QUESTIONS

Display devices

Short Questions.

1.        Define pel / pixel

2.        Are printers raster scan systems or random scan systems

3.        Are pen plotters raster scan systems or random scan systems

4.        How many vertical retrace are there in a raster system with a refresh rate of 60 Hz

5.        What is interlacing

6.        What is Progressive Scan

7.        Instead of phosphor what is used in DVST

8.        What are vector displays

9.        What is refresh display file

10.    Distinguish between shadow mask method & beam penetration method

11.    What are FPDs

12.    Distinguish emissive and non-emissive display systems

13.    What is the role of negatively charged electrodes in CRT

14.    What is the role of positively charged anodes in CRT

15.    What is the role of Shadow Mask or Aperture Grill in CRT

16.    What are phosphor triads

17.    Differentiate between Raster Scan & Random Scan Systems.

18.    Which four colors a random scan system can produce

19.    “More the resolution, Less is the refresh rate”, Comment.

20.    What is Horizontal retrace

21.    What is vertical retrace

22.    How is the image drawn on DVST, erased

23.    DVSTs can produce only one color.

24.    How is the vacuum tube used to store image in a DVST

25.    What is Refresh rate

26.    What is resolution

27.    What is aspect ratio

28.    What is the aspect ratio of human eyes

29.    What is dot pitch

30.    What is persistence

31.    What is contrast ratio

32.    At what voltage the gas used in plasma displays is ionized into PLASMA State

33.    Which Gases are used in plasma displays

34.    What is the sustain voltage in Plasma Displays

35.    How many sub pixels are there for each pixel in plasma displays

36.    “There is one address for all the sub pixels of a single pixel in plasma displays”. True or False.

37.    What excites the phosphors in a plasma display

38.    Why Plasma Displays do not work at very high altitude

39.    What is the role of Indium Tin Oxide (ITO) in LCDs.

40.    Differences between TN and STN which are used in LCDs

41.    “When a pixel is turned on the liquid crystals are arranged in helical shape”. True or False

42.    LCDs do not produce any light of their own. Then how do they work?

43.    ‘LCD, Plasma or CRT’, which of these uses the least amount of electricity.

44.    What are passive displays

45.    What are active displays / TFTs

46.    Discuss Advantages & Disadvantages of LCDs, Plasma, CRT.

47.    Why LCDs cannot be used with Light Pens.

48.    Is it possible that a display device displays a picture even after the current is switched off.

49.    How do you flush a DVST.

50.    What is polarized light

Long Questions.

1.        Explain the working of Monochrome CRT in detail. Also Draw a neat Diagram

2.        Explain the working of Coloured CRT in detail. Also Draw a neat Diagram

3.        What are the different kinds of CRT based on Refreshing Technology

4.        Explain the working of Random scan systems and DVSTs

5.        Discuss Working of Plasma Panel Displays

6.        Discuss Working of Liquid Crystal Displays

7.        Comparatively analyze CRT, Plasma and LCD Displays

HardCopy Devices

Short Questions.

1.      Differentiate Hardcopy and Softcopy Output Devices

2.      Differentiate Impact & Nonimpact Printers

3.      List Different Impact Printers

4.      List Different Non-Impact Printers

5.      Which printers can be used to take carbon copies of same printout

6.      Can a daisy wheel printer be used to print different fonts of characters

7.      What are piezoelectric printers

8.      What are bubble jet printers

9.      What is the use of guard droplets

10.  What colors are used in cartridge of colored inkjet printers

11.  Which printer uses solid ink

12.  What is the use of laser beam in a laser printer

13.  What is the use of fuser in a laser printer

14.  Printers are raster scan systems or random scan systems

15.  Plotters are raster scan or random scan in nature

Long Questions.

1.      Explain the working of Dot-Matrix Printers

2.      Explain the working of Daisy wheel Printers

3.      How the Drum printers work

4.      Discuss working of Inkjet printers

5.      Discuss working of Bubble jet printers

6.      Discuss working of Peizo-electric Inkjet printers

7.      How do the Laser Printers work

Display Devices

Modern day computers use, many kind of display devices, which are softcopy devices, and are used to display the output in a virtual form. There are many kind of display devices used these days, most common of these are CRT, LCD, Plasma, LED etc.

CRT (Cathode Ray Tube) :

Cathode Ray Tube monitors are the most commonly used display devices, though they are being replaced with other technologies very fast. Following figure depicts major parts of a CRT.

Working of CRT is as follows:

Ø  CRTs have a distinctive funnel shape. These are glass vacuum tubes.

Ø  At the very back of a monitor is an electron gun.

Ø  The electron gun fires electrons towards the front through a vacuum which exists in the tube of the monitor. The gun can also be referred to as a cathode - hence the electrons fired forward are called Cathode Rays. These are negatively charged rays.

Ø  The cathode rays then pass through negatively charged electrodes, which accelerate and concentrate the cathode rays.

Ø  At the neck of the funnel-shaped monitor are one or more anodes, which is magnetised or positively charged. As negatively charged electrons pass the anode, they are pulled in one direction or the other. This moves the electrons towards the correct part of the screen.

Ø  The electrons then pass through a mesh, and this mesh defines the individual pixels and resolution on the screen. This mesh is called “Shadow Mask”. Modern CRT displays use aperture grill instead.

Ø  Electrons then hit the phosphor coating which is on the inside of the glass screen. When the negative particles hit the phosphor, they immediately light up - causing the light to shine through the front of the monitor, thus making up the picture on the screen.

Ø  In coloured monitors there are three differently coloured phosphorus (Red, Green, Blue) for each pixel (known as phosphor triads), and depending on which phosphor the electron hits, will give the pixel its colour. Whereas in monochrome (Black/White) monitors the phosphors emit only white light.

Ø  Some CRT monitors use a single electron gun at the rear of the monitor to produce the electrons that will become the red, green and blue pixel after hitting the correct phosphor. However, higher quality monitors have an individual gun for each colour.

Ø  The entire front area of the tube is scanned repetitively and systematically in a fixed pattern called a raster. An image is produced by controlling the intensity of the electron beams.

Ø  The surface of the CRT glows for a fraction of a second before it starts to fade. This means that the picture has to be redrawn many times per second to avoid flicker.

Different Kinds of CRT based on REFRESHING TECHNOLOGY.

Based on how the CRT refreshes the screen once it is fully drawn, different CRT can be categorized as:

·         Raster Scan

·         Random Scan

·         Direct View Storage Tube (DVST)

Raster Scan

Ø  In a raster scan, an image is subdivided into a sequence of (usually horizontal) strips known as "Scan Lines".

Ø  This ordering of pixels by rows is known as raster order, or raster scan order.

Ø  In raster scanning, the electron beam sweeps a row of pixels horizontally left-to-right, then stops and rapidly moves back to the left, where it turns back on and sweeps out the next line of pixels. This is called “Horizontal Retrace”.

Ø  This process continues until the electron beam reaches the end of the bottom line of pixels.

Ø  The electron beam then stops and goes to the first pixel of the first line.This is called “Vertical Retrace”.

Ø  These systems produce realistic graphics.

Ø  They can produce many different colours.

Ø  Major disadvantage of these systems is that they are

low in resolution.

Ø  These are expensive systems.

Ø  Raster scan system are use shadow mask methods to

produce colours images.

Raster scan system -->

Shadow Mask Method

§  In this, CRT has three phosphor colour 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 grill as shown in figure below

§  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 the dot triangle.

§  Different colours can be produced as combination of red, blue and green.

An example of Raster scans system is the Television. With every vertical retrace the next frame starts. Scanning happens so fast that human eye cannot distinguish between one frame and the other. Every vertical retrace leads to refreshing the screen. Thus refresh rate is also roughly equal to number of vertical retrace per second.

Interlacing is the technique used in standard CRT monitors when refreshing the screen. In this technique, with every horizontal retrace, the electron gun skips one line and moves to the alternate line. So in one frame every odd line of pixels is refreshed and in the next frame every even line of pixels is refreshed.

Progressive Scan in this technique all the lines of pixels are scanned in each frame. This technique is used in better quality TVs these days.

Random Scan

Ø  In this system images are defined in term of Line segments instead of pixels.

Ø  The electron beam is directed to only that part of the screen where the image is to be drawn.

Ø  Random Scan monitor draw one picture at a time therefore they are also known as vector display.

Ø  Refresh rate of random scan system depends on the number of lines to be displayed.

Ø  Picture definition is stored in an area of memory called refresh display file.

Ø  These systems produce smooth lines.Random-scan displays are designed to draw all the component lines of a picture 30 to 60 times each second.

Ø  These systems are high in resolution.

Ø  These systems are less expensive.

Ø  These systems are designed only for drawing applications.

Ø  These cannot produce realistic images.

Ø  These cannot produce all the different colours.

Ø  Random scan monitors use the beam penetration method for displaying colour picture.

Beam Penetration Method

§  In this, the inside of CRT screen is coated with two layers of phosphor namely red and green.

§  A beam of slow electrons excites only 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 is emitted to show two additional colours- orange and yellow.

Direct View Storage Tube (DVST).

Ø  DVST is a modification of the Random scan system.

Ø  Much cheaper than Random scan systems.

Ø  Do not require a CPU for drawing image.

Ø  In DVST phosphor is replaced by Potassium Chloride (KCl).

Ø  KCl has a property that when its crystal is struck by an electron beam, it changes its colour to magenta.

Ø  Further it does not lose its brightness, so an image can be retained for a longer period as compared to a phosphor.

Ø  The image thus drawn can be erased by flooding the screen with high intensity infrared light.

Ø  An image can be retained even after the current to CRT is switched off.

  

There are some definitions that need to be understood in order to understand display devices. These are:

1.       Pixel: “The smallest displayable point on the screen is called the pixel.”

2.       Resolution: “The maximum number of pixels per inch that can be displayed without overlap is the resolution”, it’s measured as dpi (dots per inch) or ppi (points per inch).

Sometimes resolution is simply measured as the “Physical number of columns and rows of pixels creating the display”. E.g. 1024 X 768

3.       Bit Depth (colour depth): Number of bits used to represent a single pixel. This concept is known as bits per pixel (bpp). RGB colour system with 24 bits per pixel is known as full colour system or true colour system.

4.       Refresh Rate: Number of times the screen is redrawn by the raster scan in a second.  I.e. how many times the electron guns are told to fire by the video source. The refresh rate is thus also limited by the resolution. This is normally more than 60Hz.

5.       Dot Pitch (Pixel Pitch): “This refers to the  the distance (usually Diagonal) between two phosphors of the same colour”

6.       Persistence: “The time taken by the light emitted from a phosphor to decay to one tenth (1/10^th) of its original intensity”. The phosphor when struck by an electron beam starts to emit light, but this light soon starts to fade. The time taken by this light to reduce to 1/10 is the persistence. It is due to this fading that the electron beam is required to strike the same pixels again and again to avoid flicker.

7.       Aspect Ratio: “Number of pixels in the horizontal direction to the number of pixels in the vertical direction that are required to produce a line of same length”. The pixels on all the display devices are not evenly distributed in horizontal and vertical direction. That is why the screens are rectangular. All latest TVs and monitors have aspect ratio of 16:9, for a better viewing experience. The aspect ratio of human eye is 21:9.

8.       Contrast Ratio: “Difference between the brightest and the darkest part of an image”. Higher the contrast ratio more realistic is the image.