Digital Capture

1. Digital Capture

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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)

16. 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

17. Scanners • As with digital cameras the core component of a scanner is a CCD array Paper Light Light Path Mirror Lens CCD Sensor Array

18. 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

19. 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

20. 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

21. Digital Video Capture Beam Splitter Image CCD Arrays

22. 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

23. 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

24. 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

25. 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

26. 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

27. 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

28. 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

29. Fin Fin