what do you mean by high definition systems? Define aspect ratio?  What is a refresh buffer? What is the difference between bitmap and pitman frame buffer? Explain the basic operations of a CRT. (b) WHAT CAN Random scan system not display realistic shaded scenes?  Explain the architecture of random scan systems. What is DVST? Explain the input device joy stick? 

Ans. (a) High definition system: The maximum number of points that can be displayed without overlap on a CRT is referred to as the resolution, of we can say the number of points per centimeter that can be plotted horizontally and vertically are called resolution. And resolution of a CRT is dependent on the type phosphor, the intensity to be displayed and the focusing and deflection systems. Typical resolution on high quality system is 1280 by 1024, systems are referred to as high definition system. 

Difference between bitmap and pitman frame buffer  Each screen point is referred to as a pixel. In a simple black and white system each screen point is rather on or off so only one bit per pixel is needed to control the intensity of screen position and such type of frame buffer is called bitmap. In this a bi level system, a bit value of 1 indicates that the electron beam intensity is to be turned off. While additional bits are needed when color and intensity variations can be displayed. So for systems with multiple bits per pixel the frame buffer is often referred to as a pitman. 

Basic operation of a CRT:  A CRT is an evacuated glass tube. An electron gun at the rear of the tube produces a beam of electrons which is directed towards the front of the tube. The inner side of the screen is coated with phosphor substance which gives off light when it is stroked by electrons. It is possible to control the point at which the electron beam strikes. The screen, and therefore the position of the dot upon the screen by deflecting the electron beam. The deflection system of the cathode ray tube consists of two pairs of parallel plates, referred to as the vertical and horizontal deflection plates. The voltage applied to vertical plates controls the vertical deflection of the electron beam and voltage applied to the horizontal deflection plates controls the horizontal deflection of the electron beam. The two techniques are used for producing images on the CRT. Ans.(b) in random scan system picture definition is stored as a set of line drawing instructions and not as a set of intensity values for all the screen points so random scan system not display realistic shaded scenes.

Architecture of random scan system: It consist of display controller, central processing unit (CPU), display  buffer  memory and a CRT. A display controller is connected as an I/O peripheral to the to the Central Processing Unit (CPU). The display buffer memory stores the computer produced   display list or display program. The program contains point and line plotting commands with (x, y) or (x ,y ,z) end point coordinates, as well as character plotting commands. The display controller interprets commands for plotting points, lines and characters and sends digital and point coordinates to a vector generator. The vector generator then converts the digital coordinate values to analog voltages for beam deflection circuits that displace an electron beam writing on the CRT,s phosphor coating.  In vector display, beam is deflected from end point to end point, hence this technique is also called random scan, we know as beam strikes phosphor it emits light, but phosphor light decays after few milliseconds and therefore it is necessary to repeat the display list to refresh the phosphor at least 30 times per second to avoid flicker. As display buffer is used to store display list and it is used for refreshing, the display buffer memory is also called refresh buffer. 

DVST (Direct- View Storage Tubes) We know that, in raster scan display we do refreshing of the screen to maintain a screen image. The direct- view storage tubes give the alternative method of maintaining the screen image. DVST uses the storage grid which stores the picture information as a charge distribution just behind the phosphor- coated screen.  DVST consists of two electron gun: (1) primary gun. (2) Flood gun. A primary gun stores the picture pattern and flood gun maintains the picture display. 

Advantage of DVST 1. Refreshing of CRT is not required. 2. Because no refreshing is required, very complex picture can be displayed at very high resolution without flicker. 3.it has flat screen.

Disadvantage of DVST 1. They do not display colors and are available with single level of line intensity. 2. It has poor contrast as a result of the comparative low accelerating potential applied to the flood electrons.  Joysticks A joystick consist of small, vertical levers (called the stick ) mounted on a base that is used to steer the screen cursor around. Most joystick select screen positions with actual stick movement; other respond to pressure on the stick. figure below shows a movable joystick. Some joysticks are mounted on a keyboard; others functions as stand- alone units. The distance that the stick is moved in any direction from its centre position corresponds to screen- cursor movement is that direction. Potentiometers mounted at the base of the joystick measure the amount of movement, and springs return the stick to the centre position when it is released. One or more buttons can be programmed to act as input switches to signal certain actions once a screen position has been selected.