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Document Title: [RasterMonitorLesson.html (html file)]

<Raster Monitor Lesson

Raster Monitor Lesson

Submitted by Rick Schieve (rls@intgp1.att.com)

Coin-op video games use one of two types of monitors, vector or raster scan. Both vector and raster scan monitors are available in black and white or color. A black and white picture tube has one electron gun that lights just one type of phosphor (usually, but not always white). Color tubes have 3 electron guns that (when the yoke and neck magnets are aligned properly) each hit their own phosphors only, either red, green, or blue (RGB). Something called a shadow mask is used so each gun hits only one set of phosphors. This is just basic TV stuff.

Raster scan techniques are what your basic TV set or computer monitor uses. Internal circuitry uses the magnets on the neck of the tube to sweep the electron beam from left to right constantly. Another circuit moves the beam from top to bottom one scan line at a time (kind of like the way you read the page of a book). This scanning goes on all the time, but the electron beam or beams are only turned on when the desired phosphor needs to be lit. This all happens so fast that a complete picture appears.

Inputs for most raster scan games include red, green, and blue (RGB) analog inputs or a single input for black and white. RGB is usually positive logic, but some Nintendo games use negative RGB logic. For positive, when the input is turned on, the electron gun is turned on lighting the phosphor where the gun is pointed at that time. While these are analog inputs they are often driven with digital logic gates in various configurations. One RGB input may be connected to several digital outputs where the intensity of the color is controlled by how many outputs are on.

The other raster input or inputs are the sync inputs which are almost always TTL (5 volt) logic levels. Most newer games use composite sync (just one input) while many old ones use separate horizontal and vertical sync inputs. Depending on the game, positive or negative sync may be used. Negative composite sync is probably the most popular sync scheme used. When converting a video game to play another games, it is the sync input or inputs that needs the most attention as the RGB inputs usually just work fine.

For instance, you wish to use an old monitor with no composite sync input (just separate horizontal and vertical inputs) to work with composite sync. Usually you can just connect the two sync inputs together and re-adjust some monitor settings or sometimes just connect to the negative horizontal sync input. Some monitors have only a negative logic sync input to use and the logic board only produces positive sync. In this case you can just add a TTL logic inverter.

Sometimes the picture comes out upside down or a mirror image. I've only seen this with games that use a vertically mounted monitor. I think the manufactures couldn't get together on which way to rotate the monitor. In this case the wires to the deflection coils on the neck of the tube have to be reversed.

There is a whole bag of tricks to use the get things going. Most newer stuff is getting more standardized (JAMMA, etc.) using RGB and a negative composite sync from what I've seen, but I work on mostly older stuff.

This information is all from my personal experiences in working on arcade video game. If I have stated something incorrectly, constructive comments would be appreciated (please no flames).