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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

Image

slide10
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
cmos vs ccd
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 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
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
resolution
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
scanners
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)
scanners1
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
scanners2
Scanners
  • As with digital cameras the core component of a scanner is a CCD array

Paper

Light

Light Path

Mirror

Lens

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

Image

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:

4:3

16:9

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

4:3

16:9

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

1

1.067

slide29
Fin

Fin