Raster Scan Display – CRT Screen
Raster Scan Display
The figure shows the architecture of Raster Scan Display. It consists of a display controller, CPU, video controller, refreshes buffer, keyboard, mouse, and CRT.
The display image is stored in the form of 1’s and 0’s in the refresh buffer.
The video controller reads this refresh buffer and produces the actual image on a screen.
It will scan one line at a time from top to bottom & then back to the top.
In this method, the horizontal and vertical deflection signals are generated to move the beam all over the screen in a pattern shown in fig.
Here beam is swept back & forth from left to the right.
When a beam moved from left to right it is ON.
Moreover, When the beam moved from right to left it is OFF and a process of moving the beam from right to left after completion of a row known as Horizontal Retrace.
Also, When a beam reaches the bottom of the screen. It made OFF and rapidly retraced back to the top left to start again and the process of moving back to the top known as Vertical Retrace.
The screen image maintained by repeatedly scanning the same image. This process known as Refreshing of Screen.
Also, In raster scan displays a special area of memory dedicated to graphics only. This memory called Frame Buffer.
Frame buffer holds set of intensity values for all the screen points Raster Scan Display
That intensity retrieved from a frame buffer and display on screen one row at a time.
Each screen point referred as pixel or Pel (Picture Element).
Also, Each pixel can specify by its row and column numbers.
It can be simply black and white system or color system.
Moreover, In simple black and white system, each pixel either ON or OFF, so only one bit per pixel needed.
Additional bits required when color and intensity variations can display up to 24-bits per pixel included in high-quality display systems.
Also, On a black and white system with one bit per pixel, the frame buffer commonly called a Bitmap. And for systems with multiple bits per pixel, the frame buffer often referred as a Pixmap. Difference between random scan and raster scan