9.

GIP Graded Index Profile a measurement shown in the form of a diagram which illustrates how the quality of glass used in fibre optics alters gradually from the densest at the core to the optically less dense cladding.

GLT Ground Loop Transformer - an isolation transformer with no direct contact between input and output.

GROUND LOOP Caused by different earth potentials clearly seen as interference or humbars on a video signal.

HARDWARE COMPRESSION Video data compression taking place in specialised purpose built microchips.

HARD WIRED Direct cable connection between one product to another, used for control of equipment in simple systems.

HAYES COMPATIBILITY When a modem complies with the Hayes AT command language used by most communication software packages.

HELICAL SCAN Recording system used by VCR's, a record/playback head used by VCR's whereby the head rotates scanning the video tape in a helical path.

HORIZONTAL RESOLUTION The amount of information that can be displayed upon any VDU (measured in lines).

HZ Hertz - Cycles per second. The measure of frequency.

IDENT Camera caption or number displayed on a video signal.

ILLUMINANCE Measurement of light in lumens per square metre, the unit of which is lux.

IMAGE INTENSIFIER A device that uses fibre optics to increase the sensitivity of a pick-up tube.

IMAGE PLANE The surface upon which an image is formed in a camera.

IMAGE SENSOR FORMAT Refers to the size of the image plane within a camera.

IMPEDANCE Measured in ohms, refers to the combined resistance and reactance in an electrical circuit.

INTERLACE (2:1) Refers to the combination of two fields, one odd, one even to produce a single frame.

INTERLEAVING Term used in multiplexing. When a camera is alarmed, be it via external means (PIR) or by activity detection, then it is recorded every other frame.

INTERLINE TRANSFER Type of CCD imager used by some cameras.

INTERNAL SYNC Crystal controlled oscillator within a camera that is DC powered.

IP RATING A rating used to provide an index of protection from elements such as water and dust ingress.

IR LIGHTING Lighting that is above the visible wavelength, used for discrete illumination in CCTV systems.

IR SHIFT Difference in focus of a camera's field of view between white and IR lighting.

IR TRANSMISSION Method of sending video signals and/or data over free space, distance and quality of transmission is affected by environmental conditions such as fog and rain.

IRIS Mechanism within a lens to regulate the amount of light that passes through, and falls upon, the image sensor.

ISDN Integrated Service Digital Network - an International standard for voice and data communcations offering greatly improved transmission speeds than the older PSTN network (Public Switched Telephone Network).

ISIT Intensified Silicon Intensified Target - usually used for extreme low light CCTV or X-Ray machines.

ISO International Standards Organization.

I/O Input/Output.

I/P Input.

KILO Order of magnitude 10 -3.

LAG Retention that occurs in a video image when rapid motion of the camera or viewed object leaves a trail.

LAN Local Area Network - Communications network for electronic equipment, limited to usually a single building or site.

LASER A source of exceptionally pure light which can consist of a single wavelength concentrated into a straight beam, used to transmit IR lighting through fibre optic cable.

LEASED LINE A telephone connection giving a permanent point to point link.

LED Light Emitting Diode.

LINE AMPLIFIER Device that prepares a signal for transmission over extended cable runs by amplifying the signal strength.

LINE FED A camera that is powered down the same cable which it uses to send the video information back to the monitor.

LINE LOCKED A camera that is synchronised to the frequency of its AC power supply.

LINE POWERED See line fed.

LOOPING A piece of video equipment that allows video signals to pass through without being terminated.

LUMINANCE Y - the monochrome element of a video signal.

LUX Unit of light measurement.

MATRIX SWITCHING A video switcher that allows any input to be switched to any monitor output.

MCL Maximum camera length.

MEGA Order of magnitude 10-6.

MEGASTREAM Communications network comprising many hundreds of individual communication channels.

MFD Mode field diameter - fibre optics.

MI Manual Iris.

MICRO Order of magnitude 10 -6 (one millionth).

MICROWAVE TRANSMISSION Method of sending video signals and/or data over free space, longer distances than IR transmission can be achieved. Licencing is usually required.

MILLI Order of magnitude 10 -3 (one thousandth).

MODE Path taken by light rays along fibre optic cable.

MODEM Modulator / Demodulator - device used for sending digital signals over an analogue network, ie. video transmission over the PSTN network.

MODULATION A process of varying a carrier signal in relation to another signal.

MONITOR A video display unit used to display images from a camera or computer.

MONOCRHOME A black and white picture.

MOS Metal Oxide Semiconductor - a form of CCD imager used in some CCD cameras.

MTBF Mean Time Between Failure - the average time between equipment or component failure.

MTTR Mean Time To Repair - the average time it takes to repair a piece of equipment.

MULTIMODE An optical fibre that supports more than one propagation mode.

MULTIPLEXER A unit that can accept a number of camera inputs and almost simultaneously display them on a single monitor and/or record them to a single video tape. Multixplexers can also be used to transmit multiple cameras over the same transmission medium.

MULTIPLEXING Process in which multiple cameras are transmitted or recorded.

MUX Multiplexer.

NANO Order of magnitude 10 -9.

ND Neutral Density.

ND FILTER A glass filter that attenuates the light evenly over the visible spectrum, used to force the iris on a lens to open in order to focus the camera for low light conditions.

ND SPOT FILTER A graduated filter fitted to the centre of a lens, restricting the amount of light that can effectively pass through to the image sensor.

NEUTRAL COMPUTING Powerful software used in motion detection to teach the computer to learn object sizes and shapes.

NOISE Appears on screen as graininess or 'snow' effects on a video image.

NON-COMPOSITE VIDEO A video signal that is complete, except for the synchronisation information.

NTSC National Television Standards Committee. Colour Video Signal, North American and Japanese television standard - 525 Lines, 60Hz.

N/C Normally Closed.

N/O Normally Open.

O/P Output.

PAL Phase Alternate Line - Colour Video Signal standard for most of Europe, Australia and the Middle East - 625 Lines, 50Hz.

PAN & TILT Pan & Tilt head - a device that will move a camera both horizontally and verticlaly from a remote location.

PATCH PANEL A panel that joins or terminates many different circuits.

PCM Pulse Coded Modulation.

PEAK TO PEAK A value based on the maximum positive and maximum negative points of a waveform, for video the standard is 1V peak to peak.

PEAK WHITE INVERTER Circuitry that will convert white highlights over a pre-set brightness threshold to black. This is useful for car registration recognition when headlights are on.

PH Pin hole, normally refers to a pin hole lens.

PHOTO DETECTOR A device fitted to a fibre optic link to convert light to electrical power.

PHOTOCELL A device used to detect changes in light level and provide an automatic switched output when lighting levels fall to a predetermined level.

PHOTON Quantum of electromagnetic energy, a particle of light is a Photon.

PICO Order of magnitude 10 -12.

PIGTAIL Short length of fibre optic cable attached to another component such as a source of coupler.

PIP Picture in Picture - device used to superimpose one video signal over another in one display.

PIXEL In a digital display a pixel is the smallest individual dot that can be separately illuminated.

PL259 UHF connector now largely replaced by the BNC connector.

PRE-SETS Pre-positioning of P/T/Z/F (Pan/Tilt/Zoom/Focus) functions on functional cameras, activated by alarm inputs or operator selection.

PROTOCOL A set of rules governing the flow of information in a communcations system.

PSTN Public Switched Telephone Network - analogue communications network used for day to day telephony and data transmission.

PSU Power Supply Unit.

QUAD SPLITTER A device used to display 4 cameras simultaneously on a single monitor.

RACK MOUNT Piece of equipment that can be housed into a rack enclosure, industry standard is 19" width (to outside of mount) with the height measured in U. 1U = 1.75 inches. (44.45 mm)

RANDOM INTERLACE Method of television scanning that does not define a strict relationship between adjacent scan lines in sequential fields.

RASTER Pre-defined scanning area for a television picture tube.

REFLECTANCE The level of light that is reflected off any given surface, normally shown as a percentage.

REFLECTED LIGHT Scene illumination multiplied by reflectance. This is the light level available for the camera and determines picture quality.

REFRACTION Deflection of light at certain angles when it enters obliquely from another medium of different density.

REGENERATORS Units placed at regular intervals along a transmission system to detect weak signals and strengthen them.

REMOTE SWITCHERS A video switcher with a main switching box whose controller can be sited remotely via a single or pair of cables.

REPEATER Device that amplifies and then re-transmits a signal.

RISE-TIME The time required for the leading edge of a pulse to rise from 10% to 90% of its total amplitude.

RS170 Video Sync Pattern for the United States - 525 lines 60Hz.

RS232 Computer terminology for communications networks.

RS422 Computer terminology for communications networks.

RS485 Computer terminology for communications networks.

RX Receiving equipment.

SCANNER Device for remotely controlling the horizontal movement of a camera - pan only.

SCART Standard European connector for carrying audio and video signals.

SCENE ILLUMINATION The density of light falling upon a scene to be viewed, measured in lux.

SCREEN SPLITTER Device used to display two cameras on a single monitor, splitting the screen horizontally or vertically.

SECAM Sequential Colour & Memory - colour television broadcast system used in France.

SENSITIVITY For a camera usually specified in lux to provide indication of light level required to gain a full video signal from the camera.

SHUTTER Electronic circuit found in many CCD cameras which allows the light gathering period of the camera to be manually altered.

SIMPLEX Term seen in video multiplexers. With a simplex multiplexer you can only perform one function of recording or playing back at a time, ie. if you are reviewing images from tape, you cannot record images or view live images in a multiscreen format.

SIT Silicon Intensified Target - form of camera used in very low light levels.

SLAVE A secondary item (control position) that performs the same, or limited functions, of the master position.

SLOWSCAN Old form of video transmission over telephone networks, replaced these days by fastscan or DVST systems.

SN RATIO Signal to Noise Ratio - the ratio of maximum peak to peak signal voltage to the RMS voltage of the noise at a given time, measured in decibels.

SOFTWARE COMPRESSION Video data compression achieved by computer program processing.

SPECTRAL RESPONSE Sensitivity of an imaging device to different frequencies of light. Visible light is 300 to 730nm, IR is 715 - 850nm and invisible IR 850nm+.

SPEED OF UPDATE The time taken to refresh a single picture.

SPIKE An amplitude surge. An undesirable momentary increase in electrical current or signal.

S-VHS Super Video Home System - a higher resolution recording medium than VHS.

SWIVEL MOUNT Used on camera and housing mounts to allow precise positioning of cameras.

SYNC GENERATOR Device that produces a synchronisation signal.

SYNCHRONISATION The occurrence of two or more electrical events at the same time - in sync.

SYNCHRONOUS DATA Data and synchronisation pulses sent at the same time, used for more sophisticated communications.

TA Terminal Adaptor - used to connect video transmission products to an ISDN digital telephone line.

TDG Time & Date generator - device used to overlay the time and date on a video signal.

TELEMETRY Electronic signalling method, in CCTV used to remotely control camera functions.

TERMINATION In CCTV the termination is 75 ohms; if a signal is being looped then equipment should be set to Hi-Z (un-terminated).

TIME BASE CORRECTION Method used to align unsynchronised camera signals, widely used by video multiplexers and quad splitters.

TIME LAPSE VCR A VCR that can slow down its recording rate, used to extend the length of recording on a standard 3 hour tape to as much as 960 hours.

TOUCH SCREEN CONTROL Ability to control camera and system controls directly from the VDU screen by simply touching the relevent object/icon.

TP Twisted Pair - an electrical wire consisting of two conductors twisted around each other.

TRANSDUCER Device used to convert a form of energy into electrical energy.

TVL Television Lines - used to describe the resolution of a camera or VDU.

TX Transmitting equipment.

UNBALANCED SIGNAL Term used for coaxial cable transmission.

UNTERMINATED Input to a unit in a CCTV system that requires looping to another piece of equipment.

VARIFOCAL A type of lens that allows manual adjustment between two focal points to provide the desired field of view.

VCR Video Cassette Recorder.

VDU Video Display Unit.

VERTICAL INTERVAL Sequential video switchers that wait until the current field has completed its cycle before

SWITCHER Switching to the next camera, even though the dwell time may have been exceeded. Thus reducing the amount of bounce seen between switching images.

VHS Video Home System - VCR format and name given to 1/2" cassette tape used in VCR's.

VIDEO LAUNCH AMPLIFIER An amplifier placed between the video source and the transmission cable, used where the signal needs to be processed before being launched over coaxial or twisted pair cables.

VMD Video Motion Detection (not activity detection) - a process of determining motion by complicated electronic analysis of picture signal and/or neutral computing techniques.

V.24 List of definitions between terminal equipment and modems.

V.34 28800 bps modem for PSTN and leased line circuits.

WAVELENGTH Distance between the same two points on adjacent waves, or the time necessary for a wave to complete a single cycle.

WDM Wavelength Division Multiplexing.

2 WIRE Transmission medium using the same two wires for transmit and receive channels. Either leased line or dial up.

4 WIRE Transmission system using 2 separate pairs of wire for the transmit and receive channels.

Leased line or dial up can only be achieved on PSTN by dialling 2 separate numbers.

WHITE BALANCE Used in colour CCD cameras. White should be referenced and determined by colour temperature for the truest colour rendition.

WHITE LEVEL The brighest part of a picture corresponding to 1V peak to peak (0.7V above black level).

X.21 General purpose interface between data terminal equipment and data circuit terminating equipment for synchronous operation on public data networks ie. ISDN.

Y/C Video signal containing separate luminance (Y) and chrominance (C) to provide greatly improved picture quality from VCR's ie. S-VHS.

ZOOM RATIO A lens with variable elements giving adjustable magnification and differing fields of view in one, ie 12:1.