Transcript lecture 2
Introduction to Graphical Hardware Display Technologies Cathode Ray Tubes (CRTs) • CRTs are the most common display device today • Evacuated glass bottle (last remaining vacuum tube) • Heating element (filament) • Electrons attracted to focusing anode cylinder • Vertical and Horizontal deflection plates • Beam strikes phosphor coating on front of tube Evacuated glass bottle (last remaining vacuum tube) Heating element (filament) Electrons attracted to focusing anode cylinder Vertical and Horizontal deflection plates Beam strikes phosphor coating on front of tube Cathode Ray Tube • The phosphor than emits a small spot of light to each position contacted by the electron beam. • The light emitted by the phosphor fades very rapidly. • One way to keep the phosphor glowing is to redraw the picture repeatedly. This type of display is called a Refresh CRT Cathode Ray Tube (CRT) 1. 2. 3. 4. Electron guns Electron beams Focusing coils Deflection coils 5. Anode connection 6. Mask for separating beams for RGB part of displayed image 7. Phosphor layer with RGB zones 8. Close-up of the phosphorcoated inner side of the screen Cathode Ray Tube • Intensity of the electron beam is controlled by setting voltage levels on the control grid • Amount of light emitted by the phosphor coating depends on the number of electrons striking the screen, we control the brightness of a display by varying the voltage on the control grid • Focusing deflection of the electron beam can be controlled either with electric fields or with magnetic fields Cathode Ray Tube • Different kind of phosphors are available for use in a CRT, besides color a major difference between phosphors is their persistence: how long they continue to emit light after the CRT beam is removed. • Persistence is defined as the time it takes the emitted light from the screen to decay to one tenth of its original intensity. • Lower persistence phosphors require higher refresh rates to maintain a picture on the screen without flicker. Raster Scan Display • The raster is a series of horizontal lines crossing the face of the CRT screen. Each horizontal line is made up of one trace of the electron beam from left to right. The raster starts at the top left corner of the crt screen. As each horizontal line is completed, the blanked electron beam is rapidly returned or retraced to the left of the screen. Vertical deflection moves the beam down, and the horizontal sweep repeats. When the vertical sweep reaches the bottom line of the raster, a vertical blanked retrace returns the sweep to the starting position of the raster, and the process is repeated. • PICTURE ELEMENTS. - The actual display of data results from the use of picture elements. A picture element is a variable dot of light derived from video signals input to the display monitor. The picture elements, often called pixels • Pictures with alphanumeric characters and graphics can be created and displayed by varying the intensity or brightness of the picture element dots. This is done in conjunction with the phosphor coating on the face of the crt. A 7 by 9 picture element character. Approximately 640 picture elements per horizontal line are required for the display of an 80 character line. Therefore, you can expect 140,000 picture elements on a raster scan display screen (80 alphanumeric characters per line and 25 lines). Random Scan Display • Refresh rate on a random scan display depends on the number of lines to be displayed. • Picture definition is stored as a set of line drawing commands in an area of memory referred to as the refresh display file or the Refresh Buffer. • Random scan displays are designed to draw all the component lines of a picture 30 to 60 times each second. Random Scan Display • Random scan system are designed for line drawing applications and cannot display realistic shaded scenes. • As the picture definition is stored as a set of line drawing instructions and not as a set of intensity values of all screen points it generally have higher resolution than raster systems. A CRT monitor contains millions of tiny red, green, and blue phosphor dots that glow when struck by an electron beam that travels across the screen to create a visible image. The illustration below shows how this works inside a CRT. • A beam penetration method for displaying color pictures has been used with random scan monitors. • Shadow-mask A shadow mask is a thin metal screen filled with very small holes. Three electron beams pass through the holes to focus on a single point on a CRT displays' phosphor surface. The shadow mask helps to control the electron beams so that the beams strike the correct phosphor at just the right intensity to create the desired colors and image on the display. The unwanted beams are blocked or "shadowed."