B C D
E F G
H I J
K L M
N O P
Q R S
T U V
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
signals, maintaining signal quality equal to that of a CD. Recent
in hardware and software have led
to a system for archiving digitized video signals.
A logarithmic ratio of two signals or values, usually refers to power,
voltage and current. When
it is calculated for power it is 10log10
and for voltage in a
constant impedance situation 20log10
Direct Broadcast Satellite. Broadcasting from a satellite
to a consumer user,
usually using a small aperture
DC. Direct current.
Current that flows in only one direction, as opposed to AC.
Discrete Cosine Transform. Mathematical algorithm used to
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
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
Demodulator. A device
that separates the signal from the carrier frequency onto which it was
Depth of field. The
area in front of and behind an object in perfect focus, that appears
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.
A change in sub-carrier amplitude of a video signal caused by a change
luminance level of the signal.
The resulting TV picture will show a change in
caused by a simultaneous change
in picture brightness.
A change in the sub-carrier phase of a video signal caused by a change
the luminance level of the signal.
The hue of colours in a scene change with
of the scene.
Digital video recorder.
A system that allows the recording of video images in a digital form
of many digital storage formats such as computer hard disc, DigitalVHS,
or DVD. It is acceptable
in English Courts for evidential purposes.
Digital signal. An
electronic signal where every different value is expressed as a
DIN. Deutsche Industrie-Normen.
Germany's standards body, equivalent to the British
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
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
Digital Signal Processing. It usually refers to the electronic
section of a camera
capable of processing or enhancing
Data Terminal Equipment
Tone Multi Frequency) is the dialling signal that you generate
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
from one recording to another which may include a change of
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
in multiplex mode. It can also refer to duplex data communication
matrix switcher and a telemetry
DVD. Digital Versatile
Disc, originally Digital Video Disc. An optical medium that stores
binary digital information by the
reflection or scattering of laser light.
A new standard proposed by JVC for recording digital signals on a VHS
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
Exchange File. A computer graphics file format, for drawings
Dynamic range. The
difference between the smallest amount and the largest amount that