1. CHAPTER TWELVE DISPLAY TECHNOLOGY (I) CRT & LCD T.J.Iskandar Abd Aziz Adapted from Notes Prepared by: Noor Fardela Zainal Abidin Revised on Sept 2012 1 CGMB113/ CITB 123: MULTIMEDIA TECHNOLOGY

2. 2 identify the terms and concept related to display technology state the different types of display technologies understand how the various display technology works At the end of this chapter, students should be able to:

3. CRT TECHNOLOGY all TVs in use today rely on a device known as the cathode ray tube, or CRT, to display their images 100 year old technology. ▫ A glass bell envelope which contains a vacuum and an electron gun. ▫ By the application of electric current, and electron stream is created, which is fired through the vacuum towards the inside face of the glass envelope. ▫ Here it strikes a phosphor layer, which converts the beam into visible light, colour being achieved through mixing varying levels of light intensity from red, green and blue phosphors. 3

4. CRT TECHNOLOGY 4 There is a cathode and a pair (or more) of anodes. There is the phosphor-coated screen. There is a conductive coating inside the tube to soak up the electrons that pile up at the screen-end of the tube. Steering coils are use to position the electron beam at any point on the screen.

5. CRT TECHNOLOGY ( diagram 1.1) 5

6. CRT TECHNOLOGY: Cathode 6 The "cathode" is a heated filament The "ray" is a stream of electrons that naturally pour off a heated cathode into the vacuum. Electrons are negative. The anode is positive, so it attracts the electrons pouring off the cathode. In a TV's cathode ray tube, the stream of electrons is focused by a focusing anode into a tight beam and then accelerated by an accelerating anode. This tight, high-speed beam of electrons flies through the vacuum in the tube and hits the flat screen at the other end of the tube. This screen is coated with phosphor, which glows when struck by the beam.

7. CRT TECHNOLOGY: Steering Coils 7 steering coils are simply copper windings These coils are able to create magnetic fields inside the tube, and the electron beam responds to the fields. One set of coils creates a magnetic field that moves the electron beam vertically, while another set moves the beam horizontally. By controlling the voltages in the coils, you can position the electron beam at any point on the screen.

8. CRT TECHNOLOGY: Phosphor 8 Phosphor is any material that, when exposed to radiation, emits visible light.visible light The radiation might be ultraviolet light or a beam of electrons.

9. CRT TECHNOLOGY: Phosphor 9 In a CRT, phosphor coats the inside of the screen. When the electron beam strikes the phosphor, it makes the screen glow. In a color screen, there are three phosphors arranged as dots or stripes that emit red, green and blue light. There are also three electron beams to illuminate the three different colors together. There are thousands of different phosphors that have been formulated.

10. CRT TECHNOLOGY: Adding Color 10 The screen is coated with red, green and blue phosphors arranged in dots or stripes If you turn on your TV or computer monitor and look closely at the screen with a magnifying glass, you will be able to see the dots or stripes. On the inside of the tube, very close to the phosphor coating, there is a thin metal screen called a shadow mask. This mask is perforated with very small holes that are aligned with the phosphor dots (or stripes) on the screen

11. CRT TECHNOLOGY: Adding Color 11

12. CRT TECHNOLOGY: Adding Color 12 When a color TV needs to create a red dot, it fires the red beam at the red phosphor. Similarly for green and blue dots. To create a white dot, red, green and blue beams are fired simultaneously -- the three colors mix together to create white. To create a black dot, all three beams are turned off as they scan past the dot. All other colors on a TV screen are combinations of red, green and blue.

13. CRT TECHNOLOGY: Painting the Screen 13 In a black-and-white TV, the screen is coated with white phosphor and the electron beam "paints" an image onto the screen by moving the electron beam across the phosphor a line at a time. To "paint" the entire screen, electronic circuits inside the TV use the magnetic coils to move the electron beam in a "raster scan" pattern across and down the screen. The beam paints one line across the screen from left to right. It then quickly flies back to the left side, moves down slightly and paints another horizontal line

14. CRT TECHNOLOGY: Painting the Screen 14 The term horizontal retrace is used to refer to the beam moving back to the left at the end of each line, while the term vertical retrace refers to its movement from bottom to top. A TV screen normally has about 480 lines visible from top to bottom

15. CRT TECHNOLOGY: Painting the Screen 15 Standard TV uses Interlacing technique when painting the screen The beam paints every other line as it moves down the screen - - for example, every odd-numbered line. Then, the next time it moves down the screen it paints the even-numbered lines, alternating back and forth between even-numbered and odd- numbered lines on each pass. The entire screen, in two passes, is painted 30 times every second. Progressive scanning, which paints every line on the screen 60 times per second. (computer monitors use progressive scanning because it reduces flicker)computer monitors Because the electron beam is painting all 525 lines 30 times per second, it paints a total of 15,750 lines per second. (Some people can actually hear this frequency as a very high-pitched sound emitted when the television is on.)

16. CRT TECHNOLOGY: Painting the Screen 16 When a television station wants to broadcast a signal to your TV, or when your VCR wants to display the movie on a video tape on your TV, the signal needs to mesh with the electronics controlling the beam so that the TV can accurately paint the picture that the TV station or VCR sends.VCR The TV station or VCR therefore sends a well-known signal to the TV that contains three different parts:  Intensity information for the beam as it paints each line  Horizontal-retrace signals to tell the TV when to move the beam back at the end of each line  Vertical-retrace signals 60 times per second to move the beam from bottom-right to top-left

17. CRT: Advantage and disadvantage 17 Advantages of CRT  robust, well-known technology  high-quality resolution and image control Disadvantages of CRT  size (footprint) on monitors  short or mini-neck tubes possible, but exacerbates distortion problems  analogue technology

18. LCD Technology 18 Liquid Crystal Display (LCD) LCDs are common because they offer some real advantages over other display technologies. They are thinner and lighter and draw much less power A simple LCD display from a calculator

19. Liquid Crystals 19 So, do liquid crystals act like solids or liquids or something else? Liquid crystals are closer to a liquid state than a solid. It takes a fair amount of heat to change a suitable substance from a solid into a liquid crystal, and it only takes a little more heat to turn that same liquid crystal into a real liquid. This explains why liquid crystals are very sensitive to temperature and why they are used to make thermometers and mood rings. thermometersmood rings It also explains why a laptop computer display may act funny in cold weather or during a hot day at the beach!laptop computer

20. Nematic Phase Liquid Crystals 20 The liquid crystals that make LCDs possible. Affected by electric current. LCDs use these liquid crystals because they react predictably to electric current in such a way as to control light passage.light

21. Creating an LCD 21 LCD is possible because  Light can be polarized.  Liquid crystals can transmit and change polarized light.  The structure of liquid crystals can be changed by electric current.  There are transparent substances that can conduct electricity

22. Creating LCD 22 LCD is made of several layers that are arranged according to the following order: 1. Polarising filter 2. Sheet of glass 3. Electrode 4. Alignment layer 5. Liquid crystals 6. Alignment layer, 7. Electrode, 8. Sheet of glass, 9. Polarising filter.

23. Creating LCD 23

24. How LCD Works? 24 The first stage of an LCD display involves passing the light through a polarizing filter. It then gets into a layer filled with liquid crystals that are controlled by the transistors. The light then passes through color filters (like CRT monitors, each LCD display pixel consists of three components – red, green and blue). Transistor applies voltage to liquid crystals, that sets their spatial alignment. Light changes its polarization angle when it passes through the ordered liquid crystal molecular structure, and depending on its new polarization angle it will be absorbed completely or partially. This allows the creation of any halftone from black to pure white.

25. Types of LCD 25 Two types of LCD 1. Passive Matrix (Dual Scan) 2. Active Matrix-TFT (Thin Film Transistor)

26. Passive Matrix (Dual Scan) 26 The matrix refers to the underlying layer of conductors, used to activate the screen elements. In passive matrix, this is usually made up of a lattice of conductive strips running from edge to edge of the display As these strips are relatively long, the time taken to activate each element is longer than in active matrix models. This means that it takes longer to refresh the screen.

27. Active Matrix-TFT (Thin Film Transistor) 27 Uses a much more complex conductor array, replacing the lattice with a grid of independent transistors that lie on a layer beneath the screen elements. Far more complex to manufacture, but much faster because it independently addresses the liquid crystal cells. Viewing angle is wider. Much more expensive and has a higher power drain.

28. Active Matrix-TFT (Thin Film Transistor) 28

29. Advantage n’ Disadvantage LCD 29 Advantages  LCD monitors consume less power  Do not produce electromagnetic radiation as CRTs do  Do not flicker like CRTs  Are light and slim in size  Full viewable size Disadvantages  More expensive than CRT (but less so now!)  Poorer resolution

30. Reference 30 Tom Harris, How LCDs Works, http://www.stuffo.com. 1998-2005 http://www.stuffo.com Lachlan L. Mackinnon. Notes: Multimedia Technology (F291G2). Heriot_Watt University. Edinburgh. Scotland