Lecture 5 by coooolone


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									INTRODUCTION TO COMPUTER                                             LECTURE 05

       There are currently just three common types of output devices, monitor,
printer and sound system.
       In three output devices monitors. The most important because they are the
output devices with which users inter act most often. Two basic types of
monitors are used with PCs. The first is the typical monitor that you see on a
desktop computer. It looks a lot like a television screen and works the same way.
This type uses a large vacuum tube, called a cathode ray tube (CRT).
How a CRT Monitor Displays an Image
       Near the back of monochrome or grayscale monitor housing is an electron
gun. The gun shoots a beam of electrons through a magnetic coil, which aims the
beam at the front of the monitor. The back of the monitor’s screen is coated with
phosphors, chemicals that glow when they are struck by the electron beam. The
screen’s phosphor coating is organized into a grid of dots. The smallest number
of phosphor dots that the gun can focus on is called a pixel, a contraction of
picture element. Modern monochrome and grayscale monitors can focus on
pixels small as single phosphor dot. A color monitor works just like a
monochrome one, except that there are three electron beams instead of just one.
The three guns represent the primary additive colors (red, green, and blue),
although the beams they emit are colorless, each pixel on the screen is made up
of three tiny red, green, and blue phosphors arranged in a triangle. When the
beams of each of these guns are combined and focused on a point on the screen,
the phosphors at that point light up to form a tiny spot of whiter light. Different
colors can be displayed by combining various intensities of the three beams.
       The resolution of a computer monitor is classified by the number of pixels
on the screen, expressed as a matrix. For example, a resolution of 640 x 480
means here are 640 pixels horizontally across the screen and 480 pixels vertically
don the screen.
Dot Pitch
       The last critical specification of a color monitor is the dot pitch, the
distance between the phosphor dots that make up a single pixel.

        A plotter is a special king of output device. It is like a printer in that it
produces images on paper, but the plotter is typically used to print large-format
imaged, such as construction or engineering drawings created in a CAD system.
Early plotters were bulky, mechanical devices that used robotic arms, which
literally drew the image on piece of paper. Table plotters (or flatbed plotters) use
two robotic arms, each of which holds a set of colored ink pens, felt pens, or
pencils. The two arms work in concert with one another, operating at right angles
as they draw on a stationary piece of paper. As well as being complex and large
INTRODUCTION TO COMPUTER                                              LECTURE 05

(some table plotters are almost as big as a billiard table), table plotters are
notoriously slow; a large, complicated drawing can take several hours to print.
       Just as microphones are now important input devices, speakers and their
associated technology are key output systems. Today, when you buy a
multimedia PC, you are getting a machine that includes a CD-ROM drive, a
high-quality video controller (with plenty of VRAM), speakers, and a sound
card. The speakers attached to these systems are similar to ones you connect to a
stereo. The only difference is that they are usually smaller, and they contain their
own small amplifiers. Otherwise, they do the same thing any speaker does; they
transfer a constantly changing electric current to a magnet, which pushes the
speakers cone back and forth. The moving speaker cone creates pressure
vibrations in the air---in other words, sound.
       Internally, a PC’s components communicate through the data bus, which
consists of parallel wires. Similarly, a parallel interface is a connection where
there are eight or more wires through which data bits can flow simultaneously.
Most computer buses transfer 32 bits simultaneously. However, the standard
parallel interface for external devices like printer usually transfers eight bits (one
byte) at a time over eight separate wires. With a serial interface, data bits are
transmitted one at a time through a single wire (however, the interface includes
additional wires of r the bits that control the flow of data). Inside the computer,
a chip called a UART converts parallel data from the bus into serial data that
flows through a derail cable. Serial ports are most often used to connect a mouse
or a modem.


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