De-multiplexing:
is the process of separating the different video, audio or data channels which were multiplexed at the source.

Depth of field:
is the distance between the nearest and furthermost points of the scene which appear in sharp focus. It depends upon the F-stop and focal length of the lens.

Dielectric:
is a material which insulates the solid centre core of a coaxial cable from the shield. The thickness of the dielectric affects the impedance of the cable.

Dome:
is a type of camera housing made of smoked or tinted plastic. It is excellent for discreet surveillance and is also available with in built pan tilt heads.

DSP chip:
Digital Signal Processing chip is a solid state device which converts an analogue video signal into a digital video signal. It is used in digital cameras and also in computers to digitise video images.

Duplex:
is a system which can handle simultaneously two channels of video, audio or data signals.

Dwell time:
is a feature in a switcher. It is the time period a picture from a given camera remains on the monitor before being replaced by a picture from the next camera. Switchers may have fixed or programmable dwell times.