CCTV Glossary
Cascadia Video Products LLC
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D
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.
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