Data Lifetime : This is the name generally given to a feature that ensures that data is not retained on the system for more than a specified amount of time. This is a legal requirement in many European countries.
Day/Night Operation : This refers to cameras that are capable of providing usable images at night by changing camera parameters to work more efficiently in low light. A basic example of day/night operation is the use of IR LEDs within the camera housing. Some very inexpensive cameras can provide usable images in total darkness. More sophisticated changes in more expensive cameras include switching the camera to black and white mode at night and reducing the image capture rate to allow more light to reach the imager.
Dealer : In reference to CCTV, this is a generic term used to indicate individuals or companies that sell products to end users. Dealers come is all sizes and capabilities. They buy from manufacturers or distributors and design CCTV systems to meet individual customer needs.
Decibel (dB) : A logarithmic measure of the ratio between two powers, voltages, currents, sound intensities, etc. Signal-to-noise ratios are expressed in decibels.
Depth of Field : This is the in-focus range of the image produced by the lens. Objects in the focus area are clear. They will become less clear as they get closer to, or further away from the camera. The distance of the area of clear focus is the depth of field.
Distribution : This is the term given to indicate the overall distribution process where manufacturer’s goods are purchased by intermediaries and eventually sold and installed at end users.
Distributors : In reference to CCTV, this is the term used to indicate companies that provide products to a wide range of dealers. This is part of the overall distribution process. Distributors buy in bulk from many manufacturers and sell to dealers at wholesale prices.
DNS : (Domain Name System): Matches Internet computer names to IP numbers. This allows you to type in the URL (www.abc.com) instead of the IP address (172.20.1.99).
Driver - (Device Driver): A driver is a software program that allows a computer to communicate with a peripheral. You need the appropriate driver to allow your printer to work with your system. Many drivers are available on a PC as part of the operating system. However, don’t depend on this as drivers for devices newer than the operating system will not be installed. You typically get a copy of the driver with the purchased device. The manufacturer’s web site is a common place to get the latest available drivers for a device. Drivers are often referred to as DLLs (dynamic link library). Virtually all drivers used in CCTV devices are proprietary. So once you get away from the standard stuff you have to rely upon the manufacturer’s good will for interface help.
DHCP : This is an acronym for Dynamic Host Configuration Protocol. This is a function that allows a network to automatically assign an IP address to a PC when the user logs on to the system. All devices on a network require an IP address. DHCP does this automatically. The alternative is to manually enter a static IP address in each device.
Dial-up : This is a slow speed network connection using ordinary telephone lines and modems. The maximum connection speed is reported as 56K but most provide closer to 30K.
Digitized Signal : This is an analog signal that has been converted to a digital form so that it can be processed and altered.
Digital : In CCTV, digital refers to devices that operate in pixel formats. Analog video devices use traditional NTSC and PAL formats. The lines of difference are blurred when you consider the analog signal is converted to digital and back again (often several times) within a traditional CCTV system. Pure end-to-end digital video is achieved using IP cameras through a network to NVRs and LCD monitors.
Digital Zoom : This refers to enlarging a portion of an image by adding additional pixels within the image to fill the larger area. It makes the picture area larger at the expense of video quality. The intelligence for this feature can be in a camera or a DVR.
Distribution Amplifier : Distribution amps take an incoming video signal and split it into separate signals that are sent to multiple devices. Dist. Amps come in a range of sizes (1x4, 16x4, etc). Despite the name, few of these devices actually amplify the Video signal. They separate and distribute a single input to several outputs. They don’t increase the maximum cable distance.
Domain : A number of computer devices administered as a group. A Domain server is set up and maintained by the network administrator (the person in charge of the network).
DSL : An acronym for Digital Subscriber Line. This is a high speed network connection typically used in homes and businesses.
DSP : Acronym for Digital Signal Processor. DSP chips compress video freeing the CPU processor for other tasks. This increases compression capabilities at an additional cost.
Dual Stream : This term is typically used to indicate a device capable of providing two different video compression methods. An example is a DVR that shows live video in M-JPEG and transmits to the remote software using MPEG-4. IP cameras often have selectable (dual stream) transmit capability. This may sometimes refers to two streams of the same compression method with different parameter settings.
DVR : An acronym for Digital Video Recorder.
DVD : An acronym for Digital Video Disk. This is the standard media used for PCs and movies.
Dynamic IP Address : This refers to IP addresses that are automatically assigned to a network device when the user logs on to the system. See DHCP.