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Digital cameras
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Digital cameras translate
the images they "see" into a digital
rather than analogue format. The images from
digital cameras is translated or encoded electronically
into discrete, discontinuous values (for example,
these could be visualised as pluses and minuses
or high and low voltages). Because the images
are digitised they can easily be linked in to
computer based editing and transmission systems.
Those that can digitise to a raw uncompressed
format (compared with MPEG2, MPEG4 or H.264)
can produce the best quality video available.
Digital cameras are generally more expensive
like for like than analogue, are larger, require
more power and can be more complex to use and
set up. They are, however, at the cutting edge
of technology.
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Analogue cameras
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Analogue cameras use an analogue
rather than digital signal. The image an analogue
camera "sees" is encoded into a range
of frequencies (rather than 1s and zeros, pluses
or minuses as in the case of digital cameras).
The decoder (e.g. a TV monitor in the case of
PAL) translates changes in frequency back into
an image. The key feature of these cameras is
that they produce an image that degrades steadily
along cables. Unlike digital cameras there is
no absolute cut off; the image quality slowly
degrades, eventually fading to give a "snowy"
or "cloudy" image with loss of all
definition. These are the older generation of
cameras, still very useful but being superseded.
They still produce high quality, broadcastable,
images, are very easy to install and configure,
can be small and discrete and are less expensive
like for like than digital cameras. Using digital
encoders you can have most of the advantages
of digital cameras.
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Digital transmission
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An important feature of digital
transmission is that there is no immediate loss
of image quality collected from digital cameras
if the signal is slightly weakened. As long
as the decoder (the unit that translates the
digital signal from the camera back into a picture
e.g. a computer or digital monitor) can see
the signal (pluses and minuses) then it can
translate this into a perfect image regardless
of signal's intensity. This is true as long
as the signal's intensity is within a certain
range; at some point the signal will becomes
too weak (e.g. when transmitted long distances),
the decoder will not be able to differentiate
the signal from background noise and you simply
will not get a continuous image; no intermediate,
"snowy" or "ghostly" partial
image, just nothing at all.
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High definition (HD)
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High definition refers to
any video system of higher resolution than standard definition
(SD) video, and most commonly involves display
resolutions of 1280×720 pixels (720p)
or 1920×1080 pixels (1080i/1080p). The
subscript i refers to interlaced and p to progressive
scan.
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Image resolution
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Image resolution describes
the detail an image holds either from digital
or analogue cameras. Higher resolution generally
means more image detail. Resolution is measured
in various ways. Basically, resolution quantifies
how close lines can be to each other and still
be visibly resolved. Resolution units can be
tied to physical sizes (e.g. lines per mm, lines
per inch) or to the overall size of a picture
(lines per picture height, also known simply
as lines, or TV lines).
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Interlacing
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Interlacing is a technique
of improving the picture quality of a video
without consuming extra bandwidth. Interlaced
video was designed for display on CRT televisions.
Interlaced scan refers to one of two common
methods for "painting" a video image
on an electronic display screen (the other being
progressive scan) by scanning or displaying
each line or row of pixels. This technique uses
two fields to create a frame. One field contains
all the odd lines in the image, the other contains
all the even lines of the image. A PAL (or 576i)
vision display or camera scans 50 fields every
second (25 odd and 25 even). The two sets of
25 fields work together to create a full frame
every 1/25th of a second, resulting in a display
of 25 frames per second.
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Raster images and quantisation
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Raster images are a representation in digital
form of the image the camera "sees"
and described using a finite set of digital
values, called picture elements or pixels. The
digital image contains a fixed number of rows
and columns of pixels. Pixels are the smallest
individual element in an image, holding quantised
values that represent the brightness of a given
colour at any specific point. The way in which
an image is sampled is generally called quantisation.
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Satellite technology
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At both ends of a satellite link there is an
earth station; a receiving or transmitting unit
connected to a satellite dish. The size of the
dish reflects the power required for transmission,
which in turn is dependant upon the amount of
data and quality of image you want to transmit;
the higher the quality the more the data, the
bigger the dish. In essence satellites orbiting
the earth reflect the signal from one point
to another. Many companies offer bandwidth on
their satellites for hire. You simply choose
the best one for you.
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Standard definition (SD)
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Standard definition is the video resolution
that is the basic standard for broadcasting,
but not considered high definition (HD); it
is roughly the same resolution that is attained
from the highest quality composite or component
analogue cameras. SD refers to digital television
broadcast in 4:3 or 16:9 aspect ratio (i.e.
the width and height of the images in pixels),
the same aspect ratio as NTSC (480i) or PAL
(576i).
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