Hardware Concepts: Display devices(ii)

Raster-Scan Display

−        Raster Scan Display  is based on television technology. In raster-scan the electron beam is swept across the screen, one row at a time from top to bottom. No of scan line per second is called horizontal scan rate.

−        As electron beam moves across each row, the beam intensity is turned on and off to create a pattern of illuminated spots. Picture definition is stored in a memory called frame buffer or refresh buffer. Frame buffer holds all the intensity value for screen points. 

 

Figure: A raster-scan system displays an object as a set of points across each screen scan line

 

 

Figure: Raster Scan display system

−        The stored intensity value is retrieved from frame buffer and painted on the scan line at a time. Home television are common examples using raster display

−        Intensity range for pixel position depends on capability of raster system. For B/W system each point on screen are either on or off, so only one bit per pixel is needed to control the pixel intensity. To display color with varying intensity level, additional bits are needed. Up to 24 to 32 bit per pixel are included in high quality systems, which require more space of storage for the frame buffer, depending upon the resolution of the system.

−        A system with 24 bit pixel and screen resolution 1024  1024 require 3 megabyte of storage in frame buffer.

1024*1024 pixel = 1024*1024*24 bits = 3 MB

−        The frame butter in B/W system stores a pixel with one bit per pixel so it is termed as bitmap. The frame buffer in multi bit per pixel storage, is called pixmap.

−        Refreshing on Raster-Scan display is carried out at the rate of 60 or higher frames per second. 60 frames per second is also termed as 60 cycle per second usually used unit Hertz (HZ)

−        Returning of electron beam from right end to deft end after refreshing each scan line is horizontal retrace . At the end of each frame, the electron beam returns to the top left corner to begin next frame called vertical retrace.

Interlaced: Display in two pass with interlacing. 

Figure: Horizontal retrace and Vertical retrace

                                                             

Question: Consider a RGB raster system is to be designed using 8 inch by 10 inch screen with a resolution of 100 pixels per inch in each direction. If  we want to store 6 bits per pixel in the frame buffer, How much storage(in bytes) do we need for the frame buffer?

Solution: Size of screen = 8 inch ´ 10 inch.

Pixel per inch(Resolution)  = 100.

Then, Total no of pixels  =  8´100 ´ 10 ´ 100 pixels

Bit per pixel storage = 8

Therefore Total storage required in frame buffer = (800 ´1000´8) bits

                                                                                        = (800 ´1000´8)/8 Bytes

                                                                                        = 800000 Bytes.