video-display devices

1.       Video Display Devices

1.1.    Refresh Cathode-Ray Tubes (CRTs)

1.2.    Raster-Scan Displays

1.3.    Random-Scan Displays

1.4.    Color CRT Monitors

1.4.1. Beam penetration method

1.4.2. Shadow mask method

1.5.    Direct View Storage Tube(DVST)

1.6.    Flat-Panel Displays

 

—  Graphics output technology may be split into two categories:

—  Non-permanent output to a screen

—  Permanent output to a paper

—  Recently all devices are digital in nature:

—  Therefore, producing images become the process of setting individual points on the screen or on paper

—  The points are laid out in a regular pattern on the output media.

Video Display Devices

Ø  Refresh Cathode-Ray Tubes (CRTs)

Ø   Raster-Scan Displays

Ø   Random-Scan Displays

Ø   Color CRT Monitors

Ø   Flat-Panel Displays

Refresh Cathode-Ray Tubes (CRT)                     

—  A beam of electrons emitted by an electron gun, passes through focusing and deflection systems that direct the beam toward specified positions on the phosphor-coated screen.

—  Because the light emitted by the phosphor fades very rabidly, the refresh process is needed to maintain the picture on the screen.

—  Refreshing is done by redrawing the picture repeatedly by quickly directing the electron beam back over the same screen points.

—  Refresh rate: the frequency at which a picture is redrawn on the screen.

—  Components of the Electron Gun :

1.       The heated metal cathode

2.       A control grid

—  Heat is supplied to the cathode by directing a current through a coil of wire (the filament) inside the cathode.

—  This causes electrons to be “boiled off” the hot cathode surface.

—  
Then, the free, negatively electrons are then accelerated towards the phosphor coating by a high positive voltage.

—  Intensity of the electron beam is controlled by the voltage at the control grid.

—  A high negative voltage applied to the control grid will shut off the beam.

—  A smaller negative voltage on the control grid decreases the number of electrons passing through.

—  The brightness of a display point is controlled by varying the voltage on the control grid.

—  The focusing system forces the electron beam to converge to a small cross section as it strikes the phosphor.

—  Deflection of the electron beam can be controlled by the deflection coils.

—  Spots of light are produced on the screen by the transfer of the CRT beam energy to the phosphor.

—  Persistence: how long phosphors continue to emit light after the CRT beam is removed.

—  Persistence is defined as the time that it takes the emitted light from the screen to decay to one-tenth of its original intensity.

—  Lower-persistence phosphors require high refresh rates to maintain a picture definition on the screen without flicker and they are useful for animation.

—  Higher-persistence phosphors  are useful for displaying highly complex, static pictures.

Raster-Scan Displays

—  The electron beam is swept across the screen one row at a time from top to bottom. Each row is referred to as a scan line.

—  Picture definition is stored in the frame buffer. This memory area holds the set of intensity values for the screen points. These stored values are then retrieved from the refresh buffer and used to control the intensity of the electron beam as it moves from spot to spot across the screen.

—  Refreshing on raster-scan display is carried out at the rate of 60-80 frames per seconds, this can be done by using following retrace techniques

—  Horizontal retrace

—  Vertical retrace

—  The scan is synchronized with the access of the intensity values held in the frame buffer.

—  The maximum resolution is determined by:

—  The characteristics of the monitor

—  Memory capacity available for storing the frame buffer

Random-Scan Displays

—  The electron beam directed only to those parts of the screen where a picture is to be displayed.

—  Sometimes called: store-writing or calligraphic displays.

—  Picture definition is stored as a set of line-drawing commands.

—  Draws all the component lines of a picture 30 to 60 times each second, with up to 100,000 “short” lines in the display list.

—  Designed for line-drawing applications and they cannot display realistic shaded scenes.

—  A pen plotter operate in a similar way.

—  Draws the components  lines of an object in any order specified.

—  Have higher resolution than raster-scan systems.

—  Produce smooth line drawing.

—  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.

—  Other names: display list, display program or refresh rate “A set of commands”.

—  After all line drawing commands have been processed, the system cycles through the set of commands in the display file.

—  All component lines of a picture are drawn 30 to 60 times each second

—  When a small set of lines is to be displayed each  refresh cycle is delayed to avoid refresh rates greater than 60 frames per second.

—  Advantages:

—  For line drawing applications

—  Higher resolution than raster scan systems

—  Smooth lines

—  Disadvantages:

—  Cannot display realistic shaded scenes

—  Faster refreshing of the set of lines could burn out the phosphor

Color CRT Monitors

—  Cathode Ray Tube(CRT) is the most common display device

—  High resolution

—  Good color fidelity

—  High contrast (400:1)

—  High update rates

Techniques for producing color :

—  Beam penetration method

—  Shadow mask method

Beam Penetration Method

—  Random scan monitors use the beam penetration method for displaying color picture. In this, the inside of CRT screen is coated two layers of phorphor namely red and green.

—  A beam of slow electrons excites ony the outer red layer, while a beam of fast electrons penetrates red layer and excites the inner green layer. At intermediate beam speeds, combination of red and green light are emitted to show two addtional colors- orange and yellow.

Advantages

—  Less expensive

Disadvantages

—  Quality of images are not good as comparatable with other methods

—  Four colors are allowed only

Shadow Mask Method

—  Raster scan system are use shadow mask methods to produced a much more range of colors than beam penetration method.

—  In this, CRT has three phosphor color dots. One phosphor dot emits a red light, second emits a green light and third emits a blue light.

—  This type of CRT has three electrons guns and a shadow mask grid as shown in figure below:

—  In this figure, three electrons beams are deflected and focused as a group onto the shadow mask which contains a series of holes. When three beams pass through a hole in shadow mask they activate dot triangle as shown in figure below

Advantages

—  produce realistic images

—  also produced different colors

—  and shadows scenes.

Disadvantages

—  low resolution

—  expensive

—  electron beam directed to whole screen

Direct View Storage Tube(DVST)

—  A cathode-ray tube in which secondary emission of electrons from a storage grid is used to provide an intensely bright display for long and controllable periods of time. Also known as display storage tube; viewing storage tube.

—  These monitors can play high resolution picture without flicker.

Flat Panel Displays

—  Thin screen displays found with all portable computers and becoming the new standard with desktop computers. Instead of utilizing the cathode-ray tube technology flat-panel displays use Liquid-crystal display (LCD) technology or other alternative making them much lighter and thinner when compared with a traditional monitor.