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 Ration: 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 (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 inch and 1 inch 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 inch, 1/2 inch, or 2/3 inch. There are two types, frame transfer and interline transfer.
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 inch, 1/2 inch, and 1/3 inch 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 less expensive than the C-mount equivalents.
Digital signal: An analog signal that has been converted to a digital form so that it can be processed by a microprocessor.
EIA: The American 525 line standard for 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 the 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 dialed up. When the link is confirmed, the picture is transmitted.
Frame: The combination of two interlaced fields, 25 frames are created every second.
Infrared light: The wavelength of light produced above 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.
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 of 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.
Multiplexer: An electronic system that can accept a number of camera inputs and record them virtually sinultaneously. They can also provide multiscreen 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 coaxial cable, microwave, infrared link, etc. This requires a multiplexer at each end of the line.
Noise: 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 monochrom standard. NTSC is the color 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 color 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.
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 systmes 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 conected to a desktop controller by a single cable for each monitor.
S/N Ration: 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, 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 a time. This means that if set to record over long periods of time, 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 at the appropriate symbol which can 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.
Video motion detection: A method of detecting movement in the view of the camera by the electronic analysis of the change in picture contrast.
White level: The brightest part of a video signal corresponding to approximately 1.0 volt.

Lens Specific Terms

Auto Iris (AI): A lens that takes a video signal reference from the camera. It uses this in an amplifier to derivea DC voltage to open or close the iris aperture. The video level control is part of the lens.
Direct Drive (DD): A lens that takes a reference DC voltage from the camera to open or close the iris aperature. The video level control is part of the camera.
Electronic Iris: This is a camera that uses its electronic shutter to control how much of the light falling on the CCD sensor is used to produce a picture, thereby emulating the iris control of the lens. Video level is normally factory pre-set.
Galvometric drive: The method by which the small DC voltages produced by either A1 or D0 systems are converted into movement of the lens iris. It is a modern, cost effective alternative to the servo drive method. It is used in both A1 and D0 lenses.

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