2.1 Video Display Devices
Cathode Ray Tubes (CRT)
A cathode ray tube (CRT) is a specialized vacuum tube in which images are produced when an electron beam strikes a phosphorescent surface.It modulates, accelerates, and deflects electron beam(s) onto the screen to create the images. Most desktop computer displays make use of CRT for image displaying purposes.
The operation of CRT :-
- The electron gun emits a beam of electrons (cathode rays).
- The electron beam passes through focusing and deflection systems that direct it towards specified positions on the phosphor-coated screen.
- When the beam hits the screen, the phosphor emits a small spot of light at each position contacted by the electron beam.
It redraws the picture by directing the electron beam back over the same screen points quickly.
Properties of CRT
- Persistence is the one of the major property of phosphorous used in CRT‟s.
- It means how long they continue to emit light after the electron 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.
- A phosphor with lower persistence is useful for animation and a higher–persistence phosphor is useful for displaying highly complex static picture.
- Graphics monitor are usually constructed with the persistence 10 to 60 microseconds.
- The maximum number of points (pixel) that can be displayed without overlap on a CRT is referred to as
- It is also defined as the number of points per unit of measure (per centimeter or per inch) that can be plotted horizontally and vertically.
- Resolution of the display device depends on the type of phosphor used and the focusing and deflection system.
- This number gives the ratio of total vertical pixels to total horizontal pixels.
Pixel or pel
Pixel is the smallest addressable screen element. Each pixel has its own intensity, name or address by which we can control. It is a measure of screen resolution.
Frame buffer (refresh)
It is the large contiguous piece of memory into which the intensity values for all pixels are placed. It contains the internal representation of the image.
Number of pixels per unit area is called pixel density.If a careen of size 4 inch by 3 inch and of resolution 800 by 700 then pixel density is, Pixel density (pd)=800/4 x 600/3
=200 x 200
It is defined as the distance from one pixel to the next pixel i.e. pixel spacing.
It is defined as the number of pixels in the horizontal and vertical directions on the screen.
- Light emitted by phosphor fades very rapidly, so to keep the drawn picture glowing constantly; it is required to redraw the picture repeatedly and quickly directing the electron beam back over the some point. This process is called refresh
- The no of times/sec the image is redrawn to give a feeling of non-flickering pictures is called refresh-rate. Refresh rates are described in units of cycles per second, or Hertz (Hz), where a cycle corresponds to one frame. It is also called a frame rate, horizontal scan rate, and vertical scan rate. A refresh rate of 70 Hz means that the image is redrawn 70 times a
Bit depth (color depth)
It is defined as the number of bits assigned to each pixel in the image. It specifies the number of colors that a monitor can display.
Bit map and pixel map
An image of two colors (generally black and white) is called as a bitmap. An image of more than two colors is called as a pixel map.
Time taken to access horizontal pixels is called scan line.
Advantages of cathode ray tube
- It operates at any resolution and at an aspect ratio.
- CRT runs at the highest pixel resolution.
- Cathode ray tube produces a very dark black and the highest contrast level.
- CRT is best suited for rapidly moving or changing the image and have a fast response time.
- CRT is less expensive than any other display technology.
Disadvantages of cathode ray tube
- It is relatively bright but not as LCD’s brightness.
- Some CRTs are round spherical or cylindrical shapes but new CRTs are flat.
- They are very large heavy and bulgy.
- They also consume a lot of electricity and produce a heat.
Two types of Refresh CRT’s
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 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 as shown in the following illustration.
Each screen point is referred to as a pixel (picture element) or pel. At the end of each scan line, the electron beam returns to the left side of the screen to begin displaying the next scan line.
Horizontal Retrace and Vertical Retrace:
- At the end of each scan line the electron beam returns to the left side of the screen to begin the
displaying the next scan line. The return to the left to the screen, after refreshing each scan line, is called
horizontal retrace of the electron beam.
- And at the end of each frame, the electron beam returns to the top left corner of the screen to begin the
next frame, is called vertical retrace.
Random Scan (Vector Scan)
In this technique, the electron beam is directed only to the part of the screen where the picture is to be drawn rather than scanning from left to right and top to bottom as in raster scan. It is also called vector display, stroke-writing display, or calligraphic display.
Picture definition is stored as a set of line-drawing commands in an area of memory referred to as the refresh display file. To display a specified picture, the system cycles through the set of commands in the display file, drawing each component line in turn. After all the line-drawing commands are processed, the system cycles back to the first line command in the list.
Random-scan displays are designed to draw all the component lines of a picture 30 to 60 times each second.
Refresh Rates in Random-Scan displays:
Generally, refreshing on random-scan display is carried out at the rate of 60 frames per second. 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 used in line-drawing applications.
Vector displays generally used to produce graphics with higher resolution
Color CRT Monitors
A color CRT monitor displays color pictures by using a combination of phosphors that emit differentcolored light. By combining the emitted light from the different phosphors, a range of colors can be
generated. The two basic techniques for producing color displays with a CRT are;
1. Beam Penetration Method
2. Shadow Mask Method
A. Beam Penetration Method
B. Shadow Mask Method
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. This technique is used in raster
scan display devices.
It gives much wider range of colors than a beam penetration method. A shadow Mask CRT has three phosphor color dots at each pixel location. One phosphor dot emits a red light, another emits green light and the last one emits a blue light. This type of CRT also has three electron guns one for each color dot. A shadow mask grid is installed just behind the phosphor coated screen. The three electron beams are deflected and focused as a group onto the shadow mask, which contains a series of very fine 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 color of pixel is controlled by light of intensity. Different colors can be obtained by varying the intensity levels.
FigureOperation of a delta–delta, shadow-mask CRT. Three electron guns, aligned with the triangular
color-dot patterns on the screen, are directed to each dot triangle by a shadow mask.
Difference between Beam Penetration and Shadow Mask method.
|Beam Penetration method||Shadow Mask method|
||It is used with Random Scan System to display color.||It is Used With Raster Scan System to display color.|
||It can displays Only four colors i.e. Red , Green, Orange and Yellow.||it can display Millions of colors.|
||Less colors are available because the colors in Beam Penetration depends on the speed of the electron beam.||Millions of colors are available because the colors in Shadow Mask depends on the type of the ray.|
|Cost||It is Less Expensive as compared to Shadow Mask.||It is More Expensive than other methods.|
|Picture Quality||Quality of picture is not so good i.e. Poor with Beam Penetration Method.||Shadow Mask gives realism in picture with shadow effect and millions of color.|
|Resolution||It gives High Resolution.||It gives Low Resolution.|
- The Flat Panel display was first invented at the University of Illinois in 1964.
- 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.
- Current uses for flat-panel displays include small TV monitors, calculators, pocket video games, laptop
computers, advertisement boards in elevators, portable monitors.
Two types of flat panel displays
1. Emissive displays
2. Non-emissive displays.
- Emissive displays
The emissive displays (or emitters) are devices that convert electrical energy into light. Plasma panels and light-emitting diodes (LED) are examples of emissive displays .
- Also called 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 (LCD).
- Also called gas-charge displays. Plasma panels are constructed by filling the region between two glass
plates with a mixture of gases usually neon.
- A series of vertical conducting ribbons is placed on one glass panel, and a set of horizontal ribbons is
built into other glass panel.
- Firing voltage is applied to a pair of horizontal and vertical conductors. It causes the gas at the
intersection of the two conductors to break down into glowing pattern.
- Picture definition is stored in the refresh buffer.
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 retrieved from the refresh buffer and converted to voltage levels that are applied to the
diodes to produce the light patterns in the display.
- LED‟s have the screen refresh rate of 60 frames per second.
Liquid-crystal Displays (LCDs)
- Liquid crystal displays are the devices that produce a picture by passing light from light source through a
liquid crystal material that transmit the light.
- Two glass plates are used. Rows of horizontal transparent conductors are built into one glass plate, and
columns of vertical conductors are put into the other plate.
- Liquid crystal material is placed between these two glass plates. And intersection of two conductors
defines the pixel position. Picture definition is stored in the refresh buffer, and the screen is refreshed at
the rate of 60 frames per second. Liquid-crystal displays (LCDs) are commonly used in small systems,
such as calculators and portable, laptop computers.