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CCTV Video Surveillance Chennai India. About CCTV Video Surveillance Chennai India. CCTV Video Surveillance Products Chennai India. CCTV Related Terms - Glossary Chennai India. CCTV Video Surveilance Sitemap, Chennai India. CCTV Video Surveillance Contact Us, Chennai India.  

Decoder A device used to recover the component signals from an encoded source.

Decibel (db)  is a logarithmic scale. In the context of CCTV it refers to voltage or signal strength. A 6db increase is equivalent to doubling the signal strength.

Depth of Field (also known as depth of focus) refers to the range of distance from the camera that is in good focus. The smaller the aperture of the lens, the longer the depth of field. However, less light coming into the lens, means that the image will be dimmer, so the camera needs to compensate by increasing the gain. For example, the best (longest) depth of field would be obtained on a bright, sunny day, with a very small aperture on the camera. In this case the imaging device still gets a reasonable amount of light, even with a small aperture. The worst (shortest) depth of field would be in a dark room, with a large aperture. To improve depth of field, the best approach is to increase the amount of light on the subject, or use a more sensitive camera.

Digital indicates information which is encoded into ones and zeros (binary code) by some computer processing element. Digital in this industry, however, can refer to several different things. Digital signal processing is a feature of all CCD cameras and many multiple camera processors. Digital video refers to digital recording of video using a DVR video recorder or DVR card.

Digital Signal Processing (DSP) is a level of image correction or enhancement which takes place within a device using a digital computer element. This digital element encodes the information into digital information for the highest level of detail and quality. Cameras with DSP imagers do not necessarily produce "digital video," but they do provide very crisp video images. Quads and multiplexers offering DSP do not necessarily produce "digital video" either, but they do deliver very high tvres and often other dynamic characteristics.

Digital Video (DV) refers to video which has been compressed into digital information for storage onto digital storage media like a hard disk drive, CD, or DVD. The quality of digital video will vary depending on factors such as the camera quality and compression method.

Digital Video Recorder (DVR) - This device is capable of accepting one or more video (and sometimes audio) input signals for recording onto digital storage media. A DVR is basically a computer specifically designed to gather and compress video into a digital video format for storage on a hard disk drive or other form of digital media. DVRs are quickly replacing VCR video recorders for security and surveillance purposes without the need for changing tapes. Key differences between DVR and VCR recorders come to available features. Advanced DVRs are capable of accepting multiple video and / audio inputs without the need for bulky and expensive multiplexers or video quads. As well, most DVRs offer built-in motion detection recording (without costly and conspicuous PIR motion sensors), and many can be remotely viewed and played back over the internet. Some models may easily be backed up onto external media for long term archival. These backup methods may include CDs, DVDs, flash media cards, or via USB to a computer or other storage device. offers several DVR models, all selected for their superior reliability, ease of use, quality of manufacture and variety of useful features. This is a device that records video signal digitally. Normally this is on a large hard drive in the machine. For this reason it is sometimes known as a hard disk recorder (HDR). Digital Video Recorder records video pictures digitally.

DIP Switches Dual Interface Poll switches usually allow you to change the configuration of a circuit board to suit your application.

Directional Antenna - An antenna for wireless equipment which sends or receives a propugated radio signal on the same axis as the corresponding antenna on the other end (with line-of-sight visibility between the two whenever possible). While omni-directional antennas are great for short range broadcasts because of their versatility, high gain directional antennas are recommended for most long range wireless video applications.

Distribution Amplifier A device that provides several isolated outputs from one looping or bridging input, and has a sufficiently high input burden and input-to-output isolation to prevent loading of the input source.


Digital Versatile Disk (DVD) Sometimes called digital video disk. This is an optical disk the same size as a CD, used for storing data of various formats. Including video, audio and computer data.

Direct Drive (DD) Some auto iris lenses require a DC signal from the camera. These are known as direct drive lenses.

Distribution Amplifier  is a device that accepts one video input and sends it out to 2 or more devices. This is often used in a situation where the signal needs to be split between a number of video monitors or recording devices. 2 or 3 devices can often (but not always) be driven just by splitting the signal with a "T" connector, but more than that, and invariably a distribution amplifier is required to give an acceptable signal quality.

Duplex An electronic device used to record and display camera images at the same time. A full-duplex DVR can record camera images while displaying images from a different camera at the same time.

DVR Card - A DVR card is a device for installation in a home PC computer which converts that PC into a DVR digital video recorder by gathering video and compressing it into digital information onto a computer's hard disk drive. When all works well, this allows a home PC to record up to several surveillance cameras. However, due to reliability concerns and software glitches associated with Windows® as an operating system platform, using PCI cards to record video is not recommended. A home PC is designed to perform many operations at once, but relatively simple functions. Recording digital video requires enormous system resources like computer memory and processor attention. Complete DVR digital video recorders designed specifically to handle recorded video (and solely recorded video) offer the benefits of reliability, as well as simple setup and use, a computer card can never achieve.

Dwell Time The length of time a switcher displays one camera before sequencing to the next.