Difference between Raster Scan Display and Random Scan Display

Difference between Raster Scan Display and Random Scan Display

Raster Scan Display

In raster scan display the electron beam scans the entire surface of CRT, one row at a time. It starts from the top left corner of the screen then moves horizontally from left to right. It then jump to left side of screen and scans another line until it reaches bottom right corner of screen. The rate at which the electron beam scans the surface of the CRT is directly proportion to the frequency of local line voltage. Therefore, in areas such as united states where 60-cycle line voltage is used, the electron beam will usually scan across the entire face of the CRT 60 times in one second.

As the electron beam is moved across each row of matrix or raster the electron beam is turned off and on to create a picture in form of illuminated spots. Picture to be displayed (i.e. picture definition) is stored in a memory area called refresh buffer or frame buffer.

Refresh buffer or frame buffer is a refresh storage area in the digital memory in which the intensity values and other parameters (called attributes) of all the pixels making up the image are stored in binary form. This area organized in form of 2D table. Every row-column entry stores information such as brightness and/or color value of the corresponding pixel on the screen.

When the electron beam scan the various lines the various intensity values that are stored in the frame buffer are then read out and displayed on the screen. The frame buffer used in black and white system is different from the one used in colored system.

Random Scan Display

In random scan display the electron beam directed straightway to the particular points on the screen where image displayed. It does not scan the entire screen from top to bottom and from left to right like raster scan CRT. The picture is formed on random scan CRT by drawing a set of random straight lines much in the same way one might move a pencil over a piece of paper to draw an image i.e. drawing strokes from one point to another, one line at a time. Because of this reason random scan display also known as stroke-writing display or vector display or calligraphic display.

In random scan display, picture information or definition for the image to be drawn stored in a memory arca called refresh buffer. This refresh buffer also known as display file or display list or display program. As random scan display draws picture in form of lines, image information stored in form of line drawing commands and not as a set of intensity values for screen positions.

Working of Random Scan CRT

The display list or display program contains the end point coordinates of each line to be drawn. The various component lines of a picture can draw and refresh in any specified order. This process is quite similar to the way a person or a plotter draws a line segment where in a pen can be moved along a certain direction for a certain length and then lifted and moved to a new direction.

In order to display a picture, display processing unit executes the various line drawing commands one by one. Once all the commands in display file process; the system cycles back to the first line command in the list. This is how refreshing is done is random scan CRT. The refresh rate on random scan system depends upon the number of lines to display. However the average refresh rate is 30-60 times per second. The major disadvantage of random scan systems is that they are designed for line drawing applications; and cannot display realistic shaded scenes.

However, these systems have higher resolution than raster scan systems as picture definition stored. As a set of line drawing commands rather than as a of intensity values for all screen points. Another advantage of random scan systems over raster scan system is that they produce; Smooth line drawings as CRT beam directly follows the line path. These kind of CRT’S were used in late fifties and early sixties to produce charts; and pictures line by line on the tube surface in any random order.

Difference between Raster Scan Display and Random Scan Display

Raster Scan Display	Random Scan Display
The electron beam scans the entire surface of CRT from top to bottom and from left to right with one row at a time.	The electron beam is directed only at those parts of the screen where picture is to be drawn.
Picture definition is stored in frame buffer or refresh buffer as a set of intensity values for all the screen points.	Picture definition stored in display list or display le as a set of line drawing commands.
Refreshing occurs when the electron beam completes the entire scan or cycle and moves back to the top left corner of screen.	Refreshing occurs when all the line drawing
Refresh rate is independent of picture complexity. It is 60 to 80 cycles per second.	Refresh rate depends directly on picture complexity. It is 30 to 60 times per second.
A picture displayed in form of pattern illuminated spots on the screen.	A picture displayed in form of lines.
It has poor or less resolution as picture definition stored as intensity values.	It has high resolution as picture stored as a set of line drawing commands.
It produces jagged lines because plotted values are discrete.	Random scan display produces smooth lines as the electron beam directly follows the line path.
It is suitable for displaying realistic images and scenes with different colors and shading patterns.	It is not suitable for displaying realistic images as it cannot display shades. Used for line drawing applications.
Raster scan uses concept of interlacing.	Random scan doesn’t use the concept of interlacing.
Scan conversion required.	Scan conversion not required.
It uses shadow-mask method.	It uses beam penetration method.
Less expensive than It random scan system.	Costlier than raster scan system.