Terminology > [D]
Video Surveillance Glossary of Terms [D]
By: Will Roberts
- » D1
D1 is a resolution of 720x486 (NTSC) or
720x576 (PAL), and was one of Sony’s first digitized
- » dB (Decibel)
A decibel is a logarithmic unit that
measures the loudness, power, or strength of a signal.
- » DC (Direct Current)
DC differs from AC (alternating
current) in that electricity always flows thorough it in the
same direction. A pair of wires has one positive wire and
one negative. Many security cameras are 12 Volt DC,
although some can operate at different voltages.
- » DD (Direct Drive)
This uses a gearless drive mechanism,
making it less prone to mechanical failure. PTZ security
cameras will often use them for pan, tilt, and zooming.
- » Default Gateway
In order to send data or video between
networks, the IP Address of the Router is required. This
address is known as the Default Gateway.
- » De-multiplexing
This refers to the procedure of
separating different channels of video, audio, or data that
were multiplexed at the source.
- » Depth of field
This is the difference between the
nearest and furthest points in a scene that remain in sharp
focus. Depth of field is dependent on the F-stop and focal
length of the lens.
- » DHCP (Dynamic Host Configuration Protocol)
A DHCP refers to the protocol used by a
host computer to obtain an IP address so that it can
communicate with other host computers. These addresses are
usually dynamic, meaning they change periodically, so a
connection cannot be obtained (or maintained) over the open
Internet. Use of both static IP addresses and dynamic DNS
helps establish a consistent connection.
- » Digital Signal
A sequence of binary bits that
represent ones and zeros makes up a digital signal.
- » Distribution Amplifier
This device amplifies and distributes
an audio or video signal to multiple outputs, such as
several video monitors or recording devices. This device
allows the maintenance of the original signal’s output
impedance to avoid mismatches which could reduce the power
required to properly drive the signal’s end point.
- » DivX
DivXNetworks created DivX, a MPEG-4
digital video technology. Among its benefits is compression
technology, which allows DivX equipped network cameras to
store a month of video on a 20’gigabyte hard drive.
- » DNS (Domain Name Service)
DNS is the system that matches server
IP addresses to web site domain names.
- » Dome Camera
A common indoor security camera, dome
cameras are mounted on the ceiling. Their two main
advantages are a more appealing visual appearance and being
easily movable. Their drawback is a lack of usefulness
during low light situations (therefore not effective when
the lights are off).
- » DSL (Digital Subscriber Line)
DSL is a digital telecommunications
protocol that allows existing copper phone lines to be used
for high-speed transfer of data between home and business
end-users. xDSL refers to the various types of Digital
Subscriber Lines which include: ADSL (Asymmetric DSL), SDSL
(Single-line DSL), HDSL (High-data-rate DSL) and VDSL
(Very-high-data-rate DSL). In theory, ADSL (the most common
of these types), allows for download speeds of up to 9 Mbps
and upload speeds of up to 640 Kbps. In reality, commercial
performance is normally up to 1.544 Mbps download and 128
- » DSP (Digital Signal Processing)
These chips can compress video
independent of the CPU, which avoids the need to draw
processing power from the CPU, allowing it to focus on other
applications and computing tasks.
- » DTMF (Dual Tone Multi-Frequency)
The scientific term for the Touch Tone
signal used on telephones, it is the existing standard for
the use of twisted wire pairs to send signals. Some PTZ
cameras use DTMF signals in the transferring of telemetry
information to the camera. This allows users to move the
camera by dialing the number for that camera and then
pressing buttons on their phone.
- » Duplex
A type of multiplexer that allows you
to simultaneously record images to tape and display live
multiple-picture (or single picture) screen images of
security cameras. Another capability that is provided is
the ability to record images on one VCR while at the same
time playing back previously recorded images on a second
VCR. Compare with a simplex multiplexer which provides less
features and capabilities.
- » DVR (Digital Video Recorder)
This device transforms analog video
signals from security cameras into digital format, suitable
for storage on a hard drive. It also helps the user manage
the stored video files, as well as providing motion
detection settings and PTZ security camera control. DVRs
can often be remotely accessed over the Internet.
- » Dwell Time
The time a multiplexer or DVR stays on
an individual camera before moving onto the next one in the
sequence is known as dwell time.
- » Dynamic IP address
This is the rotation of IP addresses
such that every time a user logs onto the Internet, their IP
address changes. This is done for Internet security
purposes, either by the user or by their ISP. This process
can interfere with the use of networked devices such as
Network IP Cameras because they normally require a static IP
address to function properly.
- » Dynamic Range
A camera with a wide dynamic range is
able to operate in variable light conditions (known as lux
levels), especially those of low light. A “dB” is usually
the unit used to measure the dynamic range of security
cameras, with more being better. A camera with a 60 dB
dynamic range would be well suited to clearly record a scene
in low or high light levels.
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