ColoR TUNiNG
UTiliTy FOR
ThE ClASSic ATARi
by JEff POTTER, AC GRAPHics EdiTOR
Screen Of Many Colors
Your Atari 8-bit computer with its GTIA chip (Graphics Television Interface Adapter, part of the custom 40-pin chipset found in all models of Atari 8-bits) can produce an amazing array of 256 colors on a composite color monitor or color television set. However, this is only true if your computer's color output has been properly adjusted. Poorly tuned computers might not produce certain colors, or might generate the same color from two different SETCOLOR commands-which means you'll miss other colors elsewhere in the spectrum.
In my quest for information on the Atari computer, I could never find a good procedure describing how to properly align the color. There's nothing about it in the Atari service manuals, and the Sams COMPUTERFACTS manuals only say, "...adjust the Color Control ...for a blue screen...". If you've ever tried this adjustment you'll observe your screen passing through a variety of different shades of blue (and other colors) as you turn the control. Which BASIC blue is the right blue? Realizing that no two people see blue the same, I came up with my own procedure for adjusting the color that doesn't rely entirely on arbitrary choices.
Hey Man, Where's Your Pot?
All 8-bit Ataris have a color tuning pot (potentiometer, or variable resistor) which you can safely adjust without opening the case. On the 800 it's located along the back side, about 3 1/4" from the left side along the seam between the ribbed top cover and the main body. On the 600XL and 800XL it's located on the bottom of the case, at a point directly beneath the left SHIFT key. For you 1200XL owners, it's the hole just beneath the serial number label on the bottom of the case, positioned between the 6 and 7 number keys on the keyboard. If you own the very rare 1450XL, you'll have to open the case and look for the pot on the lower right side of the board (Atari placed the tuning access hole in the same position as it is on the 1200XL, but the board location doesn't match!). On the 130XE you'll also find it on the bottom of the case, but more near the center (directly beneath a point one inch above the "3" key and about two inches to the right of the "ATARI" label).
In all instances, you'll see a hole not plugged by a Phillips head screw or a black rubber foot. You should be able to fit a very narrow (5/64" or so) flat screwdriver blade into this hole to engage the slot in the tuning control. But... don't go fiddling with it just yet!
A Little BASIC First
Boot up your computer with BASIC installed. Type in SETCOLOR 1,0,0 to get black characters, then type SETCOLOR 2,1,10. Adjust your color TV or monitor to produce a clean, bright yellow (about the color of a number 2 pencil) using the HUE or TINT controls. This is the same color my APACVIEW program creates as a reference background. It shouldn't be too orange, or show any tint of green. Now type in the following BASIC program:
4 REM COLRTUNE BY JEFF POTTER
5 REM ATARI CLASSICS COLOR TUNING
6 REM UTILITY: ADJUST POT UNTIL
7 REM BEST TRANSITION BETWEEN GREEN
8 REM AND ORANGE OCCURS IN CENTER
9 REM OF SCREEN.
10 GRAPHICS 11:N=8
20 FOR K=0 TO 79
30 N=N+0.2
40 IF N>15.99 THEN N=1
50 COLOR INT(N)
60 PLOT K,0:DRAWTO K,191
70 NEXT K
80 GOTO 80
Using COLRTUNE
As always, save a copy to disk, then type RUN. COLRTUNE should draw a bar pattern of all 15 colors available from the GTIA chip (except for black which has intentionally been skipped). Just left of center should be a rather sick orange-green color, and just right of center should be a rather pure orange. Now insert the screwdriver in the tuning pot you located earlier, and adjust it carefully.
You should try to match the color just left of center so that it is just a tiny bit greener than the color just right of center. When perfectly tuned, the screen will be a continuum of colors, with no abrupt change at the center, no missing colors, and no duplicate colors. Your tuning pot only goes once around, so don't try to turn it too far left or too far right.
On some Ataris, a metal screwdriver detunes the color circuit, so don't try to make a decision on the colors with the screwdriver inserted. Even though you're poking into your Atari with a metal screwdriver, you stand very little chance of getting shocked: only very low voltages (+5 volts) exist inside your computer. (This is about what three fresh flashlight batteries in series generates and isn't considered dangerous.)
If you follow these simple instructions, your Atari will produce all the colors and hues it was designed for. Keep the program handy and "tweak" it up periodically, as the setting will tend to drift as your machine gets older. If you're interested in details on how the color adjust circuit in your computer works, see Ben Poehland's article "Color And The Clock" immediately after this one. Good luck!
[Editor's Note: Several of the Editors tried Jeff's COLRTUNE and found it effective and easy to use. We'll include it on the February '94 AC Software Disk, although this program is so short we think most of you will want to just go ahead and type it in. -BP]
