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D/A (also DA). Opposite to A/D, i.e., digital to
analog conversion. Dark current. Leakage signal from a CCD sensor in the absence of incident light. Dark noise. Noise caused by the random (quantum) nature of the dark current. DAT (digital audio tape). A system developed initially for recording and playback of digitized audio signals, maintaining signal quality equal to that of a CD. Recent developments in hardware and software might lead to a similar inexpensive system for video archiving, recording and playback. dB. Decibel. A logarithmic ratio of two signals or values, usually refers to power, but also voltage and current. When power is calculated the logarithm is multiplied by 10, while for current and voltage by 20. DBS. Direct broadcast satellite. Broadcasting from a satellite directly to a consumer user, usually using a small aperture antenna. DC. Direct current. Current that flows in only one direction, as opposed to AC. DCT. Discrete cosine transform. Mathematical algorithm used to generate frequency representations of a block of video pixels. The DCT is an invertible, discrete orthogonal transformation between time and frequency domain. It can be either forward discrete cosine transform (FDCT) or inverse discrete cosine transform (IDCT). Decoder. A device used to recover the component signals from a composite (encoded) source. Degauss. To demagnetize. Delay line. An artificial or real transmission line or equivalent device designed to delay a wave or signal for a specific length of time. Demodulator. A device that strips the video and audio signals from the carrier frequency. Depth of field. The area in front of and behind the object in focus that appears sharp on the screen. The depth of field increases with the decrease of the focal length, i.e., the shorter the focal length the wider the depth of field. The depth of field is always wider behind the objects in focus. Dielectric. An insulating (nonconductive) material. Differential gain. A change in sub-carrier amplitude of a video signal caused by a change in luminance level of the signal. The resulting TV picture will show a change in color saturation caused by a simultaneous change in picture brightness. Differential phase. A change in the sub-carrier phase of a video signal caused by a change in the luminance level of the signal. The hue of colors in a scene change with the brightness of the scene. Digital disc recorder. A system that allows recording of video images on a digital disc. Digital signal. An electronic signal where every different value from the real-life excitation (sound, light) has a different value of binary combinations (words) that represent the analog signal. DIN. Deutsche Industrie-Normen. Germany’s standard. Disk. A flat circular plate, coated with a magnetic material, on which data may be stored by selective magnetization of portions of the surface. May be a flexible, floppy disc or rigid hard disk. It could also be a plastic compact disc (CD) or digital video disc (DVD). Distortion. Nonproportional representation of an original. DMD. Digital micro-mirror device. A new video projection technology that uses chips with a large number of miniature mirrors, whose projection angle can be controlled with digital precision. DOS. Disk operating system. A software package that makes a computer work with its hardware devices such as hard drive, floppy drive, screen, keyboard, etc. Dot pitch. The distance in millimeters between individual dots on a monitor screen. The smaller the dot pitch the better, since it allows for more dots to be displayed and better resolution. The dot pitch defines the reaslution of a monitor. A high-resolution CCTV or computer monitor would have a dot pitch of less than 0.3 mm. Drop-frame time code. SMPTE time code format that continuously counts 30 frames per second, but drops two frames from the count every minute except for every tenth minute (drops 108 frames every hour) to maintain synchronization of time code with clock time. This is necessary because the actual frame rate of NTSC video is 29.94 frames per second rather than an even 30 frames. DSP. Digital signal processing. It usually refers to the electronic circuit section of a device capable of processing digital signals. Dubbing. Transcribing from one recording medium to another. Duplex. A communication system that carries information in both direction is called a duplex system. In CCTV, duplex is often used to describe the type of multiplexer that can perform two functions simultaneously, recording in multiplex mode and playback in multiplex mode. It can also refer to duplex communication between a matrix switcher and a PTZ site driver, for example. DV-Mini. Mini digital video. A new format for audio and video recording on small camcorders, adopted by the majority of camcorder manufacturers. Video and sound are recorded in a digital format on a small cassette (66×48×12 mm), superseding S-VHS and Hi 8 quality. D-VHS. A new standard proposed by JVC for recording digital signals on a VHS video recorder. Dynamic range. The difference between the smallest amount and the largest amount that a system can represent. EBU. European Broadcasting Union. EIA. Electronics Industry Association, which has recommended the television standard used in the US, Canada and Japan, based on 525 lines interlaced scanning. Formerly known as RMA or RETMA. Encoder. A device that superimposes electronic signal information on other electronic signals. Encryption. The rearrangement of the bit stream of a previously digitally encoded signal in a systematic fashion to make the information unrecognizable until restored on receipt of the necessary authorization key. This technique is used for securing information transmitted over a communication channel with the intent of excluding all other than authorized receivers from interpreting the message. Can be used for voice, video and other communications signals. ENG camera. Electronic News Gathering camera. Refers to CCD cameras in the broadcast industry. EPROM. Erasable and programmable read only memory. An electronic chip used in many different security products that stores software instructions for performing various operations. Equalizer. Equipment designed to compensate for loss and delay frequency effects within a system. A component or circuit that allows for the adjustment of a signal across a given band. Ethernet. A local area network used for connecting computers, printers, workstations, terminals, etc. within the same building. Ethernet operates over twisted wire and coaxial cable at speeds up to 10 Mbps. Ethernet specifies a CSMA/CD (carrier sense multiple access with collision detection). CSMA/CD is a technique of sharing a common medium (wire, coaxial cable) among several devices. External synchronization. A means of ensuring that all equipment is synchronized to the one source. |