CCTV Glossary and DIY

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Glossary of CCTV Technology and DIY Advice

Glossary A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

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DIY	Simple do it yourself advice
FAQ	Favorite questions

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 - 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 have AGC off.
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, i.e.. 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 autodialing 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.
AUTOMATIC IRIS	A lens that adjusts automatically to allow the right amount of light to fall on the imaging device. There is a tiny motor and amplifier built in which receives a control signal from the camera to maintain a constant one volt peak to peak (pp) video level.
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. This is important when a zoom lens is fitted.
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.
BANDWIDTH	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, i.e.. 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	CCD: Charge coupled device; It is light sensitive and forms the imaging device of most modern cameras. Size is measured diagonally and can be 1/3",1/2",etc". There are two types, frame transfer and interline transfer.
What is CCD sensor?	Our video cameras are piece of technology and at the heart of it is the CCD (charge coupled device) imager chip. A CCD consists of several hundred thousand individual picture elements (pixels) on a tiny 1/2", 1/3", or 1/4" chip. Each pixel responds to light falling on it by storing a tiny charge of electricity. The pixels are arranged on a precise grid, with vertical and horizontal transfer registers carrying the signals to the camera's video processing circuitry. This transfer of signals occurs sixty times per second. The CCD camera's electronic shutter is not really a moving shutter, but a clever piece of signal processing. Under low light conditions, the CCD is allowed to gather signal for the full 1/60th of a second. Under brighter lighting conditions, the video processing chip automatically responds by reading the CCD and then immediately "early purging" it, resulting in precise control over the video level. Even at a shutter speed of 1/100,000 of a second, the CCD camera is still delivering 60 images per second, but each image is gathered over a much shorter period of time. It doesn't end with the CCD...the image is constantly monitored and optimised by advanced on board signal processing circuitry. The end result is a fantastic picture, with no fiddling, no adjustments, and best of all, unbelievable reliability.
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.
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 smaller and cheaper than the C-mount equivalents.
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. 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 adapter ring to reduce this distance to 12.5mm.
20mA CURRENT LOOP	Electronic signaling 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 i.e.. 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 magnetic tape playback head or worn or damaged tapes.
DTMF	Dial Tone Multi Frequency used by some manufacturers for telemetry signaling 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, i.e.. 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	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 dialed up. When the link is confirmed, the picture is transmitted.
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 signaling 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	Multiple 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.

Glossary H to O H I J K L M N O

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	The internal generation of sync pulses in a camera using a crystal controlled oscillator. This is needed on non-mains powered cameras.
IP RATING	A rating used to provide an index of protection from elements such as water and dust ingress. (see page 34)
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 communications 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.
Lens calculator	Lens calculator
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.
LOOP FRAME STORE	The principle is that a series of video frames is compressed and stored in a continuous loop. This records a certain number of frames and then records over them again and again until an alarm signal is received. When this happens it carries on recording for a dozen frames or so and then stops. This means that frames before and after the incident are recorded. This eliminates the boring searching through hours of video tape and concentrates on the period of activity.
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, i.e.. video transmission over the PSTN network.
MODULATION MODULATOR	Modulation is the impression of a baseband video and audio signal to a Radiowave (RF Frequency) that can be processed (demodulated) by a television tuner. This modulated RF frequency can then be combined with an existing feed of local channels and be distributed to all the televisions on a coaxial network as a new channel.
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	An electronic system that can accept a number of camera inputs and record them virtually simultaneously. They can also provide multi screen displays with four, nine, sixteen etc. cameras on the screen at once. 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.
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.

Glossary P to Z P Q R S T U V W X Y Z

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 vertically 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.
PIR	PIR is the abbreviation of “Passive Infra Red”, which is a motion sensor to detect heat from moving objects, such as human bodies and animals. When any motion is detected, P.I.R. will trigger the alarm system. The TV will show you what your cameras see. With P.I.R., you can always keep an eye on visitors at the front door, intruders in the garage, or any place within its detection coverage. With it, our products can detect moving objects within wider detection coverage to provide our customers optimum security and protection.
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-SET CONTROLLER	Pre-positioning of P/T/Z/F (Pan/Tilt/Zoom/Focus) functions on functional cameras, activated by alarm inputs or operator selection. A function contained within a telemetry system that, on receipt of a signal, causes a particular camera to pan, tilt and zoom to a predetermined field of view. Most systems can accommodate up to sixteen pre-set positions for each camera. This is an especially useful feature on larger systems with alarmed areas.
PROTOCOL	A set of rules governing the flow of information in a communications 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. REMOTE SWITCHER: A video switcher which is connected to the camera cables 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.
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 in LUX falling on the area to be viewed. For best results the ratio of the lightest to the darkest areas should not be more than a factor of two.
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, i.e.. 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, 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.
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	The system by which a signal is transmitted to a remote location in order to control CCTV equipment e.g. to control pan and 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.
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. Achieved by the tape moving in steps and recording one frame at time. This means that if set to record over long periods much information can be lost. On receipt of an alarm signal these machines can be automatically switched to real time mode.
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 a 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.
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
VIDEO MOTION DETECTION	A method of detecting movement in the view of the camera by the electronic analysis of the change in picture contrast.
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 brightest 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 i.e.. ISDN.
Y/C	Video signal containing separate luminance (Y) and chrominance (C) to provide greatly improved picture quality from VCR's i.e.. S-VHS.
ZOOM RATIO	A lens with variable elements giving adjustable magnification and differing fields of view in one, i.e. 12:1.