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This glossary is intended to provide a quick reference to many terms used
in closed circuit television. Most of them are explained in much greater
detail in the appropriate sections.
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2:1 INTERLACE: The precise combination of two
fields of 312 1/2 lines to create a single frame of 625 lines. (CCIR)
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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
should always be considered with AGC. off.
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ALARM ACTIVATED VCR: From selecting 'record',
a normal V.C.R. would take from 15 to 21 seconds before it actually starts
recording usable pictures. With this type of 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 from an alarm or any other input.
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ALGORITHM: Mathematics, a rule or procedure
for solving a problem
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ANALOGUE SIGNAL: In video, the representation
of a camera scene by varying voltages in the video signal, the voltage being
directly proportional to the light level.
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APERTURE: The light gathering area of a lens.
The iris controls the size of the aperture.
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ARMOUR: Extra protection for a cable that
improves resistance to cutting and crushing. The most common material used
is steel.
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ASPECT RATIO: The ratio of the vertical to the
horizontal image size. This is 3:4.
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ATTENUATION: A term that refers to signal loss
in a transmission system or light loss through a lens system.
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AUTOMATIC IRIS: A lens that automatically adjusts
to allow the correct amount of light to fall on the imaging device. There
are a tiny motor and amplifier built in which generally receives a control
signal from the camera to maintain a constant one volt peak to peak (pp)
video level. There are two manual controls on the lens to allow compensation
for varying conditions of 'peak' and 'average' light.
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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.
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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.
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BANDWIDTH: The amount of space in a given part
of the spectrum needed to carry communication signals.
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BUFFER: The material surrounding the fibre to
protect it from physical damage
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BLANKING PERIOD: The period of the composite
video at black level and below when the retrace occurs, making it invisible
on the screen.
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BLACK LEVEL: The dark parts of a video signal
corresponding to approximately 0.3 volts.
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BIFURCATOR: An adapter with which a loose tube
containing two optical fibres can be split into two single fibre cables.
(See loose tube)
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C-MOUNT: The standard screw mounting for 2/3"
and 1" camera lenses. The distance from the flange surface to the focal point
is 17.526 mm. A C-mount lens can be used on a camera with a CS- mount by
adding an adapter ring to reduce this distance to 12.5 mm. (See CS-mount
)
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CABLE EQUALISER: An amplifier to increase a
video signal to the optimum value. This is usually to compensate for cable
losses.
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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.
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CCIR: The European 625 line standard for the
video signal.
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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
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CHROMINANCE: The part of a colour video signal
that carries the colour information.
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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.
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COMPOSITE VIDEO: The complete video signal
comprising the sync and video information. The sync pulse should be .3 volts
and the video signal should be .7 volts.
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CORE: The central region of an optical fibre
through which signal carrying infrared is transmitted. Manufactured from
high density silica glass.
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CS-MOUNT: A new generation of lenses designed
for, 1/2", 1/3" , 1/4" and 1/8" cameras incorporating CS-mounts. The distance
from the flange surface to the focal point is 12.5 mm. CS-mount lenses cannot
be used on cameras with C-mount configuration. These lenses are more compact
and cheaper than the C-mount equivalents.
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dB: Decibel, a logarithmic ratio between two
signals.
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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.
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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.
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DIGITAL SIGNAL: An analogue signal that has
been converted to a digital form so that it can be processed by a micro
processor.
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EIA: The American 525 line standard for the
video signal.
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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.
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fc: Foot candles used in some USA specifications
to define sensitivity. 10 fc is approx. 1 lux.
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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.
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FIELD OF VIEW: The relationship between the
angle of view and the distance of the object from the lens.
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FIELD: One half of a frame consisting of 312
1/2 lines, 50 fields are created every second.
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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.
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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.
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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.
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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.
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FRAME: The combination of two interlaced fields,
25 frames are created every second.
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GAMMA CORRECTION: An electronic correction carried
out in the camera circuitry to balance the brightness seen by the camera
to that of the monitor.
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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.
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GRADED INDEX: (Graded index profile). A measurement
shown in the form of a diagram which illustrates how the quality of glass
used in this type of optical fibre alters gradually. From the densest at
the core to the optically less dense cladding.
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GROUND LOOP TRANSFORMER: An isolation transformer
so that there is no direct connection between input and output.
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GROUND LOOP: An AC current 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. The result is interference on the signal.
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HARDWIRED: Controlling remote equipment by direct
voltage transmitted along a multicore cable from the main controller. This
is very labour intensive to install and is only used in simple systems with
short cable runs.
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HERTZ (Hz): The number of variations or cycles
per second.
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ILLUMINANCE: The measurement of light in lumens
per square metre, the unit of which is the lux.
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IMPEDANCE: A measure of the total opposition
to current flow in an alternating current circuit, measured in Ohms.
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INFRA RED LIGHT: The wavelength of light produced
above the visible part of the spectrum.
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INFRA RED TRANSMISSION: A method of transmitting
video and telemetry signals across free space along an infra red beam. This
opens possibilities for using C.C.T.V. where it had been previously impossible
to run cables. Distance can be limited and the signal can be degraded in
adverse weather conditions.
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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.
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INTERNAL SYNC: The internal generation of sync
pulses in a camera without reference to external sources. This uses a crystal
controlled oscillator and is needed on non mains powered cameras.
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IP RATING: Index of protection, a number combination
that defines the protection afforded from outside influences by an
enclosure.
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IR. SHIFT: The difference in the field of view
in focus between daylight and infra red light.
