D Channel (Delta
Channel) |
This 16-kbps channel is used to signal the telephone company computer to make calls, put them on hold and activate features such as conference calling and call forwarding. It also receives information about incoming calls, as in Caller ID. May also be used for low-speed transactions with x.25 protocol. |
| D.D. (Direct Drive) |
An Auto Iris lens requiring a D.C. reference from the camera rather than the traditional video reference. |
| Daisychain |
To connect devices in a series, one after the other where the transmitted signals go to the first device, then to the second and so on. |
| DAT/DAT Drive |
Digital Audio Tape. Despite its name this is currently the most popular method for long term storage of any digital data. |
| Data cable |
Cable used for transmitting low level signals between system units. |
| dB (decibel) |
A logarithmic unit for comparing two voltages, each 6dB doubles the voltage. |
| DDS3 |
The third generation "Digital Data Storage" tape drive system. Used to archive the digitally compressed pictures from many hard disk video recorders, 12Gb native memory. Specified at 1,000 tape passes (recommended VHS tape use is 10 passes for security applications). The main draw back with this technology for security use is the drives need for frequent head cleaning. |
| DDS4 |
The fourth generation "Digital Data Storage" tape drive system, 20Gb native memory. Specified at 1,000 tape passes. The main draw back with this technology for security use is the drives need frequent head cleaning. |
| Default |
An assumption made by the system when no specific choice is given by the programme or the user. |
| Depth of Field |
The area of acceptable focus of an image. The wider you set the aperture of a lens the lower the depth of field becomes. |
| Dialup Line |
A two way line used in Public Switched Telephone Network (PSTN) |
| Digital |
A signal that levels are represented by binary numbers. These can be kept in a store. |
Digital Video
Recorder - DVR |
See Hard Disk Recorder. |
Distribution
Amplifier |
A device that accepts a video signal and sends it out over a number of independent outputs. |
| Distribution Code |
A code allocated to each Cotag distributor to ensure security. Only cards carrying this code will work with the system supplied by your distributor. |
| Door Open Time |
The time allowed for a controlled door to remain open after a valid entry before raising an alarm on the system. |
| Door Strike |
An electro-mechanical device used to lock and unlock a door under electrical control. |
| DOS |
A disk operating system in which programmes are stored on disk. |
| Dry Contact |
A contact that is isolated or unconnected from any electrical source. |
| Duplex (Multiplexer) |
A multiplexer with two frame stores allowing it to show multi-screen pictures while performing time multiplex recording. |
DVD (Digital Video
Disk, Digital
Versatile Disk) |
A de facto standard agreed on by the computer, consumer electronics and entertainment industries for storing 4.7 or more GB of data on a single optical disc the size of a CD. |
| Dwell Time |
The length of time a switcher will hold a camera before moving on to the next in sequence. |
