Transcript Multimedia Systems - University of Sussex

Digital Capture
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Digital Capture
 Video
and static images are
intrinsically analogue in nature
 Visual
scenes are continually changing
 Our eyes are analogue devices
• Even though they are based on
a finite number of receptors, the
signals from these receptors are
inherently analogue
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Digital Capture
 Capturing
video or still images
electronically is now predominantly a
digital process
 This
is similar to the capture of sound
where we capture an analogue
quantity and then convert it to a
digital signal
 Both video and still images are
captured using an array of sensors
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Digital Capture
 Digital
image capture originated in the
late 1960s when various scientists
discovered that Complementary Metal
Oxide Semiconductors (CMOS) could be
made photosensitive
 The Charge Coupled Device (CCD) was
invented in late 1969 at Bell Labs by
William Boyle and George Smith
 CCD image quality was so superior that
it quickly eclipsed CMOS
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Charge Coupled Device
 When
photons hit silicon, the electrons
in the silicon are excited and covalent
bonds that hold the electrons to the
silicon atoms are broken
 The number of electrons that are
released is directly proportional to the
energy or intensity of the light
 The CCD is a collection of tiny lightsensitive diodes (photosites), which
convert photons into electrons
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Charge Coupled Device
light-sensitive diodes do not
provide any colour information on
their own
 Thus filters must be used to
produce a coloured image
 There are several methods of
producing colour images from the
non-colour sensitive CCD
 The
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Digital Colour Capture
 The
common way to record the three
primary colours in an image is to
permanently place a filter over each
individual photosite
 It is possible to get enough information
in the general vicinity of each sensor to
make very accurate guesses about the
true colour at that location
 This process of averaging neighbouring
pixels is called interpolation
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Digital Colour Capture
 The
most common pattern of filters is
the Bayer filter pattern
 This pattern alternates a row of red and
green filters with a row of blue and
green filters
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CCD Capture Process
 The
bottom row of the CCD is read out
serially and then the row above is
copied down and so on
Analogue to
Digital
Converter
Image
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CMOS
 Both
CCD and CMOS image sensors
convert light into electrons at the
photosites
 In most CMOS devices, there are
several transistors at each pixel which
amplify and move the charge using
more traditional wires
 The CMOS approach is more flexible
because each pixel can be read
individually
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CMOS vs. CCD
 CCD
sensors create high-quality, lownoise images
 CMOS sensors, traditionally, are more
susceptible to noise
 Each CMOS sensor has several
transistors located next to it and many
of the photons hit these transistors
instead of the chip reducing light
sensitivity
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CMOS vs. CCD
 CMOS
sensors are low-power, CCDs
consume as much as 100 times more
power than an equivalent CMOS sensor
 CMOS chips can be fabricated on just
about any standard silicon production
line, so they tend to be very cheap
 CCD sensors have been mass produced
for a longer period of time, giving higher
quality pixels, and better resolution
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Still Image Capture Future
 At
the moment CCD technology is the
most widely used, because of its high
quality and low noise susceptibility
 In the future as CMOS fabrication
quality improves it will become the
dominant technology, because it is
cheaper and uses less power
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Resolution

The number of pixels and the maximum
resolution numbers on digital cameras are
slightly different
 For
example, a 2.1-megapixel camera is capable of
producing images with a resolution of 1600 X 1200
 However, a 1600 x 1200 image contains 1,920,000
pixels not 2,100,000 pixels
 2.1-megapixel refers to the actual number of
photosites on the CCD

Some of the photosites are not being used for
imaging, but as additional circuitry for the
interface to the ADC
 This circuitry is dyed black so that it doesn't
absorb any light and distort the image
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Scanners
 An
alternative
method of digitising
still images is by
using a scanner
 Home use flatbed
scanners presently
have resolutions of
up to 1200x2400
dots per inch (dpi)
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Scanners
 As
with digital cameras the core
component of a scanner is a CCD array
 In most scanners this array consists of
three lines of CCD sensors, one for
each colour
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Scanners
 As
with digital cameras the core
component of a scanner is a CCD array
Paper
Light
Light Path
Mirror
Lens
CCD Sensor
Array
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Scanner Resolution
 The
number of physical elements in the
CCD array determines the x-direction
resolution
 This can be increased by:
Using
multiple lines of CCDs and precision
optics
Software interpolation between sensors
 The
precision of the stepper motor
determines the y-direction resolution
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Colour Depth
 This
the number of colours that the
scanner is capable of reproducing
All
colour scanners support 24-bit true colour
 Many
scanners offer bit depths of up to
36 bits
These
still only output 24-bit colour, but
perform internal processing to select the best
possible choice from the colours available in
the increased palette
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Digital Video Capture
 Most
digital video cameras use CCD
technology similar to that of digital still
image cameras
 High-end video cameras use 3 separate
CCD arrays rather than using a Bayer
filter with one CCD
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Digital Video Capture
Beam
Splitter
Image
CCD Arrays
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Digital Video Capture
 The
CCD array takes two passes to
create one frame of video
i.e.
first the even rows are scanned then the
odd rows
 This
is known as Interlacing
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Frame Rate
 Video
streams are made up of a series
of still images played one after another
at high speed
 This fools the eye into believing that it is
observing a continuous stream
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Frame Rate
 This
is the number of frames per second
that are displayed
 PAL (Phase Alternating Line) in the UK
produces 25fps (50 fields per second)
 When producing video for the web we
can save data by using lower frame
rates
e.g.
20fps, 15fps etc
Below 15fps playback becomes noticeably
jerky
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Video Camera Resolution
 Resolution
of a video camera is usually
much less than that of a digital still
image camera
 DV-PAL resolution is 720x576 visible
pixels
 PAL is broadcast at 625 lines an extra 49
lines for moving the beam and
information such as teletext
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Frame Aspect Ratio
 Frame
aspect ratio describes the ratio of
width to height in the frame dimensions
of an image
 The two most common aspect ratios
are:
4:3
16:9
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Frame Aspect Ratio
 High-end
video cameras can capture
video in both 4:3 and 16:9
 This is usually done by Hard Matting
4:3
16:9
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Pixel Aspect Ratio
 Specifies
the ratio of width to height of
one pixel in an image
 You may have noticed that PAL 720x576
is slightly narrower than 4:3
 PAL uses rectangular pixels with a pixel
aspect ratio of 1.067
1
1.067
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Fin
Fin
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