of CCTV Terminology
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.
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
beging recording can be from an alarm or any other input.
The light gathering area of a lens, controlled by the iris.
RATIO: The ratio of the vertical to the horizontal image
size. This is 3:4.
This refers to signal loss in a transmission system.
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.
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.
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
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.
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.
The European 625 line standard for the video signal.
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.
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
SIGNAL: An analogue signal that has been converted to a digital
form so that it can be processed by a micro processor.
LENS: An auto-iris lens with an internal circuit which receives
voltage and a video signal from the camera to adjust the iris.
American 525 line standard for the video signal.
LENGTH: The distance betwen 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.
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.
The combination of two interlaced fields, 25 frames are
created every second.
The f-number indicates the brightness of the image formed by
lens, controlled by the iris. A smaller f-number means a
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.
RED LIGHT: The wavelength of light produced above the
visible part of the spectrum.
SYNC: The internal generation of sync pulses in a camera
using a crystal controlled oscillator. This is needed on
non-mains powered cameras.
The mechanism that can be adjusted to vary the amount of
light falling on the imaging device.
LOCKED: The sync pulses of cameras are locked to the AC
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 circult 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
POWERED: A camera in which the power is supplied along the
same coaxial cable that carries the video signal.
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
Measurement of light. The lower the lux, the better to see in
low light conditions.
IRIS LENS: A lens with a manual adjustment to set the iris
opening (F-stop) in a fixed position. Generally used for fixed
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.
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.
In a video signal this appears as snow or graininess in the
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.
European similar standards are PAL and CCIR. The European
and American standards do not work together.
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
TILT: A device that can be remotely controlled to provide
both vertical and horizontal movement for a camera.
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.
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.
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.
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.
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.
Used to express the fineness of an image.
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'
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.
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
TO NOISE: The ratio, expressed in decibels, of the signal
voltage to the noise voltage in an electronic circuit.
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.
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.
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 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.
The ability of a zoom lens to remain in focus during the entire
zoom range from wide angle to telephoto position.
MOTION DETECTION: A method of detecting movement in the view
of the camera by the electronic analysis of the change in
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.
LEVEL: The brightest part of a video signal corresponding to
approximately 1.0 volt.
A lens system that may be effectively used as a wide angle,
standard, or telephoto lens by varying the focal length of the
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.