Automatic Gain Control (AGC)
This is a feature of many CCTV cameras
that helps keep the strength of the
output signal constant, even when the
light level changes. In other words, it
boosts the signal strength at low light
levels, and caps it at higher levels.
Cameras with an Auto Iris feature, have
the ability to compensate for large
variations in light levels. Particularly
useful for cameras that need to
compensate for changes from bright
sunlight to dark shadows. The auto iris
circuitry is normally linked to a
motorised iris drive that physically
opens and shuts the iris on the lens.
Closing a physical iris is a much better
way to protect a camera from being
damaged by bright sunlight then simply
using electronics to reduce the signal
Some cameras and video servers have the
ability to accept alarm inputs. These
are inputs from standard sensor devices
such as, Passive Infra-red (PIR)
detectors, door contacts, active beams
etc. or relay outputs from intruder
alarm panels, or access control panels.
Alarm input circuits can come in a
number of forms. Most commonly they are
self powered, so will operate with a
passive switch style circuit. However,
in some cases, they expect to be
provided with power (normally 12V) as
the signal of open or closed. In a few
cases manufactures only supply TTL (very
low voltage) level inputs. In that case,
they can only be safely used with normal
alarm sensors in conjunction with some
additional isolation circuitry. In this
case, it is best to contact the
manufacturer for advice.
This refers to the amount of background
light in any given situation.
Most CCTV cameras, although internally
working with digital components, produce
a standard analogue signal. This is
where the image is represented by a
variable voltage level and frequency
timings. In the UK and Europe, the
standard for the analogue signal output
from CCTV cameras is called PAL. In the
USA, the standard used is called NTSC.
The aperture of a lens, is the area that
allows the light to pass into the lens.
Lenses with large apertures allow more
light in than lenses with small
This is the ratio between the the height
and width of an image.
Automatic Level Control (ALC)
This is a feature of some cameras, that
allows the camera to either bring out
detail in bright areas of the image, or
bring out more detail in the darker
areas, depending on how it is set up.
This is often useful where the view has
a window in the background, where the
sunlight is causing the image of the
room to be much darker than required.
Automatic White Balance
This is a feature of some cameras that
automatically adjusts the colour
settings to maintain the quality of the
white areas of the image.
This is the alignment of the rear of the
lens to the imaging device.
This is a feature of cameras that
automatically adjusts the image to
compensate for bright sunlight or bright
lights, to give more detail on the
darker areas of the image. For example
to focus on the detail of a face of a
person that has the sunlight shining
This is a method of transmitting audio
or video signals over a pair of wires,
often twisted pair cable. By sending two
equal, but opposite signals, this system
minimises external interference, and
maintains signal quality.
Balun stands for Balanced - Unbalanced.
It is a device used to interface between
balanced lines and unbalanced lines. For
example, twisted pair to co-axial.
BNC is a bayonet style connector for
coaxial cable that is most commonly used
for CCTV installations.
Coupled Device (CCD)
One of the two main types of image
sensing device used in cameras. It
operates by converting light energy into
This refers to the part of the video
signal that contains the colour
Lens & CS Mount Lens
There are two main types of lens used in
CCTV cameras. The C mount lens has a
flange back distance of 17.5mm. The CS
mount lens has a flange back distance of
12.5mm. C mount lenses therefore have a
longer focal distance. CS mount became
widely used, because it its more
practical for many of today's more
compact cameras. Lenses are often
supplied with a 5mm spacer ring
(sometimes called a C ring) that allows
a C mount lens to be used on a CS
camera. Most modern cameras are CS.
This refers to cable that has a central
conductor, surrounded by a shield
sharing the same axis. The shield can be
made from a variety of materials
including, braided copper, or lapped
foil. There are various standards for
specific types of co-axial cable. The
cable used for normal CCTV installations
is called RG59.
A full video signal that combines
picture signal and synchronisation
Compression is the method by which the
amount of digital data required to
represent an image may be minimised.
This generally involves eliminating
redundant information, and/or removing
information that the human eye does not
notice. Compression techniques can
broadly be split into two categories:
- Lossless, where the original
image may be reproduced exactly from
the compressed data (such as PNG and
- Lossy, where the original image
cannot be reproduced exactly, but
the reduction in the picture quality
is tolerated in order to increase
the compression ratio.
Common types of compression for digital
video pictures include: JPEG and
JPEG-2000 (for still images), M-JPEG and
MPEG (for moving pictures).
Video Recorder (DVR)
This is a device that records video
signal digitally. Normally this is on a
large hard drive in the machine. For
this reason it is sometimes known as a
hard disk recorder (HDR).
Direct Drive (DD)
Some auto iris lenses require a DC
signal from the camera. These are known
as direct drive lenses.
Depth of Field
Depth of field (also known as depth of
focus) refers to the range of distance
from the camera that is in good focus.
The smaller the aperture of the lens,
the longer the depth of field. However,
less light coming into the lens, means
that the image will be dimmer, so the
camera needs to compensate by increasing
the gain. For example, the best
(longest) depth of field would be
obtained on a bright, sunny day, with a
very small aperture on the camera. In
this case the imaging device still gets
a reasonable amount of light, even with
a small aperture. The worst (shortest)
depth of field would be in a dark room,
with a large aperture. To improve depth
of field, the best approach is to
increase the amount of light on the
subject, or use a more sensitive camera.
The decibel is a logarithmic scale. In
the context of CCTV it refers to voltage
or signal strength. A 6db increase is
equivalent to doubling the signal
This is a device that accepts one video
input and sends it out to 2 or more
devices. This is often used in a
situation where the signal needs to be
split between a number of video monitors
or recording devices. 2 or 3 devices can
often (but not always) be driven just by
splitting the signal with a "T"
connector, but more than that, and
invariably a distribution amplifier is
required to give an acceptable signal
Versatile Disk (DVD)
Sometimes called digital video disk.
This is an optical disk the same size as
a CD, used for storing data of various
formats. Including video, audio and
Electronic Iris (EI)
This is an electronic implementation of
an auto iris. It uses electronics to
simulate the effect of opening and
closing the iris, by increasing or
decreasing the effective shutter time of
An external sync allows a piece of
equipment to take its video
synchronisation from another unit, so
that it can align itself with the system
as a whole.
The distance between the centre of a
lens, or its secondary principal point
and the imaging sensor. Lower lengths
give a greater field of view and less
magnification. Longer lengths give a
narrower field of view and greater
magnification. The table below gives an
approximate value for the angle of the
field of view for lenses of various
focal lengths. 30˚ is considered to be a
normal view, telephoto (longer) lenses
have lower angles. Most CCTV cameras
have one of the 3 sizes of imaging
devices listed below, 1/4", 1/3" or
1/2". As you can see, this makes a big
impact when choosing lenses.
This refers to a full TV picture. The
PAL signal transmits 25 full frames per
This is an adjustment to ensure that
variations in light input, produce a
corresponding output in light intensity.
This refers to a poor image, where the
primary signal is weak, allowing a
secondary signal to interfere with the
PAL video signals transmit odd and even
lines alternately. This is a 2:1
interlace. The two sets of lines are
combined to form each single frame.
Devices with internal sync, have an
internal crystal to provide sync pulses,
without needing reference from any
Infra Red (IR)
Low frequency light below the visible
spectrum. This is often used for covert
or semi-covert surveillance to provide a
light source for cameras to record
images in dark or zero light conditions.
This is a mechanical device that adjusts
to vary the amount of light passing
through the lens of a camera.
JPEG is a standard for
coding/compression of still pictures. It
is used in the CCTV systems to compress
and store individual frames of video.
JPEG was developed by the Joint
Photographic Experts Group.
JPEG-2000 is a successor to the JPEG
compression standard, that uses
wavelet-based compression as opposed to
the Discrete Cosine Transformation (DCT).
It provides a higher compression ratio
than the JPEG standard, for the same
Liquid Crystal Display (LCD)
This is a technology used for flat
screen displays. Aside from being
smaller and lighter, it also has the
advantage of using less power than
traditional cathode ray tube screens.
This device takes a weak video signal as
its input and outputs a new stronger
version of the same signal. These are
useful/necessary for transmitting video
over very long cable runs. This is
sometimes known as a video line
Line Fed Camera
This refers to the use of equipment that
can provide power to the camera and take
the video signal along the same cable.
It normally involves placing an
"encoder" unit at one end and a
"decoder" unit at the other. These units
effectively combine and then split the
power from the video signal at either
end of the cable run.
This refers to connecting an additional
device in parallel with an existing
video cable. For example, when driving a
video recorder as well as a monitor from
the same video signal, simply by T'ing
off the cable.
This is a measure of the amount of light
striking a surface. i.e. the luminus
flux density at a surface. One lux is
one lumen per square metre. Cameras for
use in good lighting conditions, or in
daylight would normally be rated at 2
Lux or more. Cameras with a Lux rating
of 0.2 Lux or less would be considered
low-light cameras. It is not possible to
get good colour definition in low light
levels, so in general low light cameras
are always monochrome. However,
day/night cameras use electronics to
switch from colour during the daytime,
to monochrome during night or low light
conditions. Many low light cameras are
also infra-red sensitive, so that
infra-red illumination can be used.
Particularly useful in zero light
This refers to the part of a video
signal that carries the monochrome
information. i.e. brightness
This is a device that allows any of its
camera inputs to be switched to one or
more of its monitor outputs. The outputs
can of course also be video recorders.
This refers to the process of manually
setting the focus on a lens.
The device used to view video pictures.
These devices do not normally have
television RF frequency receivers. They
normally have composite, or component
This refers to a black and white image
rather than a colour one.
MPEG is a standard used for coding and
compression of moving images. It was
developed by the Moving Pictures Experts
Group. It is now used widely for the
compression of video images. However
MPEG isn't just one standard. They have
developed several standards for
different uses. For example MPEG-2 is
used for DVD's and set top boxes. MPEG-4
was developed for multi-media
applications for fixed and mobile web
This is a device that takes inputs from
2 or more video channels and combines
them into one signal. This is often done
by using time division multiplexing,
which interleaves frames from each
channel in such a way that they can be
split out again. Frequency division
multiplexing uses different frequencies
to achieve the separation of the
This refers to a camera that is designed
to record pictures and transmit them
directly over a computer network or
dialup internet connection. Network
cameras normally do not have any
analogue video outputs. The images are
encoded directly in one of the standard
compression techniques, such as JPEG or
This is standard for TV signals
developed by the National Television
Standards Committee in the USA. The UK
and Europe, use a similar, but different
standard known as PAL.
This is the standard for TV signals used
in the UK. It stands for Phase
This is a type of lens with a very small
aperture. Normally used for covert
applications, where it can easily hide
behind or within another object.
A pixel refers to an individual area on
the surface of the imaging device,
normally a CCD. It is made from
photosensitive material which converts
light into electrical energy. In the
context of a display monitor, a pixel is
also referred to as an individual area
on the surface of the screen which
converts electrical energy to visible
This is the system used for TV signals
This is a standard used in Europe for a
20 pin connector used to carry video and
audio signals. It is most widely used in
domestic TV appliances.
The shutter in a camera is the device
that controls the time period for which
light is gathered by the collecting
device. Old cameras used mechanical
shutters. However modern cameras use
electronic circuits to perform the same
function with no moving parts.
Signal to Noise Ratio (S/N Ratio)
This is the ratio between the signal
strength and the noise levels on an
audio or video signal.
Television Lines (TVL)
This is a measure of the resolution of a
video device. Higher number is higher
resolution. 380 TVL is considered medium
resolution. 480 TVL or greater is
considered high resolution.
This refers to a type of lens that has
the facility to change the focal length.
This allows adjustment of the
magnification and field of view of the
This is a feature that detects motion
within a video signal. Normally this is
used to trigger recording of images.
Advanced video motion detection systems
have the facility to adjust the
sensitivity and object size that will
trigger the system. They also allow the
image to be blocked out, such that only
certain areas of the image are taken
into account when scanning for motion.
Video Server (Network Video Server)
This is a device that accepts inputs
from CCTV cameras using the PAL,
composite video signal. It digitises the
signal and then transmits images via a
network, dialup or GSM connection.
Advanced video servers have built-in
video motion detection, and can buffer
images while they are being uploaded to
a remote server, where they are