Some cameras, dvrs 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.
The first, most general component of a
lighting model is ambient light. Ambient light is diffuse, non-directional
light that is the result of multiple reflections from surrounding surfaces. Put
simply it is light that has no obvious source; it is 'everywhere'. When a
picture or video has a low ambient light level, it is going to be rendered as a
'dark' scene (although this may be offset by more specific point sources).
Most CCTV cameras, although internally
working with digital components, produce a standard analog signal. This is
where the image is represented by a variable voltage level and frequency
timings.
In the USA, the standard used is called
NTSC. In the UK and Europe, the standard for the analog signal output from CCTV
cameras is called PAL.
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 apertures.
The aspect ratio of an image is its
displayed width divided by its height (usually expressed as "x:y").
For instance, the aspect ratio of a
traditional television screen is 4:3, or 1.33:1. High definition television
uses an aspect of 16:9, or about 1.78:1. Aspect ratios of 2.35:1 or 1.85:1 are
frequently used in cinematography, while the aspect ratio of a standard 35mm
film frame is around 1.37:1.
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 strenth.
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.
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.
Video signals are normally transmitted
along co-axial cable, which require a terminating resistor at either end. If
the signal is looped through a piece of equipment that has an input and an
output, then it should be terminated if it is the final piece of equipment, but
not terminated if it is in between other equipment.
Some appliances have a manual switch for
this setting, other equipment automatically detects if termination is required
and applies the additional resistor as necessary.
This is a feature of some cameras that
automatically adjusts the color settings to maintain the quality of the white
areas of the image.
When a lens does not hold focus at both
ends of the zoom range, you may need to adjust the back focus. By adjusting the
back focus, you are changing the distance of between the pick-up device and the
rear element of the lens. Technically, it's the distance from the vertex of the
rearmost element of the lens to the focal plane.
In most cases, changing a lens will not
require adjusting the back focus.
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 from
behind.
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.
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.
One of the two main types of image sensing
device used in cameras. It operates by converting light energy into electrical
charge.
This refers to the part of the video signal
that contains the color information.
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 pulses.
Digital video pictures can be compressed
with a number of techniques. These include, JPEG, M-JPEG, MPEG and Wavelet.
The scale for measuring intensity is the
decibel scale. Since the range of intensities which the human ear can detect is
so large, the scale which is frequently used by physicists to measure intensity
is a scale based on multiples of 10. This type of scale is sometimes referred
to as a logarithmic scale.
In the context of CCTV it refers to voltage
or signal strength. A 6db increase is equivalent to doubling the signal
strength.
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.
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 computer data.
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).
Some auto iris lenses require a DC signal
from the camera. These are known as direct drive lenses.
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
quality.
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 the camera.
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 center 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. A NTSC
signal transmits 30 full frames per second. A PAL signal transmits 25 full
frames per second.
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
picture.
Cycles per second.
The total opposition offered by a device to
the flow of an alternating current. Measured in Ohms Ω
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.
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
external device.
IP ratings are a BSi standard measurement
for how waterproof something is. Many cameras or camera housings are designed
for outdoor use, and therefore need to be waterproof to some degree. The
details of the tests are defined in BS EN 60529 : 1992. The IP number has two
digits, and optional letters after them. These have the following meaning:
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 www.jpeg.org
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 corrector.
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 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 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 refers to the part of a video signal
that carries the monochrome information. i.e. brightness information.
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 meter.
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 color definition in low light levels, so in general
low light cameras are always monochrome. However, day/night cameras use
electronics to switch from color 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
conditions.
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 video inputs.
Something which is monochromatic has a
single color.
For an image, the term monochrome is
essentially the same as black-and-white, but the monochrome may be preferred to
indicate that combinations such as green-and-white, green-and-black, etc., are
not excluded.
A monochrome computer display is capable of
displaying only a single color, often green, amber, red or white, and often
also shades of that color.
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 applications.
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 signals.
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 analog video
outputs. The images are encoded directly in one of the standard compression
techniques, such as JPEG or MPEG.
NTSC is the analog television system in use
in the United States and many other countries, including most of the Americas
and some parts of East Asia. It is named for the National Television System(s)
Committee, the industry-wide standardization body that created it.
PAL, short for phase-alternating line,
phase alternation by line or for phase alternation line, is a color encoding
used in broadcast television systems, used throughout the world except in most
of the Americas, some East Asian countries (which use NTSC), parts of the
Middle East and Eastern Europe, and France (which use SECAM, though most of
them are in the process of adopting PAL).
Peak to Peak (P-P) refers to the
measurement of voltage of a signal between the most negative and most positive
points.
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 light.
This is a fault condition on a video signal
which refers to loss of vertical synchronization. It causes the picture to
continuously move up or down
SÉCAM (Séquentiel couleur avec mémoire,
French for "sequential colour with memory") is an analog color
television system first used in France. SÉCAM was invented by a team led by
Henri de France and working at Thomson. It is historically the first European
color television standard.
SECART 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.
This is the ratio between the signal
strength and the noise levels on an audio or video signal.
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 75 Ohm terminator that is
used to terminate each end of a video line.
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 camera.
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.
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 recorded.
This is a compression technique used to
give high compression without degrading the image quality.