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Digital Capture. 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 capture1
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
digital capture2
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
digital capture3
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
charge coupled device
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 light-sensitive diodes (photosites), which convert photons into electrons
charge coupled device1
Charge Coupled Device
  • The 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
digital colour capture
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
digital colour capture1
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
ccd capture process
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


  • 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
cmos vs ccd
  • CCD sensors create high-quality, low-noise 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
cmos vs ccd1
  • 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
still image capture future
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
  • 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
  • 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)
  • 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
  • As with digital cameras the core component of a scanner is a CCD array



Light Path



CCD Sensor Array

scanner resolution
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
colour depth
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
digital video capture
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
digital video capture1
Digital Video Capture

Beam Splitter


CCD Arrays

digital video capture2
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
frame rate
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
frame rate1
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
video camera resolution
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
frame aspect ratio
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:



frame aspect ratio1
Frame Aspect Ratio
  • High-end video cameras can capture video in both 4:3 and 16:9
  • This is usually done by Hard Matting



pixel aspect ratio
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