[Coco] Re: CoCo Monitor

Robert Gault robert.gault at worldnet.att.net
Thu Jun 24 19:09:15 EDT 2004

Kevin Diggs wrote:
> Hi,
> 	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?
> 					kevin

Artifact colors are not "generated" in the Coco, so the 555 timer is not 
"creating" them. Artifact colors are a property of the NTSC color system.

On US TVs, there is a color circuit running at a frequency similar to 
the pixel spread on a PMODE4 graphics screen. When the Coco is in PMODE4 
SCREEN1,1 the color carrier in the Coco is turned off, turning off color 
on the TV. Under these conditions, the rapidly changing dot patterns at 
the PMODE4 frequency cause the TV to think it is seeing color carrier 
information and a false (artifact) color is generated. These colors 
can't be seen on an RGB monitor.

Take your Coco2 and in PMODE3,1:SCREEN1,1 draw vertical lines at every 
other horizontal position. Now issue a PMODE4,1 command without a 
SCREEN. You should see either a full red or blue screen.

By choosing various horizontal dot patterns, it is possible to get on a 
PMODE4 screen, white, black, red, blue, yellow, green, and several other 

I'm not sure but artifact colors may not work on the PAL TV system. At 
the very least, the colors would be quite different.

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