ITFT-Video display devices
- 2. Video Display Devices • A Video display device is an output device for presentation of information in visual way . When the input information is supplied as an electrical signal, the display is called an electronic display. • Common applications for electronic visual displays are televisions or computer monitors. Different types of Video display Devices • Cathode Ray Tube • Raster Scan displays • Random Scan displays • Color CRT-monitors • Direct View Storage Tube • Flat-Panel Displays • Light-emitting Diode (LED) • Liquid-crystal Displays (LCDs)
- 4. CRT • Classical output device is a monitor. • Cathode-Ray Tube (CRT) • Invented by Karl Ferdinand Braun (1897) • Beam of electrons directed from cathode (-)to phosphor-coated (fluorescent) screen (anode (+)) • Directed by magnetic focusing and deflection coils (anodes) in vacuum filled tube • Phosphor emits photon of light, when hit by an electron, of varied persistence (long 15-20 ms for texts / short < 1ms for animation) • Refresh rate (50-60 Hz / 72-76 Hz) to avoid flicker / trail • Phosphors are organic compounds characterized by their persistence and their color (blue, red, green).
- 6. Cathode-Ray Tube (CRT) Horizontal deflection and vertical deflection direct the electron beam to any point on the screen Intensity knob: regulates the flow of electrons by controlling the voltage at the control grid (high voltage reduces the electron density and thus brightness) Accelerating voltage from positive coating inside screen (anode screen) or an accelerating anode Image maintenance Charge distribution to store picture information OR Refresh CRT: refreshes the display constantly to maintain phosphor glow.
- 7. • Characteristics of Cathode-Ray Tube (CRT) • Intensity is proportional to the number of electrons repelled in beam per second (brightness) • Resolution is the maximum number of points that can be displayed without overlap; is expressed as number of horizontal points by number of vertical points; points are called pixels (picture elements); example: resolution 1024 x 768 pixels. Typical resolution is 1280 x 1024 pixels. • High-definition systems: high resolution systems.
- 8. Categories of CRT CRT Based on Technology Based on Color Raster Scan System Random Scan System Beam Penetration Method Shadow Mask Method
- 9. Random Scan Display/ Calligraphic displays Random scan systems are also called vector, stroke-writing, or calligraphic displays. Random scan system uses an electron beam which operates like a pencil to create a line image on the CRT. The image is constructed out of a sequence of straight line segments. Each line segment is drawn on the screen by directing the beam to move from one point on screen to the next, where each point is defined by its x and y coordinates. After drawing the picture, the system cycles back to the first line and design all the lines of the picture 30 to 60 time each second.
- 10. When operated as a random-scan display unit, a CRT has the electron beam directed only to the parts of the screen where a picture is to be drawn. Random-scan monitors draw a picture one line at a time and for this reason are also referred to as vector displays (or stroke-writing or calligraphic displays
- 12. • Refresh rate on a random-scan system depends on the number of lines to be displayed. • Picture definition is now stored as a set of line- drawing commands in an area of memory referred to as the refresh display file. • Random-scan systems are designed for line- drawing applications and can-not display realistic shaded scenes. • Since picture definition is stored as a set of line- drawing instructions and not as a set of intensity values for all screen points, vector displays generally have higher resolution than raster systems.
- 13. • Advantages: • High resolution • Easy animation • Requires little memory • Disadvantages: • Requires intelligent electron beam (processor controlled) • Limited screen density, limited to simple, line-based images • Limited color capability. • Improved in the 1960’s by the Direct View Storage Tube (DVST) from Tektronix.
- 14. Raster-Scan Displays • In raster scan approach, the viewing screen is divided into a large number of discrete phosphor picture elements, called pixels. • Each pixel on the screen can be made to glow with a different brightness. Color screen provide for the pixels to have different colors as well as brightness.
- 16. • In a raster-scan system, the electron beam is swept across the screen, one row at a time from top to bottom. As the electron beam moves across each row, the beam intensity is turned on and off to create a pattern of illuminated spots. • Picture definition is stored in a memory area called the refresh buffer or frame buffer. • This memory area holds the set of intensity values for all the screen points. Stored intensity values are then retrieved from the refresh buffer and "painted" on the screen one row (scan line) at a time (Fig. 2.6). • Each screen point is referred to as a pixel or pel
- 17. Color CRT Monitors • 1. Beam Penetration Method • 2. Shadow Mask Method
- 18. Beam Penetration Method • The beam-penetration method for displaying color pictures has been used with random-scan monitors. • Two layers of phosphor, usually red and green, are coated onto the inside of the CRT screen, and the displayed color depends on how far the electron beam penetrates into the phosphor layers. • A beam of slow electrons excites only the outer red layer. A beam of very fast electrons penetrates through the red layer and excites the inner green layer.
- 19. • At intermediate beam speeds, combinations of red and green light are emitted to show two additional colors, orange and yellow. • The speed of the electrons, and hence the screen color at any point, is controlled by the beam-acceleration voltage. • Beam penetration has been an inexpensive way to produce color in random-scan monitors, but only four colors are possible, and the quality of pictures is not as good as with other methods.
- 21. • Shadow-mask methods are commonly used in raster-scan systems (including color TV) because they produce a much wider range of colors than the beam-penetration method. • A shadow-mask CRT has three phosphor color dots at each pixel position. One phosphor dot emits a red light, another emits a green light, and the third emits a blue light. • This type of CRT has three electron guns, one for each color dot, and a shadow-mask grid just behind the phosphor-coated screen.
- 22. • The three electron beams are deflected and focused as a group onto the shadow mask, which contains a series of holes aligned with the phosphor-dot patterns. • When the three beams pass through a hole 'in the shadow mask, they activate a dot triangle, which appears as a small color spot on the screen. • The phosphor dots in the triangles are arranged so that each electron beam can activate only its corresponding color dot when it passes through the shadow mask.
- 23. . • Another configuration for the three electron guns is an in-line arrangement in which the three electron guns, and the. • Corresponding red-green-blue color dots on the screen, are aligned along one scan line instead of in a triangular pattern. • This in-line arrangement of electron guns is easier to keep in alignment and is commonly used in high-resolution color CRTs. • It provides the variety of color range
- 24. DVST • This is an alternative method to monitor a screen image, as it sores the picture information inside the CRT instead of refreshing the screen. • A direct-view storage tube (DVST) stores the picture information as a charge distribution just behind the phosphor-coated screen. • Two electron guns are used in a DVST. One, the primary gun, is used to store the picture pattern; the second, the flood gun, maintains the picture display. • A DVST monitor has both disadvantages and advantages compared to the refresh CRT. Because no refreshing is needed, very complex pictures can be displayed at very high resolutions without flicker.
- 26. Disadvantages of DVST systems • do not display color and that selected parts of a picture cannot be erased. • To eliminate a picture section, the entire screen must be erased and the modified picture redrawn. • The erasing and redrawing process can take several seconds for a complex picture.
- 27. Flat-Panel Displays • The term flat panel display refers to a class of video device that have reduced volume , weight and power requirement compared to a CRT. • A significant feature of flat-panel displays is that they are thinner than CRTs, and we can hang them on walls or wear them on our wrists. Categories of flat-panel displays: • emissive displays: The emissive displays (or emitters) are devices that convert electrical energy into light. • non emissive displays: Non emmissive displays (or non emitters) use optical effects to convert sunlight or light from some other source into graphics patterns. The most important example of a non emissive flat-panel display is a liquid-crystal device.
- 28. Light-emitting Diode (LED) • In LED, a matrix of diodes is arranged to form the pixel positions in the display and picture definition is stored in a refresh buffer. Information is read from the refresh buffer and converted to voltage levels that are applied to the diodes to produce the light patterns in the display
- 29. Liquid-crystal Displays (LCDs) • Liquid crystal displays are the devices that produce a picture by passing polarized light from the surroundings or from an internal light source through a liquid crystal material that transmit the light. • Liquid-crystal displays (LCDs) are commonly used in small systems, such ' as calculators and portable laptop computers. • These non-emissive devices produce a picture by passing polarized light from the surroundings or from an internal light source through a liquid-crysta material that can be aligned to either block o transmit the light