What is Output device and its classification?

Figure: Display mechanism of a CRT monitor.
Figure: Display mechanism of a CRT monitor.

Output Device

An output device is used to take output from a computer.

The most common output devices are:




4.Speaker  and so on


A monitor is used to display visualize form of data.

Types of Monitor:

Monitors are following types:

1.CRT (Cathode Ray Tube) monitor.

2.Flat Panel monitor:

  1. LCD (Liquid Crystal Display) monitor.
  2. LED (Light Emitting Diode) monitor.

    Question: Mention a comparative study  among CRT, LCD and LED monitors.


    Table: Comparative study among CRT, LCD, and LED monitor.
    CRT Monitor
    Question: Draw a diagram of display mechanism of a CRT monitor.






    Figure: Display mechanism of a CRT monitor.

    Discussion: The display mechanism of a CRT monitor is very much like a home TV. There is an electron gun made of filament (coil) and cathode at the end of long glass tube. A very high voltage of electricity is passed through the filament and the electrons of cathode are boiled off. Then the boiled electrons are emitted.  A continuous flow of emitted electrons  make an electron beam and passed toward the screen. The glass screen is inside coated with phosphor. For a monochrome (Black/White) monitor only white color phosphor is used where as a color monitor three different colors (Red, Green, Blue) of phosphors  are used. In a monochrome monitor only a single electron gun is used but for a color monitor three electron guns are used for three colors of phosphors. When the electron beam strikes on the phosphor then an illumination of light is taken place for a fraction of time. To sustain the display of an object (picture) on the monitor the electron beam strikes on the same position phosphor  again and again (45 to 72 or more times per second). For  a color monitor the intensity of electron beams of three electron guns are different. As a result, a combination of the illumination of three different color of phosphors make a final color of a pixel. When the intensity of all the three electron beams are highest then a white color is produced for a pixel. But for no electron beams (zero intensity) a black color is produced. To display a complete object, the electron beams scan the monitor left to right and top to bottom.

    Color monitor :

    1.Beam Penetration method  [ 1 electron gun, 3 phosphors (R,G,B) are overlapped fashion ]

    2.Shadow Masking method [ 3 electron guns, 3  phosphors (R,G,B) are triangular form.

    Specification of a Monitor:



    3.Refresh Rate

    4.Dot Pitch.

    Size: Diagonal distance of a monitor. May be 15, 17, 21, or more inch.

    Resolution:  The number of pixels scattered on monitor at a particular moment. It is considered in row by column (mxn) that is matrix form . It is changeable.

    Example: 600X480, 800X600, 1024X768 etc.

    There is an inversely proportional relationship between the resolution and size of pictures.

    Refresh Rate:  The number of times the electron beam scans (strikes on pixels) the entire monitor per second is called refresh rate. It is measured in hertz (cycle/sec.) and it has a range 45 Hz to 72 Hz. A 60 Hz refresh rate is standard. A lower refresh rate makes picture flickering and higher makes harm for phosphor coating.

    Dot Pitch: Dot pitch is the distance of the three color (R,G,B) phosphors. It has a standard measurement and it is 0.28mm.

    Flat Panel Monitor

    LCD Monitor

    A LCD monitor is made by liquid crystal which is transparent in normal condition but it becomes opaque when charged with electricity. There are two types of LCD monitors such as (a) passive matrix LCD monitor  (b) active matrix LCD monitor.

    Passive Matrix LCD Monitor:  A passive matrix LCD monitor  is made by an array of transistors which are arranged in a row and in a column. The intersection of a row and a column makes a pixel. To emit any pixel , current is passed through the top of the column and right of the row of the associated pixel.

    Advantage :

    • Low cost.


    • Viewing angle problem.
    • Submarining problem.
    • Low contrast problem.

    Dual Scan Passive Matrix LCD:

    The submarining problem is overcome by scanning the monitor two times than normal scanning.

    Active Matrix LCD Monitor

    The viewing angle problem is overcome by a matrix of pixels. Here each pixel is represented by a transistor.  Some active LCD monitor has four transistors (thin film transistors) to represent each pixel.


    • No viewing angle problem.
    • No low contrast problem.


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