Video Surveillance Glossary of Terms [D]

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• » D1 [top]
  D1 is a resolution of 720x486 (NTSC) or 720x576 (PAL), and was one of Sony’s first digitized videotape formats. 
  
• » dB (Decibel) [top]
  A decibel is a logarithmic unit that measures the loudness, power, or strength of a signal.
  
• » DC (Direct Current) [top]
  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) [top]
  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 [top]
  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 [top]
  This refers to the procedure of separating different channels of video, audio, or data that were multiplexed at the source.
  
• » Depth of field [top]
  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) [top]
  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 [top]
  A sequence of binary bits that represent ones and zeros makes up a digital signal.
  
• » Distribution Amplifier [top]
  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 [top]
  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) [top]
  DNS is the system that matches server IP addresses to web site domain names. 
  
• » Dome Camera [top]
  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) [top]
  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 Kbps upload.
  
• » DSP (Digital Signal Processing) [top]
  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) [top]
  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 [top]
  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) [top]
  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 [top]
  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 [top]
  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 [top]
  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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