Week 9 Monitors and output to the screen

Monitors, also known as Visual display units (V.D.Us) Desktop computers contain a Cathode Ray Tube (C.R.T.) and relevant power supplies. However Liquid Crystal Displays (L.C.Ds), as used in laptops and Desktops these days.

A black and white C.R.T. contains a gun that can shoot an electron beam against a phosphorescent screen near the front of the tube. Colour Monitors have three guns, one each for red, green, and blue. The gun scans almost horizontally across the screen then executes a horizontal retrace (Flyback) before executing the next sweep. When the gun reaches the bottom of the screen vertical retrace (Flyback) occurs.

A device like this that produces an image line by line is called a raster scan device. Both horizontal and vertical movement are applied by increasing voltages on deflection plates then reversing the voltages during the flyback periods. The screen image is repainted between 60 and 100 times per second (measured in Hz ( 75Hz, or 75 times/ sec)

To produce a pattern of dots on the screen a grid is present between the gun and screen. When positive voltages are applied to the grid the electrons are accelerated causing the beam to hit the screen and making it glow briefly. When negative voltages are applied to the grid, the electrons are repelled and the screen does not glow. Therefore a binary electrical signal can be converted to a visual display consisting of bright and dark spots.

On a personal computer there are two ways to organise output to the screen. One way is as a character map. Another is as a bitmap.

Character map. To display characters the C.P.U. sends the ASCII codes to video Ram (held on a graphics board) in alternate bytes. One byte for the character and one for its attribute. Attributes can include colour, intensity, whether character is blinking or not. If the screen is divided into an array of 25 * 80 characters it requires 4000 bytes of video memory. That is 2000 for the characters and 2000 for the attributes.

Bit map Here the screen is not regarded as an array of 25 * 80 characters but as an array of picture elements called pixels. Screens operating as bit mapped terminals come in various screen resolutions or graphics mode.

Bit map Screen resolution is measured in pixels, or picture elements. One pixel is the smallest area that can be manipulated by the computer in a specific graphics mode( an example of a graphics mode is VGA ). Screen resolution is always stated as the horizontal number of pixels by the vertical number of pixels. A screen displaying 800 x 600 pixels has 600 rows, each 800 pixels wide.

Screen Resolution

Dot Pitch In most colour monitors the dots are built of triads, groups of three phosphor dots, one for Red, Green and Blue. Here the dot pitch is the distance, centre to centre, between two dots of the same colour. Three electron guns are needed to illuminate the red green and blue phosphors on the screen.

Dot Pitch These dots are very close together and one measure of the quality of a screen is a distance between the dots called dot pitch. You will see adverts for screens with dot pitches from 0.22mm to 0. 28mm, the smaller the distance, the better the quality. You cannot have more pixels than dots available.

Early colour monitors allowed each gun to be either on or off therefore allowing a maximum of 8 colours only (see handout) Later analogue technology allowed the intensity of the guns to be infinitely variable. This allows for as many colours as you can address using video memory. If you allow 1 byte for each gun you have 24 bits assigned to the colours or 24 bit colour graphics. With 24 bits available you have 2^24 or 16,777,216 colours available.

Graphics Cards To get close to true colour, 8 bits are needed for each of the three primary colours or three bytes per pixel. Thus a 1024 * 768 screen resolution display needs 1024 *768 * 3 = 2.3 MB of Video Ram for a single coloured image. To alter the image requires a great deal of processing power. Therefore graphics cards are provided with most modern computer systems containing dedicated video memory ( often 256MB or 512MB) and an onboard dedicated graphics processing unit (G.P.U.)

Refresh Rates Higher screen resolutions require more lines to be painted on to the screen each time the screen is refreshed by the raster scan. The higher the screen resolution the higher the horizontal refresh rate required to prevent flicker.

TRUTH TABLE FOR AND

TRUTH TABLE FOR OR

Suppose we had the following Data inside a database table What is the result of the following queries? Give the row number only.

Answer ANSWER 1 EXAM < 40 AND COURSEWORK < Row ANSWER 2 EXAM >= 55 OR COURSEWORK >= 60...Row..1,2,4..

Thank you