[Coco] Re: CoCo Monitor

Roger Taylor rtaylor at bayou.com
Sat Jun 26 18:27:52 EDT 2004

At 05:20 PM 6/24/2004, you wrote:
>         What are "artifact colors"? Does this have anything to do with
>the 555 timer chip that is hooked to one of the color lines from the
>6847 in my deuce?

I'm sure you'll get lots of answers for this one.  I first really noticed 
artifact colors with my first CoCo 2 hooked to a color TV.  Even though the 
PMODE4 (256x192) graphics mode was a 2-color mode, usually black and white 
only or black and green, the DITHERING of pixels would simulate wild 
colors, depending on the spatial frequency/pattern, etc.  The most famous 
dithering pattern we've seen on the CoCo is the "every other pixel" pattern 
which will produce red or blue.  In other words, if you plot the colors 
from left to right and fill the entire screen with 
black/white/black/white/black/white.... and so on, the resulting image 
would appear to be solid blue (or red).  This is why some games ask you to 
hit the reset button if the screen is red (or blue) and hit ENTER when it 
appears to be a certain color that the game was designed to look best 
with.  Why the CoCo outputs red or blue instead of shades of gray when you 
dither black and white, I don't know, but the color TVs and monitors are 
somehow seeing false colors in the video signal.

Roger Taylor

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