Refresh Rate: refreshing on raster - scan displays is carried out at the rate of 60 to 80 frames per second, although some system are designed for higher refresh rates. Sometimes, refresh rates are described in units of cycles per second, or Hertz (Hz), where a cycle corresponds to one frame. Using these units, we would describe a refresh rate of 60 frames per second as simply 60 Hz.
Refresh buffer: in a raster-scan system, the electron beam is swept across the screen, one row at a time from top to bottom. As the 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 area called the refresh buffer or frame buffer. This memory area holds the set of intensity values for all the screen points. Stored intensity values are then retrieved from the refresh buffer and ''pointed'' on the screen on row at a time. Raster - scan systems interactive raster graphics system typically employ several processing units. In addition to the central processing unit, or CPU, a special - purpose processor called the video - controller or display controller, is used to control the operation of the display devise. Organization of a simple raster scan system shown in figure below. Here the frame buffer can be anywhere in the system memory and the video controller accesses the frame buffer to refresh one screen. In addition to the video controller, more sophisticated raster systems employ other processors as coprocessor and accelerators to implement various graphics operations.