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D
D/A (also DA).  Opposite to A/D, i.e., digital to analogue 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 have led to a system for archiving digitized video signals.
dB.  Decibel.  A logarithmic ratio of two signals or values, usually refers to power, but also
voltage and current.  When it is calculated for power it is 10log₁₀ (P₁/P₂) and for voltage in a
constant impedance situation 20log₁₀ (V₁/V₂).
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).  It is part of
the JPEG compression algorithm.
DD.  Direct Drive.  A method of controlling one form of auto iris lens (the other is video iris),
where the electronic has been removed from the lens and is built into the camera.  To use a
DD lens with a camera, the camera must have a DD output.
Decoder.  A device used to recover the component signals from an encoded source.
Degauss.  To demagnetize or randomize magnetic particles in a tape.
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 separates the signal from the carrier frequency onto which it was
modulated.
Depth of field.  The area in front of and behind an object in perfect 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 longer the depth of field.  The depth of field decreases with an
increase in aperture ie a smaller F No.  The depth of field is always greater behind the objects
in focus, than in front of it (roughly 1/3 in front and 2/3 behind).
Dielectric.  An insulating (non-conductive) material usually when used in a capacitor or specific
insulating situation such as in a coaxial cable.
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 colour 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 colours in a scene change with the brightness
of the scene.
Digital video recorder.  A system that allows the recording of video images in a digital form
on one of many digital storage formats such as computer hard disc, DigitalVHS, DV, DAT
or DVD.  It is acceptable in English Courts for evidential purposes.
Digital signal.  An electronic signal where every different value is expressed as a different
binary code.
DIN.  Deutsche Industrie-Normen.  Germany's standards body, equivalent to the British
Standards Institution.
Disk (as in hard or floppy disc).  A flat circular plate, coated with a magnetic material, on
which data may be stored by selective magnetization of portions of the surface, representing
binary ones or zeroes.  May be a flexible, floppy disc or rigid hard disk.  It can also be a
plastic sandwich of a metal to be optically read by reflection or scattering of laser light eg. a
compact disc (CD) or Digital Versatile Disc (DVD).
Distortion.  Nonproportional representation of an original.
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 centres of individual phosphor dots on a
monitor screen.  The smaller the dot pitch the better, since it allows for more dots to be
displayed per inch and thus better resolution.  The dot pitch defines the resolution of a
monitor.
DSP.  Digital Signal Processing.  It usually refers to the electronic circuit section of a camera
capable of processing or enhancing signals.
DTE.  Data Terminal Equipment
DTMF (Dual Tone Multi Frequency)  is the dialling signal that you generate when you press
an ordinary telephone's keys.  It is known as "Touchtone" phone (formerly a registered
trademark of AT&T).  With DTMF, each key you press on your phone generates two
tones of specific frequencies. So that a voice can't imitate the tones, one tone is generated
from a high-frequency group of tones and the other from a low frequency group.  It is
sometimes used as a telemetry control signalling system.
Dubbing.  Transcribing from one recording to another which may include a change of
medium.
Duplex.  A communication system that can carry information in both directions, at the same
time 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 data communication between a
matrix switcher and a telemetry receiver.
DVD.  Digital Versatile Disc, originally Digital Video Disc.  An optical medium that stores
binary digital information by the reflection or scattering of laser light.
D-VHS.  A new standard proposed by JVC for recording digital signals on a VHS video
recorder.
DWDM.  Dense Wavelength Division Multiplexing, a recently introduced system for
increasing the number of channels in a fibre optic system.  The spacing between
wavelengths can be as little as 10nm.
DXF.  Drawing Exchange File.  A computer graphics file format, for drawings created by
Autodesk.
Dynamic range.  The difference between the smallest amount and the largest amount that
a system can process.
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