DEFINITIONS & TERMS
as used in this Catalogue
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- Active CCD Elements
- Elements within the CCD Sensor area upon which the image is focussed by
the lens and actually used to create the video information. Excludes Dummy
elements.
- AGC
- Automatic Gain Control, increases the video amplifier gain as light levels
fall, decreases gain as light levels increase.
- Alarm activated VCR
- After pressing 'record' a common domestic VCR may take 3 to 10 seconds
before it starts recording usable pictures. With an alarm activated recorder
the tape is 'loaded' (wrapped around drum, etc) and will be ready to start
recording in about one second. The recording start and stop signals may be
from a Quad, Multiplexer, Alarm sensor etc.
- Analog
- The representation of numerical values by physical variables such as
voltage, current, etc. A physical variable that is proportionally similar to
another variable over a specified range.
- Aperture
- An opening or hole through which matter or radiation may pass.
- Aperture (Lens Stop)
- A physical constraint that limits the diameter of axial light passing
through a lens. Affects the light transmission capability of a lens, may be
adjustable and controlled by an iris. Expressed as an F-Stop number, the
lower the number the greater the transmission of light.
- Aspect ratio
- The ratio of the vertical to the horizontal image size. This is usually
3:4 for a standard TV, Video or Computer Monitor and for most common CCTV
Cameras. (1/3" CCD = 3.6 mm x 4.8 mm. 1/4" CCD = 2.4 mm x 3.2 mm)
- Attenuation
- The loss of average power in a transmission system, usually expressed in
dB (ie: loss of video signal through a coaxial cable or light intensity
through optical fibre).
- Automatic Iris
- A device for automatic exposure control that adjusts lens iris size
(aperture) usually via a motor driven diaphragm and in response to changing
video signal level. Video Drive type samples the composite video output
signal and has an inbuit amplifier usually with adjustable Level and
Peak-Average metering potentiometers. DC Drive type has Motor and Damping
Coil inputs for use with cameras that have an inbuilt amplifier.
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- Back Focal Length
- The distance from the surface of the rear-most lens element to the image
plane, CCD Sensor Area (video camera) or Film Emulsion (film camera).
- Back focus
- The mechanical adjustment on a camera that varies the relative distance
between the imaging device and the rear lens element. This is an important
adjustment that needs to be set correctly when lens is first fitted to a
camera, it is particularly important when a zoom lens is fitted.
- Balanced Signal
- A video signal may be converted to a balanced signal for transmission via
'twisted pair' (balanced) cable. Sometimes used for long cable runs where
the signal level would be excessively attenuated using unbalanced coaxial
type cable. Balanced unshielded cable is usually less lossy than shielded
coaxial cable.
- Bandwidth
- The range of frequencies over which a particular device is designed to
function within specified limits.
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- C-Mount
- A standard screw mounting for 1/4", 1/3", 2/3" and 1"
Video and some 16 mm Film camera lenses. Thread size is 1" diameter, 32
threads-per-inch, the distance from the flange surface to the image plane is
17.526 mm. A C-Mount lens can be used on a CS-Mount camera by adding an ~
5mm spacer adapter ring.
- CCD
- Charge Coupled Device. A light sensitive integrated circuit that is used
in most most modern cameras. Size is measured diagonally and may be
1/3", 1/2" or 2/3".
- CCIR
- Commite Consultatif International Radiotelegraphique. This is a standards
committee of the International Telecommunications Union (ITU), who made the
technical recommendation for the European (also used in Australia) 625 line,
50 fields/second (25 fps) standard for video signals.
- Composite Video
- The complete visual wave form of the colour video signal composed of
chrominance and luminance picture information: blanking pedestal, field,
line and colour sync pulses, and field equalizing pulses.
- CRT
- Cathode Ray Tube. The main part of a normal monitor or television (the
screen).
- CS-Mount
- A modern and more compact lens type (similar to C-Mount) designed for
1/4", 1/3" and 1/2" CS-Mount CCD cameras. Thread size is
1" diameter, 32 threads-per-inch, the distance from the flange surface
to the image plane is 12.5 mm. CS-mount lenses cannot be used on C-Mount
cameras, these lenses are smaller and usually cheaper than an equivalent
C-mount.
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- Decibel (dB)
- The standard unit used to express gain or loss and relative power levels.
It is a logarithmic measure (dB) = 10log (p2/P1) of the ratio between two
powers, voltages, currents, sound intensities, etc. Signal-to-Noise ratios
are expressed in decibels.
- Digital Signal
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- EIA
- American 525 line, 60 fields/second (30 fps) video signal standard.
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- Field
- In a raster scan television, one of the two scans that is interlaced to
make a frame. One field consists of odd numbered lines in a frame and the
other field consists of even numbered lines. Each field viewed independently
forms a picture but when 'interlaced' they form a frame.
- Focal length
- The distance between the secondary principal point in the lens and the
plane of the imaging device. The longer the focal length (numerical value),
the narrower the angle of view.
- Frame Store
- An electronic method of capturing and storing a single frame of video.
- Frame
- The combination of two interlaced fields. The frame frequency is half the
field frequency.
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- Genlocking
- The process of aligning the data rate of a video image with that of a
digital device to digitise the image and enter it into computer memory. The
machine that performs this function is known as a genlock.
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- Infra red light
- The wavelength of light produced above the visible part of the spectrum.
- Interlace
- The pattern described by two separate field scans when they join to form a
complete video frame. As the video picture is transmitted, the first field
picks up even-numbered scan lines - the second, odd-numbered ones. The two
interleave together to form a single, complete frame..
- 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.
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- 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 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 of activity.
- Luminance
- Brightness.
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- Multiplexer
- 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 multiple 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.
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- Noise
- Any unwanted signal that affects the video signal. Usually random
electrical energy or interference. In video, noise can produce a random salt
and pepper pattern over the picture. Heavy video noise is called snow.
- NTSC
- National Television Systems Committee of the Electronics Industries
Association (EIA) which prepared the NTSC format specifications approved by
the Federal Communications Commission, for US commercial colour
broadcasting. `NTSC' also refers to a colour television format having 525
scan lines, a field frequency of 60 Hz, a broadcast bandwidth of 4 MHz, line
frequency of 15.75 KHz, frame frequency of 1/30 of a second, and a colour
subcarrier frequency of 3.58 MHz.
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- Overscanning
- A technique used in consumer display products that extends the deflection
of a CRT's electron beam beyond the physical boundaries of the screen to
ensure that images will always fill the display area. See also underscanning.
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- PAL
- Phase Alternation Line; the European standard colour television system,
except for France. PAL's image format is 4:3, 625 lines, 50 Hz and 4 MHz
video bandwidth with a total 8 MHz of video channel width.
- Pan Tilt Zoom (PTZ)
- A device that can be remotely controlled to provide both vertical and
horizontal movement for a camera, with zoom.
- Peak to peak
- The measurement of any signal from the lowest value to the highest. In a
composite or full video signal, this should be 1 Volt.
- Pixel
- Picture element.
- 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 systems can accommodate up to sixteen preset positions
for each camera. This is an especially useful feature on larger systems with
alarmed areas.
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- Quad screen
- Display where 4 cameras are viewed on the same screen, each camera image
occupying a quarter of the display area. Other `multi screen' modes are
possible such as 9, 16 and 25 way. The splits do not always have to be equal
and other configurations can be possible.
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- Real time recording
- Refers to the top speed of a video recorder; governed by the monitor -
pictures are available as fast as the video can accept them.
- 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
connected to a desk top controller by a single cable for each monitor.
- Resolution
- Number of pixels per unit of area. A display with a finer grid contains
more pixels and thus has a higher resolution, capable of reproducing more
detail in an image.
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- Scene illumination
- The density of light 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. Normally the camera syncs need to be locked
together.
- SECAM
- Sequential Couleur A Memoire (sequential colour with memory), the
French colour TV system also adopted in Russia. The basis of operation is
the sequential recording of primary colours in alternate lines. The image
format is 4:3, 625 lines, 50 Hz and 6-MHz video bandwidth with a total 8 MHz
of video channel width.
- Signal to Noise Ratio (SNR)
- A measurement of the noise level in a signal expressed in dB (decibels).
The higher the Signal to Noise ratio, the better the quality of the signal.
- SVHS or Super VHS
- A higher quality extension of the VHS home videotape format, featuring
higher luminance and the ability to produce better copies.
- S-Video
- Type of video signal used in the Hi8 and SVHS videotape formats. S-video
transmits luminance and colour portions separately, using multiple wires,
thus avoiding the colour encoding process and its inevitable loss of picture
quality.
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- Tecton
- THE company for CCTV, multiplexers, video transmission, video switchers,
antennae, precision time, text overlay and much more. See our Home
Page.
- Telemetry
- 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.
- 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 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.
- 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 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.
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- Underscanning
- A technique generally used by some TV and video systems as a way of
ensuring that the complete image is always visible within a display area;
the opposite of overscanning.
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- VCR
- Video Cassette Recorder aka video, video recorder.
- Video Motion Detection
- A method of detecting movement in the view of the camera by the electronic
analysis of the change in picture contrast.
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- White level
- The brightest part of a video signal corresponding to approximately 1.0
Volt.
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- Y/C Video
- See S-video.
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REFERENCES used in compilation of this document: "The Photonics
Dictionary"