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D-1

A digital video format that stores uncompressed digitized component video, encoded at Y'CbCr 4:2:2 using the CCIR 601 raster format, along with PCM audio tracks as well as time code on a 19 mm (3/4") cassette tape.  Commonly called  "full resolution" for TV specifications.  Normally D1 can mean one of the following video resolutions: 704x576 (TV PAL),  704x480 (TV NTSC), 720x576 (DVD-Video PAL) or 720x480 (DVD-Video NTSC.

DC Drive

DC driven lenses do not have the electronics for video processing, but rather only the motor that opens and closes the iris. The whole processing process in a DC driven lens is done by the cameras Auto Iris electronic section. The output from the camera is a DC voltage which opens and closes the iris.  In summary, the difference is that video drive lenses have the electronics onboard to analyze the video signal, whereas a DC drive lens opens and closes the iris based on the DC voltage being provided by the camera.  A DC drive lens relies on the camera to analyze the video signal.  Video drive lenses cannot be used with cameras that provide DC Auto Iris output, and DC drive lenses cannot be used with cameras that provide video Auto Iris output. They are not cross compatible!

Decibel, dB

In connection with digital and video image sensors, decibels generally represent ratios of video voltages or digitized light levels, using 20 log of the ratio, even when the represented optical power is directly proportional to the voltage or level, not to its square. Thus, a camera signal-to-noise (S/N) ratio of 60 dB represents a power ratio of 1000:1 between signal power and noise power, not 1,000,000:1.  In electronics, the decibel is used to express power or amplitude ratios (gains), in preference to arithmetic ratios or percentages. One advantage is that the total decibel gain of a series of components (such as amplifiers and attenuators) can be calculated simply by summing the decibel gains of the individual components. Similarly, in communications, decibels are used to account for the gains and losses of a signal from a transmitter to a receiver through some medium (free space, wave guides, coax, fiber optics, etc.) using a link budget.

Direct Current, DC

It is the electrical current whose electrons flow in one direction only and is generally constant.

Dispersion

The property of materials which have a refractive index that varies according to the wavelength of light, i.e., bend the rays of some colors more than others; a prism placed in the path of a ray of white light bends the blue and violet rays more than the orange and red, so that it spreads out or "disperses" the colors as a continuous spectrum.

Distortion

Even if the other possible aberrations were totally eliminated, images could result that still have a distorted appearance. For an example, a rectangle may appear as a barrel or pin cushion-shaped object.  A lens aberration which does not affect the sharpness of the image, but alters the shape of objects; the inability of a lens to render straight lines perfectly straight; does not improve by stopping down the lens; there are two types of distortion:

Barrel: Straight lines are bowed in at the edges of the picture frame resembling the sides of a barrel; present in small amounts in some wide angle or wide angle-zoom lenses, bu~uncorrected in fisheye lenses.

Pincushion: The opposite of barrel distortion; straight lines are bowed in toward the middle to resemble the sides of a pincushion; present in small amounts in some telephoto and telephoto-zoom lenses.

Dielectric

It is an insulating (non-conducting) medium.  It is the insulating material between conductors carrying a signal in a cable.  In coaxial cables it is between the center conductor and the outer conductor.  In twisted pair cables it is the insulation between conductors plus any surrounding air or other material.

Digital Signal Processor, DSP

DSP is concerned with the representation of the signals (motion, sound, CDs, pictures, videos, Biological membrane potentials, etc) by a sequence of numbers or symbols and the processing of signals.  These signals are any time-varying or spatial-varying quantities that are measurable through time or over space. Digital signal processing algorithms typically require a large number of mathematical operations to be performed quickly on a set of data. Signals are converted from analog to digital, manipulated digitally, and then converted again to analog form. Normally, an analog-to-digital converter is required to convert continuous signals (an input analog voltage (or current) to discrete digital numbers. The reverse operation is performed by a digital-to-analog converter.  At the heart of DSP lies computer microchips, or 'chip sets' which have replaced the conventional integrated circuits in the camera head. This enables DSP camera manufacturers to offer installer friendly, feature-rich products.

The market for DSP technology falls into two broad categories: 'standard' and 'premium' DSP.  Standard DSP cameras generally offer more consistent picture quality than their analogue counterparts, operating over a wider range of lighting conditions.  Premium DSP cameras, however, have much richer functionality.  This includes programmable intelligent backlight compensation (BLC), Video Motion Detection, remote set-up and control using a serial data link; built-in character generator and on-screen menus.  These features make Premium DSP cameras the ideal choice for complex surveillance conditions such as those encountered in town centres.

Digital Noise Reduction, DNR

The image in a CCTV camera is sourced from a non-digital component: the CCD image sensor.   An image sensor is typically comprised of a matrix of light sensors.  A light sensor can be thought of as simply a device that converts light into an electric charge.  Each square of the image sensor matrix is a photosite, usually with one light sensor 'painted' on it.  A photosite generally corresponds to one pixel in your digital image.  When light (photons) strike the image sensor, electrons are produced.  These "photoelectrons" give rise to analog signals which are then converted into digital pixels by an Analog to Digital (A/D) Converter.

There are a number of sources of noise contamination.  Heat generated might free electrons from the image sensor itself, thus contaminating the "true" photoelectrons. These "thermal electrons" give rise to a form of noise called thermal noise or dark current.  Another type of noise is more akin to the 'grain' obtained by using a high ISO film.  When we use a higher ISO, we are amplifying the signal we receive from the light photons.  Unfortunately, as we amplify the signal, we also amplify the background electrical noise that is present in any electrical system.   In low light, there is not enough light for a proper exposure and the longer we allow the image sensor to collect the weak signal, the more background electrical noise it also collects. In this case the background electrical noise may be higher than the signal.

CCTV Camera manufacturers have therefore incorporated in their firmware noise reduction algorithms that kick in when a slow shutter speed and/or high ISO is used to try to reduce the noise.  Depending on the quality of the algorithms, these work to a large extent: they do remove lots of noise and the smoothening effect of noise reduction is frequently accomplished from fine image detail.

Digital Video Disc, DVD

It is a type of optical disc storage technology.  A DVD looks like a CD-ROM disc, but it can store greater amounts of data. DVDs are often used to store full-length movies and other multimedia content that requires large amounts of storage space.

Digital Zoom

This is simulating a longer focal length in a digital camera via software. The digital zoom is accomplished via image enhancement software within the camera itself rather than the inherent mechanics of the physical zoom lens.

DirectX

It is an extension of the Microsoft Windows Operating System. DirectX technology helps games and other programs to use the advanced multimedia capabilities of your hardware.

Display Resolution

It is simply the physical number of columns and rows of pixels (usually means pixel dimensions) creating the display (e.g., 1280×1024).  The display resolution of a digital television or computer display typically refers to the number of distinct pixels in each dimension that can be displayed.   For example, a 15 inch display, whose dimensions work out to 12 inches wide by 9 inches high, capable of a maximum 1024 by 768 (or XGA) pixel resolution can display around 85 PPI in both the horizontal and vertical directions. This figure is determined by dividing width (or height) of the display area in pixels, by width (or height) of the display area in inches. It is possible for a display's horizontal and vertical PPI measurements to be different. 

Dynamic Host Configuration Protocol (DHCP)

A TCP/IP service protocol that offers dynamic leased configuration of host IP addresses and distributes other configuration parameters to eligible network clients. DHCP provides safe, reliable, and simple TCP/IP network configuration, preventing address conflicts, and helps conserve the use of client IP addresses on the network.  DHCP uses a client/server model where the DHCP server maintains centralized management of IP addresses that are used on the network.  DHCP-supporting clients can then request and obtain lease of an IP address from a DHCP server as part of their network boot process.