Glossary of CCTV Terminology

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.

ALARM ACTIVATED VCR: After pressing 'record', a normal VCR takes about 20 seconds before it starts recording usable pictures. With an alarm activated recorder it can be set so that the tape is ready to start recording in about one second. The signal to begin recording can be from an alarm or any other input.

APERTURE: The light gathering area of a lens, controlled by the iris.

ASPECT RATIO: The ratio of the vertical to the horizontal image size. This is 3:4.

ATTENUATION: This refers to signal loss in a transmission system.

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.

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. This is important when a zoom lens is fitted.

BALANCED SIGNAL: A video signal is converted to a balanced signal to enable it to be transmitted along a 'twisted pair' cable. Used in situations where the cabling distance is too great

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.

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" or 2/3". There are two types, frame transfer and interline transfer.

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

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

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.

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

DC TYPE LENS: An auto-iris lens with an internal circuit which receives voltage and a video signal from the camera to adjust the iris.

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

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

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

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

F-NUMBER: The f-number indicates the brightness of the image formed by lens, controlled by the iris. A smaller f-number means a brighter image.

FLANGE BACK: The distance from the flange of the lens (beginning of the lens mount) to the focal plane. C-Mount lenses have a flange back distance of 17.526mm vs. 12.5 for CS-Mount.

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

INTERNAL SYNC: The internal generation of sync pulses in a camera using a crystal controlled oscillator. This is needed on non-mains powered cameras.

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

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

LEVEL CONTROL: Main iris control. Used to set the auto-iris circuit to a video level desired by the user. After set-up, the circuit will adjust the iris to maintain this video level desired by the user. After set-up, the circuit will adjust the iris to maintain this video level in changing lighting conditions. Turning the control towards high will open the iris, towards low will close the iris.

LINE POWERED: A camera in which the power is supplied along the same coaxial cable that carries the video signal.

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.

LUX: Measurement of light levels. The lower the lux, the darker the scene.

MANUAL IRIS LENS: A lens with a manual adjustment to set the iris opening (F-stop) in a fixed position. Generally used for fixed lighting applications.

MINIMUM OBJECT DISTANCE (M.O.D.): The closest distance a given lense will be able to focus on an object. This is measured from the vertix (front) of the lens to the object. Wide angle lenses generally have a smaller M.O.D. than large focal length lenses.

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.
NOISE: Any unwanted electrical signal. In a video signal this appears as snow or graininess in the picture.

NTSC/EIA: The video standards produced and used by the USA (though not exclusively). EIA is the monochrome standard. NTSC is the colour standard which adds on top of the EIA. The European similar standards are PAL and CCIR. The European and American standards do not work together.

PAL/CCIR: The video standards used throughout Europe. CCIR is the monochrome standard. PAL is the colour standard which adds on top of the CCIR. The USA similar standards are NTSC and EIA. The European and American standards do not work together.

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.

PINHOLE LENS: Lens used for applications where the camera/lens must be hidden. Front of lens has a small opening to allow the lens to view an entire room through a small hole in the wall.

PRE-POSITION LENSES: Zoom lenses which utilize a variable-resistor (potentiometer) to indicate zoom/focus position to the lens controller. After initial set-up, this allows the operator to view different pre-set areas quickly without having to readjust the zoom and focus each time.

PRE-SET CONTROLLER: 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 preset positions for each camera. This is an especially useful feature on larger systems with alarmed areas.

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.

RESOLUTION: Used to express the fineness of an image indicated by the number of T.V. lines that make up the image. The higher number of lines, the higher the resolution.

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

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

SIGNAL TO NOISE: The ratio, expressed in decibels, of the signal voltage to the noise voltage in an electronic circuit.

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

TIME LAPSE VCR: A type of video recorder that can be set to record continuously over long periods. This can be anything from three hours to 480 hours, achieved by the tape moving in steps and recording one frame at 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.

TRACKING: The ability of a zoom lens to remain in focus during the entire zoom range from wide angle to telephoto position.

VIDEO MOTION DETECTION: A method of detecting movement in the view of the camera by the electronic analysis of the change in picture contrast.

VIDEO TYPE LENS: An auto-iris lens without an internal circuit to control the iris. All iris control voltages come from a circuit located within the camera.

WHITE LEVEL: The brightest part of a video signal corresponding to approximately 1.0 volt.
ZOOM LENS: A lens system that may be effectively used as a wide angle, standard, or telephoto lens by varying the focal length of the lens.

ZOOM RATIO: The ratio of the starting focal length (wide position) to the ending focal length (telephoto position) of a zoom lens. A lens with a 10X zoom ratio will magnify the image at the wide angle end by 10 times.