Product guide

68
AGC (Automatic Gain Control)
An electr
onic circuit that tries to keep signals at a constant level; more
useful in cameras at low light levels. The higher the number the better
,
typical values 12 - 20dB i.e. a gain of appr
ox 4x - 10x.
A.I. (Auto Iris
An electr
onic circuit fitted to the iris of a lens to help compensate for
large changes in light levels.
A.L.C. (Automatic Level Contr
ol)
On AI lenses, also known as the peak/average control. Adjusting this
contr
ol allows the auto iris circuitry to either take bright spots more into
consideration (peak), bringing out detail in bright areas, or less into
consideration (average) bringing out detail in shadows.
ADSL (Asymmetrical Digital Subscriber Line)
A telephone line with uneven transmission rates; half the transmission
can attain speeds up to 640 kbps; the other half can attain speeds of
2.048 Mbps or gr
eater in the opposite direction.
Ah
Ampere hours. e.g. 1A for 6 hours = 6Ah. Always de-rate the battery
capacity by 66%
Alar
ming
The ability of CCTV equipment to r
espond to an input signal, normally
a simple switch closure. The response varies depending on equipment
type, most common is for switcher to 'hold' on the camera
corresponding to the alarm input.
Ambient Light Level
Background or general light level of a given area.
Analogue
A signal in which any level is represented by a directly proportional
voltage; not digital.
Angle of V
iew
This is what alters (not the focal length) when you use a lens designed
for one format on another format. It decreases with format size.
Aperture
The 'opening' of a lens, a measure of its light gathering capability.
Relative Aperture is a ratio between its focal length and effective
aperture, measured in F numbers, generally the lower the better.
Archive
Long ter
m storage of digital files to a removable media. At this time
DAT is the most common media for archiving of digital recordings.
Aspherical Lens
A lens designed with a non spherical shape so that it refracts the light
passing thr
ough it to either lower the lens apertur
e so that it passes
more light or decrease barrel distortion on wide angle lenses.
Asynchr
onous T
ransmission
T
ransmission in which time intervals between transmitted characters
may be of unequal length. T
ransmission is controlled by start and stop
bits at the beginning and end of each character. Compare with
synchronous transmission.
A
TM (Asynchronous Transfer Mode)
A high-speed cell-switching network technology that handles data
and real-time voice and video. ATM is defined in the Broadband ISDN
(BISDN) standar
d and pr
ovides "bandwidth on demand" by char
ging
customers for the amount of data they send. Data rates ar
e scalable,
starting as low as 2.048 Mbps with intermediate speeds of 25,51 and
100 Mbps, to high speeds of 155,622 Mbps and up into the gigabit
range.
Auto White Balance
Feature on colour cameras whereby the camera constantly monitors
the light and adjusts its colour to maintain white areas.
Auto-terminating
Feature whereby the equipment automatically selects the correct
termination depending on whether the video output BNC is
connected.
B.L.C. (Back Light Compensation)
A feature of CCD cameras which electronically compensates for high
backgr
ound lighting to give detail which would normally be
silhouetted.
Back Focus
The mechanical aligning of the imaging device with the focal point
of the lens. Most important on zoom lenses to ensur
e the image stays
in focus throughout the zoom range.
Balanced Signal
Method of transmitting video, usually over twisted pair cable, that
consists of two equal but opposite signals being sent down two
conductors.
Balun (Balanced Unbalanced)
A device that connects a balanced line, such as twisted pair
, to an
unbalanced line, such as coax cable.
Band Width
The fr
equency range that an electronic circuit can accept.
Bandwidth
The range of fr
equencies available for signalling the difference
between the highest and lowest frequencies of a band, measured in
Hertz.
Base-Band V
ideo
Unmodulated video signal suitable for display on a monitor but not a
domestic TV.
Baseband
A full-channel bandwidth transmission wher
e digital signals ar
e
placed onto a transmission line with no change in their modulation.
Baud
Unit of signalling speed. The speed in baud is the number of line
changes (in frequency, amplitude, etc.) or events per second. At low
speeds, each event represents only one bit condition, and baud rate
equals bps. As speed increases, each event represents more than
one bit and baud rate does not truly equal bps.
Baud rate
Data transmission speed.
Bit (Binary Digit)
The smallest unit of information in a binary system; a one or zero
condition.
Black Level
The level on a video signal, just above the sync pulses, wher
e black
portions of the picture are represented.
Blanking (Field and line flyback blanking)
The operation of tur
ning of
f the monitor display, or pick-up device,
during sync pulses to avoid thin white lines appearing on the picture.
BNC: Video connector, the most commonly used in CCTV.
C Mount
Long established industrial standar
d for connecting cameras and
lenses, widely used in the CCTV industry.
C.C.D. (Char
ged Coupled Device)
A 'chip' that per
for
ms the same function as a camera tube.
C.C.I.R. (Comite Consultatif International des Radiocommunications)
The European TV standard 625 lines 50 fields.
Cable scr
een/shield
A conductor which forms a sheath around one or more insulated
conductors. This reduces the effect of interference onto the
conductors. This shield is usually metallic foil or stranded wir
e wrapped
ar
ound the conductors.
CD-ROM (Compact Disc Read-Only Memory)
A compact disk format used to hold text, graphics and hi-fi stereo in
excess of 650 MB of data, which is equivalent to about 250,000 pages
of text or 20,000 medium-resolution pages.
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Glossary of Terms