Standard televisions utilize a raster scan line to display data on a screen. This works because within the unit, an electronic beam (Atari Raster Scan – for Atari users) shoots from the back of the tube aimed at the television screen. During this distance the beam passes through horizontal and vertical coils that can deflect the beam if they are energized. This allows it to strike any point on the screen. Next the beam is moved across the television. An intensity of the beam is managed to make areas glow more bright or otherwise it will be dim.
The beam begins its journey at the top, left of the screen and scans horizontally across the screen. During this trace, the intensity provides the ability to paint an image on the screen. After it arrives at the right edge, it shuts off and returns back to the left. Then it moves up just a little, turns on again, and begins the new sweep across the screen. This continues for 262 scans across the screen. The lines will fill from the top to the bottom. Once it reaches the bottom, the electron beam powers off, returns to the upper left corner, and repeats the same cycle. This occurs 60 times each second.
Image Generation on a Monitor
The “Atari Raster Scan” that travels across the screen from left to right is known as a “horizontal scan line”. According to De Re Atari, ‘a horizontal scan line is the fundemental unit of measurement of vertical distance on the screen’. The height of an image is measured by the total of horizontal lines it spans. When the beam returns to the top of the screen after traveling to the right this is known as the ‘vertical blank’. Drawing the entire screen takes 16,684 microseconds, and the ‘vertical blank’ is 1400 microseconds. A single horizontal lines takes 64 microseconds.
Televisions include what is known as an “overscan”, which allows an image to expand off the edges of the television tube. This prevents any borders from appearing in a picture on the TV. The computer manages a smaller image, so overscan has no value, which is the reason why borders were generated occupying 192 horizontal scan lines instead of the normal 262.
Horizontal Television Scan
A “color clock” measures the horizontal distance. This is done by determining the width of an image and record how wide it is in color clocks. Each horizontal scan occupies 228 color clocks, but only 176 can actually be seen. A color television has a horizontal resolution limit of 176 pixels. The Atari has the ability to control individual half-clocks to expand up to 352 pixels with its horizontal resolution. This will also produce what are known as “color artifacts” and create more colors.
Ths issue when generating a raster scan line on the screen, is that the television has no recall of the image. The image must be recorded by the computer and transfer a signal to the television display to advise it what needs to be updated. This is the reason why internal hardware circuits manage the television inside a computer system.
The Atari raster scan lines are responsible for creating some of the GRAPHICS modes on the Atari 65xe and moving characters known as Player Missile Graphics.
The screenshot on this page demonstrates the raster display image. Notice that it is made up of finite pixels that occupy each character area. Although color artifact is not evident here, it is possible when writing display lines in Graphics mode 8.